Neural Circulatory Responses To Simulated Diving

Although less marked than in mammals, bradycardia due to increased vagal activity during simulated diving has been observed in humans. Little is known about the sympathetic neural responses. The goal of the present study was to measure the neural and circulatory responses to simulated diving (simultaneous facial cold and apnea). We studied 56 healthy volunteers (24 female and 32 male). Muscle sympathetic nerve activity (MSNA), blood pressure and systemic vascular resistance increased markedly during simulated diving. Significant reductions in heart rate were observed only during the late phase of simulated diving. The MSNA response during simulated diving was greater than the combined MSNA responses to the individual stimuli of either facial cold or apnea. Diving is a powerful stimulus to sympathetic nerve traffic with significant bradycardia. Diving elicits synergistic sympathetic and parasympathetic responses, greater than the sum of the individual responses to facial cold exposure and apnea. These data provide insight into autonomic triggers that could help explain catastrophic cardiovascular events which may occur during asphyxia or swimming, such as in patients with congenital long QT syndrome.