Effects of postural change from supine to head-up tilt on the skin sympathetic nerve activity component synchronised with the cardiac cycle in warmed men

Human temperature regulation under heat stress in health, disease, and injury

The human body constantly exchanges heat with the environment. Temperature regulation is a homeostatic feedback control system that ensures deep body temperature is maintained within narrow limits despite wide variations in environmental conditions and activity-related elevations in metabolic heat production. Extensive research has been performed to study the physiological regulation of deep body temperature. This review focuses on healthy and disordered human temperature regulation during heat stress. Central to this discussion is the notion that various morphological features, intrinsic factors, diseases, and injuries independently and interactively influence deep body temperature during exercise and/or exposure to hot ambient temperatures. The first sections review fundamental aspects of the human heat stress response, including the biophysical principles governing heat balance and the autonomic control of heat loss thermoeffectors. Next, we discuss the effects of different intrinsic factors (morphology, heat adaptation, biological sex, and age), diseases (neurological, cardiovascular, metabolic, and genetic), and injuries (spinal cord injury, deep burns, and heat stroke), with emphasis on the mechanisms by which these factors enhance or disturb the regulation of deep body temperature during heat stress. We conclude with key unanswered questions in this field of research.

Rapid saline infusion and/or drinking enhance skin sympathetic nerve activity components reduced by hypovolaemia and hyperosmolality in hyperthermia

KEY POINTS

In hyperthermia, plasma hyperosmolality suppresses both cutaneous vasodilatation and sweating responses and this suppression is removed by oropharyngeal stimulation such as drinking. Hypovolaemia suppresses only cutaneous vasodilatation, which is enhanced by rapid infusion in hyperthermia. Our recent studies suggested that skin sympathetic nerve activity (SSNA) involves components synchronized and non-synchronized with the cardiac cycle, which are associated with an active vasodilator and a sudomotor, respectively. In the present study, plasma hyperosmolality suppressed both components; drinking removed the hyperosmolality-induced suppressions, simultaneously with increases in cutaneous vasodilatation and sweating, while not altering plasma volume and osmolality. Furthermore, a rapid saline infusion increased the synchronized component and cutaneous vasodilatation in hypovolaemic and hyperthermic humans. The results support our idea that SSNA involves an active cutaneous vasodilator and a sudomotor, and that a site where osmolality signals are projected to control thermoregulation is located more superior than the medulla where signals from baroreceptors are projected.

ABSTRACT

We reported that skin sympathetic nerve activity (SSNA) involved components synchronized and non-synchronized with the cardiac cycle; both components increased in hyperthermia and our results suggested that the components are associated with an active vasodilator and a sudomotor, respectively. In the present study, we examined whether the increases in the components in hyperthermia would be suppressed by plasma hyperosmolality simultaneously with suppression of cutaneous vasodilatation and sweating and whether this suppression was released by oropharyngeal stimulation (drinking). Also, effects of a rapid saline infusion on both components and responses of cutaneous vasodilatation and sweating were tested in hypovolaemic and hyperthermic subjects. We found that (1) plasma hyperosmolality suppressed both components in hyperthermia, (2) the suppression was released by drinking 200 mL of water simultaneously with enhanced cutaneous vasodilatation and sweating responses, and (3) a rapid infusion at 1.0 and 0.2 ml min^-1  kg^-1 for the first 10 min and the following 20 min, respectively, increased the synchronized component and cutaneous vasodilatation in diuretic-induced hypovolaemia greater than those in a time control; at 0.1 ml min^-1  kg^-1 for 30 min no greater increases in the non-synchronized component and sweating responses were observed during rapid infusion than in the time control. The results support the idea that SSNA involves components synchronized and non-synchronized with the cardiac cycle, associated with the active cutaneous vasodilator and sudomotor, and a site of osmolality-induced modulation for thermoregulation is located superior to the medulla where signals from baroreceptors are projected.

