Mechanical Ventilation

I. Indications for respiratory intervention

A. protection of upper airway

B. relief of airway obstruction

C. improved pulmonary toilet

D. refractory cardiogenic pulmonary edema

1. hypotension in the face of acute cardiogenic pulmonary edema

2. left ventricular failure unresponsive to medical support

3. non-resolving ischemia

E. respiratory failure

1. there are no hard and fast rules to determine who should be intubated or receive non-invasive support (CPAP or BiPAP) and in general, there is no substitute for an experienced physician or respiratory therapist at the bedside

a) during a respiratory arrest, the need is apparent

b) if the history suggests rapid deterioration, then it is better to intubate early before the patient's condition deteriorates, making intubation more likely to be associated with complications

c) in the face of myocardial ischemia, the added work of breathing in even moderate respiratory failure can substantially worsen ischemia; a young person with a healthy heart can tolerate tachypnea and moderate hypoxemia much better than an older patient with an impaired heart

d) in general, a pO2 < 50 while the patient is receiving supplemental oxygen is an indication for ventilatory support

II. Non-invasive ventilation (Hillberg, 1997; Meyer, 1994)

A. advantages:

1. allows speech

2. ideal for patients with nocturnal hypoventilation

3. avoids many of the complications of endotracheal intubation

B. disadvantages:

1. patients must be NPO while receiving ventilation

2. lack of airway protection

a) if emesis occurs, aspiration is a risk

b) secretion removal can be a problem

3. patient must be alert with normal respiratory drive

4. cannot fully sedate patients

5. slower correction of blood gas abnormalities

6. upper airway must be intact

7. cannot provide full (100%) ventilatory support

8. some patients find the apparatus intolerably uncomfortable

C. patients likely to benefit:

1. respiratory failure likely to be reversible within 24-48 hours:

a) acute reversible cardiogenic pulmonary edema

b) acute exacerbation of COPD

c) post-op respiratory failure

d) laryngeal edema

2. chronic use in progressive respiratory failure:

a) neuromuscular disease with respiratory muscle weakness

b) lung transplant candidates awaiting donor

3. acute respiratory failure in patients who do not wish to be intubated:

a) terminally ill with respiratory failure

b) AIDS

D. patients less likely to benefit:

1. adult respiratory distress syndrome

2. acute pneumonia with rapidly progressive course

3. obtunded patients

4. unstable or severe acute systemic disease

a) severe hypoxemia (ie requiring 100% oxygen) - these patients are very difficult to safely intubate if their condition deteriorates

b) diseases accompanied by hemodynamic instability

(1) hypotension

(2) significant arrhythmia

c) acute intraabdominal process

d) upper GI bleed

e) sepsis

E. supportive evidence:

1. Brochard, 1990

a) 13 patients with exacerbation of COPD using modified BiPAP

b) pCO2 fell from a mean of 68 to 55

c) 8% of patients required mechanical ventilation vs. 85% of controls

d) non-invasive ventilation associated with fall in respiratory rate, rise in pO2, and fewer days in the ICU

e) death rate similar to intubated patients

2. Meduri, 1991

a) 19 patients with a variety of causes of respiratory failure (13 had exacerbation of COPD) using modified BiPAP

b) overall, 5/19 required intubation

c) mean pCO2 fell from 75-59

3. Bott, 1993

a) randomized, controlled study using modified volume ventilation via a mask

(1) 30 patients with exacerbation of COPD

(2) 30 controls (conventional treatment)

b) mean pCO2 fell from 64- 54

4. Meyer, 1994

a) review of published data regarding non-invasive ventilation to 1994

b) overall success rate in avoiding intubation Å 67%

c) mean pCO2 was reduced from 70 to 55

d) total of 10 patients with congestive heart failure

e) success rate in avoiding intubation in patients with CHF = 70%

5. Confalonieri, 1996

a) historical case control study of 24 patients receiving full face mask non-invasive ventilation

b) 2/24 non-invasive ventilation patients required intubation

c) 9/24 standard treatment patients required intubation

d) long term survival was lower in the non-invasive ventilation group (50% vs. 71%)

6. Antonelli, 1998

a) prospective, randomized controlled study using intubation and conventional mechanical ventilation compared to early use of non-invasive ventilation with pressure support plus CPAP in patients with hypoxemic acute respiratory failure

b) survival was better in the non-invasive group (72% vs. 53%; p = 0,19)

c) complications were fewer in the non-invasive group (38% vs. 66%; p = 0.02)

d) ICU stays were shorter in the non-invasive ventilation group (6.6 days vs. 14 days; p = 0.002)

F. specific types:

1. CPAP

a) background:

(1) this is the best technique for patients with obstructive sleep apnea

(2) this is usually the best non-invasive technique for most cardiac patients who require ventilatory assistance but do not require immediate intubation

(3) patient continuously receives a set air pressure, during both inspiration and expiration

(4) the patient has full control over respiratory rate, inspiratory time, and depth of inspiration

(5) there are a number of advantageous effects of the increase in pressure at end-expiration:

(a) increase in the functional residual capacity (FRC)

(b) decreased afterload to the left ventricle

(c) increased patency of alveoli (especially those atelectatic due to pulmonary edema or poor inspiratory effort)

b) technique:

(1) select mode of delivery: nasal vs. face mask

(a) nasal is often better tolerated for chronic use but in the acute setting, face masks may give more uniform pressure delivery

(2) begin at 5 cm H2O and increase in increments of 5

(3) if the patient requires 20 cm pressure or more, they probably should be intubated

2. BiPAP

a) background:

(1) this system (marketed by Respironics) provides a set inspiratory pressure and a different set expiratory pressure

(2) the patient has full control over the respiratory rate, inspiratory time, and depth of inspiration

(3) the unit senses the beginning and end of inspiration so that the delivered pressure can be changed from the expiratory pressure to the inspiratory pressure and vice versa

(4) the simplistic way of conceptualizing BiPAP is CPAP plus a boost during inspiration (much like you are inhaling in front of a strong fan which is set at high speed during inspiration and low speed during exhalation)

(5) because of the higher cost and complexity, CPAP is usually preferable to BiPAP in patients except in the following situations:

(a) CPAP alone is insufficient to improve dyspnea or hypoxemia

(b) the patient has respiratory muscle fatigue

(c) the patient has underlying COPD

(d) the patient has a high pCO2

b) technique:

(1) select a properly fitting mask; in general, this will be the smallest mask which comfortably fits the patient

(a) some air leak around the mask is okay

(2) avoid NG tubes - the lower esophageal sphincter will usually remain closed as long as the airway pressure is < 30 cm H2O

(3) place a piece of wound care dressing on the bridge of the nose to prevent abrasion

(4) select and place a headstrap and attach it to the patient and the mask

(a) do not set this too tight; you should be able to fit two fingers underneath the strap

(5) initial settings:

(a) inspiratory airway pressure (IPAP): 8 cm H2O

(b) expiratory airway pressure (EPAP): 4 cm H2O

(c) most patients will tolerate the unit best if placed on the "spontaneous" mode rather than the "spontaneous/timed" mode or the "timed" mode

i) the "timed" modes deliver the increased inspiratory pressure independent of the patient and can be uncomfortable for the awake, spontaneously breathing patient

