The oxygen-conserving potential of the diving response: A kinetic-based analysis

We investigated the oxygen-conserving potential of the human diving response by comparing trained breath-hold divers (BHDs) to non-divers (NDs) during simulated dynamic breath-holding (BH). Changes in haemodynamics [heart rate (HR), stroke volume (SV), cardiac output (CO)] and peripheral muscle oxygenation [oxyhaemoglobin ([HbO(2)]), deoxyhaemoglobin ([HHb]), total haemoglobin ([tHb]), tissue saturation index (TSI)] and peripheral oxygen saturation (SpO(2)) were continuously recorded during simulated dynamic BH. BHDs showed a breaking point in HR kinetics at mid-BH immediately preceding a more pronounced drop in HR (-0.86bpm.%(-1)) while HR kinetics in NDs steadily decreased throughout BH (-0.47bpm.%(-1)). By contrast, SV remained unchanged during BH in both groups (all P>0.05). Near-infrared spectroscopy (NIRS) results (mean +/- SD) expressed as percentage changes from the initial values showed a lower [HHb] increase for BHDs than for NDs at the cessation of BH (+24.0 +/- 10.1 vs. +39.2 +/- 9.6%, respectively; P<0.05). As a result, BHDs showed a [tHb] drop that NDs did not at the end of BH (-7.3 +/- 3.2 vs. -3.0 +/- 4.7%, respectively; P<0.05). The most striking finding of the present study was that BHDs presented an increase in oxygen-conserving efficiency due to substantial shifts in both cardiac and peripheral haemodynamics during simulated BH. In addition, the kinetic-based approach we used provides further credence to the concept of an oxygen-conserving breaking point in the human diving response.