Skin sympathetic nerve activity as a biomarker for syncopal episodes during a tilt table test

BACKGROUND

Autonomic imbalance is the proposed mechanism of syncope during a tilt table test (TTT). We have recently demonstrated that skin sympathetic nerve activity (SKNA) can be noninvasively recorded using electrocardiographic electrodes.

OBJECTIVE

The purpose of this study was to test the hypothesis that increased SKNA activation precedes tilt-induced syncope.

METHODS

We studied 50 patients with a history of neurocardiogenic syncope undergoing a TTT. The recorded signals were band-pass filtered at 500-1000 Hz to analyze nerve activity.

RESULTS

The average SKNA (aSKNA) value at baseline was 1.38 ± 0.38 μV in patients without syncope and 1.42 ± 0.52 μV in patients with syncope (P = .77). On upright tilt, aSKNA was 1.34 ± 0.40 μV in patients who did not have syncope and 1.39 ± 0.43 μV in patients who had syncope (P = .65). In all 14 patients with syncope, there was a surge of SKNA before an initial increase in heart rate followed by bradycardia, hypotension, and syncope. The peak aSKNA immediately (<1 minute) before syncope was significantly higher than baseline aSKNA (2.63 ± 1.22 vs 1.39 ± 0.43 μV; P = .0005). After syncope, patients were immediately placed in the supine position and aSKNA dropped significantly to 1.26 ± 0.43 μV; (P = .0004). The heart rate variability during the TTT shows a significant increase in parasympathetic tone during syncope (low-frequency/high-frequency ratio: 7.15 vs 2.21; P = .04).

CONCLUSION

Patients with syncope do not have elevated sympathetic tone at baseline or during the TTT except immediately before syncope when there is a transient surge of SKNA followed by sympathetic withdrawal along with parasympathetic surge.

Interactions between body fluid homeostasis and thermoregulation in humans

Humans are unique in their ability to control body temperature with a large amount of skin blood flow and sweat rate while exercising in an upright position. However, cutaneous vasodilation in the body reduces total peripheral resistance and blood pooling in cutaneous veins decreases venous return to the heart and cardiac filling pressure. In addition, hypovolemia by sweating accelerates the reduction in cardiac filling pressure. These may threaten the maintenance of blood pressure if they are not compensated for. To prevent this, cutaneous vasodilation and sweat rate are suppressed by baroreflexes or hyperosmolality with dehydration. These mechanisms suppress heat dissipation, accelerate the increase in body temperature, and sometimes cause heat stroke. As a countermeasure to prevent this, we have recommended glucose electrolyte solutions but recently found that aerobic training with carbohydrate + whey protein supplementation markedly improves heat dissipation mechanisms by plasma volume expansion. In this article, we will discuss the importance of improving body fluid homeostasis for thermoregulation under heat stress in humans and the strategy to attain this.

Interval Walking Training Can Increase Physical Fitness in Middle-Aged and Older People

No long-term exercise training regimen with high adherence and effectiveness for middle-aged and older individuals is currently broadly available in the field. To address this problem, we developed an exercise training system comprising interval walking training and an information technology network that requires only minimal staff support. We hypothesized that our training system could increase physical fitness in older people.

Measuring and quantifying skin sympathetic nervous system activity in humans

Development of the technique of microneurography has substantially increased our understanding of the function of the sympathetic nervous system (SNS) in health and in disease. The ability to directly record signals from peripheral autonomic nerves in conscious humans allows for qualitative and quantitative characterization of SNS responses to specific stimuli and over time. Furthermore, distinct neural outflow to muscle (MSNA) and skin (SSNA) can be delineated. However, there are limitations and caveats to the use of microneurography, measurement criteria, and signal analysis and interpretation. MSNA recordings have a longer history and are considered relatively more straightforward from a measurement and analysis perspective. This brief review provides an overview of the development of the technique as used to measure SSNA. The focus is on the utility of measuring sympathetic activity directed to the skin, the unique issues related to analyzing and quantifying multiunit SSNA, and the challenges related to its interpretation.