27 This chapter presents an overview of the problem of respiratory failure and discusses the different pathophysiologic types and consequences of respiratory insufficiency. Chapter 28 addresses a specific form of acute respiratory failure known as acute respiratory distress syndrome (ARDS), which does not require the presence of preexisting lung disease. Chapter 29 considers some principles of management of respiratory failure, as well as specific modalities of current therapy. In practice, it is most convenient to classify acute respiratory failure into two major categories on the basis of the pattern of gas exchange abnormalities. In the first category, hypoxemia is the major problem. The patient’s PCO2 is normal or even low. This condition is the hypoxemic variety of acute respiratory failure. For example, localized diseases of the pulmonary parenchyma (e.g., pneumonia) can result in this type of respiratory failure if the disease is sufficiently severe. However, an even broader group of etiologic factors causes hypoxemic respiratory failure by means of a generalized increase in fluid within the alveolar spaces, often as a result of leakage of fluid from pulmonary capillaries. The latter problem is frequently called ARDS and can be the consequence of a wide variety of disorders that cause an increase in pulmonary capillary permeability.* Because of the importance of this syndrome as a major form of acute respiratory failure, Chapter 28 focuses entirely on the problem of ARDS. In the second category, hypercapnia is present. For the respiratory failure to be considered acute, the pH must show absent or incomplete metabolic compensation for the respiratory acidosis. From the discussion of alveolar gas composition and the alveolar gas equation in Chapter 1, it is apparent that hypercapnia is associated with decreased arterial PO2 because of altered alveolar PO2. Therefore, even if ventilation and perfusion are relatively well matched and the fraction of blood shunted across the pulmonary vasculature is not increased, arterial PO2 falls in the presence of hypoventilation and consequent hypercapnia. In fact, many cases of hypercapnic respiratory failure have marked ventilation-perfusion mismatch as well, which further accentuates the hypoxemia. With these concepts in mind, it is clear the hypercapnic form of respiratory failure generally involves not just hypercapnia; it may be more appropriately considered the hypercapnic/hypoxemic form of respiratory failure.
Classification and Pathophysiologic Aspects of Respiratory Failure
Classification of Acute Respiratory Failure
Hypercapnic/Hypoxemic Type