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  • • Insidious nonspecific symptoms include headache and memory impairment.
  • • Elevated Pco2 on blood gas analysis is a late finding and is necessary for diagnosis.
  • • Vital capacity and maximal inspiratory pressure may help with etiology and severity.
  • • Important causes include neuromuscular disease, chronic obstructive pulmonary disease (COPD), and central hypoventilation.

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The term respiratory failure specifies the failure of the entire integrated respiratory system, including the lungs, chest wall, and brain, to maintain adequate oxygenation of arterial blood and proper elimination of carbon dioxide (Figure 26–1). Ultimately, respiratory failure interferes with respiration at the cellular level. Somewhat arbitrary values for arterial oxygen tension (Pao2)and arterial carbon dioxide tension (Paco2) define respiratory failure, as gas exchange at the tissue level cannot be easily measured. These values are influenced by altitude of residence, age, and metabolic processes as well as by breathing itself. At altitude, oxygenation and Paco2 are both lower than at sea level. With advanced age, oxygenation worsens and Pco2 may rise slightly. Respiratory failure for a patient at rest at sea level is defined by a Pao2 or Paco2 far outside the normal range: usually below 60 mm Hg for Pao2 or above 45–50 mm Hg for Paco2.

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Figure 26–1.
Graphic Jump Location

Mechanisms of chronic ventilatory failure.

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Ventilatory failure is generally diagnosed by the presence of an arterial Pco2 above 45 mm Hg. This may result from a decreased central nervous system respiratory drive to breathe or be secondary to impaired function of the respiratory muscles and/or mechanical abnormalities of the chest wall. Most often, however, ventilatory failure is caused by diseases affecting the lung mechanics, such as chronic obstructive lung disease (Figure 26–1). Hypoxemia in the setting of ventilatory failure results from low alveolar oxygen tension (Pao2) and worsens with increasing arterial carbon dioxide tension (Paco2). Progressive worsening of ventilation (progressive hypoventilation), even in the absence of intrinsic lung disease, will not only increase arterial Paco2 but also decrease alveolar (Pao2) and arterial (Pao2) oxygen tension.

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The alveolar gas equation relates ventilatory failure to hypoxemia, as an increased Paco2 has a reciprocal relation to Pao2[Appendix, Eq. (1)]. A derivative of this equation, the alveolar–arterial oxygen difference [Appendix, Eq. (2)], should be used at the bedside to distinguish ventilatory failure from hypoxemic respiratory failure in which ventilation is preserved but oxygen levels are low. Hypoxemic respiratory failure is usually acute and is caused by conditions such as pneumonia, acute respiratory distress syndrome (ARDS), or pulmonary embolism (see Chapter 15: Acute Respiratory Distress Syndrome).

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Ventilatory respiratory failure,...

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