Comparison of Indices Used to Detect Hypoventilation during Sleep

Because there is no uniform method of measuring the severity of sleep apnea, we compared respiratory indices calculated from airflow and oxyhemoglobin saturation (SaO2) signals in 16 subjects during a night's sleep. Airflow was measured with a loosely fitting pneumotachograph or thermister and the following indices calculated manually: total apneas (A) and hypopneas (H); A and H per hour of sleep; total A H time; average duration of A H episodes, and A H index (the product of 2 and 4). SaO2 was measured with a Hewlett-Packard ear oximeter and the following indices calculated with a microcomputer; total number of desaturation (D) episodes; D episodes per hour of sleep; average maximum D; D index (the product of 7 and 8); SaO2 50 (1), and SaO2 10(1). There was a significant correlation among all indices; the highest correlation was between total A and H and total D (r = 0.97). We conclude that SaO2 indices calculated with a microcomputer correlate well with flow indices.