Sympathetic nerve activity and simulated diving in healthy humans

Abstract

The goal of our study was to develop a simple and practical method for simulating diving in humans using facial cold exposure and apnea stimuli to measure neural and circulatory responses during the stimulated diving reflex. We hypothesized that responses to simultaneous facial cold exposure and apnea (simulated diving) would be synergistic, exceeding the sum of responses to individual stimuli. We studied 56 volunteers (24 female and 32 male), average age of 39 years. All subjects were healthy, free of cardiovascular and other diseases, and on no medications. Although muscle sympathetic nerve activity (MSNA), blood pressure, and vascular resistance increased markedly during both early and late phases of simulated diving, significant reductions in heart rate were observed only during the late phase. Total MSNA during simulated diving was greater than combined MSNA responses to the individual stimuli. We found that simulated diving is a powerful stimulus to sympathetic nerve traffic with significant bradycardia evident in the late phase of diving and eliciting synergistic sympathetic and parasympathetic responses. Our data provide insight into autonomic triggers that could help explain catastrophic cardiovascular events that may occur during asphyxia or swimming, such as in patients with obstructive sleep apnea or congenital long QT syndrome.

Keywords: Arrhythmias; Bradycardia; Diving; Long QT syndrome; Microneurography; Sympathetic nervous system.

Figure 1

Original recordings of ECG, BP, MSNA, and respiration during baseline, apnea, facial cold, and simulated diving (facial cold and apnea). Note increase in MSNA with bradycardia during simulated diving.

Figure 2

Neural circulatory responses to facial cold and simulated diving. BP, MSNA, and VR increase significantly during facial cold and simulated diving with significant bradycardia during the late phase of simulated diving. Data are means ±SD. * P<0.05 vs baseline

Figure 3

Average neural and circulatory changes during baseline, apnea, facial cold, and simulated diving. Mean BP, MSNA, and VR increase significantly during facial cold and simulated diving without significant changes in HR. Data are means ±SD. *, P<0.05 vs baseline; †, P<0.05 vs apnea; ‡, P<0.05 vs facial cold

Figure 4

Neural circulatory changes from baseline during early and late phases of apnea, facial cold, and simulated diving. MSNA increases in the early phase of facial cold and increases further in the late phase of simulated diving. BP and VR also increase during the late phase of facial cold and into the late phase of simulated diving. A significant fall in HR was observed only during the late phase of simulated diving. The effect of simulated diving was synergistic, greater than the sum of the individual effects of either apnea or facial cold. Data are means ±SD. *, P<0.05 vs baseline; †, P<0.05 vs apnea; ‡, P<0.05 vs facial cold