DEFINITION AND CLASSIFICATION OF RESPIRATORY FAILURE
Respiratory failure is defined as inadequate gas exchange due to malfunction of one or more components of the respiratory system. There are two main types of acute respiratory failure: hypoxemic and hypercarbic. Hypoxemic respiratory failure is defined by arterial O2 saturation <90% while receiving an increased inspired O2 fraction. Acute hypoxemic respiratory failure can result from pneumonia, pulmonary edema (cardiogenic or noncardiogenic), and alveolar hemorrhage. Hypoxemia results from ventilation-perfusion mismatch and intrapulmonary shunting.
Hypercarbic respiratory failure is characterized by alveolar hypoventilation and respiratory acidosis. Hypercarbic respiratory failure results from decreased minute ventilation and/or increased physiologic dead space. Conditions associated with hypercarbic respiratory failure include neuromuscular diseases (e.g., myasthenia gravis), disease processes causing diminished respiratory drive (e.g., drug overdose, brainstem injury), and respiratory diseases associated with respiratory muscle fatigue (e.g., exacerbations of asthma and chronic obstructive pulmonary disease [COPD]). In acute hypercarbic respiratory failure, PaCO2 is typically >50 mmHg. With acute-on-chronic respiratory failure, as is often seen with COPD exacerbations, considerably higher PaCO2 values may be observed. The degree of respiratory acidosis, the pt’s mental status, and the pt’s degree of respiratory distress are better indicators of the need for mechanical ventilation than a specific PaCO2 level in acute-on-chronic respiratory failure. Two other types of respiratory failure are commonly considered: (1) perioperative respiratory failure related to atelectasis, and (2) hypoperfusion of respiratory muscles related to shock.
MODES OF MECHANICAL VENTILATION
Respiratory failure often requires treatment with mechanical ventilation to decrease the work of breathing and reverse severe hypoxemia and respiratory acidosis. There are two general classes of mechanical ventilation: noninvasive ventilation (NIV) and conventional mechanical ventilation. NIV, administered through a tightly fitting nasal or facial mask, is widely used in acute-on-chronic respiratory failure related to COPD exacerbations. NIV typically involves a preset positive pressure applied during inspiration and a lower pressure applied during expiration; it is associated with fewer complications such as nosocomial pneumonia than conventional mechanical ventilation through an endotracheal tube. However, NIV is contraindicated in cardiopulmonary arrest, severe encephalopathy, severe GI hemorrhage, hemodynamic instability, unstable coronary artery disease, facial surgery or trauma, upper airway obstruction, inability to protect the airway, and inability to clear secretions.
Most pts with acute respiratory failure require conventional mechanical ventilation via a cuffed endotracheal tube. The goal of mechanical ventilation is to optimize oxygenation while avoiding ventilator-induced lung injury. Various modes of conventional mechanical ventilation are commonly used; different modes are characterized by a trigger (what the ventilator senses to initiate a machine-delivered breath), a cycle (what determines the end of inspiration), and limiting factors (operator-specified values for key parameters that are monitored by the ventilator and not allowed to be exceeded). Three of the common modes of mechanical ventilation are described below; additional information is provided in Table 15-1.
TABLE 15-1CLINICAL CHARACTERISTICS OF COMMONLY ...