Cardiopulmonary Adaptations and Maladaptations to the Chronic Intermittent Hypoxia Associated With Repetitive Apnea Diving

Abstract

Hypoxia (low oxygen) induces a reversible form of pulmonary hypertension which can be studied to elucidate the etiology of pulmonary hypertension and right heart dysfunction. Breath-hold divers (BHD) routinely place themselves into extremely hypoxemic (low blood oxygen) conditions and therefore repeatedly expose themselves to acute intermittent hypoxia and bouts of pulmonary hypertension and increased right heart work. The purpose of this study was to 1) determine if pulmonary arterial pressure and right heart dysfunction in hypoxia were greater in BHD compared to controls, and 2) determine the role of a pulmonary vasodilator in alleviating the increased pulmonary pressure in response to hypoxia. The hypotheses under hypoxic conditions were as followed 1) Right heart dysfunction will be the most severe amongst subjects with the greatest degree of pulmonary hypertension, regardless of group and 2) Sildenafil administration will result in a reduction of pulmonary arterial pressure compared to a placebo during hypoxia, regardless of group. Subjects (n=24, 12 BHD, 12 Control) completed two 30-minute isocapnic normobaric hypoxic breathing challenges, after receiving either 50mg sildenafil or placebo, with a 48-hour minimum washout period between visits. Pulmonary arterial systolic pressure (PASP) and right heart function via tricuspid anular plane systolic excursion (TAPSE) measures were made using Doppler ultrasound, and total pulmonary resistance (TPR) was calculated as PASP / cardiac output (Qt). Compared to placebo, BHD had a reduction in TPR with sildenafil in normoxia placebo (312.5 ± 92.8 vs 385 ± 122.6 dynes/sec/cm-5, respectively, p = .04). Compared to normoxia, PASP was increased with hypoxia in BHDc with placebo (22.9±6.0 vs. 32.4±8.9 mm Hg, respectively, p=0.0002) but not with sildenafil. Conversely, BHD had no changes in PASP with placebo, but had an increase in PASP from normoxia to hypoxia with sildenafil (22.3±4.8 vs. 31.5±6.9 mm Hg, respectively, p=0.0002). There was no effect of group or treatment on TAPSE. Our data suggests breath hold divers may have a previously unrecognized chronic pulmonary vasoconstriction in room air that is prevented with administration of sildenafil.