The Oxygen-Conserving Potential of the Diving Response, A Kinetic-Based Analysis
Authors: Guillaume Costalat, Jeremy Coquart, Ingrid Castres, Fabrice Joulia, Olivier Sirost, Eric Clua, Frédéric Lemaître
DOI / Source: 10.1080/02640414.2016.1183809
Date: 11 May 2016
Reading level: Intermediate
Why This Matters for Freedivers
Freedivers often feel the diving response kick in, but this paper shows that trained divers don’t just have “more bradycardia”—they may have a distinct mid-apnea shift where the body becomes more aggressive about saving oxygen. That helps explain why experienced divers can keep working (finning) while their heart rate and muscle oxygen use drop in a way that looks like the body is actively protecting the brain and vital organs.
Synopsis
The diving response is your body’s built-in “oxygen saving mode” during apnea: heart rate drops, blood flow is redirected away from the limbs, and oxygen is rationed for organs that matter most. This study asked a simple but powerful question: Do trained breath-hold divers conserve oxygen more effectively than non-divers during a dynamic-style breath-hold?
To test this, the researchers compared trained breath-hold divers with people who had never trained apnea. Everyone did a maximal breath-hold while lightly pedaling (to simulate the “working” demand of dynamic apnea) with cold face immersion—a strong trigger for the diving response. They continuously recorded heart and circulation variables (heart rate, stroke volume, cardiac output), finger oxygen saturation (SpO₂), and muscle oxygenation in the leg using near-infrared spectroscopy (NIRS). They also took blood lactate after the breath-hold.
The standout finding is that trained divers didn’t just show a stronger overall bradycardia—they showed a clear “kink” in the curve: about halfway through the breath-hold, their heart rate pattern changed and began dropping faster. The authors call this an “oxygen-conserving breaking point.” After this point, trained divers showed a deeper reduction in heart rate and cardiac output than non-divers, even though stroke volume stayed roughly stable in both groups.
The muscle data helps explain what this might mean in real-world terms. As the breath-hold progressed, trained divers showed less rise in deoxygenated hemoglobin in the working leg muscle and a noticeable drop in total hemoglobin signal near the end—consistent with reduced muscle blood flow (less oxygen delivery to the legs) and therefore less oxygen being “spent” in the periphery. At the same time, trained divers had higher lactate after the breath-hold, which fits the idea that if you restrict oxygen delivery to working muscle, you push more toward anaerobic energy production.
Put together, the results support a practical picture: trained breath-hold divers may have a more “decisive” oxygen-saving response during working apnea—first a controlled phase, then a mid-apnea shift where the body clamps down harder on peripheral circulation and slows the heart more sharply. The authors argue this improves overall oxygen-conserving efficiency and strengthens the idea that there’s a meaningful “switch point” in the human diving response.
Abstract
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 ([HbO2]), deoxyhaemoglobin ([HHb]), total haemoglobin ([tHb]), tissue saturation index (TSI)] and peripheral oxygen saturation (SpO2) 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 (−0.86 bpm.%−1) while HR kinetics in NDs steadily decreased throughout BH (−0.47 bpm.%−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.