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At least 19 recordsLinked to original sources
PubMed1994

Dynamic leg exercise improves tolerance to lower body negative pressure.

Lower body negative pressure (LBNP) of 100 mm Hg generates a footward force approximating one body weight for most subjects, yet LBNP may also produce syncope by pooling blood in the lower body. We hypothesized that dynamic leg exercise would improve LBNP tolerance by reducing lower body fluid accumulation. Seven healthy male subjects underwent four supine LBNP tolerance tests: 1) the control condition: conventional, resting LBNP with a saddle (SADL); 2) resting LBNP against a footplate instead of a saddle (FP); 3) cyclic, non-load-bearing ankle plantar- and dorsiflexion movements during LBNP with a saddle (SADLEX); and 4) LBNP with cyclic ankle plantar- and dorsiflexion exercise against the footplate (FPEX). Tolerance tests consisted of reducing chamber pressure in 10 mm Hg decrements every 3 min to -100 mm Hg or presyncope. Tolerance was assessed by integrating the area under the LBNP x time step function. Exercise doubled LBNP tolerance relative to resting LBNP (SADL: 771 +/- 162 mm Hg x min; FP: 819 +/- 212; SADLEX: 1461 +/- 175; FPEX: 1656 +/- 160; p < 0.05). During FPEX, footward force oscillations averaged 215 +/- 14 N (21.9 +/- 1.4 kg), and mean peak force of 872 +/- 26 N (88.9 +/- 2.7 kg; n = 5) was produced by plantarflexing against 100 mm Hg LBNP. Calf volume increased slightly less when exercise accompanied LBNP (SADLEX and FPEX), while heart rate increased more. Therefore, exercise improved tolerance in part by skeletal muscle pumping of venous blood from the legs.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed2001

Differential sympathetic nerve and heart rate spectral effects of nonhypotensive lower body negative pressure.

Lower body negative pressure (LBNP; -5 and -15 mmHg) was applied to 14 men (mean age 44 yr) to test the hypothesis that reductions in preload without effect on stroke volume or blood pressure increase selectively muscle sympathetic nerve activity (MSNA), but not the ratio of low- to high-frequency harmonic component of spectral power (P(L)/P(H)), a coarse-graining power spectral estimate of sympathetic heart rate (HR) modulation. LBNP at -5 mmHg lowered central venous pressure and had no effect on stroke volume (Doppler) or systolic blood pressure but reduced vagal HR modulation. This latter finding, a manifestation of arterial baroreceptor unloading, refutes the concept that low levels of LBNP interrogate, selectively, cardiopulmonary reflexes. MSNA increased, whereas P(L)/P(H) and HR were unchanged. This discordance is consistent with selectivity of efferent sympathetic responses to nonhypotensive LBNP and with unloading of tonically active sympathoexcitatory atrial reflexes in some subjects. Hypotensive LBNP (-15 mmHg) increased MSNA and P(L)/P(H), but there was no correlation between these changes within subjects. Therefore, HR variability has limited utility as an estimate of the magnitude of orthostatic changes in sympathetic discharge to muscle.

Adult↗
PubMed1993

Dynamics of transcapillary fluid transfer and plasma volume during lower body negative pressure.

Lower body negative pressure (LBNP) is a stimulus frequently used to study reflex circulatory responses in humans. Studies have provided data on LBNP-induced blood pooling; however, the possibility that LBNP also might be associated with an important loss of plasma fluid has attracted little attention. Therefore this problem was analysed in male volunteers exposed to prolonged (10 min) high (70-75 mmHg) LBNP. Data on LBNP-induced blood pooling that were more reliable than in previous literature were also provided. LBNP caused early pooling of more than 870 ml of blood. Rapid filtration of plasma into the exposed tissues occurred throughout LBNP. The cumulative oedema in the legs and buttocks averaged as much as 460 ml, and additional quite large volumes of plasma apparently accumulated in other parts of the lower body. Concomitantly, there was compensatory absorption of extravascular fluid in the upper body. The net decrease in plasma volume (PV) was still large and averaged 491 +/- 29(SE) ml. Two aspects of the demonstrated process of transcapillary fluid fluxes and PV decline may be emphasized. Firstly, in conjunction with the primary large redistribution of intravascular volume, it certainly implies that LBNP is a potent stimulus as also indicated by a progressive increase in heart rate (HR) and a progressive decline in systolic pressure throughout experimental intervention. In fact, LBNP-induced circulatory stress clearly has bearings on the extreme hypovolaemic situation provided by the pressure-bottle haemorrhage technique used in animals. Secondly, it not only offers an interesting example of the dynamics of PV but appears to have more general validity with regard to states characterized by gravitational shifts of blood (hydrostatic load), like upright exercise and quiet standing.

