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504 PART 4: Pulmonary Disorders

outcome. If Pplat is less than 30 cm H O and pH is less than 7.20, RR a total dose of 2.5 mg reduced the inspiratory flow-resistive pressure
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2
can be safely increased for the purpose of lowering Pa CO 2 and elevating 18%. Increasing the nebulized dose to a total of 7.5 mg reduced airway
arterial pH until Pplat nears the threshold pressure. Commonly, patients resistance further in 8 of 10 patients, but caused side effects in half of the
can be ventilated to a pH >7.20 with a Pplat <30 cm H O. patients. Thus if MDIs are used during mechanical ventilation, use of a
2
Of note, in a previously cited manuscript, Anzueto and colleagues spacer on the inspiratory limb of the ventilator improves drug delivery. 201
reported the incidence, risk factors, and outcomes of barotrauma in a Regardless of whether an MDI with spacer or nebulizer is used, higher
cohort of patients that were mechanically ventilated using a strategy of drug dosages are required and the dosage should be titrated to achieve
limited tidal volumes and airway pressures. Of a total of 5183 patients a fall in the peak-to-pause airway pressure gradient. Nebulizers should
172
ventilated for more than 12 hours, 79 were asthmatics. Five (6.3%) of be placed close to the ventilator, and in-line humidifiers stopped during
these patients developed barotrauma, which was associated with worse treatments. Inspiratory flow should be reduced to approximately 40 L/min
outcomes. Interestingly, patients with and without barotrauma did not during treatments to minimize turbulence, although this strategy may
differ in any ventilator parameter. worsen DHI and must be time limited. Patient-ventilator synchrony is
crucial to optimize drug delivery. When no measurable drop in airway
Sedation and Paralysis: Sedation improves comfort, safety, and patient- resistance occurs, other causes of elevated airway resistance such as a
ventilator synchrony, particularly when hypercapnia serves as a potent kinked or plugged endotracheal tube should be excluded. Bronchodilator
stimulus to respiratory drive. Some patients (such as those with sudden- nonresponders should also be considered for a drug holiday. Data from
onset asthma) may be ready for extubation within hours. In these patients, randomized controlled trials are needed to determine the effects of bron-
propofol is an attractive sedative because it can be rapidly titrated to chodilators in intubated patients and to provide evidence for or against
a deep level of sedation, and there is quick reversal of sedation after usual clinical recommendations for bronchodilator use. 202
discontinuation. Lorazepam and midazolam are less attractive alterna-
187
tives, since time to awakening is generally longer and less predictable than ■ OTHER CONSIDERATIONS
with propofol. 188,189 The addition of an opioid to propofol or a benzodiaz-
epine achieves the most desirable combination of amnesia, sedation, analge- Rarely, the above strategies are unable to stabilize the patient on the venti-
lator. In these situations other therapies are available. Inhalational general
sia, and suppression of respiratory drive. For all patients, daily interruption 203,204
of sedatives and analgesics avoids unwanted drug accumulation. 190 anesthetic bronchodilators acutely reduce Ppk and Pa CO 2 . These agents
cause myocardial depression, arterial vasodilation, and arrhythmias, and
Ketamine, an IV anesthetic with sedative, analgesic, and bronchodila-
ting properties, is reserved for use in intubated patients with severe their benefit does not last after drug discontinuation. Heliox delivered
. However,
205
bronchospasm that precludes safe mechanical ventilation. 191-193 Ketamine through the ventilator circuit may decrease Ppk and Pa CO 2
must be used with caution because of its sympathomimetic effects and safe use of heliox requires institutional expertise and careful planning.
Flow meters (which are gas-density dependent) must be recalibrated
propensity to cause delirium, and even psychosis.
When safe and effective mechanical ventilation cannot be achieved for heliox, and a spirometer should be placed on the expiratory port of
by sedation alone, short-term muscle paralysis is indicated. Short- to the ventilator during heliox administration to measure tidal volume. A
intermediate-acting agents include atracurium, cis-atracurium, and trial of heliox use in a lung model is recommended prior to patient use.
