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Acute respiratory failure (ARF) is one of the leading causes of admission to the intensive care unit (ICU). Recently, incidence ranges for ARF, acute lung injury (ALI), and acute respiratory distress syndrome (ARDS) in adults were found to be 77.6–88.6, 17.9–34.0, and 12.6–28.0 cases/100,000 population per year, respectively.1,2 Mortality rates of approximately 40% were reported for patients with ARF, and similar or slightly lower rates for those with ALI and ARDS.3,4
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The respiratory system primarily functions to provide adequate blood oxygenation and carbon dioxide elimination for the purposes of sustaining aerobic metabolism and pH homeostasis, respectively. Although the etiologies of respiratory failure are far too numerous to list, the underlying pathophysiologic mechanisms are similar and usually lead to a final common pathway. A consensus definition has not been established for ARF; however, several large studies have defined ARF as a Pao2/FiO2 ratio <200, or Pao2 <60 with either an FiO2 of >0.6 (hypoxemic) or a Paco2 >50 (hypercapnic). Irrespective of the criterion used to establish ARF, it can generally be stated that all patients with respiratory impairment will have either primary ventilatory or primary oxygenation impairment (Figure 8-1).
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This chapter will discuss the basic pathophysiologic mechanisms of respiratory failure and the approach to the management of these patients. The most common diseases for respiratory failure are discussed in other chapters and will not be discussed in great detail here. However, two disease processes not discussed elsewhere will be covered in greater detail here as they present their own unique challenges, that is, cervical spinal cord injury (SCI) and neuromuscular diseases.
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Hypoxemic respiratory failure is usually the result of hypoventilation, a disorder of alveolar oxygen diffusion, shunting of systemic venous blood to the arterial circuit, or a ventilation–perfusion (V/Q) mismatch. These descriptions provide an accurate depiction of the physiologic mechanisms for hypoxemic respiratory failure and are most useful for understanding how a particular disease causes hypoxemia.5 In a large multicenter international prospective cohort study of patients requiring mechanical ventilation (MV), the most common reported causes of ARF were postoperative respiratory failure, pneumonia, congestive heart failure, sepsis, and trauma.6 In a small prospective cohort study that included 41 patients with hypoxemic respiratory failure, chronic obstructive pulmonary disease and pneumonia were the most common causes.7 Other data from small, randomized controlled trials of noninvasive ventilation identified congestive heart failure, pneumonia, trauma, ARDS, and mucous plugging as the most common causes of respiratory failure.8,9
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Hypoventilation is a reduction in the volume of gas delivered to the alveoli per unit time (alveolar ventilation). Assuming oxygen consumption remains unchanged, hypoxia then results. Hypoventilation always causes a rise in Paco2. Alveolar hypoventilation of a nonpulmonary etiology is ...