Adult↗
PubMed1986

Cardiovascular responses of women to lower body negative pressure.

Lower body negative pressure (LBNP) has provided a method for studying cardiovascular responses in men while simulating a return to the stresses of 1-G following space flight. In this study, we have monitored responses of women to the stresses provided by LBNP. There were 20 women, 23-43 years, each tested in the follicular and luteal phases of the menstrual cycle. Variables were recorded during supine control; at -30, -40, -50 mm Hg LBNP; immediately after pressure release; and after 5 min recovery. There were no significant differences in response to LBNP between the two menstrual phases. During LBNP calf circumference was enlarged; transthoracic impedance was increased; stroke volume, left ventricular ejection time, the Heather Index of contractility and systolic pressure were reduced; total peripheral resistance was elevated; and cardiac output fell despite a rise in heart rate. Differences in cardiovascular variables between 0 mm Hg LBNP and -50 mm Hg LBNP were generally similar to reported differences between supine and standing. The responses of these women to LBNP were qualitatively similar to those reported for the Apollo astronauts and other male subjects. These women appeared to compensate with a greater heart rate response; however, the net cardiovascular compensation as determined from arterial pressure appears to be similar in men and women.

Adult↗
PubMed1998

Reflex responses to regional venous pooling during lower body negative pressure in humans.

Lower body negative pressure is frequently used to simulate orthostasis. Prior data suggest that venous pooling in abdominal or pelvic regions may have major hemodynamic consequences. Therefore, we developed a simple paradigm for assessing regional contributions to venous pooling during lower body negative pressure. Sixteen healthy men and women underwent graded lower body negative pressure protocols to 60 mmHg while wearing medical anti-shock trousers to prevent venous pooling under three randomized conditions: 1) no trouser inflation (control), 2) only the trouser legs inflated, and 3) the trouser legs and abdominopelvic region inflated. Without trouser inflation, heart rate increased 28 +/- 4 beats/min, mean arterial pressure fell -3 +/- 2 mmHg, and forearm vascular resistance increased 51 +/- 9 units at 60 mmHg lower body negative pressure. With inflation of either the trouser legs or the trouser legs and abdominopelvic region, heart rate and mean arterial pressure did not change during lower body negative pressure. By contrast, although the forearm vasoconstrictor response to lower body negative pressure was attenuated by inflation of the trouser legs (delta forearm vascular resistance 33 +/- 10 units, P < 0.05 vs. control), attenuation was greater with the inflation of the trouser legs and abdominopelvic region (delta forearm vascular resistance 16 +/- 5 units, P < 0.05 vs. control and trouser legs-only inflation). Thus the hemodynamic consequences of pooling in the abdominal and pelvic regions during lower body negative pressure appear to be less than in the legs in healthy individuals.

Abdomen↗
PubMed1989

Sympathetic nerve activity in arm and leg muscles during lower body negative pressure in humans.

Nonhypotensive lower body negative pressure (LBNP) is reported to decrease forearm but not calf blood flow as measured by strain-gauge plethysmography. This suggests that unloading of cardiopulmonary receptors increases sympathetic outflow to arm but not to leg. To test this hypothesis we measured muscle sympathetic nerve activity (MSA) in the arm (radial nerve) and leg (peroneal nerve) simultaneously during LBNP. In eight healthy subjects, we measured heart rate, blood pressure, and radial and peroneal MSA during LBNP at 10 and 20 mmHg. There was no difference between radial and peroneal MSA at rest, and there were successive parallel increases of MSA in both nerves during LBNP at 10 and 20 mmHg. These data indicate that there are nearly identical increases of sympathetic outflow to the arm and leg during mild to moderate degrees of orthostatic stress.

