Pulmonary Vascular Adaptations to Hypoxia in Elite Breath-Hold Divers
Authors: Thomas Kjeld, Anders Brenøe Isbrand, Henrik Christian Arendrup, Jens Højberg, Jacob Bejder, Thomas O. Krag, John Vissing, Lars Poulsen Tolbod, Johannes Hendrik Harms, Lars Christian Gormsen, Dan Fuglø, Egon Godthaab Hansen
DOI / Source: https://doi.org/10.3389/fphys.2024.1296537
Date: 31 July 2024
Reading level: Advanced
Why This Matters for Freedivers
This study suggests that during a long max breath-hold, elite divers may reduce blood flow through the lungs, which changes how efficiently oxygen is taken up from the remaining air in the lungs. It’s a fascinating “diving-mammal-like” strategy that could help explain how top divers tolerate very low oxygen near the end of long apneas — and it also reminds us that the lungs and heart are working as one system during extreme breath-holds.
Synopsis
Most freedivers think of the lungs as the main oxygen store — but what really matters is how well that oxygen gets into the blood as the breath-hold goes on. This paper tests a bold idea: elite breath-hold divers might protect oxygen uptake during apnea by gradually reducing lung blood flow, increasing the “ventilation to perfusion” ratio (VA/Q). In simple terms, that means the lungs become more like a carefully managed oxygen-exchange surface rather than a fully perfused sponge — similar to strategies proposed in dolphins.
The researchers measured pulmonary blood volume (PBV) during maximum dry apneas using three different methods (echocardiography, PET/CT, and cardiac MRI) in a small group of elite divers. Across methods, PBV fell substantially as the apnea continued. After about 2 minutes, PBV was already down (roughly a quarter to two-fifths depending on method). After about 4 minutes, PBV fell even further (roughly half to two-thirds in the reported measurements). The main driver was a drop in cardiac output during apnea, meaning less blood was being pushed through the lungs.
They also measured lung function and compared divers to matched athletic controls. Divers had bigger lung volumes (especially alveolar volume), but when oxygen diffusion was adjusted for that bigger lung size, the divers showed a lower diffusion constant (KCO). That combination supports the idea that elite divers aren’t just “bigger lungs = better,” but may also be changing how the lungs are perfused during apnea.
Finally, arterial blood gases during a long pool apnea showed how extreme the hypoxia can get: the fraction of oxygenated hemoglobin dropped dramatically from normal resting values to very low levels by the end of the breath-hold. The authors interpret the overall pattern as a possible human version of a diving-mammal trick: reduce lung blood volume during apnea to shift VA/Q and preserve alveolar oxygen delivery as oxygen stores run down.
Abstract
Elite breath-hold divers tolerate profound hypoxia during long apneas. This study tested whether they show a diving-mammal-like pattern of reducing lung perfusion during apnea to shift the ventilation–perfusion relationship. Pulmonary blood volume was assessed during and after maximum apneas using echocardiography, PET/CT, and cardiac MRI. Across methods, pulmonary blood volume decreased during apnea, largely explained by a fall in cardiac output. Compared with matched controls, divers had larger lung volumes (including alveolar volume) but a lower diffusion constant when adjusted for lung size. Arterial blood gases during prolonged pool apneas demonstrated marked reductions in oxygenated hemoglobin by the end of apnea. The findings support the idea that elite breath-hold divers may reduce pulmonary blood volume during apnea, potentially increasing VA/Q and supporting oxygen delivery from the alveoli as oxygen stores decline.