title: Heart rate response to breath holding at 18.6 ATA.
abstract: The heart rate (HR) responses to breath-holding (BH) with the without face immersion (FI) in 31 or 27 degrees C water was studied in 1 ATA air and hyperbaric He-O2 environments in 4 male subjects during a dry saturation dive to simulated depth of 580 ft (18.6 ATA). When a 60 sec BH or FI was performed while leaning forward, there was a significant linear correlation between the maximal bradycardial response (delta HRmax) and ambient pressure for simple BH (r-0.08, P less than 0.05) and 31 degrees C FI (r = 0.91, P less than 0.01), but not for 27 degrees C FI. A similar trend was seen during 30 sec BH's while seated erect. The facial cold-dependent component of the FI bradycardia was not significantly altered by pressure. In general, there were significant correlations between the initial HR and the initial thoracic conductive volume (TCV; measured by the four-electrode Minnesota impedance cardiograph), and between the initial TCV and delta HRmax observed during seated erect BH's. Since the TCV was generally higher at depth, it is suggested that a mechanical effect due to increased TCV at depth, possibly related to increased gas density, is at least partly responsible for the pressure dependence of BH bradycardia.
mesh_terms: D001274:Atmospheric Pressure; D004242:Diving; D005145:Face; D006321:Heart / Q000502:physiology; D006339:Heart Rate; D006371:Helium; D006801:Humans; D006931:Hyperbaric Oxygenation; D007101:Immersion; D008297:Male; D011187:Posture; D012119:Respiration; D013696:Temperature
chemical_list: D006371:Helium