Adult↗
PubMed1993

Transcapillary fluid responses to lower body negative pressure.

The effect of lower body negative pressure (LBNP) on transcapillary fluid balance is unknown. Therefore, our objective was to assess leg interstitial fluid pressures (IFP), leg circumference, plasma volume (PV), and net whole body transcapillary fluid transport (TFT) during and after supine LBNP and to evaluate the addition of oral saline ingestion on transcapillary exchange. Six healthy men 23-41 yr old underwent 4 h of 30 mmHg LBNP, followed by 50 min of supine recovery on two separate occasions, once with and once without ingestion of 1 liter of isotonic saline. IFP was measured continuously in subcutis as well as superficial and deep regions of the tibialis anterior muscle by slit catheters. TFT was calculated by subtracting urine production and calculated insensible fluid loss from changes in PV. During exposure to LBNP, IFP decreased in parallel with chamber pressure, foot venous pressure did not change, leg circumference increased by 3 +/- 0.35% (SE) (P < 0.05), and PV decreased by 14 +/- 2.3%. IFP returned to near control levels after LBNP. At the end of minute 50 of recovery, PV remained decreased (by 7.5 +/- 5.2%) and leg circumference remained elevated (by 1 +/- 0.37%). LBNP alone produced significant movement of fluid into the lower body but no net TFT (-7 +/- 12 ml/h). During LBNP with saline ingestion, 72 +/- 4% of the ingested fluid volume filtered out of the vascular space (TFT = 145 +/- 10 ml/h), and PV decreased by 6 +/- 3%.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1982

Influence of training on the blood pressure changes during lower body negative pressure in rats.

The responses of non-trained and endurance-trained rats to conditions of lower body negative pressure (LBNP) was evaluated in normotensive, borderline hypertensive, and genetic hypertensive groups, as well as in sub-groups subjected to conditions of ventilation with 100% oxygen, systematic hemorrhaging, or sino-aortic denervations. Compared to their non-trained controls, normotensive trained rats exhibited significantly greater and faster falls in arterial blood pressure. This finding suggested a change in baroreceptor sensitivity. Related, but not statistically significant trends were observed with the hypertensive groups. Borderline hypertensive rats (DOCA injections) did not demonstrate any of these differences. Measurements of blood changes during the LBNP procedure and the effects of inspiring 100% oxygen indicated that the aortic and carotid chemoreceptors were not responsible for this training effect. After baroreceptor denervation, the group differences were abolished. In addition, the training effects were generally absent when hemorrhaging was performed, a result suggesting a difference in compliance. We have concluded from these results that endurance training will be associated with greater decreases in arterial blood pressure during LBNP than will be experienced by non-trained populations. However, the responsible mechanisms are unclear and will require further investigation.

Animals↗
PubMed1997

Venous and arterial reflex responses to positive-pressure breathing and lower body negative pressure.

We examined the relative importance of arteriolar and venous reflex responses during reductions in cardiac output provoked by conditions that increase [positive end-expiratory pressure (PEEP)] or decrease [lower body negative pressure (LBNP)] peripheral venous filling. Five healthy subjects were exposed to PEEP (10, 15, 20, and 25 cmH2O) and LBNP (-10, -15, -20, and -25 mmHg) to induce progressive but comparable reductions in right atrial transmural pressure (control to minimum): from 5.9 +/- 0.4 to 1.8 +/- 0.7 and from 6.5 +/- 0.6 to 2.0 +/- 0.2 mmHg with PEEP and LBNP, respectively. Cardiac output (impedance cardiography) fell less during PEEP than during LBNP (from 3.64 +/- 0.21 to 2.81 +/- 0.21 and from 3.39 +/- 0.21 to 2.14 +/- 0.24 l.min-1.m-2 with PEEP and LBNP, respectively), and mean arterial pressure increased. We observed sustained increases in forearm vascular resistance (i.e., forearm blood flow by venous occlusion plethysmography) and systemic vascular resistance that were greater during LBNP: from 19.7 +/- 2.91 to 27.97 +/- 5.46 and from 20.56 +/- 2.48 to 50.25 +/- 5.86 mmHg.ml-1.100 ml tissue-1.min (P < 0.05) during PEEP and LBNP, respectively. Venomotor responses (venous pressure in the hemodynamically isolated limb) were always transient, significant only with the greatest reduction in right atrial transmural pressure, and were similar for LBNP and PEEP. Thus arteriolar rather than venous responses are predominant in blood volume mobilization from skin and muscle, and venoconstriction is not intensified with venous engorgement during PEEP.