(d) match the previous supplemental oxygen rate; in general, 2-5 liters/minute is a reasonable starting point

(6) adjustments:

(a) increase IPAP by 2 cm H2O at a time to reduce pCO2, reduce dyspnea, reduce accessory muscle use, or reduce stridor

(b) increase EPAP by 2 cm H2O at a time to increase FRC and thus increase pO2 in patients with poor pulmonary compliance

(c) the maximum IPAP (and EPAP) is 20 cm H2O, however, few patients will tolerate this high of pressure

i) in general, if the patient requires an IPAP of more than 15 cm H2O, strong consideration should be given to intubation

3. Full face mask ventilation

a) background:

(1) adminstered with any ventilator capable of doing pressure-support mode

(a) can also be done as a volume cycled ventilator but with volume ventilators there is a higher incidence of barotrauma, aspiration, and gastric insufflation

(2) generally used with a full face mask as opposed to BiPAP which is usually used with a nose mask

(3) as with any non-invasive mode of ventilation, patients should be reasonably alert and able to protect their airway

(4) because of higher cost and greater complexity than BiPAP, it is generally relegated to patients patients in an ICU who require greater ventilatory support than can be achieved with BiPAP

(a) higher flow rates and higher pressures can be achieved with full face mask ventilation compared to BiPAP

b) technique:

(1) select a properly fitting mask

(2) NG tubes can be placed if necessary but are generally preferred to be avoided

(3) place a piece of wound care dressing (such as duoderm) on the bridge of the nose to minimize tissue necrosis from the tightly fitting mask

(4) initial settings:

(a) oxygen at percentage sufficient to keep the pO2 > 55

(b) start at a pressure support level of 10 cm

(c) CPAP can be administered at an initial setting of 5 cm if desired

(5) adjustments:

(a) adjust the pressure support level up or down by 2 cm per adjustment to achieve an average tidal volume of 7-10 cc/kg body weight

(b) generally, the pressure support should not be > 25 cm

i) there is a greater chance of tissue necrosis and gastric insufflation at this level of pressure support

(1) an NG should be placed to minimize gastric insufflation and aspiration if this degree of pressure support is required

ii) if greater pressure support levels are required, consider intubating the patient

III. Positive pressure mechanical ventilators

A. modes:

1. assist-control

a) background:

(1) ventilator provides full tidal volume at a minimum preset rate

(2) additional full tidal volumes given if the patient initiates extra breaths

b) advantages:

(1) provides near complete resting of ventilatory muscles

(2) can be effectively used in awake, sedated, or paralyzed patients

c) disadvantages:

(1) patients can hyperventilate and become alkalotic

(2) patients can "stack" breaths (air trapping) and develop barotrauma

(3) patients can develop "autoPEEP" with barotrauma or hypotension

d) initial settings:

(1) tidal volume = 8-10 ml/kg (6 ml/kg in ARDS)

(2) rate = 12

(3) FiO2 = 1.0

(4) peak inspiratory flow = 60 liters/minute

2. synchronized intermittent mandatory ventilation (SIMV)

a) background:

(1) ventilator provides set tidal volume at a preset rate

(2) when a ventilator breath is programmed to occur, the ventilator waits a predetermined trigger period; any patient-initiated breath during this trigger period results in a programmed ventilator delivered breath

(3) the patient can take additional breaths but tidal volume of these extra breaths is dependent on the patient's inspiratory effort

b) advantages:

(1) in theory, results in improved blood return to the right ventricle owing to intermittent negative pressure (spontaneous) breaths

(2) patients often more comfortable since they have more control over their ventilatory pattern and minute ventilation

c) disadvantages:

(1) can result in chronic respiratory fatigue if set rate is too low; in this situation, the following may be seen:

(a) high respiratory rate

(b) rising pCO2

(2) air trapping can occur

d) initial settings:

(1) tidal volume = 8-10 ml/kg (6 ml/kg in ARDS)

(2) rate = 12

(3) FiO2 = 1.0

(4) peak inspiratory flow = 60 liters/minute

3. pressure support (Dekel, 1996)

a) background:

(1) not a volume-cycled mode

(2) when the patient triggers the ventilator, a set pressure (1-100 cm) during the patient's inspiration; this air pressure is stopped when the patient ceases to inspire

(3) tidal volume and minute ventilation are dependent on the patient

b) advantages:

(1) avoids patient-ventilator aschrony

(2) patients often more comfortable since they have full control over their ventilatory pattern and minute ventilation

(3) often avoids breath stacking and autoPEEP (especially in patients with COPD)

(4) ability to permit self-determination of respiratory rate and to permit forced exhalation offers substantial advantages in status asthmaticus (Meduri, 1996)

c) disadvantages:

(1) cannot be used in heavily sedated, paralyzed, or comatose patients

(2) respiratory muscle fatigue can develop if the pressure support is set too low

d) initial settings:

(1) set pressure support at the pressure required to generate a tidal volume of 8-10 ml/kg

(a) this will usually be about the same as the plateau pressure

(b) 30-35 cm will usually give full ventilatory support

(2) FiO2 = 1.0

4. pressure control (Marik, 1997)

a) background:

(1) the breath is pressure limited rather than volume limited

(2) best reserved for patients with ARDS

b) advantages:

(1) in ARDS, pO2 may increase 10-15%

c) disadvantages:

(1) there is no guaranteed tidal volume and thus there is no guaranteed minute ventilation

(2) you must be very attentive to changes in the patients respiratory mechanics since unstable reactive airways disease can dramatically affect minute ventilation

(3) air trapping can be a problem

(4) CO2 retention frequently occurs (although this may be acceptable in "permissive hypercapnia" strategies for ventilation of some patients with acute respiratory failure)

(5) in general, patients must be heavily sedated since this is an uncomfortable mode for most patients

d) initial settings:

(1) set PEEP (generally 7 - 12)

(2) set maximum inspiratory pressure (generally < 35 cm)

(3) set rate (this will vary in order to achieve a desired minute ventilation)

(a) real time flow-time curves can facilitate optimization of pressure control settings

(4) FiO2 = 1.0

5. positive end-expiratory pressure (PEEP)

a) background:

(1) ventilator provides a fixed positive airway pressure at the end of expiration

(2) when used with assist-control ventilation, the term PEEP is used

(3) when used with spontaneous breathing, the term CPAP (continuous positive airway pressure) is used

b) advantages:

(1) opens closed alveolar units thus improving lung compliance and oxygenation

(2) to a point, peak and plateau airway pressures actually decrease since there are more alveoli open at the beginning of inspiration

(3) may improve secretion drainage from otherwise closed alveoli

(4) can reduce right ventricular venous return and also lower left ventricular afterload

(5) can be given on the ventilator or via a CPAP mask in the non-intubated patient

c) disadvantages:

(1) barotrauma

(2) can be risky and counterproductive in patients with obstructive airways disease

(3) may worsen hypoxemia in patients with localized (as opposed to diffuse) lung disease (eg, pneumonia)

(4) hypotension and reduced cardiac output

(5) increased cardiac shunt (especially PFOs)

(6) increased intracranial pressure

(7) decreased renal perfusion

(8) hepatic congestion

d) initial settings:

(1) 5 cm is fairly standard and in many hospitals is used on most patients initially placed on the ventilator

