Sustained mild hypergravity reduces spontaneous cardiac baroreflex sensitivity

Neuro-Cardiovascular Responses to Sympathetic Stimulation in Fighter Pilots

INTRODUCTION: The chronic effects of regular exposure to high acceleration levels (G-force) on the neuro-cardiovascular system are unclear. We compared the mean arterial pressure (MAP) and cardiac autonomic modulation between nonpilots (NP) vs. military fighter (FP) and transport (TP) pilots. Additionally, we correlated the cardiac autonomic indices with the cardiorespiratory fitness and flight experience of FP.METHODS: A total of 21 FP, 8 TP, and 20 NP performed a tilt test (TT), during which beat-to-beat blood pressure and heart rate were recorded.RESULTS: No difference was detected between groups for changes in MAP and heart rate variability indices during the TT. However, the analysis of areas under the curves showed a greater increase in MAP in FP vs. TP and NP. Conversely, there was a greater decrease in indices reflecting vagal modulation in TP vs. FP and NP (rMSSD, pNN50, and SDNN), and a greater increase in heart rate and sympathovagal balance in TP vs. other groups (LF/HF). The maximal oxygen uptake was strongly correlated with the vagal reserve in FP (r = -0.74). Moreover, the total flying hours of FP were positively correlated with resting HFnu (r = 0.47) and inversely correlated with resting LFnu (r = -0.55) and LF/HF (r = -0.46).CONCLUSION: FP had a higher pressor response to TT than TP and NP. Vagal withdrawal and sympathovagal increase induced by TT in FP were similar vs. NP and attenuated vs. TP. Greater cardiorespiratory fitness and accumulated flying hours in FP seemed to favor lower sympathetic and greater vagal modulation at rest.dos Santos Rangel MV, de Sá GB, Farinatti P, Borges JP. Neuro-cardiovascular responses to sympathetic stimulation in fighter pilots. Aerosp Med Hum Perform. 2023; 94(10):761-769.

Human cardiovascular adaptation to hypergravity

Despite decades of experience from high-G exposures in aircraft and centrifuges, information is scarce regarding primary cardiovascular adaptations to +Gz loads in relaxed humans. Thus, effects of G-training are typically evaluated after regimens that are confounded by concomitant use of anti-G straining maneuvers, anti-G suits and pressure breathing. Accordingly, the aim was to evaluate cardiovascular adaptations to repeated +Gz exposures in the relaxed state. Eleven men underwent 5 weeks of centrifuge G training, consisting of 15 × 40 min +Gz exposures at G levels close to their individual relaxed G-level tolerance. Before and after the training regimen, relaxed G-level tolerance was investigated during rapid (ROR) and gradual (GOR) onset-rate G exposures, and cardiovascular responses were investigated during orthostatic provocation and vascular pressure-distension tests. The G training resulted in: (i) a 13% increase in relaxed ROR G tolerance (P < 0.001), but no change in GOR G tolerance, (ii) increased pressure resistance in the arteries and arterioles of the legs (P < 0.001), but not the arms, (iii) a reduced initial drop in arterial pressure upon ROR high G, but no change in arterial pressure under basal resting conditions or during GOR G loading, or orthostatic provocation. The results suggest +Gz adaptation via enhanced pressure resistance in dependent arteries/arterioles. Presumably this reflects local adaptations to high transmural pressures, resulting from the +Gz-induced exaggeration of the intravascular hydrostatic pressure gradients.

The Omnipresence of Autonomic Modulation in Health and Disease

The Autonomic Nervous System (ANS) is a critical part of the homeostatic machinery with both central and peripheral components. However, little is known about the integration of these components and their joint role in the maintenance of health and in allostatic derailments leading to somatic and/or neuropsychiatric (co)morbidity. Based on a comprehensive literature search on the ANS neuroanatomy we dissect the complex integration of the ANS: (1) First we summarize Stress and Homeostatic Equilibrium - elucidating the responsivity of the ANS to stressors; (2) Second we describe the overall process of how the ANS is involved in Adaptation and Maladaptation to Stress; (3) In the third section the ANS is hierarchically partitioned into the peripheral/spinal, brainstem, subcortical and cortical components of the nervous system. We utilize this anatomical basis to define a model of autonomic integration. (4) Finally, we deploy the model to describe human ANS involvement in (a) Hypofunctional and (b) Hyperfunctional states providing examples in the healthy state and in clinical conditions.

