Human baroreflex rhythms persist during handgrip and muscle ischaemia.

AIM: To determine whether physiological, rhythmic fluctuations of vagal baroreflex gain persist during exercise, post-exercise ischaemia and recovery. METHODS: We studied responses of six supine healthy men and one woman to a stereotyped protocol comprising rest, handgrip exercise at 40% maximum capacity to exhaustion, post-exercise forearm ischaemia and recovery. We measured electrocardiographic R-R intervals, photoplethysmographic finger arterial pressures and peroneal nerve muscle sympathetic activity. We derived vagal baroreflex gains from a sliding (25-s window moved by 2-s steps) systolic pressure-R-R interval transfer function at 0.04-0.15 Hz. RESULTS: Vagal baroreflex gain oscillated at low, nearly constant frequencies throughout the protocol (at approx. 0.06 Hz - a period of about 18 s); however, during exercise, most oscillations were at low-gain levels, and during ischaemia and recovery, most oscillations were at high-gain levels. CONCLUSIONS: Vagal baroreflex rhythms are not abolished by exercise, and they are not overwhelmed after exercise during ischaemia and recovery.

Nine months in space: effects on human autonomic cardiovascular regulation.

We studied three Russian cosmonauts to better understand how long-term exposure to microgravity affects autonomic cardiovascular control. We recorded the electrocardiogram, finger photoplethysmographic pressure, and respiratory flow before, during, and after two 9-mo missions to the Russian space station Mir. Measurements were made during four modes of breathing: 1) uncontrolled spontaneous breathing; 2) stepwise breathing at six different frequencies; 3) fixed-frequency breathing; and 4) random-frequency breathing. R wave-to-R wave (R-R) interval standard deviations decreased in all and respiratory frequency R-R interval spectral power decreased in two cosmonauts in space. Two weeks after the cosmonauts returned to Earth, R-R interval spectral power was decreased, and systolic pressure spectral power was increased in all. The transfer function between systolic pressures and R-R intervals was reduced in-flight, was reduced further the day after landing, and had not returned to preflight levels by 14 days after landing. Our results suggest that long-duration spaceflight reduces vagal-cardiac nerve traffic and decreases vagal baroreflex gain and that these changes may persist as long as 2 wk after return to Earth.

Cardiovascular control with artificial neural networks.

NASA: This paper describes the use of artificial neural networks to model cardiovascular autonomic control in a study of the hemodynamic changes associated with space flight. Cardiovascular system models were created including four parameters: heart rate, contractility, peripheral resistance, and venous tone. Artificial neural networks were then designed and trained. A technique known as backpropagation networking was used and the results of the application of this technique to heart rate control are presented and discussed.^ieng

Applied potential tomography shows differential changes in fluid content of leg tissue layers in microgravity.

Absence of hydrostatic forces in the human cardiocirculatory system normally leads to an overall body fluid deficit. It was hypothesized that this is mainly due to a loss of interstitial fluid. An experiment was performed on board the Russian MIR station. Cuffs were positioned around both thighs and inflated up to suprasystolic values. This maneuver took place just before and after immediately a lower body negative pressure session (LBNP). The redistribution of fluids underneath the cuffs was assessed by means of cross-sectional impedance tomography (Applied Potential Tomography, APT). A microgravity induced loss of interstitial fluid was measured in all layers of the observed cross-section. The APT-readings changed significantly (SD approximately +/- .9) from 3.0 at 1g to 1.7 at 0g for the outer layer and from 2.7 at 1g to 2.0 at 0g for the middle layer (expressed in arbitrary units). The LBNP maneuver was able to fill the interstitial space but only at levels higher than -15 mmHg LBNP. This suggests that the superficial tissues in the legs are as much affected as the deeper ones by changing g-conditions and LBNP can be used to counteract interstitial fluid loss in this area.

Body fluid distribution in man in space and effect of lower body negative pressure treatment.

The lack of hydrostatic forces in space eventually produces a fluid deficit within the circulatory system. This deficit may alter the circulatory regulation patterns. The aim of the present study was to determine how much of this fluid deficit is attributable to interstitial fluid losses and to determine the effects of lower body negative pressure (LBNP) treatment on fluid distribution. The body fluid distribution of one subject was assessed before, during, and after weightlessness using two electrical impedance methods: (a) standard quadripole impedance for the segments of upper torso, lower torso, thigh, and calf and (b) an electrical impedance tomography technique (applied potential tomography) for a thigh cross-section. To assess the content of interstitial free fluid a thigh cuff overlying the electrodes for applied potential tomography was inflated to suprasystolic values to ascertain how much fluid can be squeezed out of blood vessels and tissue of skin and muscle. After the first thigh cuff maneuver (CUFF I) the subject performed a cardiovascular stress test with LBNP to mimic the gravity-induced blood shift to the lower part of the body. Then the compression maneuver was repeated (CUFF II). (a) This experimental sequence demonstrated a reduction in interstitial fluid in weightlessness of roughly 40% at the thigh. (b) The CUFF I and LBNP experiment demonstrated a reduced ability to cope with blood pooling in microgravity. (c) The CUFF II experiment suggests that LBNP in microgravity can refill the interstitial spaces and counteract the associated cardiovascular deterioration. The impedance measurements provided estimates of the contribution of different body sections to the observed body weight loss of more than 6 kg. The chest contributed nothing of significance, the lower torso more than 0.5 l, and both calves roughly 1.5 l. The thigh segments of both legs contributed between 1.5 l and 2.0 l with an interstitial free fluid reduction in muscle and skin by 40%.

Orthostatic stress by lower body negative pressure and its body fluid distribution kinetics under microgravity.

To support the hypothesis that interstitial fluid loss is one of the components of the "cardiovascular deconditioning" induced by space flight, a series of three experiments was performed in one subject, after 5 days in space: 1) standard thigh cuffs were inflated up to suprasystolic pressure values (CUFF I) for two minutes, 2) Half an hour later a lower body negative pressure test (LBNP) followed with -15 mmHg for 15, -30 mmHg for 5, and -40 mmHg for 15 minutes, 3) Twenty minutes after that, the CUFF maneuver was repeated (CUFF II). Body fluid shifts were detected by quadripole segmental electrical impedance (BIM) and by electrical impedance tomography (APT). The APT electrodes were placed directly under one of the thigh cuffs. Cardiovascular reaction patterns were observed by continuous ECG-recordings and by minute-by-minute arm cuff blood pressure measurements. By means of segmental impedance the expected body fluid change in the abdominal, thigh, and calf regions could be detected. The thoracic segment showed no significant changes in microgravity. However, the calf increased its electrical impedance roughly by 50%. The interstitial fluid in the skin and musculature, detected by APT during CUFF I, was reduced more than 50%. The LBNP-maneuver was able to refill this interstitial space. However, only the higher levels (LBNP > 15 mmHg) showed the outward filtration component during the constant LBNP levels. In addition, the LBNP-experiment clearly showed the reduced ability to cope with orthostatic stress. Pulse pressure dropped and heart rate increased much more than on ground. The fluid displacement during CUFF I showed a similar hemodynamic response, which had not been seen on ground before. The CUFF II maneuver after LBNP showed that the reaction had returned to normal. This leads to the conclusions that LBNP can counteract cardiovascular deconditioning, but only higher LBNP-levels are able to transfer fluid into the interstitial spaces of the legs, if they are emptied by the loss of gravitational forces.