Nocturnal Hypoxaemia in Chronic Obstructive Pulmonary Disease

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Abstract
Day and night arterial and O2 saturation (SaO2) has been measured in 41 patients with chronic obstructive pulmonary disease (COPD), mean FEV1 [functional expiratory volume in 1 s] 0.84 (range 0.4-1.4) liters, and with a range of daytime SaO2 values of 67-95%. The mean and biggest falls in SaO2 at night were much greater in the patients with lower daytime saturations. When falls in arterial O2 tension (PaO2) were estimated from the decreases in SaO2, there was no correlation between the estimated biggest fall in PaO2 and daytime SaO2 and only a weak correlation between estimated mean fall in PaO2 and daytime SaO2. Measurement of ventilation in 4 hypoxemic patients with COPD (range 60-90% SaO2) by respiratory inductance plethysmography showed that nocturnal hypoxemic dips were accompanied by diminished ventilation, which was not always shown by nasal thermistors. Because nocturnal hypoxemic dips are transient the ideal alveolar-arterial O2 difference, which assumes a constant respiratory exchange ratio, cannot be used to assess the mechanism of hypoxemia. Erythrocyte mass was strongly correlated with daytime SaO2 but this correlation was not significantly improved by including nocturnal hypoxemia in the regression. Greater falls in SaO2 at night are related to lower initial SaO2 values, and the cause may be a reduction in ventilation.