(2) changes in PEEP may not be reflected by changes in arterial blood gases for 20-30 minutes so changes in the PEEP setting should usually not be made faster than this

(3) the ventilator circuit should not be broken unless absolutely neccessary; disconnecting the patient (for example, to transport him/her) can result in an immediate loss of the benefit of PEEP which can require 20-30 minutes or more to restore

(4) PEEP is added in increments of 2-5 cm until the "best PEEP" is obtained

(a) there is no optimal way to assess "best PEEP"

(b) some PEEP authorities choose the level which provides the highest static compliance (in other words, the lowest airway plateau pressure)

(c) in general, use the lowest amount which gives the desired effect on pO2 without lowering blood pressure, reducing cardiac output, or increasing the plateau pressure on the ventilator

(5) PEEP over 20 cm is rarely beneficial and usually results in additional pressure-induced lung injury

6. airway pressure release ventilation (APRV) (Downs, 1987)

a) background:

(1) ventilator supplies a low level of CPAP alternating with a relatively high level of CPAP

(2) it can be coupled with pressure support

(3) at intermittant times the higher CPAP level is reduced for 1 or more breaths, permitting rapid exhausting of the gas in the expiratory reserve volume and resulting in a higher tidal volume

(4) when used in a patient who is not spontaneously breathing, this mode is no different than pressure control ventilation

b) advantage:

(1) potential for barotrauma and overdistension is reduced

(2) venous return is preserved

(3) permits spontaneous breaths

c) disadvantages:

(1) requires the use of relatively high CPAP levels

(2) CPAP reductions can result in hypoxemia

(3) better for post-op and mildly diseased lungs and role in severe respiratory failure is unclear

B. strategies for the difficult to ventilate patient:

1. patients with high plateau airway pressures (> 35 cm)

a) causes:

(1) excessively high tidal volume

(2) pneumothorax or massive pleural effusion

(3) microatelectasis (due to ARDS, etc.)

(4) cardiogenic pulmonary edema

(5) secretions or foreign body in the airway

(6) kinking or obstruction of the endotracheal tube or ventilator tubing

(7) ascites or abdominal distension

(8) bronchospasm

(9) severe intrinsic lung disease (pulmonary fibrosis, etc.)

(10) patient "fighting" the ventilator

b) treatment:

(1) insure that the patient is optimally suctioned and that all bronchospasm is being treated

(a) when giving bronchodilators by MDI to ventilated patients, more puffs should be given - generally 4 puffs of albuterol; attention should be given to the manner that MDIs are introduced into the ventilator circuit since directing the MDI spray toward the wall of the ventilator tubing drastically reduces the delivered dose (Fink, 1996)

(2) check a chest x-ray to insure that there is no pneumothorax

(3) treat the underlying cause of stiff lungs (diuresis for pulmonary edema, antibiotics for diffuse pneumonia, etc.)

(4) insure that total body oxygen requirements are maximally reduced:

(a) treat fever

(b) sedate patients who are moving a great deal

(c) treat infection

(5) optimize oxygen delivery

(a) transfuse patients with severe anemia (the optimal hemoglobin in acute respiratory failure has not been determined and probably depends on the individual patient but pending definitive studies, a useful rule is to keep the hemoglobin > 9 gm)

(6) add PEEP to the "best PEEP" level to optimally open otherwise closed alveoli during expiration and therefore reduce the pressure required to open these alveoli during inspiration

(7) reduce peak inspiratory flow rate

(a) this will effectively increase inspiratory time and decrease expiratory time

(b) in its extreme, it results in "inverse ratio ventilation" with an I:E ratio < 1:1 (a normal ventilated patient has an I:E ratio of about 1:2 or 1:3)

(8) reduce tidal volume

(a) if the respiratory rate is unchanged, this can result in so called "controlled hypoventilation" when the minute ventilation is allowed to decrease below the patient's natural minute ventilation (by either a decrease in the tidal volume or a decrease in the respiratory rate); this can result in an increase in pCO2 ("permissive hypercapnia") and usually requires the patient to be heavily sedated since hypercapnia can be quite uncomfortable

(b) the lowest tolerable pH during controlled hypoventilation is unclear but pending definitive studies, it is probably desirable to keep the pH > 7.20 during hypoventilation

(9) sedation and/or paralysis (see below)

(10) change to pressure control ventilation mode

2. patients with persistent hypoxemia despite FiO2 > 50%

a) causes:

(1) underlying intrinsic lung disease (pneumonia, pulmonary edema, etc.)

(2) central airway obstruction (bronchospasm, secretions, foreign body, neoplasm, etc.)

(3) intra cardiac shunt (especially patent foramen ovale)

(4) pulmonary embolus

(5) other pulmonary vascular disease (pulmonary hypertension, etc.)

(6) autoPEEP

b) treatment:

(1) suction the airways to remove secretions, etc.

(2) administer bronchodilators if indicated

(3) therapeutic bronchoscopy if there is evidence of atelectasis or lobar collapse

(4) add PEEP to the optimal level

(5) diagnose and treat underlying causes (above), especially pulmonary embolus

(6) in patients with severe ARDS or alveolar hemorrhage, consider placing the patient in the prone position as this can improve oxygenation by up to 15% in these situations (Chatte, 1997)

3. patients with autoPEEP

a) causes:

(1) fundamentally, this is the pressure associated with an end-expiratory volume which is higher than the normal functional residual capacity (FRC)

(a) this is most commonly seen in patients with obstructive airways disease in whom there is insufficient time for full exhalation resulting in "air trapping" and associated progressive hyperinflation

(b) it can also occur in persons without obstructive airways disease who have insufficient time for normal exhalation (eg, hyperventilation or too high of a tidal volume)

(2) autoPEEP should be suspected when:

(a) any patient with obstructive airways disease is receiving mechanical ventilation

(b) any patient with unexplained hypotension after initiating mechanical ventilation

(c) any patient with unexplained tachycardia after initiating mechanical ventilation

(3) autoPEEP can be inferred by any of three findings:

(a) an expired air volume that is less than the inspired volume (trapped air)

(b) a flow-time graphic (available on many newer ventilators) showing that flow never reaches 0 before the next breath (in other words, the patient is still expiring when the next breath is delivered)

(c) chest auscultation demonstrating that expiratory noises (wheezing, etc.) are audible all the way up until the next breath is delivered

(4) cannot be measured directly from the ventilator dials

(a) autoPEEP represents the pressure in the alveoli and not the pressure in the ventilator circuit; therefore, there can be life-threatening autoPEEP despite an end-expiratory pressure of 0 on the ventilator pressure meter (Leatherman, 1996)

(b) the expiratory port of the ventilator circuit must be occluded in order to allow the pressure within the airway to equilibrate and register on the pressure dial

(c) autoPEEP can be heterogeneous so that some areas of the lung have normal emptying of alveolar air and other areas can have severe effects of autoPEEP

b) treatment:

(1) reduce tidal volume

(2) increase peak inspiratory flow (allowing more expiratory time)

(3) reduce respiratory rate

(4) sedate the hyperventilating patient

(5) permit controlled hypoventilation (see below)

(6) treat any bronchospasm

(7) consider switching to pressure support mode

4. patients with ARDS

a) a low tidal volume strategy of 6 ml/kg was associated with a 22% improvement in mortality (31% vs. 39%) and a greater number of ventilator-free days in the first 4 weeks of hospitalization (12 vs. 10) compared to a traditional/high tidal volume ventilation strategy of 12 ml/kg. There was also less multiple organ system failure and lower blood interleukin-6 levels in the low tidal volume group (ARDS network, N Engl J Med 2000)

b) pressure control ventilation can be useful on occasion

c) PEEP needs to be judiciously adjusted and can result in substantially improved oxygenation

d) prone ventilation can result in improved oxygenation

C. Sedation and Paralysis in the Intensive Care Unit

1. Introduction:

a) Appropriate use of sedatives and analgesics can greatly facilitate the care of patients in the ICU by improving rest and relieving suffering.

b) Appropriate use of paralytics can greatly facilitate mechanical ventilation in these many of these patients.

c) Over-dosing of these agents or selection of an inappropriate agent can result in a prolongation of mechanical ventilation and or the ICU stay which in turn can result in thousands of dollars of extra costs to the patient.

d) Accurate knowledge of the onset of action, duration of action, side effects, monitoring, and dosing of these agents is vital to the appropriate use of them.

