This chapter discusses the indications for mechanical ventilation in adult patients. We focus on patients who are already in the intensive care unit (ICU) or who are considered for transfer to the ICU, that is, patients with new onset of signs and symptoms over minutes or hours. We do not deal with the indications for mechanical ventilation for chronic respiratory failure or in pediatric patients; these subjects are covered in Chapters 23, 18, 33, and 34.
There is a paucity of research—and no clinical trials—on the indications for mechanical ventilation. This situation contrasts with the growing amount of research on the discontinuation of mechanical ventilation. Although it is tempting to apply indices used for predicting the outcome of weaning trials as indices to identify patients who require mechanical ventilation, such an approach has not been tested. It is also probably unwise.
Two factors account for the limited research on indications for mechanical ventilation. First, such patients are extremely ill. Any intervention—such as careful collection of physiologic measurements—that delays institution of ventilation might be viewed as unethical. Second, the nosology of respiratory failure is unsatisfactory (see “Nosology” below). In everyday practice, clinicians do not decide to institute mechanical ventilation because a patient meets certain diagnostic criteria. Instead, clinicians typically decide to institute ventilation based on their assessment of a patient’s signs and symptoms. This decision is also grounded on a foundation of solid biomedical theory, specifically principles of pulmonary pathophysiology. Accordingly, we develop our discussion of ventilator indications along these two lines: physical examination and pathophysiologic principles.
Clinical presentations that cause a physician to institute mechanical ventilation are protean. They range from patients presenting with frank apnea to patients with clinical signs of increased work of breathing with or without laboratory evidence of impaired gas exchange.1
Apneic patients, such as those who have suffered catastrophic central nervous system (CNS) damage, need immediate institution of mechanical ventilation. To advocate controlled trials to determine the need for mechanical ventilations in apneic patients is unethical.
Clinical Signs of Increased Work of Breathing
Asthma, chronic obstructive pulmonary disease (COPD), pneumonia, cardiogenic pulmonary edema, and acute respiratory distress syndrome (ARDS) are just a few of the many conditions that cause an increase in work of breathing and, with it, increased energy expenditure by the respiratory muscles.
The energy expenditure of the respiratory muscles can be quantified in terms of pressure-time product2—the time integral of the difference between the esophageal pressure tracing and the estimated recoil pressure of the chest wall3,4 (Fig. 4-1). The pressure-time product of patients in acute respiratory failure is about four times5–7 the normal value (100 cm H2O·s/min), and it can be increased sixfold in individual patients.5,6 The inspiratory pressure-time product ...