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Article Dans Une Revue Journal of Applied Physiology Année : 2023

Pitfalls in indirect assessment of ectopic beats and parasympathetic modulation during voluntary breath-holding

Résumé

We read the recent case study by Mulder et al. (1) with interest. Their study aimed to investigate the effect of prolonged static breath-holding on beat-to-beat heart rate kinetics, using four elite breath-hold divers as participants. The investigators’ primary finding was that ectopic beats, speculatively inferred from a basic heart rate monitor (Polar T31), were potential contributors to blackouts. In our view, it is important to acknowledge both methodological and scientific limitations in the work conducted by Mulder et al. (1). Within the context of a maximal static voluntary breath-holding, the authors suggest that the observed alteration in R-R intervals indicate the presence of cardiac arrhythmias, specifically ectopic beats. Among the four studied breath-hold divers, we acknowledge that the sole diver who experienced a blackout exhibited a significantly higher proportion of artifacts, particularly during the second half of breath-holding. However, in our opinion, the authors’ interpretation that these artifacts result from ectopic beats remain speculative. It is widely recognized that breath-holding induces a progressive increase in carbon dioxide tension, triggering initial involuntary contractions of the diaphragm during breath-holding (2). Therefore, such “mechanical noise” might have significantly disrupted the accurate detection of R-R intervals by the heart rate monitor. The study conducted by Mulder et al. (1) did not include measurements of involuntary breathing movements. This is worth noting considering that 1) diaphragmatic contractions exhibit substantial interindividual variability among divers (2), and 2) their intensity varies in accordance with an individual’s level of training (3). Consequently, it is important not to disregard their potential implications. Next, the authors support their interpretation by relying on a noninvasive measurement of cardiac autonomic modulation. Specifically, the root mean square of successive difference (RMSSD) of R-R intervals was used to quantify cardiac parasympathetic modulation. In the early stage of breath-holding, a significant increase in RMSSD was found in the syncope-prone breath-hold divers. However, a careful examination of their methodological approach preclude drawing any conclusions regarding the effect of breath-holding on cardiac autonomic regulation. Indices derived from heart rate variability are highly sensitive to physiological artifacts such as ectopic beats or missed beats detections, especially during short recording periods (4). The study lacks a description of the methodology used for detecting and addressing potential artifacts, which is crucial. Considering the pronounced noise present in the R-R signal of the syncope-prone breath-hold diver, it is likely that the increase in RMSSD results from a biased analysis, thereby unrelated to any actual elevation in parasympathetic modulation. In addition, an increase in RMSSD at the beginning of apnea would contradict the existing literature on the subject. Our research group (3, 5) and others (6) consistently found no changes in RMSSD during the early stage of breath-holding. This can be explained by the strong dependence of heart rate variability on respiratory frequency and tidal volume. More intriguingly, the study by Muller et al. (1) suggested that the supposed increase in RMSSD contributed to the genesis of abnormal beats. This proarrhythmic effect is very unlikely, as parasympathetic stimulation lengthens the effective refractory period of cardiac tissue, ultimately reducing the likelihood of abnormal electrical pathways reactivating and causing ectopic beats (7). Prolonged breath-holding is often characterized by biphasic bradycardia, wherein the transition from the normoxic phase to the hypoxemic phase of breath-holding is marked by a significant additional drop in heart rate (3, 5, 8). Among the breath-hold divers examined in the study conducted by Muller et al. (1), three out of the four participants exhibited evident biphasic diving bradycardia. Notably, the breath-hold diver prone to syncope stood out as the only participant who did not display a distinct biphasic pattern. Thus, the lack of parasympathetic modulation, rather than its increase, would be an explanatory factor in the genesis of ectopic beats. In the study conducted by Mulder et al. (1), peripheral oxygen saturation (SpO2) was continuously recorded throughout breath-holding; unfortunately, only the nadir values were documented. For instance, the extend of the hypoxemic phase of breath-holding cannot be differentiated among the divers, which may potentially contribute to the occurrence of ectopic beats (8). Finally, we believe that relying on heart rate monitors to extrapolate ectopic beats has limited perspectives in the present context. Instead, the availability of chest straps that enable single-lead ECG recordings presents a more favorable approach, since allowing for direct assessment of any abnormal electrophysiological events. For example, the chest strap sensor developed by Movesense company has recently obtained certification as a Class IIa medical device, complying with the European Medical Device Regulation. Hence, we strongly advocate for their utilization in future studies involving real-world apnea contexts.
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Dates et versions

hal-04257661 , version 1 (25-10-2023)

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Frédéric Lemaître, Guillaume Raphaël Costalat. Pitfalls in indirect assessment of ectopic beats and parasympathetic modulation during voluntary breath-holding. Journal of Applied Physiology, 2023, 135 (4), pp.726-727. ⟨10.1152/japplphysiol.00363.2023⟩. ⟨hal-04257661⟩
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