2. Sedatives (Society of Critical Care Medicine: "Practice Parameters For Systemic Intravenous Analgesia And Sedation For Adult Patients In The Intensive Care Unit"; September, 1995)

a) General principles:

(1) use non-pharmacologic measures whenever possible:

(a) establish regular sleep-wake cycles

(b) minimize stimulation during sleep

(c) reassure patient and provide general comfort

(d) use opiates as needed for pain; sedatives are a poor substitute for analgesics when pain is the primary problem facing the patient

(2) identify withdrawl symptoms which can contribute to agitation

(a) narcotics

(b) benzodiazapines

(c) caffeine

(d) nicotine

i) nicotine patches are underutilized in the ICU

(3) there are no sedation scales which have been validated for use in adults making objective assessment of the level of sedation difficult

(a) the "Ramsey Score" is often used as a bedside estimate of the degree of sedation

(b) bispectral EEG may be useful in the future to titrate sedation

(4) the concurrent use of narcotics can enhance the effectiveness of these agents and often allow for reductions in the dose of both classes of drugs

(5) Sedative infusions should be stopped once daily in all patients. In a study of 128 patients receiving mechanical ventilation who are receiving sedative infusions, half the patients had a once daily interruption of sedative infusions and half had management of sedation as per the attending physician's routine. Daily sedative infusion interruption resulted in a reduction in duration of mechanical ventilation (4.9 vs. 7.3 days) and a reduced length of stay in the ICU (6.4 vs. 9.9 days).

b) Specific agents:

(1) Benzodiazapines:

(a) general properties:

i) provides sedation

ii) induces antegrade amnesia

iii) no analgesic properties

iv) can be reversed with flumazenil (0.3 - 0.5 mg IV)

(b) specific agents:

i) lorazepam (Ativan)

(1) general properties:

(a) slow onset of action (10-20 minutes)

(b) intermediate half life (6 hours)

(c) less lipophilic than diazapam and therefore does not accumulate in tissues as much as diazepam and is less likely to exhibit prolonged sedation

(d) no active metabolites

(e) elimination not affected by renal or hepatic failure

(f) price recently fell dramatically after the patent ran out and generic lorazapam became available

(2) side effects:

(a) respiratory depression

(3) use:

(a) good for intermittent bolus administration; can also be used as a drip - especially if prolonged sedation is anticipated

(b) usual starting dose = 2-4 mg Q 2-4 hours (can go up to 10 mg/hr)

ii) midazolam (Versed)

(1) general properties:

(a) rapid onset of action (usually 2-5 minutes)

(b) relatively short half life ( < 2 hours), however, in critically ill patients, it can accumulate and result in sedation for many hours or even days after discontinuation

(2) side effects:

(a) respiratory depression

(b) there is no difference between lorazepam and midazolam in terms of time to awakening after prolonged continuous IV infusion because midazolam accumulates in fat (Deppe, 1994)

(3) use:

(a) can be used as a continuous drip but it is costly

(b) usual dose = 2 mg IV, increase dose by 2 mg every 3-5 minutes until desired level of sedation achieved

iii) diazepam (Valium)

(1) general properties:

(a) peak effect seen in Å 3-5 minutes

(b) duration of action extended because of active metabolites

(2) side effects:

(a) respiratory depression

(b) effects of an initial dose abate rapidly because of redistribution to peripheral tissues; sustained effect seen with repeated administration

(c) active metabolites have half lives of up to 200 hours

(3) use:

(a) rarely used because of:

i) long half life

ii) difficulty maintaining in solution for prolonged drip administration

(b) best reserved for intermittent administration when patient is anticipated to require very prolonged sedation

(c) dose = 2-5 mg Q 5-10 minutes until desired level of sedation achieved

(2) Propofol (Diprovan):

(a) general properties (Mirenda, 1995):

i) no analgesic properties

ii) very rapid onset of action (1-2 minutes)

iii) very short half life (10-15 minutes)

(b) side effects:

i) very lipophilic; requires a dedicated central IV line

ii) respiratory depression

iii) bradycardia

iv) pain at infusion site (central lines preferred)

v) in high doses, it can result in clinically significant hypertriglyceridemia due to the emulsifying lipids

(1) care must be taken to avoid accidental contamination of the lipid by unusual infectious organisms (Bennett, 1995)

(a) change bottles and tubing Q 12 hours

vi) may cause a 20-30% fall in systolic blood pressure in some patients

vii) use cautiously in patients with increased intracranial pressure

(c) use:

i) only useful as a continuous infusion

ii) usual dose:

(1) 0.5 mg/kg/hr

(2) increase by 0.3-0.6 mg/kg/hr every 5 or 10 minutes

(3) usual maintenance dose = 0.5 - 3 mg/kg/hr

(3) Barbiturates:

(a) general properties:

i) anticonvulsants

ii) may reduce intracranial pressure in some patients

iii) cause sedation

iv) lack analgesic properties and lack amnestic properties

(b) side effects:

i) cause histamine release & depression of central vasomotor centers

(1) tachycardia

(2) hypotension

(3) myocardial suppression

ii) stimulate hepatic enzymes to degrade other drugs

iii) causes respiratory depression

iv) patients can develop tolerance to effects

(c) use:

i) rarely used for sedation purposes because of hemodynamic effects

(4) Etomidate:

(a) general properties:

i) analgesic and hypnotic properties

ii) minimal cardiovascular and respiratory depression

(b) side effects

i) causes adrenal suppression in as little as 1 or 2 doses

(c) use:

i) mainly for short procedures, such as intubation

ii) adrenal suppression limits its use in the ICU

(5) Ketamine:

(a) general properties

i) analgesic properties

ii) little respiratory depression

iii) intrinsic bronchodilator properties

(b) side effects:

i) associated with bad dreams ("disassociative state")

ii) increases intracranial pressure

iii) increases blood pressure

iv) excess secretions (can be blocked with pre-medication using atropine)

(c) use:

i) primarily used in conjunction with a benzodiazapine in patients with status asthmaticus in whom the bronchodilator properties can be useful

ii) also useful for temporary analgesia & sedation during brief, painful procedures such as dressing changes

iii) dose:

(1) load with 0.75 mg/kg

(2) maintenance = 0.15 mg/kg/hr

(6) Haloperidol:

(a) general properties:

i) lessens agitation, anxiety, aggression

ii) has no sedative effects

iii) does not depress respiratory centers

iv) onset of action usually 30-60 minutes

v) half-life = 10-25 hours

(1) prolonged in hepatic [but not renal] failure

(b) side effects:

i) extrapyramidal symptoms (less if benzodiazapine is concurrently used)

ii) prolongation of QT interval

(c) use:

i) not FDA approved for IV use although it is usually used this way in the ICU

ii) 2-10 mg every 30 minutes until delirium controlled or maximum dose of 50-60 mg is reached

iii) use about 50-75% of loading dose every 24 hours to control delirium

(7) sedative costs per day (average wholesale costs)

(a) diazepam $11-44

(b) lorazepam $45-69

(c) midazolam $90-180

(d) propofol $54-325

(e) haloperidol $6-72

3. Analgesics

a) Opiods:

(1) general properties:

(a) provides sedation

(b) no amnestic properties

(c) good analgesic properties

i) tolerance often develops after Å 3-4 days

(d) can be reversed with naloxone

(e) side effects:

i) hypotension can be a problem

(1) causes:

(a) vagal-induced bradycardia

(b) increased venous capacitance

(c) increased histamine release

(2) worse if the patient is hypovolemic

ii) gastric slowing can be a problem

iii) respiratory depression

(1) in non-intubated patients in the ICU, consideration should be given to continuous pulse oximetry monitoring

iv) central vagal stimulation often results in bradycardia

v) withdrawl symptoms can occur if the drugs are used for more than 7 days; this can be avoided by tapering the drug by 25% of the original dose every 12-24 hours

(2) specific agents:

(a) morphine

i) general properties:

(1) relatively slow onset of action (Å 30 minutes)

(2) half-life = 1/5 - 2 hours

(a) may be prolonged in renal or hepatic failure

(b) active metabolite (morphine-6-glucuronide) can accumulate in renal failure

ii) side effects:

(1) histamine release

(2) respiratory depression

(3) active metabolites accumulate in renal failure

iii) use:

(1) this is the preferred analgesic agent in the ICU

(2) initial dose = 2-5 mg IV

(3) subsequent dose = 2-10 mg/hr

(b) hydromorphone

i) general properties:

(1) more potent than morphine

(2) more sedating than morphine

(3) safer to use in renal failure than morphine

ii) side effects:

iii) use:

(1) 1-2 mg Q 1-2 hours

(c) fentanyl

i) general properties:

(1) more potent (100 times) than morphine

(2) more rapid onset of action and longer duration of action than morphine

(3) half-life = 30-60 minutes

(a) with prolonged administration, half-life can increase to 9-16 hours

(b) accumulates in patients with hepatic dysfunction

(4) less histamine release than morphine (and thus fewer cardiovascular side effects)

(5) dose usually does not require alteration in renal or hepatic failure

ii) side effects:

(1) chest wall rigidity unresponsive to mechanical ventilation but reversible with naloxone when administered by rapid IV push - avoidable when administered over 30 seconds

iii) use:

(1) preferred over morphine for analgesia when the patient is hemodynamically unstable or having bronchospasm

(2) load with 25-100 ug then 25-100 ug/hr

(d) meperidine (Demerol)

i) general properties:

(1) less respiratory depression than morphine

ii) side effects:

(1) histamine release

(2) it's metabolite (normeperidine) can accumulate with repeated dosing causing CNS excitation (apprehension, tremors, seizures)

(a) accumulates more in renal failure

iii) use:

(1) no advantage over morphine for ICU sedation for most patients and excessive side effects when used long term substantially limit the usefulness of Demerol in the ICU

(3) narcotic costs per day (average wholesale costs)

(a) morphine $6-9

(b) hydromorphone $4-15

(c) fentanyl $23-69

b) Tricyclic antidepressants:

(1) mainly used for neuropathic pain (eg, Guillian-Barré syndrome)

c) Non-steriodal anti-inflammatory agents

(1) general properties:

(a) analgesic

(2) side effects:

(a) anti-platelets effects

(b) gastrointestinal ulceration

(c) renal insufficiency

(d) cover up fever

(3) use:

(a) rarely used in the ICU for analgesia because of undesirable side effects

(b) may be occasionally useful in select patients, especially COX-2 inhibitors

4. Neuromuscular blocking agents (Society of Critical Care Medicine: "Practice Parameters For Systemic Intravenous Analgesia And Sedation For Adult Patients In The Intensive Care Unit"; September, 1995)

a) The neuromuscular junction:

(1) acetylcholine is produced in nerve terminals by acetylation of choline by choline acetylase

(2) nerve impulses cause release of acetylcholine

(3) acetylcholine binds to nicotinic cholinergic receptors on muscle

(4) depolarizing agents bind to receptor and prevent normal closure of receptor with loss of intracellular potassium

(a) these agents cause fasiculations as the drug initially takes effect

(5) non-depolarizing agents block acetylcholine from interacting with the receptor

(a) these agents do not cause fasiculations

(6) extrajunctional receptors proliferate during inactivity (trauma, immobilization, stroke, spinal cord injury, burns) and can result in massive potassium shifts out of muscle cells when channels are opened with depolarizing blockers

(a) avoid succinylcholine in these settings

b) Uses:

(1) facilitating intubation

(2) facilitating ventilation

(3) minimizing increases in intracranial pressure by coughing, suctioning, etc.

(4) treating neuroleptic malignant syndrome & tetanus

c) Caution: patients are awake and not sedated by these agents

(1) ALL patients receiving these drugs need concurrent use of sedatives

(2) lack of sedation may be manifest as unexplained hypertension, tachycardia, or diaphoresis

(3) use of a daily neuromuscular blocker holiday facilitates assessment of the adequacy of concurrent sedation

d) Types (Isenstein, 1992):

(1) Depolarizing neuromuscular blockers (succinylcholine):

(a) general properties:

i) onset of action = 1 minute

ii) duration of action = 5-10 minutes

(b) side effects:

i) hyperkalemia, especially in patients with:

(1) burns

(2) flaccid paralysis

(3) patients with malignant hyperthermia

ii) bradycardia

iii) increased secretions

iv) histamine release

(c) use:

i) dose: 1 mg/kg bolus

(1) should NOT be used for continuous paralysis

(2) Non-depolarizing neuromuscular blockers (Hunter, 1995):

(a) advantages:

i) hyperkalemia not a problem

(b) disadvantages:

i) usually have a slower onset of action than succinylcholine

ii) some patients can suffer from a prolonged paralysis syndrome which appears to be more common with vecuronium and more common in patients with either concurrent renal failure or concurrent corticosteroid use (Watling, 1994)

(1) this may be heralded by a rise in the CK and for this reason, the CK should be measured daily in patients receiving prolonged administration

iii) extra caution should be taken to insure that ventilator alarms are always on since the patient is unable to ventilate on their own and can rapidly die if there is an unrecognized ventilator malfunction

iv) patients require specialized nursing care:

(1) eye lubricant & taping lids shut to prevent corneal ulcers

(2) frequent repositioning to avoid decubitus ulcers and compression neuropathy

(3) liberal use of DVT prophylaxis

(4) frequent range of motion activities to prevent contractures

v) peripheral nerve stimulation is recommended for all patients receiving prolonged paralysis

