Vascular responses to simulated breath-hold diving involving multiple reflexes.

Breath-hold diving evokes a complex cardiovascular response. The degrees of hypertension induced by the diving reflex are substantial and accentuated by the underwater swimming. This condition provides a circulatory challenge to properly buffer and cushion cardiac pulsations. We determined hemodynamic changes during the diving maneuver and hypothesized that central artery compliance would be augmented during simulated breath-hold diving. A total of 20 healthy young adults were studied. Hemodynamics were measured during exercise on a cycle ergometer, apnea, face immersion in cold water (trigeminal stimulation), and simulated breath-hold diving. Arterial compliance was measured by recording the carotid artery diameter from images derived from an ultrasound machine at the cephalic portion of the common carotid artery 1-2 cm proximal to the carotid bulb, whereas arterial pressure waveforms were obtained using an arterial tonometry placed on the contralateral carotid artery and recorded on a data acquisition software. The change in diameter was divided by the change in blood pressure to calculate arterial compliance. Arterial compliance increased with simulated diving compared with rest (P = 0.007) and was elevated compared with exercise and apnea alone (P < 0.01). A significant increase in heart rate was observed with exercise, apnea, and facial immersion when compared with rest (P < 0.001). However, simulated diving brought the heart rate down to resting levels. Cardiac output increased with all conditions (P < 0.001), with an attenuated response during simulated diving compared with exercise and facial immersion (P < 0.05). Mean blood pressure was elevated during all conditions (P < 0.001), with a further elevation observed during simulated diving compared with exercise (P < 0.001), apnea (P = 0.016), and facial immersion (P < 0.001). Total peripheral resistance was decreased during exercise and facial immersion compared with rest (P < 0.001) but was increased during simulated diving compared with exercise (P < 0.001), apnea (P = 0.008), and facial immersion (P = 0.003). We concluded that central artery compliance is augmented during simulated breath-hold diving to help buffer cardiac pulsations.