Free Pulmonary Course on Mechanical Ventilation
by David J Pierson MD
Although mechanical ventilation is a key component of intensive care, unfamiliar jargon and technical detail render it confusing and formidably difficult for many clinicians. The rapidity and complexity of change in this area of respiratory medicine in recent years adds to the problem. Most of the current literature and nearly all the controversy in mechanical ventilation apply to only a small fraction of the patients who are intubated and ventilated in acute care hospitals. This small fraction consists of those with severe respiratory failure due to acute diffuse lung injury—acute lung injury (ALI) or the acute respiratory distress syndrome (ARDS)—and also those with severe obstructive lung disease (COPD or asthma). For the other 80 or 90 percent of ventilated patients the issues are much less difficult.
At best, mechanical ventilation can support gas exchange and lung inflation in a reasonably normal fashion. It is supportive, replacement therapy only; there is no disease that intubating and ventilating a patient will cure. When done as well as we know how, mechanical ventilation can come close to replacing the normal functions of the lungs and chest bellows. Considering the many adverse consequences of intubation and positive-pressure ventilation, the clinician should always be striving to make the period of ventilatory support as short as possible.
This primer summarizes the main issues in mechanical ventilation, explains many of the modes, settings, and terms involved, and reviews aspects of monitoring and complications with which the clinician should be familiar. It focuses on invasive mechanical ventilation (that is, on ventilating intubated patients), and on the care of adults rather than children or infants. Parts of it are adapted from the chapter on invasive mechanical ventilation in Clinical Respiratory Medicine, by Albert, Spiro, and Jett [ref-1].
Table 1[27k PDF*] lists a number of factors that are key to developing a rational approach to mechanical ventilation in acute illness. It is also important that the fundamental goals and objectives of this therapy (Table 2 [28k PDF*]) be understood, both in general and relative to the individual patient.
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Indications for Mechanical Ventilation
In the absence of a contraindication, mechanical ventilation is indicated whenever any of the circumstances listed under "clinical objectives" in Table 2 [28k PDF*] exists to a degree that threatens the life of the patient. Apart from apnea, however, few individual symptoms, signs, or laboratory findings by themselves always mandate the initiation of ventilatory support. Rather, this therapy becomes necessary in the presence of the right combination of clinical setting, severity of abnormality, and rapidity of development or worsening of physiologic abnormality. A summary of the main categories of indications for invasive ventilatory support is provided in Table 3 [31k PDF*].
Apnea and Impending Respiratory Arrest
No one would argue with providing ventilatory support for a patient who has stopped breathing altogether. On the other hand, "impending respiratory arrest" is difficult to define prospectively, and attempts to study it as an indication for intubation and mechanical ventilation have so far proved unsuccessful. These things are commonly carried out because the clinician judges the patient to be in severe respiratory distress, "tiring," or "about to arrest," but inter-observer variation and the extent to which these subjective impressions predict an unfavorable outcome if intubation is not performed have not been investigated.
Exacerbation of Chronic Obstructive Pulmonary Disease
The substantial number of studies of noninvasive positive-pressure ventilation (NPPV) in severe exacerbations of COPD has not directly addressed the validity of the criteria used for intubation. The writing committee for the Global Initiative for Obstructive Lung Disease (www.goldcopd.org), using best available evidence, recommends invasive mechanical ventilation when patients with acute exacerbations of COPD have cardiovascular instability, somnolence or other altered mental state, uncooperativeness, a high risk of aspiration, copious or very viscous respiratory tract secretions, any craniofacial condition (such as recent trauma or
surgery) potentially rendering NPPV difficult, or extreme obesity. Very severe or progressive respiratory acidosis is also accepted as an indication, but agreement is lacking as to whether a pH of 7.25, a PaCO2 of 60 mm Hg, or some other thresholds should be used.
Acute Severe Asthma
Retrospective studies have shown that relatively few patients with acute severe asthma require invasive mechanical ventilation, but no clinical trials to define the specific indications have been reported. These indications may be similar to those for acute COPD exacerbations, although the potential for more rapid physiologic improvement in asthma, and the fact that patients with asthma are typically younger and healthier than those with severe COPD raise doubt about this assumption. Unlike the situation with COPD exacerbations, the benefit of NPPV in acute severe asthma has not yet been clearly established.
In acute respiratory insufficiency complicating neuromuscular disorders such as the Guillain-Barré syndrome and myasthenia gravis, there is agreement among experienced clinicians that invasive mechanical ventilation is best initiated before the patient develops frank respiratory acidosis. Although vital capacity and the maximum inspiratory pressure generated against an occluded airway have been used in assessing the need for intubation in such patients, the thresholds shown in Table 3 have not been established through prospective studies.
Acute Hypoxemic Respiratory Failure
Severe hypoxemia by itself is seldom an indication for invasive mechanical ventilation. For example, isolated hypoxemia in patients who have diffuse pneumonia or pulmonary edema can often be managed with high-flow oxygen by mask, with or without continuous positive airway pressure (CPAP). Typically, patients who demonstrate severe hypoxemia in the setting of severe acute illness have other indications for ventilatory support, such as evidence for excessive work of breathing or diminished ventilatory drive. There is no evidence to indicate what threshold of PaCO2/FIO2 or other measure of oxygenation failure should be used as an independent indication for intubation and mechanical ventilation, or in fact whether such a threshold exists.
Several studies of NPPV in various forms of acute hypoxemic respiratory failure have yielded inconclusive results, although available data suggest that it may be possible to avoid intubation in some immunocompromised patients with the use of this modality. The presence of cardiovascular instability, altered mental status, or evidence of the inability to adequately protect the lower airway are clear indications for intubation in acute hypoxemic respiratory failure; in the absence of these findings it may be reasonable to attempt NPPV if there is single-organ failure in an otherwise healthy patient and reason to believe that the patient's condition will improve rapidly.
Heart Failure and Cardiogenic Shock
Available evidence on the management of cardiac pulmonary edema indicates that CPAP or NPPV may improve gas exchange--possibly with a reduced need for intubation--but that rates of clinical recovery and other outcomes may not be different compared to management without these interventions. Cardiogenic shock, however, may represent a separate indication for invasive mechanical ventilation, to decrease the oxygen cost of breathing at a time of severely impaired cardiac function. While this has not been subjected to a prospective clinical trial, retrospective studies of patients managed with intra-aortic balloon pumps have found higher rates of weaning from the pump and improved hospital mortality in patients who were intubated and ventilated.
Acute Brain Injury
Short-term hyperventilation can rapidly decrease intracranial pressure in patients with traumatic brain injury by constricting cerebral blood vessels and decreasing both cerebral blood flow and cerebral blood volume. However, available evidence indicates that routine hyperventilation in such patients does not improve survival or neurologic outcome, and may in fact worsen the latter. Although brief periods of hyperventilation are used acutely to reduce sudden increases in intracranial pressure while more definitive measures are undertaken, the presence of acute brain injury is not by itself an indication for hyperventilation.
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Contraindications to Invasive Mechanical Ventilation