CIH: from sleep apnea to breath-hold diving

Abstract

Recurrent apneas associated with sleep-disordered breathing increase sympathetic nerve activity and arterial blood pressure, and enhance sympathetic and ventilatory responses to acute hypoxia [14,19]. Chronic intermittent hypoxia (CIH) and activation of arterial carotid body chemoreceptors are primarily responsible for driving the increase in sympathetic activity [4,17]. The excessive sympathetic activity and hypertension are sustained and contribute to high morbidity and mortality in afflicted patients [15,19].Certain recreational and vocational activities involve episodes of volitional apnea. One example is underwater breath-hold diving. In this issue of Clinical Autonomic Research, Breskovic and colleagues [1] address the question: Do elite breath-hold divers exhibit exaggerated increases in muscle sympathetic nerve activity (MSNA) and ventilation during exposure to acute hypoxia? Such a finding would raise concern that the CIH imposed by these types of activities may increase cardiovascular risk in otherwise healthy individuals.The investigators measured arterial oxygen saturation, ventilation, MSNA, blood pressure, heart rate and stroke volume in elite divers and healthy control subjects at baseline (while breathing room air) and during progressive isocapnic hypoxia [1]. Resting blood pressure and MSNA were not significantly different in divers versus control subjects. Furthermore, hypoxia-induced increases in ventilation and MSNA were also not different between the groups. While good news for the divers, these negative results raise several important questions.Is the degree of CIH in divers sufficient to induce sustained sympathetic activation?The frequency of apneas in elite breath-hold divers, at least during the diving season, has been reported to approach that seen in patients with sleep apnea [1][2][3]7]. Furthermore, while a single maximum end-inspiratory breath-hold has little effect on arterial oxygenation in control subjects, the longer maximum breath-hold times in elite divers (up to 6 min, world record = *10 min) are associated with marked arterial oxygen desaturation and pronounced increases in MSNA and blood pressure [5,7]. The chemoreceptor reflex is surely activated when elite divers breath-hold. Thus, it appears likely that the frequency of apneas and degree of hypoxemia were sufficient to induce sympathoexcitation and hypertension in the elite divers in this study. Nonetheless, the present study is limited by the lack of information on the frequency of near-maximum apneas experienced over time by the elite divers.Of greater significance is the 1-2 month delay between intense apnea training and the day of the study [1]. CIHinduced increases in chemoreceptor sensitivity and sympathetic activity in animal models can be reversed as soon as 10 days after terminating the CIH [17]. Effects of CIH may have dissipated before the divers were studied. On the other hand, the same group of investigators have reported