Cardiac responses to lower body negative pressure and dynamic leg exercise

Cardiac responses to dynamic leg exercise at 0, 50, and 100 W in the supine position were investigated with and without the lower portion of the body exposed to a pressure of -6.6 kPa (Lower Body Negative Pressure, LBNP). Resting values for heart rate (HR) and stroke volume (SV) were considerably higher and lower, respectively, during LBNP than in the control condition. At the transition from rest to the mildest exercise during LBNP SV showed a prompt increase by about 40%, but no significant change in the control condition. HR, which increased by 17 beats X min-1 in the control condition, showed during LBNP no change initially and subsequently a small but significant drop below its resting value. Steady-state values for HR at the various levels of exercise were not significantly affected by LBNP, whereas corresponding values for SV were considerably lowered, so that exercise values for cardiac output were about 3 l X min-1 less during LBNP than in the control condition. The reductions in SV and cardiac output indicate residual pooling of blood in intra- and extramuscular capacitance vessels of the legs. With a change from rest to exercise at 100 W during LBNP mean systolic ejection rate (MSER) increased by 67%, the relations between SV and MSER suggesting that ventricular performance was maintained by a combination of the Frank-Starling mechanism and enhanced contractile strength.     

Effect of hindlimb suspension on cardiovascular responses to sympathomimetics and lower body negative pressure

To determine whether hindlimb suspension is associated with the development of cardiovascular deconditioning, male rats were studied before and after undergoing one of three treatment conditions for 9 days: 1) cage control (n = 15, CON), 2) horizontal suspension (n = 15, HOZ), and 3) head-down suspension (n = 18, HDS). Testing included lower body negative pressure administered during chloralose-urethan anesthesia and graded doses of sympathomimetic agents (norepinephrine, phenylephrine, and tyramine) administered to conscious unrestrained animals. Both HDS and HOZ were associated with a small decrease in the hypotensive response to lower body negative pressure. The HOZ group, but not the HDS group, exhibited augmented reflex tachycardia. Furthermore, both HDS and HOZ groups manifested reduced pressor responses to phenylephrine after treatment. These reductions were associated with significantly attenuated increases in mesenteric vascular resistance. However, baroreflex control of heart rate was not altered by the treatment conditions. Collectively, these results indicate that 9 days of HDS in rats does not elicit hemodynamic response patterns generally associated with cardiovascular deconditioning induced by hypogravic conditions.     

Effects of lower body negative pressure on sympathetic discharge to leg muscles in humans

Recent studies indicate that nonhypotensive orthostatic stress in humans causes reflex vasoconstriction in the forearm but not in the calf. We used microelectrode recordings of muscle sympathetic nerve activity (MSNA) from the peroneal nerve in conscious humans to determine if unloading of cardiac baroreceptors during nonhypotensive lower body negative pressure (LBNP) increases sympathetic discharge to the leg muscles. LBNP from -5 to -15 mmHg had no effect on arterial pressure or heart rate but caused graded decreases in central venous pressure and corresponding large increases in peroneal MSNA. Total MSNA (burst frequency X mean burst amplitude) increased by 61 +/- 22% (P less than 0.05 vs. control) during LBNP at only -5 mmHg and rose progressively to a value that was 149 +/- 29% greater than control during LBNP at -15 mmHg (P less than 0.05). The major new conclusion is that nonhypotensive LBNP is a potent stimulus to muscle sympathetic outflow in the leg as well as the arm. During orthostatic stress in humans, the cardiac baroreflex appears to trigger a mass sympathetic discharge to the skeletal muscles in all of the extremities.     

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

Halliwill, John R., Lori A. Lawler, Tamara J. Eickhoff,Michael J. Joyner, and Sharon L. Mulvagh. Reflex responses toregional venous pooling during lower body negative pressure in humans.J. Appl. Physiol. 84(2): 454-458, 1998.Lower body negative pressure is frequently used to simulateorthostasis. Prior data suggest that venous pooling in abdominal orpelvic regions may have major hemodynamic consequences. Therefore, we developed a simple paradigm for assessing regional contributions tovenous pooling during lower body negative pressure. Sixteen healthy menand women underwent graded lower body negative pressure protocols to 60 mmHg while wearing medical antishock trousers to prevent venous poolingunder three randomized conditions:1) no trouser inflation (control),2) only the trouser legs inflated, and 3) the trouser legs andabdominopelvic region inflated. Without trouser inflation, heart rateincreased 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 eitherthe trouser legs or the trouser legs and abdominopelvic region, heartrate and mean arterial pressure did not change during lower bodynegative pressure. By contrast, although the forearm vasoconstrictorresponse to lower body negative pressure was attenuated by inflation ofthe trouser legs (forearm vascular resistance 33 ± 10 units,P < 0.05 vs. control), attenuation was greater with the inflation of the trouser legs and abdominopelvic region (forearm vascular resistance 16 ± 5 units,P < 0.05 vs. control and trouserlegs-only inflation). Thus the hemodynamic consequences of pooling inthe abdominal and pelvic regions during lower body negative pressureappear to be less than in the legs in healthy individuals.

