Skin sympathetic nerve activity component synchronizing with cardiac cycle is involved in hypovolaemic suppression of cutaneous vasodilatation in hyperthermia

Abstract

Non-technical summary Thermoregulatory responses during exercise are reduced following thermal dehydration. If individuals do not rehydrate adequately, it could lead to heat exhaustion or stroke with the worst case scenario being death. Plasma volume loss during dehydration has been suggested to suppress cutaneous vasodilatation in response to hyperthermia via a baroreflex-mediated reduction in active vasodilator activity rather than enhanced active vasoconstrictor activity. However, no changes in the electrical signals of the efferent neural pathway have ever been identified. In the present study, we found a component of efferent skin sympathetic nerve activity that was synchronized with the cardiac cycle in thermally stressed individuals. This nerve activity increased with an increase in oesophageal temperature and the increase was significantly suppressed by hypovolaemia. Thus, this component of skin sympathetic nerve activity might represent the active vasodilator signals that regulate skin blood flow during hyperthermia in humans.Abstract Although cutaneous vasodilatation in hyperthermia was suppressed during hypovolaemia, the efferent neural pathway mediating this suppression has not been identified. To determine the electrical nerve signals which account for the suppression of cutaneous vasodilatation during hypovolaemia, skin sympathetic nerve activity (SSNA; microneurography) from the peroneal nerve, laser-Doppler blood flow (LDF) on the ipsilateral dorsal foot, mean arterial pressure (MAP; sonometry) and oesophageal temperature (T oes ) were measured before and during 45 min of passive warming in 20 healthy subjects during normovolaemia (n = 10) or hypovolaemia (n = 10) conditions. Hypovolaemia was achieved by diuretic administration. Cutaneous vascular conductance (CVC = LDF/MAP), SSNA burst frequency and total SSNA obtained from rectified and filtered SSNA signal increased as T oes increased by ∼0.5 • C by the end of warming in both groups. The increase in CVC was significantly lower in hypovolaemia than normovolaemia (P < 0.0001), but with no significant difference in the increase in burst frequency and total SSNA between groups (P > 0.32). However, using an alternative analysis that constructed spike incidence histograms from the original signal using 0.05 s bins during the 5 s following a given R-wave, we found a SSNA component synchronized with the cardiac cycle with a 1.1-1.3 s latency. This component increased with an increase in T oes and the increase was significantly suppressed by hypovolaemia (P < 0.0001). In conclusion, hypovolaemic suppression of cutaneous vasodilatation during hyperthermia might be caused by a reduction in the SSNA component synchronized with cardiac cycle. , plasma noradrenaline concentration; P osmol , plasma osmolality; PP, pulse pressure; PV, plasma volume; RH, relative humidity; RR, respiratory rate; SBP, systolic blood pressure; SkBF, skin blood flow; SR, sweat rate; SSNA, skin sympathetic nerve activity; T a , ambient temperature; T oes , oesophageal temperature;...

Sex differences in postsynaptic sweating and cutaneous vasodilation

“…Even if such a measurement could be obtained, the interpretation of skin sympathetic nerve activity is limited when used between independent groups, as bursts do not display synchronicity, have variable amplitudes, may contain multiple peaks, and the measurement may not be reproducible even within the same individual (31). Nonetheless, recent findings suggest that some components of the skin sympathetic nerve activity signal can be reliably used when assessing cutaneous vasodilation (18). Therefore, future studies are warranted to reliably investigate potential sex differences in skin sympathetic nerve activity to determine whether the lower sudomotor activity in females is also modulated by a central component of temperature regulation.…”

Subcutaneous nerve activity is more accurate than heart rate variability in estimating cardiac sympathetic tone in ambulatory dogs with myocardial infarction

“…In addition, SCNA morphology was similar to filtered skin and muscle sympathetic nerve activity obtained in microneurography studies (Online supplement Figure 1). 10, 11 The SCNA signals are more likely to contain unfiltered ECG signals as compared with the SGNA channel.…”

Plasma hyperosmolality attenuates skin sympathetic nerve activity during passive heat stress in humans

“…), possibly by affecting a cardio‐synchronous component of SSNA (Kamijo et al . ). In contrast, the physiological mechanism(s) by which plasma hyperosmolality modulates heat loss in humans has remained speculative.…”

“…), evidence suggests that hypovolaemia, and therefore baroreceptor unloading, suppresses SSNA (Kamijo et al . ). Therefore, the greater plasma volume expansion observed following i.v.…”