Mental stress may cause high gas consumption and accelerated heart rate in fast-descending divers

The descent is a critical part of a dive, both physically and mentally. Pulmonary ventilation, expressed as respiratory minute volume (RMV) and heart rate (HR) were recorded during fast and slow descents to 35 meters (m) in open water while breathing compressed air, and during swimming horizontally at moderate velocity at an 11-m depth. Values of both types of descents were compared with reference values recorded at 11 m, the "plateau" phase, halfway through the 35-m dives. It is hypothesized that the "slow-descent" and "plateau-phase" values will be less than 'fast-descent values. Depth, cylinder pressure, water temperature and HR were recorded with a dive computer yielding time-averaged means (mRMV and mHR) for the descent and for plateau. Of the 18 divers included, 16 performed the fast descents and 11 made the slow descents. The fast descents (23 m·min-1 vertically), performed with 0-8 fin kicks, yielded mRMVDescent=28 ambient L (aL)·min-1, which is 82% higher (P⟨0.001) than mRMVPlateau of 15 aL·min-1. Further, mHRDescent was121 beats·min-1 23% higher (P⟨0.001), than mHRPlateau of 100 bpm. Slow descents (2.4 m·min-1 vertically) yielded 17 aL·min-1 with mHR=101 beats·min-1, values only slightly higher than at Plateau. The 11-m dive (swimming horizontally) yielded 24 m·min-1 with 32 fin kicks·min-1, mRMV=35 aL·min-1 and mHR=115 beats·min-1. Fast descents cause a higher RMV and HR that cannot be explained by physiology alone. Presumably mental stress is a main contributor. For dives deeper than 20 m, a descent velocity of 10 m·min-1 is recommended to reduce cardiac stress, in particular for older divers.