Cardio-respiratory function during sleep.
Respiratory function undergoes sleep-associated changes which in normal subjects leave it unaffected. However in some cases they may be more marked than usual or may be superimposed on a pre-existing disease, thus giving rise to sleep-related ventilation disorders. These include obstructive sleep apnea syndrome (OSAS), nocturnal desaturation events of chronic obstructive pulmonary disease (COPD) and restrictive syndromes, as well as nocturnal asthmatic attacks. OSAS is a condition characterized by the frequent recurrence of interruptions of oronasal flow (greater than 10 s.) due to upper airway occlusion induced by a reduction in pharyngeal muscle tone. This phenomenon, particularly prominent in REM sleep, results in oxyhemoglobin desaturation and marked cardiovascular consequences (arrhythmias, increases in pulmonary and systemic arterial pressure), as well as symptoms (loud intermittent snoring, daytime sleepiness, intellectual deterioration etc.). Obesity is often associated with OSAS or may lead to a sleep-related hypoventilation syndrome. Treatment is based on weight loss, surgery of upper airway abnormalities, if present, and on splinting of the upper airway by the application of nasal continuous positive airway pressure. In COPD and restrictive disorders, nocturnal hypoxemia is mainly due to REM-associated loss of respiratory muscle tone, as well as in the sleep-related exaggeration of functional defects due to COPD (low chemoreceptor sensitivity, high closing volume etc.). Treatment is based on oxygen administration, provided that possible side-effects are carefully monitored. Nocturnal asthma is due to circadian changes in hormonal secretion (catecholamines, cortisol), as well as supine posture, reduced muco-ciliary clearance, gastro-esophageal reflux etc. Sleep itself plays some role through a depressed arousal reaction in slow wave sleep, resulting in more marked and prolonged attacks in this stage. Slow-release theophylline or beta-mimetic medications, as well as new chromones and antimuscarinic drugs are therapeutic alternatives.