Sueko Sagawa

Cardiovascular Responses to Lower Body Negative Pressure in the Elderly: Role of Reduced Leg Compliance

Background: It is warranted to test the hypothesis that the orthostatic tolerance does not diminish in the aging process per se in healthy individuals. Objective: The purpose of the present study was to examine the effects of aging on cardiovascular response and baroreflex sensitivity during lower body negative pressure (LBNP) with a special reference to leg compliance. Methods: Fifteen healthy old male subjects [mean age 68.2 ± (SE) 0.8 years] and 22 young male subjects [mean age 21.4 ± (SE) 0.3 years] underwent a 21-min bout of ramped LBNP (from 0 to –60 mm Hg, 10 mm Hg each for 3 min). Heart rate (HR), blood pressure, stroke volume (SV), forearm blood flow, and leg volume were measured throughout the experimental period. The arterial baroreflex sensitivity was calculated from spontaneous changes in beat-to-beat arterial pressure and HR during LBNP. Results: The leg compliance was lower, and the orthostatic tolerance index was higher in old than in young participants. The LBNP-associated increases in leg volume and HR and the decreases in SV were lower in old subjects, suggesting that the reduction of venous return was less in magnitude in old subjects during LBNP. The baseline value of baroreflex sensitivity evaluated by the sequence analysis was smaller, and no LBNP-related change was observed in old subjects, whereas a gradual LBNP-related reduction was observed in young subjects. The slope of regression between ΔSV and change in forearm vascular resistance during LBNP was identical in both age groups. Conclusions: We conclude that: (1) aging per se does not increase the intolerance to orthostatic stress induced by LBNP; (2) a low magnitude of venous return reduction during LBNP contributes to a higher tolerance in the old because of lower leg compliance, and (3) the sensitivity of baroreflex control of the HR is attenuated in the old; however, there is no deterioration of the sensitivity of the peripheral vasoconstriction during LBNP.

Carotid baroreflex control of heart rate during acute exposure to simulated altitudes of 3,800 m and 4,300 m

To examine the baroreflex response in humans during acute high-altitude exposure, the carotid baroreflex cardiac responsiveness was studied using a neck chamber in seven unacclimatized male subjects. Measurements were made in a high-altitude chamber on separate days at sea level and during 1-h exposure at two different altitudes of 3,800 m [partial pressure of oxygen in inspired air (Pi O2 ) = 90 mmHg] and 4,300 m (Pi O2  = 82 mmHg). R-R intervals were plotted against neck chamber pressures, and the baroreceptor response was analyzed by applying a four-parameter sigmoidal logistic function. The baroreceptor response curve shifted downward in either altitude, reflecting a tachycardic response at high altitude, and the magnitude of the shift was greater at 4,300 m than at 3,800 m. There was no change in the sigmoidal parameters at 3,800 m compared with sea level except for a reduction ( P < 0.05) of the minimum R-R interval. At 4,300 m the maximal R-R range, slope coefficient, minimum R-R interval, and maximal gain of the curve decreased significantly ( P < 0.05) compared with sea level values, whereas the centering point of the curve remained unchanged. These results suggest that hypoxia (Pi O2  = 82 mmHg) reduces the sensitivity of carotid baroreflex cardiac response.To examine the baroreflex response in humans during acute high-altitude exposure, the carotid baroreflex cardiac responsiveness was studied using a neck chamber in seven unacclimatized male subjects. Measurements were made in a high-altitude chamber on separate days at sea level and during 1-h exposure at two different altitudes of 3,800 m [partial pressure of oxygen in inspired air (PI(O2)) = 90 mmHg] and 4,300 m (PI(O2) = 82 mmHg). R-R intervals were plotted against neck chamber pressures, and the baroreceptor response was analyzed by applying a four-parameter sigmoidal logistic function. The baroreceptor response curve shifted downward in either altitude, reflecting a tachycardic response at high altitude, and the magnitude of the shift was greater at 4,300 m than at 3,800 m. There was no change in the sigmoidal parameters at 3,800 m compared with sea level except for a reduction (P < 0.05) of the minimum R-R interval. At 4,300 m the maximal R-R range, slope coefficient, minimum R-R interval, and maximal gain of the curve decreased significantly (P < 0.05) compared with sea level values, whereas the centering point of the curve remained unchanged. These results suggest that hypoxia (PI(O2) = 82 mmHg) reduces the sensitivity of carotid baroreflex cardiac response.

