Combined index of heart rate variability and oximetry in screening for the sleep apnoea/hypopnoea syndrome

Many sleep centres employ a preliminary screening test in order to reduce the number of polysomnographies required in the routine diagnosis of the sleep apnoea/hypopnoea syndrome (SAHS). We investigated the combination ofheart rate and oximetry information as a means of performing this test. A retrospective study of 100 patients with suspected SAHS was made. All patients had in-hospital polysomnography on one night. We estimated the number of respiratory event-related arousals by counting the number of autonomic arousals (assessed on the basis of changes in the heart interbeat interval) that were coincident with a rise in oximetry. The hourly index of such events was denoted the 'cardiac-oximetry disturbance index' (CODI). The median apnoea/hypopnoea index (AHI) was 16.5 (range 1.0-93.6) h - 1 . The CODI correlated significantly with the AHI (Spearman correlation coefficient r s = 0.88, P < 0.01), and the area (′ standard error) under the receiver operating characteristic (ROC) was 0.94 ′ 0.05. Oximetry alone (based on 4% dips) was a less effective screening test (r s = 0.80. P < 0.01; area under ROC 0.83 ′ 0.06). Using 2% dips in oximetry offered comparable performance with the CODI (r s = 0.91, P < 0.01; area under ROC 0.93 ′ 0.04). The CODI was better correlated with the electroencephalograph arousal index (r s = 0.84, P < 0.01) than was oximetry (2% dips, r s =11.57. P < 0.01). The CODI algorithm also offers an informal measure of self-validation: a large discrepancy between the number of autonomic arousals and the number of rises in oximetry indicates the presence of autonomic arousals without changes in oximetry (or vice versa). This self-validation mechanism identified several patients in this study, and may be useful in identifying sleep disruption due to chronic pain or other causes.