It is also possible to use extracorporeal carbon dioxide removal for the
vecuronium. Of these, cis-atracurium is preferred because it is essen-
tially free of cardiovascular effects, does not cause release of histamine, extremely rare patient in whom even permissive hypercapnia and opti-
mization of routine mechanical ventilation result in either unacceptable
and does not require hepatic and renal function for clearance. 194
Paralytics may be given intermittently by bolus or continuous IV acidosis, unacceptable DHI, or both. These circuits and the dual lumen
infusion. If a continuous infusion is used, a nerve stimulator should be catheters placed for achieving extracorporeal circulation have evolved
used or the drug should be withheld every 4 to 6 hours to avoid drug significantly in recent years and it is likely future trials will test their util-
ity in status asthmaticus as well as other causes of respiratory failure.
206,207
accumulation. Paralytic agents should be minimized whenever possible
because of the risk of postparalytic myopathy. 195-198 In one study of ■ EXTUBATION
25 ventilated asthmatics, 19 (76%) patients had an increase in serum cre-
atine kinase, and 9 (36%) had clinically detectable myopathy. Elevated Recommendations for weaning and extubation of asthmatic patients
198
creatine kinase was associated with prolonged mechanical ventilation have not been validated. Patients with sudden-onset asthma may
whether or not there was clinically detectable myopathy. In a retrospective respond quickly to bronchodilators and be eligible for extubation within
cohort study of 107 episodes of asthma requiring intubation, the concur- hours. More often several days of support are required before patients
rent use of steroids and a paralytic was associated with muscle weakness are ready for a spontaneous breathing trial. In general a spontaneous
in 29% of episodes, and steroid treatment alone was not associated with breathing trial should be considered when Pa CO 2 normalizes at a minute
weakness. Importantly, this study demonstrated that the duration of ventilation that does not cause significant DHI, airway resistive pressure
195
paralysis correlated with the incidence of myopathy, which is rare when is <20 cm H O, external PEEP is <5 cm H O, mental status is intact and
2
2
paralytics are used for less than 24 hours. Findings of a separate study significant weakness has not been identified. Extubation can usually fol-
confirm this correlation. Most patients with postparalytic myopathy low a successful spontaneous breathing trial, after which bronchodilators
196
recover, but may require weeks of rehabilitation. Use of neuromuscular should be given. The patient should continue to be observed in the ICU
blockers further increases the risk of ventilator-associated pneumonia. 197 for 12 to 24 hours during which time the focus switches to safe transfer
■ ADMINISTRATION OF BRONCHODILATORS education, environmental control measures, and use of controller agents.
to the medical ward and maximizing outpatient management through
DURING MECHANICAL VENTILATION
Questions remain regarding the administration of inhaled bronchodi- KEY REFERENCES
lators to intubated patients. In one study, only 2.9% of a radioactive
199
aerosol delivered by nebulizer was deposited in the lungs of mechanically • Adnet F, Dhissi G, Borron SW, et al. Complication profiles of adult
ventilated patients. Manthous and colleagues compared the efficacy of asthmatics requiring paralysis during mechanical ventilation.
albuterol delivered by MDI via a simple inspiratory adapter (no spacer) Intensive Care Med. 2001;27(11):1729-1736.
to nebulized albuterol in intubated patients. Using the peak-to-pause • Anzueto A, Frutos-Vivar F, Esteban A, et al. Incidence, risk factors
200
pressure gradient at a constant inspiratory flow to measure airway resis- and outcome of barotrauma in mechanically ventilated patients.
tance, they found no effect (and no side effects) from the administration Intensive Care Med. 2004;30(4):612-619.
of 100 puffs (9.0 mg) of albuterol. Albuterol delivered by nebulizer to

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