Adult↗
PubMed1996

Differential effects of low- and high-intensity lower body negative pressure on noradrenaline and adrenaline kinetics in humans.

1. Lower body negative pressure provides a means to examine neurocirculatory reflexive responses to decreases in venous return to the heart. We assessed whether the pattern of catecholaminergic responses to lower body negative pressure depends on the intensity of the stimulus (-15 versus -40 mmHg). 2. In 14 healthy subjects, responses of forearm blood flow and noradrenaline spillover and of total body noradrenaline and adrenaline spillover were assessed during infusion of [3H]noradrenaline and [3H]adrenaline during -15 and -40 mmHg of lower body negative pressure. 3. During lower body negative pressure at -15 mmHg, heart rate and pulse pressure did not change, but forearm vascular resistance increased by 25-50%. Forearm noradrenaline spillover increased by about 50%, from 0.63 +/- 0.16 to 0.94 +/- 0.23 pmol min-1 100 ml-1 (P < 0.05). Total body noradrenaline spillover did not change, and total body adrenaline spillover increased significantly by about 30%. Clearances of noradrenaline and adrenaline were unchanged. 4. During lower body negative pressure at -40 mmHg, heart rate increased and pulse pressure decreased. Forearm vascular resistance increased by about 100%, and forearm noradrenaline spillover increased by 80%, from 0.73 +/- 0.19 to 1.32 +/- 0.36 pmol min-1 100 ml-1 (P < 0.05). Total body noradrenaline spillover increased by 30%, and total body adrenaline spillover increased by about 50%. Clearances of both noradrenaline and adrenaline decreased. 5. The results are consistent with the view that selective deactivation of cardiopulmonary baroreceptors during low-intensity lower body negative pressure increases sympathoneural traffic to forearm skeletal muscle and increases adrenomedullary secretion without a concomitant generalized increase in sympathoneural outflows. Concurrent deactivation of cardiopulmonary and arterial baroreceptors during high-intensity lower body negative pressure evokes a more generalized increase in sympathoneural activity, accompanied by further increased adrenomedullary secretion and decreased plasma clearances of noradrenaline and adrenaline. The findings support differential increases in skeletal sympathoneural and adrenomedullary outflows during orthostasis, with more generalized sympathoneural responses to systemic hypotension.

Adult↗
PubMed1994

Simultaneous transcranial Doppler and arterial blood pressure response to lower body negative pressure.

Microgravity induces fluid shifts which can alter the cardiovascular responses of astronauts both during space flight and on return to Earth. The decrease in orthostatic tolerance in astronauts returning from a weightless environment can be modelled in ground-based studies using lower body negative pressure (LBNP). This study examined the physiological changes induced by LBNP and determined a reliable method of predicting the onset of presyncope to enable evaluation of countermeasures for loss of orthostatic tolerance, such as glycerol-induced hyperhydration. Six healthy male subjects, aged 18 to 45 years, were each subjected to two LBNP tests, with or without glycerol ingestion. Continuous, non-invasive measurements of middle cerebral artery blood flow velocities (CBF) by transcranial Doppler, arterial blood pressure (Finapres ABP), ECG and LBNP box pressures were recorded during each test. Negative pressure was increased in three minute intervals until symptoms of presyncope were observed. An increase in heart rate (HR), a relatively constant mean ABP and a steady decline in mean CBF were consistently observed as the box pressure was decreased. The continuous on-line measurements clearly showed consistent dynamic changes in both CBF and ABP waveforms in response to changes in LBNP. At the onset of presyncope, sudden drops in mean ABP, HR and mean CBF were typically noted, the latter providing the earliest indication of presyncope. The time required to re-establish original baseline values of CBF and ABP after release of box pressure varied widely from six to over ten minutes.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1995

Application of multivariate statistical analysis to estimate +Gz tolerance based on the changes of hemodynamic parameters during lower body negative pressure (LBNP).

