Reduction in respiratory muscle strength following a series of preparatory and maximal static, dry breath-holds

Breath-hold diving poses significant physiological challenges to maintaining O2 homeostasis. During a maximal breath-hold, numerous, subsequently augmenting, involuntary respiratory muscle contractions against a closed glottis (i.e. involuntary breathing movements, IBMs) may elicit respiratory muscle fatigue, especially under conditions of arterial hypoxemia. Thus, the purpose of this study was to determine whether respiratory muscle fatigue occurs following a series static breath-holds in 14 trained male divers. To do so, we measured transdiaphragmatic twitch pressure (PDI,TW), maximal inspiratory pressure (MIP), and maximal expiratory pressure (MEP) before and 8 min after three preparatory breath-holds (separated by 2.5 min; average duration = 185 ± 69 s and range = 62-309 s) and three maximal breath-holds (separated by 5 min; average duration = 308 s and range = 179-733 s). We calculated a pseudo O2 delivery as heart rate × peripheral O2 saturation. There was a significant reduction of PDI,TW following the breath-hold series (−8.5 ± 13.7%; range: +7 - (−37)%; p = 0.03). Similarly, we found that MIP and MEP decreased by 6.3 ± 10.6% (p = 0.04) and 8.6 ± 10.4% (p = 0.002), respectively. Using an individualized means of determining the presence of fatigue, we observed that some (~50%), but not all divers incurred respiratory muscle fatigue. Those who developed fatigue had a lower pseudo O2 delivery during the struggle phase than those who did not (p=0.01). Our findings suggest that respiratory muscle fatigue was more common in those divers who experienced a greater severity of arterial hypoxemia during the breath-holding protocol.