     

Comparison of cardiovascular responses between lower body negative pressure and head-up tilt.

To investigate local blood-flow regulation during orthostatic maneuvers, 10 healthy subjects were exposed to -20 and -40 mmHg lower body negative pressure (LBNP; each for 3 min) and to 60 degrees head-up tilt (HUT; for 5 min). Measurements were made of blood flow in the brachial (BF(brachial)) and femoral arteries (BF(femoral)) (both by the ultrasound Doppler method), heart rate (HR), mean arterial pressure (MAP), cardiac stroke volume (SV; by echocardiography), and left ventricular end-diastolic volume (LVEDV; by echocardiography). Comparable central cardiovascular responses (changes in LVEDV, SV, and MAP) were seen during LBNP and HUT. During -20 mmHg LBNP, -40 mmHg LBNP, and HUT, the following results were observed: 1) BF(brachial) decreased by 51, 57, and 41%, and BF(femoral) decreased by 40, 53, and 62%, respectively, 2) vascular resistance increased in the upper limb by 110, 147, and 85%, and in the lower limb by 76, 153, and 250%, respectively. The increases in vascular resistance were not different between the upper and lower limbs during LBNP. However, during HUT, the increase in the lower limb was much greater than that in the upper limb. These results suggest that, during orthostatic stimulation, the vascular responses in the limbs due to the cardiopulmonary and arterial baroreflexes can be strongly modulated by local mechanisms (presumably induced by gravitational effects).     

Effects of lower body negative pressure on cardiac and vascular responses to carotid baroreflex stimulation

The aim of this study was to assess carotid baroreflex responses during graded lower body negative pressure (LBNP). In 12 healthy subjects (age 29+/-4 years) we applied sinusoidal neck suction (0 to -30 mmHg) at 0.1 Hz to examine the sympathetic modulation of the heart and blood vessels and at 0.2 Hz to assess the effect of parasympathetic stimulation on the heart. Responses to neck suction were determined as the change in spectral power of RR-interval and blood pressure from baseline values. Measurements were carried out during progressive applications (0 to -50 mmHg) of LBNP. Responses to 0.1 and 0.2 Hz carotid baroreceptor stimulations during low levels of LBNP (-10 mmHg) were not significantly different from those measured during baseline. At higher levels of LBNP, blood pressure responses to 0.1 Hz neck suction were significantly enhanced, but with no significant change in the RR-interval response. LBNP at all levels had no effect on the RR-interval response to 0.2 Hz neck suction. The unchanged responses of RR-interval and blood pressure to neck suction during low level LBNP at -10 mmHg suggest no effect of cardiopulmonary receptor unloading on the carotid arterial baroreflex, since this LBNP level is considered to stimulate cardiopulmonary but not arterial baroreflexes. Enhanced blood pressure responses to neck suction during higher levels of LBNP are not necessarily the result of a reflex interaction but may serve to protect the circulation from fluctuations in blood pressure while standing.     

Effects of posture on cardiovascular responses to lower body positive pressure at rest and during dynamic exercise

We tested thehypothesis that cardiovascular responses to lower body positivepressure (LBPP) would be dependent on the posture of the subject andalso on the background condition (rest or exercise). We measured heartrate (HR), mean arterial blood pressure (MAP), and cardiac strokevolume in eight subjects at rest and during cycle ergometer exercise(76 ± 3 W) with and without LBPP (25, 50, and 75 mmHg) inthe supine and upright positions. At rest, the increase in MAP wasproportional to the increase in LBPP and was greater in the supine (6 ± 2, 15 ± 3, and 26 ± 3 mmHg) than in the upright (2 ± 3, 9 ± 3, and 17 ± 3 mmHg) position. During dynamic exercise,the increases in MAP evoked by 25, 50, and 75 mmHg LBPP were greater inthe supine (13 ± 2, 28 ± 3, and 40 ± 3 mmHg) than in theupright (7 ± 3, 12 ± 3, and 25 ± 3 mmHg)position. We conclude that the systemic pressure response to LBPP isclearly dependent on the body position, with the larger pressureresponses being associated with the supine position both at rest andduring dynamic leg exercise.

