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

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 detect