Sleep hypoxaemia in non-rapid eye movement (non-REM) and rapid eye movement (REM) sleep was examined in 20 patients with various neuromuscular disorders with reference to the relation between oxygen desaturation during sleep and daytime lung and respiratory muscle function. All the patients had all night sleep studies performed and maximum inspiratory and expiratory mouth pressures (PI and Pemax), lung volumes, single breath transfer coefficient for carbon monoxide (KCO), and daytime arterial oxygen (PaO2) and carbon dioxide tensions (PaCO2) determined. Vital capacity in the erect and supine posture was measured in 14 patients. Mean (SD) PI max at RV was low at 33 (19) cm H2O (32% predicted). Mean PE max at TLC was also low at 53 (24) cm H2O (28% predicted). Mean daytime PaO2 was 67 (16) mm Hg and PaCO2 52 (13) mm Hg (8.9 (2.1) and 6.9 (1.7) kPa). The mean lowest arterial oxygen saturation (SaO2) was 83% (12%) during non-REM and 60% (23%) during REM sleep. Detailed electromyographic evidence in one patient with poliomyelitis showed that SaO2% during non-REM sleep was maintained by accessory respiratory muscle activity. There was a direct relation between the lowest SaO2 value during REM sleep and vital capacity, daytime PaO2, PaCO2, and percentage fall in vital capacity from the erect to the supine position (an index of diaphragm weakness). The simple measurement of vital capacity in the erect and supine positions and arterial blood gas tensions when the patient is awake provide a useful initial guide to the degree of respiratory failure occurring during sleep in patients with neuromuscular disorders. A sleep study is required to assess the extent of sleep induced respiratory failure accurately.
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