(1) this is performed by respiratory therapy using the "train of four" protocol

(a) ulnar nerve (use lateral face if hand is edematous)

(b) patients rarely require paralysis associated with less than 2-3 twitches on nerve stimulation

(c) if there are 0 twitches, the patient is overparalyzed

(2) this is only recommended as an adjunct to clinical assessment

(a) in general - use the lowest amount of an agent that provides the desired end-point (ie, facilitating ventilation); this may be associated with a full 4 twitches on train of four assessment but the true end point is the clinical assessment

i) titrate to weakness NOT paralysis

(3) patients should be given a "neuromuscular blockade holiday" and the dose held once per day while receiving non-depolarizing blockers to insure that they are not over paralyzed

vi) these agents provide NO sedation and unless patients are concurrently given a sedative (preferably a benzodiazapine with amnestic properties) then they will be paralyzed but completely awake

(1) unexplained tachycardia in the paralyzed patient should be considered as evidence of inadequate sedation

(2) appropriate use of sedatives will generally reduce the amount of a neuromuscular blocker required for clinical effect by relieving anxiety and causing hypnosis

vii) prolonged use of non-depolarizing agents will result in proliferation of acetylcholine receptors on the myocyte (similar to neuromuscular disease or stroke) - this can result in massive potassium release if succinylcholine is subsequently used; therefore, if a patient has been receiving non-depolarizing agents, they should NOT subsequently receive succinylcholine

(c) specific agents:

i) pancuronium

(1) general properties:

(a) onset of action = 1.5 - 2 minutes

(b) duration of action = 60 minutes

(c) clearance = renal (80%) hepatic (20%)

(d) inexpensive

(e) unless there are contraindication: this is the drug of choice for prolonged paralysis

(2) side effects:

(a) histamine release can be a problem in patients with asthma, left ventricular failure, or arrhythmias

(b) tachycardia, hypotension or hypertension limits use in hemodynamically unstable patients

(c) prolonged paralysis can be seen

(3) use:

(a) best used as an intermittent bolus agent

(b) dose:

i) loading = 100 ug/kg

ii) maintenance =

(1) 10 ug/kg Q 1-2 hours intermittent bolus

(2) not recommended for continuous infusion

ii) vecuronium

(1) general properties:

(a) onset of action = 1.5 minutes

(b) duration of action = 30 minutes

(c) very little histamine release

(d) clearance = hepatic (80%), renal (20%)

(2) side effects:

(a) prolonged paralysis can be seen

(b) occasional hypotension and bradycardia

(3) use:

(a) give as either intermittent bolus or continuous infusion

(b) dose:

i) loading = 100 ug/kg

ii) maintenance =

(1) 10 ug/kg Q 20-30 minutes intermittent bolus

(2) 0.8 - 1.2 ug/kg/min continuous infusion

iii) atracurium

(1) general properties:

(a) onset of action = 2 minutes

(b) duration of action = 30 minutes

(c) expensive

(d) clearance = Hofmann elimination (self destructs in plasma)

i) may be used in renal failure

(e) prolonged paralysis rare

(2) side effects:

(a) significant histamine release

(b) hypotension, tachycardia or bradycardia

(3) use:

(a) less frequently used in the ICU since the availability of cisatracurium

(b) dose

i) loading = 400-500 ug/kg

ii) maintenance =

(1) 100-200 ug/kg Q 20-30 minutes intermittent bolus

(2) 2-15 ug/kg/minutes continuous infusion

iv) cisatracurium

(1) general properties:

(a) onset of action = 3-5 minutes

(b) duration of action = 60-80 minutes

(c) very little histamine release

(d) no dosage adjustments in renal or hepatic failure

(e) if pancuronium is contraindicated, this is the drug of choice for most patients requiring prolonged neuromuscular blockade

(2) side effects:

(a) very rare

(3) use:

(a) dose:

i) loading = 150-200 ug/kg

ii) maintenance =

(1) 30 ug/kg Q 1 hour intermittent bolus

(2) 1-5 ug/kg/minute continuous infusion

v) doxacurium

(1) general principles:

(a) onset of action = 6 minutes

(b) duration of action = 90 minutes

(c) clearance = predominately renal

(d) little or no histamine release

(e) little or no cardiac side effects

(2) side effects:

(a) hypotension

(3) use:

(a) dose:

i) load = 50-80 ug/kg

ii) maintenance =

(1) 5-10 ug/kg Q 30-45 minutes intermittent bolus

(2) not usually used for continuous infusion

(d) approximate daily cost comparisons (based on Ohio State University Hospitals cost)

i) pancuronium = $4-6

ii) vecuronium = $130-200

iii) atracurium = $200-365

iv) cisatracurium = $70-350

v) doxacurium = $50-150

5. Summary - Drugs of Choice:

a) Analgesia:

(1) morphine

(2) fentanyl if the patient is hemodynamically unstable or at risk of histamine release

b) Sedation:

(1) propofol or midazolam - short term sedation (< 24 hours):

(2) lorazepam - long term sedation (> 24 hours):

c) Delerium:

(1) haloperidol

d) Neuromuscular blockade:

(1) pancuronium

(2) cisatracurium if the patient is hemodynamically unstable, has cardiac disease, or at risk of histamine release

IV. Complications of mechanical ventilation

A. complications of intubation:

1. early:

a) hypoxemia due to prolonged attempts

b) right mainstem intubation

c) intubation of esophagus

d) upper airway trauma

e) aspiration

f) hypotension immediately following intubation

(1) this is usually multifactorial and etiologies include:

(a) effect of sedatives and/or paralytics used for intubation

(b) diminished blood return to the thorax due to positive pressure ventilation

(c) auto-PEEP with breath stacking (especially in patients with COPD)

(d) diminished LV output due to compression of the LV by the interventricular septum

(2) treatment:

(a) reduce tidal volume (often gives the most immediate improvement)

(b) fluid bolus (usually all that is required unless breath stacking occurs)

(c) reverse Trendlenberg position (until fluid bolus is given)

(d) reduce PEEP

(e) vasopressors with predominately alpha adrenergic properties (eg, levarterenol)

2. late:

a) cuff leak

b) sinusitis

c) upper airway injury/stenosis

d) unplanned (self) extubation

B. ventilator-related complications:

1. disconnection

2. malfunction

C. suctioning-related complications:

1. hypoxemia

a) patients should always be pre-oxygenated with 100% oxygen prior to suctioning

b) suction time should be limited

2. arrhythmias

D. ventilation-related complications:

1. nosocomial infections (Kollef, 1999)

a) usually gram negative rods and staph

b) factors decreasing pneumonia probability:

1) early removal of NG tubes

2) hand washing

3) semirecumbant positioning of patient

4) adequate nutrition

5) avoid gastric overdistension

6) avoid nasal intubation (as opposed to oral intubation)

7) continuous subglottic suctioning

8) stress ulcer prophylaxis

(a) sucralfate may be associated with less pneumonia than H2 blockers

c) early diagnosis of the specific organism(s) is critical for optimal survival

1) in a multicenter study of 413 patients with ventilator-associated pneumonia, compared to use of tracheal aspirates to guide antibiotic selection, early use of quantitative bronchoalveolar lavage or quantitative protected brush cultures to guide antibiotic selection was associated with (Ann Intern Med 2000; 132:621-30):