A closed-loop approach to the study of the baroreflex dynamics during posture changes at rest and at exercise in humans

We hypothesized that during rapid uptilting at rest, due to vagal withdrawal, arterial baroreflex sensitivity (BRS) may decrease promptly and precede the operating point (OP) resetting, whereas different kinetics are expected during exercise steady state, due to lower vagal activity than at rest. To test this, eleven subjects were rapidly (<2 s) tilted from supine (S) to upright (U) and vice versa every 3 min, at rest and during steady-state 50 W pedaling. Mean arterial pressure (MAP) was measured by finger cuff (Portapres) and R-to-R interval (RRi) by electrocardiography. BRS was computed with the sequence method both during steady and unsteady states. At rest, BRS was 35.1 ms·mmHg^-1 (SD = 17.1) in S and 16.7 ms·mmHg^-1 (SD = 6.4) in U (P < 0.01), RRi was 901 ms (SD = 118) in S and 749 ms (SD = 98) in U (P < 0.01), and MAP was 76 mmHg (SD = 11) in S and 83 mmHg (SD = 8) in U (P < 0.01). During uptilt, BRS decreased promptly [first BRS sequence was 19.7 ms·mmHg^-1 (SD = 5.0)] and was followed by an OP resetting (MAP increase without changes in RRi). At exercise, BRS and OP did not differ between supine and upright positions [BRS was 7.7 ms·mmHg^-1 (SD = 3.0) and 7.7 ms·mmHg^-1 (SD = 3.5), MAP was 85 mmHg (SD = 13) and 88 mmHg (SD = 10), and RRi was 622 ms (SD = 61) and 600 ms (SD = 70), respectively]. The results support the tested hypothesis. The prompt BRS decrease during uptilt at rest may be ascribed to a vagal withdrawal, similarly to what occurs at exercise onset. The OP resetting may be due to a slower control mechanism, possibly an increase in sympathetic activity.

Cellular channelopathy mediated by hypergravity: IL-6-mediated Nkcc1 activation and enhanced Trpm2 expression in rat atrium

Although cardiac tissue is considered a target of gravitational force (g-force), the mechanism of hypergravity on the ion modulation or identification of ion transporters is still unknown. Thus, we determine the effect of hypergravity on a physical force-sensitive cytokine, IL-6 and its related channel activity to investigate rat cardiac function changes in response to accelerated g-force. Serum IL-6 levels and intracellular calcium levels of the right atrium were moderately increased under hypergravity stimulation (4g). IL-6 was involved in the modulation of sodium-potassium-chloride cotransporter (Nkcc) activity. Surprisingly, the right atrium under 4g revealed significantly enhanced Nkcc1 activity. The use of IL-6 on the NKCC1-overexpressed or native NKCC-expressing cells also showed enhanced NKCC1 activity. Hypergravity conditions were also involved in the oxidative stress activated Trpm2 channel and revealed an enhanced expression of the Trpm2 channel under 4g in the rat right atrium. In conclusion, hypergravity revealed that moderate increases in serum IL-6 and enhanced Nkcc1 activity was modulated by IL-6. In addition, enhanced Trpm2 channel expression could be involved in the increased intracellular calcium levels of the right atrium under hypergravitational force. We therefore address that enhanced physical force-sensitive cytokine and oxidative stress by the gravitational force mediate activation of the cotransporter involved in possibilities of edema and calcium loading in cardiac tissue.

Comparison of Autonomic Control of Blood Pressure During Standing and Artificial Gravity Induced via Short-Arm Human Centrifuge

Autonomic control of blood pressure is essential toward maintenance of cerebral perfusion during standing, failure of which could lead to fainting. Long-term exposure to microgravity deteriorates autonomic control of blood pressure. Consequently, astronauts experience orthostatic intolerance on their return to gravitational environment. Ground-based studies suggest sporadic training in artificial hypergravity can mitigate spaceflight deconditioning. In this regard, short-arm human centrifuge (SAHC), capable of creating artificial hypergravity of different g-loads, provides an auspicious training tool. Here, we compare autonomic control of blood pressure during centrifugation creating 1-g and 2-g at feet with standing in natural gravity. Continuous blood pressure was acquired simultaneously from 13 healthy participants during supine baseline, standing, supine recovery, centrifugation of 1-g, and 2-g, from which heart rate (RR) and systolic blood pressure (SBP) were derived. The autonomic blood pressure regulation was assessed via spectral analysis of RR and SBP, spontaneous baroreflex sensitivity, and non-linear heart rate and blood pressure causality (RR↔SBP). While majority of these blood pressure regulatory indices were significantly different (p < 0.05) during standing and 2-g centrifugation compared to baseline, no change (p > 0.05) was observed in the same indices during 2-g centrifugation compared to standing. The findings of the study highlight the capability of artificial gravity (2-g at feet) created via SAHC toward evoking blood pressure regulatory controls analogous to standing, therefore, a potential utility toward mitigating deleterious effects of microgravity on cardiovascular performance and minimizing post-flight orthostatic intolerance in astronauts.

