Real-time heart rate variability analysis as a means of hypercapnia detection

Carbon dioxide builds up silently in a closed-circuit diver, and by the time symptoms arrive it can be too late. This study asked whether the heartbeat itself gives earlier warning. In fifteen subjects breathing progressively higher CO2, the beat-to-beat pattern grew measurably more erratic before any symptoms appeared, and returned to baseline on clean air, a signal in under two minutes. It establishes that the signal exists. It does not establish that a wearable using it would hold up in cold water, under exertion, with a working diver’s task loading.