Hypoxemia is defined as deficient oxygenation of the blood, whereas hypoxia implies a problem with tissue oxygenation. Severe hypoxemia and hypoxia ( PaO2 below about 4.5 kPa) results in anaerobic tissue metabolism and eventually cell death. Different organs are differentially sensitive to hypoxia; the cerebral cortex is most immediately dependent on oxygen and stops functioning after less than 1 min of deprivation, whereas the heart can survive for up to 5 min and skeletal muscle for approximately 2 h. Irreversible loss of function only occurs after four times the period of anoxia that results in cessation of function, and partial recovery can occur after intermediate anaerobic periods.

In the intensive care setting, it is useful to consider several discrete causes of severe hypoxemia: hypoventilation, true shunt, impaired diffusion, ventilation-perfusion inequality, and decreased mixed venous oxygen saturation. In reality, the pathophysiological processes commonly found in critically ill patients result in combinations of the above components. Acute respiratory distress syndrome, for example, affects both the pulmonary vasculature and the alveolar membrane, and therefore can

cause both ventilation-perfusion ( VlQ) mismatch and a barrier to diffusion. Each process is best understood using physiological principles ( Nunn 1993; West 1998).

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Sleep Apnea

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