Whole body heating reduces the baroreflex response of sympathetic nerve activity during Valsalva straining

The present experiment was carried out to examine the possible effects of whole body heating on the baroreflex control of muscle sympathetic nerve activity (MSNA) and heart rate (HR). Nine healthy male volunteers underwent the Valsalva maneuver during exposure to heat stress using a hot water-perfused suit. To exclude an effect from the unloading of the cardiopulmonary baroreceptors during hyperthermia, the central hypovolemia was counterbalanced by head-down tilt (HDT) at 8 degrees. The baroreflex slopes of MSNA and HR against mean arterial pressure were determined by regression analysis during the early phase II (falling blood pressure period) of the Valsalva maneuver. The core temperature increased from 36.9 +/- 0.1 degrees C at thermoneutral to 37.7 +/- 0.1 degrees C during heat exposure, whereas the left atrial diameter (LAD) decreased. Both the MSNA and HR increased (P < 0.05) during hyperthermia. The baroreflex slopes of MSNA and HR in the phase II decreased 30 +/- 8% (P < 0.05) and 24 +/- 9% (P < 0.05), respectively, during hyperthermia. The HDT increased LAD, but there was no change in HR, arterial pressure and the baroreflex slopes. These results suggest that hyperthermia reduces the baroreflex responses of MSNA and HR during a transient falling period of blood pressure. These reduced baroreflex responses may not be attributable to an unloading of the cardiopulmonary baroreceptors during heat stress.

Hypovolemic intolerance to lower body negative pressure in female runners

PURPOSE An attenuated baroreflex response and orthostatic intolerance have been reported in endurance-trained male athletes; however, it is still unknown whether this occurs also in females. The purpose of the present study was to examine whether endurance exercise-trained women had a predisposition to orthostatic compromise, and if so, what causative factor(s) may induce orthostatic intolerance. METHODS We studied cardiovascular and hormonal responses to graded lower body negative pressure (LBNP) (0 to -60 mm Hg) in 26 middle-distance female runners (18.6 +/- 0.1 yr) as the exercise-trained (ET) subjects and 23 age-matched untrained (UT) control subjects. On the basis of the occurrence of syncope episodes during LBNP, ET and UT subjects were further allocated to two groups; ET with presyncope (ET+syncope) and without presyncope (ET-syncope) and UT with presyncope (UT+syncope) and without presyncope (UT-syncope). RESULTS Occurrence of presyncope episodes during LBNP was higher in ET (65.4%, P < 0.05) than that for UT (34.8%). Leg compliance was higher (P < 0.05) in ET than in UT. LBNP reduced stroke volume (SV) more (P < 0.05), increased heart rate (HR) higher (P < 0.05), and increased forearm vascular resistance (FVR) more in ET+syncope as compared with the other groups. Response of vasoactive hormones to LBNP was higher in ET+syncope (P < 0.05) than that of the other groups except for norepinephrine (NE); high in both ET+syncope and UT+syncope. The relationship between SV and NE, an index of sympathetic neuronal response, had no training-related changes during LBNP. CONCLUSION We conclude that exercise-trained females have a high incidence of orthostatic intolerance during LBNP, with a greater reduction of SV independent of changes in baroreflex and neurohumoral function. A lower incidence of LBNP intolerance in UT may be accounted for by a lower reduction of SV during LBNP. An increase in leg compliance in the exercise-trained females may play an important role in inducing pronounced reduction of SV and hence the intolerance to LBNP.

Muscle Sympathetic Nerve Activity During Acute Increase in Core Temperature in Humans

The effect of heat stress on muscle sympathetic nerve activity was studied in nine men. Transient heat loading was given through water-perfused suits for 40 min. Esophageal temperature (Tes), blood pressure (BP), cardiac output (CO), forearm blood flow (FBF), and sympathetic nerve activity in the peroneal muscle nerve fascicles (MSNA) were measured throughout the experimental period. The average Tes increased to 38.73° ± 0.10°C (P < 0.05) at the termination of the experiment. MSNA rose linearly in proportion to the rise in Tes. FBF increased to about 20 m1/100 ml/min (P < 0.05) with the increase in Tes up to 37.6°C and plateaued thereafter. CO increased by 20% (P < 0.05) 15 min after heat exposure and remained elevated at 8.3 1/min toward the termination of the experiment while leaving BP unchanged. Based on the fact of a constant muscle blood flow during heat stress and a plateaued increment of FBF, we conclude that the Tes-dependent increase in MSNA is an indicator for the evaluation of peripheral sympathetic nerve activities in regions other than the skin.

Fragility of red blood cells under 31 atmosphere absolute for 14 days.

The present investigation was undertaken during the course of a 14-day dry saturation dive at 31 ATA to study hematologic changes and osmotic fragility of red blood cells in relation to lipid contents of the plasma and red cells. Increase in the hematocrit and blood cell count was observed in the early phase at 31 ATA, which was attributed to the hemoconcentration caused by the hyperbaric diuresis. Osmotic fragility of red blood cells fluctuated during the course of the experiment, but the change was not related to the atmospheric pressure. Cholesterol and phospholipid contents in red blood cells and the plasma also fluctuated but the change was not caused by the pressure. However, an overall correlation between the osmotic fragility and the phosphatidyl choline of red cells and the correlation between the osmotic fragility and the total cholesterol level in the plasma were observed at both 1 and 31 ATA. These correlations revealed an intimate relation between the red cell fragility and lipid composition of the cell membrane and the plasma within intra- and interindividual variations during the course of the dive. The present result did not indicate any significant alteration of red blood cell function due to prolonged exposure to 31 ATA He-O2 environment.