The Effect of Lower-Body Positive Pressure on the Cardiorespiratory Response at Rest and during Submaximal Running Exercise

Anti-gravity treadmills facilitate locomotion by lower-body positive pressure (LBPP). Effects on cardiorespiratory regulation are unknown. Healthy men (30 ± 8 y, 178.3 ± 5.7 cm, 70.3 ± 8.0 kg; mean ± SD) stood upright (n = 10) or ran (n = 9) at 9, 11, 13, and 15 km.h⁻¹ (5 min stages) with LBPP (0, 15, 40 mmHg). Cardiac output (CO), stroke volume (SV), heart rate (HR), blood pressure (BP), peripheral resistance (PR), and oxygen uptake (VO₂) were monitored continuously. During standing, LBPP increased SV [by +29 ± 13 (+41%) and +42 ± 15 (+60%) ml, at 15 and 40 mmHg, respectively (p < 0.05)] and decreased HR [by −15 ± 6 (−20%) and −22 ± 9 (−29%) bpm (p < 0.05)] resulting in a transitory increase in CO [by +1.6 ± 1.0 (+32%) and +2.0 ± 1.0 (+39%) l.min⁻¹ (p < 0.05)] within the first seconds of LBPP. This was accompanied by a transitory decrease in end-tidal PO₂ [by −5 ± 3 (−5%) and −10 ± 4 (−10%) mmHg (p < 0.05)] and increase in VO₂ [by +66 ± 53 (+26%) and +116 ± 64 (+46%) ml.min⁻¹ (p < 0.05)], suggesting increased venous return and pulmonary blood flow. The application of LBPP increased baroreflex sensitivity (BRS) [by +1.8 ± 1.6 (+18%) and +4.6 ± 3.7 (+47%) at 15 and 40 mmHg LBPP, respectively P < 0.05]. After reaching steady-state exercise CO vs. VO₂ relationships remained linear with similar slope and intercept for each participant (mean R² = 0.84 ± 0.13) while MAP remained unchanged. It follows that (1) LBPP affects cardiorespiratory integration at the onset of exercise; (2) at a given LBPP, once reaching steady-state exercise, the cardiorespiratory load is reduced proportionally to the lower metabolic demand resulting from the body weight support; (3) the balance between cardiovascular response, oxygen delivery to the exercising muscles and blood pressure regulation is maintained at exercise steady-state; and (4) changes in baroreflex sensitivity may be involved in the regulation of cardiovascular parameters during LBPP.

Speed ratio but cabin temperature positively correlated with increased heart rates among professional drivers during car races

OBJECTIVES

The present study measures heart rate (HR) on a number of professional race-car drivers during actual car races through annual seasons to test hypotheses that faster relative speed and higher cabin temperature would induce higher HR.

METHODS

Heart rates in fifteen male drivers (31.2 ± 5.5 years old) were obtained by chest-strap sensors during official-professional 13 races. Average HR was calculated while the driver was racing from the start to the end of each race.

RESULTS

The average HR during races was 164.5 ± 15.1 beats min-1 and the average amount of time each driver spent driving per race was 54.2 ± 13.7 min. Average HR significantly and positively correlated with mean speed ratio (P < 0.001), but not with the average cabin temperatures (P = 0.533, range 25.6-41.8 °C) by the multiple linear regression analysis. Both average HR and mean speed ratio were significantly lower under wet, than dry conditions (151.9 ± 16.5 vs. 168.3 ± 12.5 beats min-1, 86.9 ± 4.4 vs. 93.4 ± 1.5 %).

CONCLUSIONS

The cardiovascular system of drivers is considerably stressed at extremely high HR. This high average HR positively correlated with mean speed ratio, suggesting that faster driving speed would induce greater cardiovascular stress to professional drivers during actual races. However, contrary to our hypothesis, cabin temperature was not significantly correlated with average HR. It is speculated that direct body cooling systems used in this professional race category work well against increases in HR by thermal stress under the temperature range found herein.

The relationship between widespread changes in gravity and cerebral blood flow

OBJECTIVES

We investigated the dose-effect relationship between wide changes in gravity from 0 to 2.0 Gz (Δ0.5 Gz) and cerebral blood flow (CBF), to test our hypothesis that CBF has a linear relationship with levels of gravity.

SUBJECTS AND METHODS

Ten healthy seated men were exposed to 0, 0.5, 1.0, 1.5, and 2.0 Gz for 21 min, by using a tilt chair and a short-arm human centrifuge. Steady-state CBF velocity (CBFV) in the middle cerebral artery by transcranial Doppler ultrasonography, mean arterial pressure (MAP) at the heart level (MAPHeart), heart rate, stroke volume, cardiac output and respiratory conditions were obtained for the last 6 min at each gravity level. Then, MAP in the middle cerebral artery (MAPMCA), reflecting cerebral perfusion pressure, was estimated.

RESULTS

Steady-state CBFV decreased stepwise from 0.5 to 2.0 Gz. Steady-state heart rate, stroke volume, estimated MAPMCA and end-tidal carbon dioxide pressure (ETCO2) also changed stepwise from hypogravity to hypergravity. On the other hand, steady-state MAPHeart and cardiac output did not change significantly. Steady-state CBFV positively and linearly correlated with estimated MAPMCA and ETCO2 in most subjects.

CONCLUSION

The present study demonstrated stepwise gravity-induced changes in steady-state CBFV from 0.5 to 2.0 Gz despite unchanged steady-state MAPHeart. The combined effects of reduced MAPMCA and ETCO2 likely led to stepwise decreases in CBFV. We caution that a mild increase in gravity from 0 to 2.0 Gz reduces CBF, even if arterial blood pressure at the heart level is maintained.