The ups and downs of heart rate during sleep: a source of heterogeneity in individuals with obstructive sleep apnoea.

STUDY OBJECTIVES: The heart rate (HR) of patients with obstructive sleep apnea (OSA) exhibits cyclical variation during sleep-disordered breathing (SDB), decreasing during apnoeas/hypopnoeas and increasing abruptly when air exchange resumes. It is not yet known whether this variation exceeds spontaneous HR variation during periods without SDB (noSDB), thereby giving rise to measurable chronotropic effects. This study aimed to address this knowledge gap by comparing the distribution of HR during SDB and noSDB. METHODS: Instantaneous HR was derived from the ECG signal recorded during standard polysomnography in 135 patients with moderate-to-severe OSA. HR distributions during noSDB and SDB were characterized using a set of relevant percentiles and probabilities for each state. Four dominant chronotropic effects were defined based on changes in these descriptors from noSDB to SDB: bradycardic (B), tachycardic (T), bradycardic and tachycardic (B-T), and neither (noB-T). RESULTS: 15.5%, 48.9%, 32.6%, and 3.0% of the patients were classified into groups B, T, noB-T and B-T (excluded due to negligible sample size). The frequency of troughs below spontaneous HR variation was higher in group B than in groups T and noB-T (p<0.0001). Furthermore, patients in group B had a higher apnoea/hypopnoea index (p<0.01), oxygen desaturation index (p<0.01) and hypoxic burden (p<0.05). There was no significant difference in age, sex, or BMI between the groups. CONCLUSIONS: Patients with OSA exhibit clear heterogeneity in the overall chronotropic effects elicited by respiratory events. These different responses are associated with different degrees of OSA severity and hypoxic burden, although anthropometric and clinical characteristics remain consistent.