Case Studies in Physiology: Is blackout in breath-hold diving related to cardiac arrhythmias?

Abstract

Syncope or "blackout" (BO) in breath-hold diving (freediving) is generally considered to be caused by hypoxia. However, it has been suggested that cardiac arrhythmias affecting the pumping effectivity could contribute to BO. BO is fairly common in competitive freediving, where athletes aim for maximal performance. We recorded heart rate (HR) during a static apnea (STA) competition, to reveal if arrhythmias occur. Four male freedivers with STA personal best (PB) of 349±43s, volunteered during national championships, where they performed STA floating face down in a shallow indoor pool. A non-coded Polar T31 chest strap recorded R-R intervals and a water- and pressure proof pulse oximeter arterial oxygen saturation. Three divers produced STA near their PB without problems, while one diver ended with BO at 5min17s, which was 12s beyond his PB. He was immediately brought up by safety divers and resumed breathing within 10s. All divers attained similar lowest diving HR (47±4bpm), but HR recordings displayed a different pattern for the diver ending with BO. After a short tachycardia the three successful divers developed bradycardia which became more pronounced during the second half of the apnea. The fourth diver developed pronounced bradycardia earlier, and at 2.5min into the apnea HR started alternating between approximately 50 and 140 bpm, until the diver lost consciousness. At resumed breathing, HR returned to baseline. Nadir oxygen saturation was similar for all divers. We speculate that arrhythmia could have contributed to BO, by lowering stroke volume leading to a systolic blood pressure drop, affecting brain perfusion.

Unlocking the depths: multiple factors contribute to risk for hypoxic blackout during deep freediving

“…The parasympathetically induced bradycardia, combined with sympathetically induced exercise tachycardia, may result in an “autonomic conflict” (Shattock and Tipton 2012 ; Costalat et al 2021 ), which is supported by the highly variable HR we observed. The observation of an irregular HR, especially during deep dives, may support the theory that arrhythmias arising as the result of an autonomic conflict can potentially cause blackout (Mulder et al 2023 ). An arrhythmic HR pattern was observed in one diver during a maximal static apnea, which resulted in a blackout despite similar dive duration and oxygen saturation as other divers in the same competition (Mulder et al 2023 ).…”

“…The observation of an irregular HR, especially during deep dives, may support the theory that arrhythmias arising as the result of an autonomic conflict can potentially cause blackout (Mulder et al 2023 ). An arrhythmic HR pattern was observed in one diver during a maximal static apnea, which resulted in a blackout despite similar dive duration and oxygen saturation as other divers in the same competition (Mulder et al 2023 ). Compared to shallow dives, an increased autonomic conflict in deep dives, arising from the greater exertion-related sympathetic stimulus versus the greater temperature- and lung compression-induced parasympathetic drive, is likelier.…”

Physiological Responses during Prolonged Immersed Static Apnea in Well-Trained Apneists

“…Blood pressure was not measured in the present study to evaluate its influence on HR response and apnea duration, but during prolonged static apnea, cardiac and vascular adjustments may be separately controlled [33,34]. Additionally, even though not evaluated in the present study, involuntary breathing movements, more probable to occur during static apnea, as well as cardiac arrhythmias, detected in the later phases of static apnea [19,35], may also contribute to the HR response.…”

“…Several studies aiming to investigate and understand apnea and its physiological responses have carried out experiments in the laboratory with face immersion (e.g., using water-filled containers) providing great knowledge to the scientific community [11][12][13][14], but there are scarce data on physiological responses of well-trained apneists during an official apnea competition or under conditions simulating an actual competition [15][16][17][18][19].…”

Effects of Water Immersion on the Internal Power of Cycling

“…A practical application of this study is evident. The need to quantify the equivalent mechanical power of cycling in water with respect to air could be crucial in several circumstances: the validation and testing of equipment for self-contained underwater breathing apparatus (29,30) and breath-hold diving (31,32), the comparison of the physiological responses between dry and wet exercise, either eupneic (13,33,34) or apneic (35)(36)(37), the proper prescription of water-based rehabilitation (33,38,39). In such cases, it could be of help to plot our overall difference in Ė between WET and DRY against f p 3 (Fig.…”