Hypoxia and Carbon Dioxide Retention Following Breath-Hold Diving
Authors: Poul-Erik Paulev, Noe Naeraa
DOI / Source: https://doi.org/10.1152/jappl.1967.22.3.436
Date: 1967
Reading level: Intermediate
Why This Matters for Freedivers
Freedivers often judge readiness for the next dive by “how normal they feel” after a few recovery breaths. This classic study shows why that can be misleading during repetitive deeper dives: CO₂ can be retained across dives and only “paid back” later, creating hidden respiratory acidosis and potentially adding risk (dizziness, confusion, poor decisions, and increased chance of loss of consciousness). It also reinforces why aggressive hyperventilation is a risky trade-off: it may extend the dive, but it can make you feel weird on descent and can mask warning signals.
Synopsis
This paper looks at something that still catches freedivers out today: what happens when you do repeated breath-hold dives with short surface intervals. Most early research focused on single dives, but real-world diving often involves many dives in a row—especially spearfishing and training sets.
Six experienced subjects performed repeated dives to 18.5 m in fresh water with total dive times ranging from 36 to 124 seconds, and surface intervals of 60–120 seconds. The divers used a specific technique: before each dive they exhaled to residual volume and then inhaled a measured volume, meaning they started each dive with a controlled (but not full) lung volume. The researchers measured oxygen uptake and CO₂ movement by collecting and analyzing expired air before and after dives, and they also sampled alveolar gases.
The results are striking: during a “mean” ~42-second dive (average of seven dives), about 650 ml of O₂ (STPD) moved from the lungs into the body, while only about 260 ml of CO₂ (STPD) moved from the body into the lungs. That mismatch implies that a meaningful amount of CO₂ is being held back in the body during each dive. The really important part is what happens next: after a seven-dive series (total submerged time ~330 seconds), the divers showed an excess CO₂ elimination over the following minutes—around 3,500 ml (STPD) in the first 16 minutes after the series—evidence of delayed “CO₂ washout.”
The authors argue that in deeper repetitive breath-hold diving, danger may not be only “low oxygen.” CO₂ retention and the resulting acidosis may add an extra layer of risk, and at depth the elevated CO₂ levels can also contribute to narcotic-like symptoms. Their conclusion is blunt for its time: CO₂ retention may be more dangerous in repetitive breath-hold diving than previously believed, especially when divers stack dives with short rests.
Abstract
Experimental data from six subjects performing repeated breath-hold dives to 62 ft (18.5 m) in fresh water are presented. Rate of descent was 0.8 m/sec, ascent 1.5 m/sec, and diving time 36–124 sec. Surface intervals varied between 60 and 120 sec. The divers submerged with a lung volume containing residual volume plus 85% of vital capacity. About 650 ml (STPD) oxygen was transferred from the lungs to the body in 42-sec dives (mean of seven dives), while 260 ml carbon dioxide was given off from the body to the lungs. During the first 16 min after a seven-dive series (representing a total submerged time of 330 sec and a total surface time of 420 sec) an excess elimination of approximately 3,500 ml (STPD) carbon dioxide was found. Carbon dioxide retention with acute respiratory acidosis may—because of the repetitive factor—be more dangerous to breath-hold divers than hitherto believed.