The application of lower body negative pressure (LBNP) is very useful method for simulation of +Gz stress and for evaluation of orthostatic reaction. The different physiological changes that occur during LBNP test and +Gz acceleration test are similar. Lategola and Trent found that supine LBNP exposure at the level of -50 mmHg may be equivalent to +2Gz in producing the changes of heart rate (HR). Polese and coworkers compared hemodynamic changes occurring during upright and supine LBNP at the levels to -70 mmHg with identical measurements made during accelerations to +2Gz, +3Gz, and +4Gz in the same subjects. They noted for example that HR changes during upright LBNP exceeded HR supine levels. Peak values of HR during +3Gz and +4Gz significantly exceeded HR levels during both kinds of LBNP, but HR values at +2Gz were equivalent to those at -40 mmHg of upright and -70 mmHg of supine LBNP. So, the present study was undertaken to evaluate adaptating responses to LBNP stimulus at the level of -60 mmHg, regulatory mechanisms of the circulatory system (central and peripheral) and to look for the possibility of +Gz tolerance prediction based on the changes of some hemodynamic parameters during LBNP.

Adaptation, Physiological↗
PubMed1990

The effects of lower body negative pressure on baroreceptor responses in humans.

In healthy human subjects the immediate responses of pulse interval and the steady-state responses of arterial blood pressure and cardiac output to changes in carotid sinus transmural pressure were determined before and during the application of a subatmospheric pressure to the lower part of the body. Increases in carotid sinus transmural pressure, effected by applications of subatmospheric pressure to the neck (neck suction) resulted in prolongation of pulse interval and decrease in blood pressure; opposite responses were obtained to application of a positive pressure (neck pressure). Application of lower body negative pressure resulted in a decrease in pulse interval (heart rate increase) but little change in blood pressure. During lower body negative pressure, the responses of pulse interval to neck pressure were reduced but those to neck suction were unaffected; the responses of blood pressure to neck suction were enhanced but those to neck pressure were unaffected. From experiments in which cardiac output was also determined, it was seen that lower body negative pressure reduced cardiac output, increased calculated total body vascular resistance and augmented the resistance response to neck suction although not to neck pressure. These results are compatible with the view that application of lower body negative pressure does not change the sensitivity of the baroreceptor reflex and that the changes in the responses are due to non-linearities of the stimulus-response curves.

Adult↗
PubMed1994

Summary of lower body negative pressure experiments during space flight.

This paper summarizes the lower body negative pressure experiments performed in space, beginning with the experiments conducted on Skylab, because this program provided the most cardiovascular physiology data for United States space flight. Data obtained during studies of lower body negative pressure for use as a countermeasure after months of Russian space flight are also presented. Lower body negative pressure experiments conducted aboard Space Shuttle flights provide data about the deadaptation response of the cardiovascular system to orthostatic stress occurring during periods of zero gravity, and about protection against postflight orthostatic intolerance. Data obtained using Russian and American lower body negative pressure devices indicate that, when a crew member stands, as opposed to being supported by a seat or saddle as in the American device, there may be a slight detrimental effect in terms of the cardiovascular response to this orthostatic stress. Comparison of heart rate and blood pressure response to entry and landing of the Shuttle indicate that, although lower body negative pressure is a different stress and is applied in a different manner, the maximum heart rates during lower body negative pressure are reached at approximately the same point that the maximum heart rates are reached during entry and landing. Thus, the use of a lower body negative pressure stress in flight is a fairly good predictor of the cardiovascular response to the actual entry and landing of the Shuttle.

Blood Pressure↗
PubMed1978

Influence of lower body negative pressure upon arginine vasopressin release.

Lower body negative pressure (9-12 kPa) was applied to ten normal subjects. Large increases in plasma arginine vasopression concentration occurred only in subjects that experienced syncopal symptoms and developed hypotension. Blood samples obtained from the superior vena cava at 1/2 min intervals during application of negative pressure showed that maximal plasma vasopressin concentrations occurred with hypotension. Chromatography of the presyncopal plasma on Sephadex G-25 gave a large peak which eluted in the position of synthetic arginine vasopressin.

Adult↗