Cardiovascular autonomic modulation during passive heating protocols: a systematic review.

Objective.To conduct a systematic review of the possible effects of passive heating protocols on cardiovascular autonomic control in healthy individuals.Approach.The studies were obtained from MEDLINE (PubMed), LILACS (BVS), EUROPE PMC (PMC), and SCOPUS databases, simultaneously. Studies were considered eligible if they employed passive heating protocols and investigated cardiovascular autonomic control by spontaneous methods, such as heart rate variability (HRV), systolic blood pressure variability (SBPV), and baroreflex sensitivity (BRS), in healthy adults. The revised Cochrane risk-of-bias tool (RoB-2) was used to assess the risk of bias in each study.Main results.Twenty-seven studies were included in the qualitative synthesis. Whole-body heating protocols caused a reduction in cardiac vagal modulation in 14 studies, and two studies reported both increased sympathetic modulation and vagal withdrawal. Contrariwise, local-heating protocols and sauna bathing seem to increase cardiac vagal modulation. A reduction of BRS was reported in most of the studies that used whole-body heating protocols. However, heating effects on BRS remain controversial due to methodological differences among baroreflex analysis and heating protocols.Significance.Whole-body heat stress may increase sympathetic and reduce vagal modulation to the heart in healthy adults. On the other hand, local-heating therapy and sauna bathing seem to increase cardiac vagal modulation, opposing sympathetic modulation. Nonetheless, further studies should investigate acute and chronic effects of thermal therapy on cardiovascular autonomic control.

The role of the dorsomedial and ventromedial hypothalamus in regulating behaviorally coupled and resting autonomic drive.

Nearly a century ago it was reported that stimulation of the hypothalamus could evoke profound behavioral state changes coupled with altered autonomic function. Since these initial observations, further studies in animals have revealed that two hypothalamic regions-the dorsomedial and ventromedial hypothalamic nuclei-are critical for numerous behaviors, including those in response to psychological stressors. These behaviors are coupled with changes in autonomic functions, such as altered blood pressure, heart rate, sympathetic nerve activity, resetting of the baroreflex and changes in pituitary function. There is also growing evidence that these two hypothalamic regions play a critical role in thermogenesis, and suggestions they could also be responsible for the hypertension associated with obesity. The aim of this chapter is to review the anatomy, projection patterns, and function of the dorsomedial and ventromedial hypothalamus with a particular focus on their role in autonomic regulation. While most of what is known about these two hypothalamic regions is derived from laboratory animal experiments, recent human studies will also be explored. Finally, we will describe recent human brain imaging studies that provide evidence of a role for these hypothalamic regions in setting resting sympathetic drive and their potential role in conditions such as hypertension.