Hemoglobin Concentration and Blood Shift During Dry Static Apnea in Elite Breath-Hold Divers
Authors: Thomas Kjeld, Thomas O. Krag, Anders Brenøe, Ann Merete Møller, Henrik Christian Arendrup, Jens Højberg, Dan Fuglø, Søren Hancke, Lars Poulsen Tolbod, Lars Christian Gormsen, John Vissing, Egon Godthaab Hansen
DOI / Source: https://doi.org/10.3389/fphys.2024.1305171
Date: 30 April 2024
Reading level: Intermediate
Why This Matters for Freedivers
This study helps explain why elite divers can tolerate extremely low oxygen: they may have a slightly higher hemoglobin concentration (more oxygen-carrying capacity) and a real “blood shift” during apnea. It also challenges a popular idea that the spleen is the main blood reservoir in humans during apnea—your legs may contribute even more to centralizing blood during a long breath-hold.
Synopsis
Freedivers often talk about “blood shift” as if it’s one simple thing: the spleen contracts, extra red blood cells enter circulation, and blood moves toward the chest to protect the lungs and support the heart and brain. This paper tests that story with unusually direct measurements in elite breath-hold divers, and the results are both clarifying and surprising.
The researchers compared 11 elite male breath-hold divers (all capable of at least 5-minute apneas, several internationally ranked) with 11 highly trained controls. They first measured hemoglobin and 2,3-BPG (a molecule in red blood cells that affects how easily oxygen is released to tissues). Then, in the divers, they measured where blood volume seems to move during a maximum dry static apnea after warm-up apneas.
They looked at three main “compartments”: - Heart muscle mass (as a proxy for blood volume in the heart wall) using cardiac MRI and ¹⁵O-H₂O PET/CT. - Spleen volume using ultrasound. - Lower-extremity volume/mass using DXA scanning (the idea: legs shouldn’t change mass during a breath-hold unless blood volume shifts out of them).
Here’s what they found: - The divers had higher hemoglobin concentration than matched controls, while 2,3-BPG was similar. That points to a potential oxygen-carrying adaptation without a major shift in oxygen affinity. - During a maximum apnea, the spleen shrank by about 102 mL (a clear spleen contraction). - The legs lost about 268 mL of volume (interpreted as blood moving out of the lower extremities), which was ~162% more than the volume change attributed to the spleen. - Despite this centralization, left ventricular myocardial mass did not increase during apnea, suggesting the blood shift is not “toward the heart” in the way earlier animal work (in sedated seals) implied.
Overall, this study reframes “blood shift” in humans: yes, the spleen participates, but the lower extremities are at least as important a blood reservoir, and the redistributed blood may be going somewhere other than the heart—potentially toward the abdomen and other organs. It also supports the idea that elite apnea training may nudge oxygen-carrying capacity upward, even if still within normal clinical ranges.
Abstract
Introduction: Elite breath-hold divers can tolerate extremely low arterial oxygen levels and show low oxygen consumption in heart and skeletal muscle, similar to diving mammals. Diving mammals often have higher hemoglobin concentration and demonstrate blood redistribution during apnea. This study tested whether similar patterns occur in elite human breath-hold divers.
Methods: Hemoglobin and 2,3-biphosphoglycerate (2,3-BPG) concentrations were measured at rest in elite breath-hold divers (n = 11) and matched controls (n = 11). In the divers, left ventricular myocardial mass was assessed at rest and during apnea using ¹⁵O-H₂O PET/CT and cardiac MRI, spleen volume was assessed with ultrasonography, and lower extremity volume changes were assessed using DXA during maximum dry static apnea following apnea warm-up.
Results: Left ventricular myocardial mass was unchanged compared with rest after 2–4 minutes of apnea. During maximum apnea (~6 minutes), lower extremity volume decreased by ~268 mL and spleen volume decreased by ~102 mL. Compared with controls matched for age, BMI, VO₂max, and spleen size, divers had similar spleen size and 2,3-BPG concentration, but higher hemoglobin concentration.
Conclusion: These findings suggest (1) elite apnea training may be associated with increased hemoglobin concentration as an oxygen-conserving adaptation, (2) blood shift during dry apnea appears to come more from the lower extremities than from the spleen, and (3) unlike previous observations in sedated diving mammals, the blood shift in humans during dry apnea does not appear to be directed toward the heart.