Characterizing the repeatability of cardiovascular responses to hypoxic apneas in adults

Apneas performed during hypoxia cause pronounced bradycardia and cardiac arrhythmias. What remains unclear is the intraindividual consistency of these responses. Therefore, we assessed the within-day and between-day repeatability of responses to hypoxic apneas, including the genesis of arrhythmias or electrocardiogram (ECG) changes. Sixteen participants (8F/8M, 25 ± 3 years, BMI 23.7 ± 3.4 kg/m², mean ± SD) completed four hypoxic apneas per day on consecutive days, time-matched to a maximal apnea on day 1 (39 ± 16 s). We quantified heart rate (HR) and mean arterial pressure (MAP) repeatability using within-subject standard deviations (WSSD), [95%CI]. We hypothesized that WSSDs would be <5 bpm and <4 mmHg for HR and MAP. We also assessed the repeatability of arrhythmias or ECG changes. Within-day, the HR WSSD was 4.4 bpm [3.7, 5.6], and the MAP WSSD was 4.1 mmHg [3.4, 5.1]. Between-day comparisons had somewhat higher WSSDs: HR was 5.7 bpm [5.0, 6.5] and MAP was 5.0 mmHg [4.4, 5.8]. Arrhythmias or ECG changes occurred during 45% of apneas. These data indicate poor repeatability of electrical conduction changes during apneas. Conversely, our WSSD data indicate good within-day and between-day repeatability, albeit somewhat higher than hypothesized. These data therefore indicate that hypoxic apneas are a repeatable laboratory stressor for HR and MAP, but that researchers should exercise caution in interpreting individual-level arrhythmia data.