     

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

Peters, Jochen K., George Lister, Ethan R. Nadel, and GaryW. Mack. Venous and arterial reflex responses to positive-pressure breathing and lower body negative pressure. J. Appl.Physiol. 82(6): 1889-1896, 1997.We examined therelative importance of arteriolar and venous reflex responses duringreductions 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 butcomparable reductions in right atrial transmural pressure (control tominimum): from 5.9 ± 0.4 to 1.8 ± 0.7 and from 6.5 ± 0.6 to2.0 ± 0.2 mmHg with PEEP and LBNP, respectively. Cardiac output(impedance cardiography) fell less during PEEP than during LBNP (from3.64 ± 0.21 to 2.81 ± 0.21 and from 3.39 ± 0.21 to 2.14 ± 0.24 l · min¹ · m²with PEEP and LBNP, respectively), and mean arterial pressure increased. We observed sustained increases in forearm vascular resistance (i.e., forearm blood flow by venous occlusionplethysmography) and systemic vascular resistance that were greaterduring LBNP: from 19.7 ± 2.91 to 27.97 ± 5.46 and from 20.56 ± 2.48 to 50.25 ± 5.86 mmHg · ml¹ · 100 mltissue¹ · min(P < 0.05) during PEEP and LBNP,respectively. Venomotor responses (venous pressure in thehemodynamically isolated limb) were always transient, significant onlywith the greatest reduction in right atrial transmural pressure, andwere similar for LBNP and PEEP. Thus arteriolar rather than venousresponses are predominant in blood volume mobilization from skin andmuscle, and venoconstriction is not intensified with venous engorgementduring PEEP.

     

The cardiovascular response to lower body negative pressure in humans depends on seal location

We tested whether seal location at iliac crest (IC) or upper abdomen (UA), before and during lower body negative pressure (LBNP), would affect thoracic electrical impedance, hepatic blood flow, and central cardiovascular responses to LBNP. After 30 min of supine rest, LBNP at -40 mm Hg was applied for 15 min, either at IC or UA, in 14 healthy males. Plasma density and indocyanine green concentrations assessed plasma volume changes and hepatic perfusion. With both sealing types, LBNP-induced effects remained unchanged for mean arterial pressure (-3.0+/-1.1 mm Hg), cardiac output (-1.0 l min(-1)), and plasma volume (-11 %). Heart rate was greater during UA (80.6+/-3.3 bpm) than IC (76.0+/-2.5 bpm) (p<0.01) and thoracic impedance increased more using UA (3.2+/-0.2 Omega) than IC (1.8+/-0.2 Omega) (p<0.0001). Furthermore, during supine rest, UA was accompanied by lower thoracic impedance (26.9+/-1.1 vs 29.0+/-0.8 Omega, p<0.001) and hepatic perfusion (1.6 vs 1.8 l.min(-1), p<0.05) compared to IC. The data suggest that the reduction in central blood volume in response to LBNP depends on location of the applied seal. The sealing in itself altered blood volume distribution and hepatic perfusion in supine resting humans. Finally, application of LBNP with the seal at the upper abdomen induced a markedly larger reduction in central blood volume and greater increases in heart rate than when the seal was located at the iliac crest.     

Comparison of muscle sympathetic responses to hemorrhage and lower body negative pressure in humans

We compared changes in muscle sympathetic nerve activity (SNA) during graded lower body negative pressure (LBNP) and 450 ml of hemorrhage in nine healthy volunteers. During LBNP, central venous pressure (CVP) decreased from 6.1 +/- 0.4 to 4.5 +/- 0.5 (LBNP -5 mmHg), 3.4 +/- 0.6 (LBNP -10 mmHg), and 2.3 +/- 0.6 mmHg (LBNP -15 mmHg), and there were progressive increases in SNA at each level of LBNP. The slope relating percent change in SNA to change in CVP during LBNP (mean +/- SE) was 27 +/- 11%/mmHg. Hemorrhage of 450 ml at a mean rate of 71 +/- 5 ml/min decreased CVP from 6.1 +/- 0.5 to 3.7 +/- 0.5 mmHg and increased SNA by 47 +/- 11%. The increase in SNA during hemorrhage was not significantly different from the increase in SNA predicted by the slope relating percent change in SNA to change in CVP during LBNP. These data show that nonhypotensive hemorrhage causes sympathoexcitation and that sympathetic responses to LBNP and nonhypotensive hemorrhage are similar in humans.     

