Although sleep results in changes to most physiological systems, attention will be directed to those affecting the respiratory and cardiovascular systems. Respiratory system
Although respiratory rate tends to increase during NREM sleep (and becomes irregular during REM sleep), minute ventilation falls by 0.5 to 1.5 l/min mainly as a result of a decrease in tidal volume. This decrease is most pronounced during REM sleep. The resultant alveolar hypoventilation produces a mean fall of 3 to 10 mmHg (0.4-1.3 kPa) in PaO2 and an increase of 2 to 8 mmHg (0.2-1.1 kPa) in PaCO2. Owing to the shape of the oxyhemoglobin dissociation curve, oxygen saturation is well maintained if the individual has a normal awake PaO2. The most important factor contributing to the decrease in minute ventilation appears to be the decrease in resting tone of the upper airway musculature, which leads to increased airway resistance and, particularly in the supine position, predisposes to intermittent upper airway obstruction. In addition, intercostal muscle tone is progressively lost as sleep deepens and virtually disappears in REM sleep. During these stages, minute ventilation is provided entirely by the diaphragm. Other factors involved include a decreased chemoreceptor sensitivity to hypercarbia and hypoxemia, the absence of voluntary control of respiration present during wakefulness, and a reduction of metabolic rate during sleep. Furthermore, the supine position during sleep is associated with a decreased functional residual capacity. Therefore airway closure may occur at resting lung volumes, resulting in ventilation-perfusion mismatch. Laryngeal reflexes, cough, and ciliary activity are also decreased during both phases of sleep, resulting in an increased tendency to pulmonary aspiration of saliva and gastric contents.
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