(a) decreased mortality (16% vs. 26%)

(b) lowered Sepsis Organ Failure Assessment scores (4.9 vs. 5.8)

(c) more antibiotic free days in the first month (11 vs. 7)

2. hemodynamic effects:

a) decreased cardiac output due to impaired venous return to the right heart and increased pulmonary venous resistance due to positive pressure alveolar distension

b) autoPEEP

3. barotrauma:

a) rare if plateau pressure is < 35

(1) for this reason, we generally use a plateau pressure < 35 as a major goal of mechanical ventilation and check plateau pressures frequently

b) often heralded by interstitial air on the chest x-ray

c) pneumomediastinum & subcutaneous emphysema

(1) does not need to be treated

(2) indicates high risk for pneumothorax

d) pneumothorax

(1) always treat with a chest tube in patients receiving positive pressure ventilation because of the risk of tension pneumothorax

(2) bronchopleural fistula can be difficult to resolve until the patient is off mechanical ventilation - the best treatment for a bronchopleural fistula is treatment of the underlying lung disease

4. oxygen toxicity

a) can occur as early as 24 hours after high oxygen exposure

b) more frequent if the FiO2 is > 0.5

c) clinically resembles adult respiratory distress syndrome

d) very important to avoid since this often results in an inescapable vicious cycle of high oxygen requirements ultimately resulting in fatal respiratory failure

5. respiratory alkalosis

6. increased intracranial pressure

7. atelectasis (especially the left lower lobe)

V. Weaning from the ventilator (Alia, 2000)

A. Factors to consider:

1. non-ventilator derived parameters

a) awake and off sedatives (as much as possible)

(1) be aware of depression as a psychological limitation to weaning

b) proper position (upright, ideally sitting)

c) adequate nutrition

d) optimal fluid status

e) no infection

f) hemodynamically stable

(1) preferably off vasopressors

(2) angina optimally controlled

(3) no active bleeding

g) no metabolic alkalosis

h) normal electrolyte status

(1) calcium

(2) phosphorus

(3) potassium

(4) magnesium

i) afebrile

j) bronchospasm controlled

k) mechanical airway obstruction treated

(1) for lung tumors, placement of a bronchial stent can permit extubation

l) normal thyroid function

m) re-establishment of normal sleep-wake cycling

2. weaning parameters (Yang, 1991)

a) f/Vt < 100 breaths/min./liter

(1) calculation:

(a) disconnect the patient from the ventilator for 1 minute

(b) measure the spontaneous minute ventilation and the respiratory rate

(c) calculate the average tidal volume (in liters) as the minute ventilation ÷ the respiratory rate

(d) f/Vt = respiratory rate ÷ average tidal volume

(2) in one study (Yang, 1991), this was the single best predictor of success in weaning patients from mechanical ventilation

(a) sensitivity 97%, specificity 64%

b) maximal inspiratory pressure (also mistakenly referred to as "NIF") more negative than -20

(1) sensitivity 100%, specificity 14%

c) minute ventilation (VE) < 10 liters/min.

(1) sensitivity 31%, specificity 61%

d) tidal volume (Vt) > 4 ml/kg

(1) sensitivity 94%, specificity 39%

e) vital capacity > 10 ml/kg

f) FiO2 < 40% with pO2 > 60 mm Hg

B. Weaning methods

1. T-piece

a) theoretic advantages:

(1) simple

(2) provides strenuous exercise to the ventilatory muscles followed by periods of full ventilatory muscle rest; this is theoretically the best way to condition the inspiratory muscles

b) technique:

(1) match pCO2 and tidal volume to the patient's baseline prior to initiating weaning attempt

(2) increase FiO2 by 10% at initiation of weaning

(3) stop trial if :

(a) resp. rate increases > 10

(b) pCO2 rises > 5

(c) pulse rises > 20

(d) blood pressure rises > 20 mm systolic

(4) attempt trials at least twice per day with a minimum of 1 hour between trials

(5) extubation can be considered if the patient breathes on their own for at least 2 hours without distress

c) disadvantages:

(1) removes the internal resistance of the ventilator but does not alter the resistance due to the endotracheal tube

(a) resistance is proportional to length and inversely proportional to the 4th power of the radius

(b) some examples of endotracheal tube resistances:

i) 6.0 tube = 3.2 cm/liter/sec

ii) 7.0 tube = 2.5 cm/liter/sec

iii) 8.0 tube = 1.4 cm/liter/sec

iv) 9.0 tube = 0.5 cm/liter/sec

(c) this added resistance can pose a great deal of extra work for the patient over and above that of breathing spontaneously without an endotracheal tube

2. SIMV

a) technique:

(1) decrease rate of support by 2-4 breaths per trial as tolerated

(2) return to full support between trials

(3) extubate when rate close to or equal to zero

b) theoretic advantages:

(1) allows gradual switch from fully supported to fully independent breaths

(2) prevents patients from "fighting" the ventilator

(3) reduces respiratory muscle fatigue

(4) allows some normal venous return to the right heart due to some negative pressure (spontaneous) breaths

c) problems:

(1) work of breathing and oxygen consumption can increase as much as 2-fold due to work necessary to activate the demand valve in older ventilators (Sasoon, 1989)

3. pressure support

a) theoretic advantages:

(1) augments a spontaneous breath with positive pressure

(2) counteracts extra work of breathing due to resistance of the tubing

(3) in patients with volume overload or left ventricular failure, rapid changing to spontaneous breathing (eg, T-piece trials) can result in lung flooding and rapid failure of weaning; pressure support avoids this by gradually reducing positive pressure and thus permitting gradual correction of fluid status by diuresis, etc.

b) technique:

(1) start at a pressure support sufficient to provide a tidal volume of 8-10 ml/kg (this will usually be about equal to the plateau pressure while the patient is on assist-control)

(2) reduce support by 2-5 cm as tolerated

(3) extubate when pressure support is equal to the calculated resistance of the ventilator circuit (eg. endotracheal tube, tubing, expiratory valves, etc.); this is usually about 5 cm

c) problems:

(1) requires physician to make frequent ventilator setting decisions

4. recent comparative studies:

a) Brochard, 1994

(1) randomized multicenter study of 456 patients

(2) all patients initially underwent a 2 hour trial of spontaneous breathing; if they had respiratory distress (109 patients) then they were assigned to 1 of 3 weaning modes:

(a) T-piece

(b) SIMV

(c) pressure-support

(3) outcome was measured as the percentage of patients extubated by 21 days:

(a) T-piece = 64%

(b) SIMV = 60%

(c) pressure-support = 90%

(4) conclusion: pressure-support resulted in the most patients extubated by 21 days

b) Esteban, 1995

(1) randomized multicenter study of 546 patients

(2) all patients initially underwent a 2 hour trial of spontaneous breathing; if they had respiratory distress (130 patients) then they were assigned to 1 of 4 weaning modes:

(a) IMV

(b) pressure support

(c) T-piece (2 or more trials per day)

(d) T-piece (once daily trial)

(3) outcome was measured by time until extubation:

(a) IMV = 5 days

(b) pressure support = 4 days

(c) T-piece (2 or more/day) = 3 days

(d) T-piece (1/day) = 3 days

(4) conclusion: T-piece trials result in the most rapid successful extubation

c) Ely, 1996

(1) randomized single hospital study of 300 patients

(2) control patients underwent daily respiratory function screening

(3) study patients underwent daily respiratory function screening and underwent a T-piece trial for up to 2 hours if the screening criteria were met; if the patient satisfactorily completed the trial, the attending physician was notified

(4) for the screening criteria to be met:

(a) pO2:FiO2 ratio > 200

(b) PEEP ² 5

(c) cough reflex intact

(d) f/Vt ² 105

(e) no continuous infusion of vasopressors or sedatives

(5) time to extubation:

(a) study group = 4.5 days

(b) control group = 6 days

(6) complication rate:

(a) study group = 20%

(b) control group = 41%

(7) ICU costs:

(a) study group = $15,740

(b) control group = $20,890

(8) conclusion: daily screening for "weanability" results in shorter days on the ventilator, fewer complications, and lower ICU costs

d) Girqult, 1999

(1) randomized single hospital study of 33 patients who failed a 2 hour T-piece trial

(2) patients were assigned to early extubation with immediate institution of non-invasive ventilation using pressure support or to continued mechanical ventilation via an endotracheal tube until a T-piece trial was successfully performed

(3) There was no difference in complication rates or survival but patients in the study group had a significantly decreased duration of mechanical ventilation

e) Henneman, 2001

(1) 124 patients had weaning facilitated by a collaborative weaning flowsheet and were compared to 77 patients weaned per the previous usual practice of the ICU

(2) Patients weaned using the "weaning board" and "weaning flowsheet" came off of the ventilator an average of 3.6 days faster than patients weaned per the previous usual practice of the ICU.

5. my personal philosophy:

a) all patients should have a weaning assessment to include f/Vt daily

b) SIMV is probably never the best mode of weaning medically complex patients; it is acceptable for uncomplicated post-operative patients because of its simplicity and because almost any weaning mode works well in these patients

c) T-piece weaning is better (more rapid) for medical patients who are rapidly improving

d) pressure-support is better for medical patients who are improving slowing and for patients with severe obstructive lung disease

e) the combination of pressure-support + SIMV as a weaning mode can be unnecessarily confusing and adds little to pressure-support alone

6. rapid weaning in patients who fail an initial T-piece trial may be warrented in COPD patients if extubated to full face mask non-invasive ventilation (Nava, 1998).

C. Causes of failure to wean:

1. hypoxemia

a) diffuse pulmonary disease

b) focal pulmonary disease (pneumonia, etc.)

c) pulmonary edema

(1) removal of positive pressure can increase preload and lead to worsened heart failure, especially in patients with diastolic dysfunction which should be suspected in patients who become unexpectedly tachycardic and/or hypertensive during weaning; these patients often require a calcium channel blocker (verapamil or diltiazem) or a beta blocker for control of diastolic dysfunction before weaning will be successful

2. insufficient ventilatory drive

a) response to pathologic metabolic alkalosis

b) inadequately functioning CNS respiratory drive center

(1) sedatives

(2) malnutrition

(3) myxedema

(4) primary CNS disease

3. excessive ventilatory demands

a) excessive CO2 production

(1) sepsis

(2) agitation

(3) fever

(4) high carbohydrate intake

4. respiratory muscle weakness

a) neuromuscular disease

b) malnutrition

c) drugs

(1) neuromuscular blocking agents

(2) corticosteroids

(3) aminoglycosides

d) low magnesium, potassium, phosphorus

5. excessive work of breathing

a) airway obstruction

(1) bronchospasm

(2) secretions

(3) bronchitis

(4) kinked endotracheal tube

(5) mechanical airway obstruction (tumor, bronchostenosis, tracheo-bronchial malacia)

b) endotracheal tube too small

c) chest motion restriction (bandages, positioning, etc.)

6. acid-base disorders

7. phrenic nerve injury

a) esp. following open heart surgery

b) unilateral diaphragm paralysis rarely symptomatic unless there is substantial contralateral pulmonary disease

8. psychological factors

a) sleep deprivation

b) anxiety

c) depression

d) confusion

e) pain

VI. Unique ventilatory problems in cardiac patients

A. patent foramen ovale (Cujec, 1993)

1. PFO found in Å 15% of all patients with acute respiratory failure

2. PEEP will not increase pO2 substantially in these patients

3. PEEP can actually worsen pO2 by increasing right to left shunting

B. congestive heart failure

1. sleep apnea

a) both central & obstructive sleep apnea are common in left ventricular dysfunction

(1) in one study from a Veterans Administration hospital, 19 of 42 patients with a left ventricular ejection fraction < 45% had 20 or more episodes of apnea or hypopnea associated with a oxygen saturation < 90% for 23% ± 24% of the total sleep time (Javaheri, 1995)

b) nasal CPAP is effective in both types of apnea when related to CHF (Bradley, 1990)

(1) Cheyne-Stokes may be associated with central apnea and can improve with CPAP (although this is controversial)

(2) in this situation, left ventricular ejection fraction can increase by 5% or more with CPAP

2. symptomatic heart failure

a) moderate CHF: non-invasive ventilatory support (usually CPAP via a mask) is highly effective and often all that is required

b) in severe CHF: intubation with mechanical ventilation can result in rapid and dramatic improvements in left ventricular function and myocardial ischemia

c) specific studies:

(1) CPAP (administered via face mask) resulted in a decrease in respiratory rate, decrease in blood pressure, and an increase in pO2 in 40 patients with acute cardiogenic heart failure (Rasanen, 1987)

(2) in patients with cardiogenic shock associated with myocardial infarction, a progressive increase in ventilatory support from 0-100% resulted in an associated progressive reduction in ventilatory effort and myocardial ischemia (Rasanen, 1984)

(3) by increasing intrathoracic pressure in patients with CHF, inspiratory muscle work is reduced and left ventricular afterload is reduced (Naughton, 1995)

(4) about half of patients with acute decompensation of chronic congestive heart failure will demonstrate an improvement in cardiac output accompanied by a reduction in pulmonary capillary wedge pressure (Baratz, 1992)

(5) the best predictor of an improvement in cardiac output using CPAP is the pulmonary capillary wedge pressure - patients with a high wedge pressure are more likely to have an improvement in cardiac output (Bradley, 1992)

C. adverse affects of mechanical ventilation:

1. hyperinflation of the lung can result in an undesirable increase in pulmonary vascular resistance

2. in general, PEEP > 15 will reduce right ventricular contractility in patients with underlying right ventricular failure

3. because the interventricular septum is the thinnest portion of the left ventricular wall, if the right ventricle is compressed (increased PEEP or increased pulmonary vascular resistance), then it will shift the septum in toward the left ventricle, reducing the left ventricular cardiac output

a) this is especially true if the patient is relatively hypovolemic

4. many of the medications used to intubate or sedate patients can have undesirable effects on blood pressure or vagal tone

5. if the patient is relatively hypovolemic, severe post-intubation hypotension can occur and should be treated with:

a) reverse Trendlenberg position

b) fluid bolus

c) reducing tidal volume

d) reducing PEEP

e) vasopressors with predominately alpha adrenergic properties (eg, levarterenol)

VII. References:

Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med 2000; 342.

 

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