Effects of posture on peripheral vascular responses to lower body positive pressure

We tested the hypothesis that peripheral vascular responses (in the lower and upper limbs) to application of lower body positive pressure (LBPP) are dependent on the posture of the subjects. We measured heart rate, stroke volume, mean arterial pressure, leg and forearm blood flow (using the Doppler ultrasound technique), and leg (LVC) and forearm (FVC) vascular conductance in 11 subjects (9 men, 2 women) without and with LBPP (25 and 50 mmHg) in supine and upright postures. Mean arterial pressure increased in proportion to increases in LBPP and was greater in supine than in upright subjects. Heart rate was unchanged when LBPP was applied to supine subjects but was reduced in upright ones. Leg blood flow and LVC were both reduced by LBPP in supine subjects [LVC: 4.8 (SD 4.0), 3.6 (SD 3.5), and 1.4 (SD 1.8) ml.min(-1).mmHg(-1) before LBPP and during 25 and 50 mmHg LBPP, respectively; P < 0.05] but were increased in upright ones [LVC: 2.0 (SD 1.2), 3.4 (SD 3.4), and 3.0 (SD 2.0) ml.min(-1).mmHg(-1), respectively; P < 0.05]. Forearm blood flow and FVC both declined when LBPP was applied to supine subjects [FVC: 1.3 (SD 0.6), 1.0 (SD 0.4), and 0.9 (SD 0.6) ml. min(-1).mmHg(-1), respectively; P < 0.05] but remained unchanged in upright ones [FVC: 0.7 (SD 0.4), 0.7 (SD 0.4), and 0.6 (SD 0.5) ml.min(-1).mmHg(-1), respectively]. Together, these findings indicate that the leg vascular response to application of LBPP is posture dependent and that the response differs in the lower and upper limbs when subjects assume an upright posture.     

间断下体负压暴露方式对下体负压耐力的影响

目的：探讨不同方式反复下体负压锻炼对下体负压耐力的影响,以期筛选最佳的负压锻炼方式。方法：27名男性健康受试者随机分成3组,分别进行－5.33kPa8min(A组）、6.67kPa4min（B组）、6.67kPa8min（C组）的下体负压锻炼后累积应激指数（CSI）、总耐受时间（DNP）较锻炼前显著提高,A、B组上述指标无显著变化,下体负压暴露时的心率较平静状态显著升高,收缩压显著降低,舒张压无显著变化。结论：经过－6.67kPa/d8min连续8d的间断下体负压可以显著提高下体负压耐力。     

Cardiovascular responses to lower body negative pressure in trained and untrained older men.

To determine whether aerobic conditioning alters the orthostatic responses of older subjects, cardiovascular performance was monitored during graded lower body negative pressure in nine highly trained male senior athletes (A) aged 59-73 yr [maximum O2 uptake (VO2 max) = 52.4 +/- 1.7 ml.kg-1 x min-1] and nine age-matched control subjects (C) (VO2 max = 31.0 +/- 2.9 ml.kg-1 x min-1). Cardiac volumes were determined from gated blood pool scintigrams by use of 99mTc-labeled erythrocytes. During lower body negative pressure (0 to -50 mmHg), left ventricular end-diastolic and end-systolic volume indexes and stroke volume index decreased in both groups while heart rate increased. The decreases in cardiac volumes and mean arterial pressure and the increase in heart rate between 0 and -50 mmHg were significantly less in A than in C. For example, end-diastolic volume index decreased by 32 +/- 4 ml in C vs. 14 +/- 2 ml in A (P < 0.01), mean arterial pressure declined 7 +/- 5 mmHg in C and increased by 5 +/- 3 mmHg in A (P < 0.05), and heart rate increased 13 +/- 3 beats/min in C and 7 +/- 1 beats/min in A (P < 0.05). These data suggest that increased VO2 max among older men is associated with improved orthostatic responses.     

Comparing cardiovascular responses during exercise between head-down tilt pedaling with lower body negative pressure and upright cycling in man

If lower body negative pressure (LBNP) loaded on exercise in weightlessness environment is able to derive a comparable cardiovascular responses to these in the ground, it should be identified as an optimal LBNP for exercise in space. To investigate the LBNP, 7 young subjects were exercised 4 work rates stepping up every 50 watts from 50 watts to 200 watts every 5 minutes in the upright position or 6 degree head down tilt position with each LBNP of 20, 40, 60, 80, and 100 mmHg. Oxygen uptake during tilt exercise with over 60 mmHg LBNP was not different from it in upright exercise. Heart rate and systolic arterial pressure responses to exercise were very similar between tilt exercise with 60 mmHg LBNP and upright exercise. In conclusion, the optimal LBNP loaded on exercise in space should be around 60 mmHg.