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

Abstract

-To investigate local bloodflow regulation during orthostatic maneuvers, 10 healthy subjects were exposed to Ϫ20 and Ϫ40 mmHg lower body negative pressure (LBNP; each for 3 min) and to 60°head-up tilt (HUT; for 5 min). Measurements were made of blood flow in the brachial (BFbrachial) 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 enddiastolic volume (LVEDV; by echocardiography). Comparable central cardiovascular responses (changes in LVEDV, SV, and MAP) were seen during LBNP and HUT. During Ϫ20 mmHg LBNP, Ϫ40 mmHg LBNP, and HUT, the following results were observed: 1) BFbrachial decreased by 51, 57, and 41%, and BFfemoral 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). blood flow; orthostatic stress; baroreflex IT IS KNOWN THAT, DURING ORTHOSTATIC stress, peripheral vascular resistance and/or heart rate (HR) increase (mainly via cardiopulmonary and arterial baroreflexes), serving to maintain arterial blood pressure (1, 3-5, 7, 8, 17, 27, 36, 37, 39, 42, 44, 47). Furthermore, it has been suggested that, during orthostatic stress, differential vascular responses occur between the arms and legs (9, 12, 16). The proposed explanations for the latter phenomenon include differences in 1) "local" mechanisms stimulated by the increments in transmural pressure (11,12,18), 2) norepinephrine spillover (13), and 3) the effectiveness of ␣-or ␤-receptors (26). However, those mechanisms are not well understood.Various ways can be used to simulate orthostatic stress in humans. These include application of lower body negative pressure (LBNP) and head-up tilt (HUT), which are known to increase the pooling of blood in the lower body, leading to a decreased blood volume in the central circulation and a consequent unloading of cardiopulmonary and arterial baroreceptors. LBNP is usually applied with the subject in the horizontal posture, and the negative pressure affects mainly the superficial veins, less so the arteries, so arterial blood pressure is almost constant throughout the whole body. In contrast, during HUT, the arterial and venous pressures increase in proportion to the distance from the heart. Thus LBNP and HUT represent two possible ways of decreasing the central blood volume and enhancing sympathetic nervous activity, but they differ in the extent of the change in hydrostatic pressure (local pressure) in the dependent ...

Limb-specific differences in the skin vascular responsiveness to adrenergic agonists

“…However, significant cutaneous vasoconstrictor responses in the forearm and palm at the heart level during orthostatic stress in these thermal conditions (6,34,45,47,48) suggest participation of sympathetic vasoconstrictor function in the responses without a detectable change in the neural signal. Unfortunately, it is unknown whether sympathetic adrenergic vasoconstrictor responsiveness in the skin shows a regional difference between the upper and lower limbs, since blood flow measured in the previous reports represents a composite of flow in both muscle and skin (15,22,38). The contributions of muscle and skin to limb blood flow may be different between arms and legs, so that any regional differences in the response of either vascular bed are not clear.…”

“…IN HUMANS, peripheral vasoconstrictor responses to upright posture show heterogeneity between the upper and lower limbs (14,16,22,48). The findings from previous studies suggest that the leg vasculature is more responsive to orthostatic stimulation than the arm vasculature.…”

“…These findings suggest that the cutaneous vasoconstrictor responsiveness to exogenous NE is greater in the legs than in the arms due to a higher ␣1-and ␣2-adrenoceptor reactivity, while the ␤-adrenoceptor function plays a minor role in regional differences in adrenergic vasoconstriction in normothermic humans. skin blood flow; exogenous norepinephrine; ␣-adrenergic receptor; ␤-adrenergic receptor; microdialysis IN HUMANS, peripheral vasoconstrictor responses to upright posture show heterogeneity between the upper and lower limbs (14,16,22,48). The findings from previous studies suggest that the leg vasculature is more responsive to orthostatic stimulation than the arm vasculature.…”

“…The findings from previous studies suggest that the leg vasculature is more responsive to orthostatic stimulation than the arm vasculature. The augmented vasoconstrictor response observed in the human lower-leg vasculature is influenced by sympathetic and local mechanisms (14,22,24,38) and could play an important regulatory role by limiting the degree to which blood accumulates in the legs during orthostatic stress (29,30).…”

Modulation of radial blood flow during Braille character discrimination task

“…The Doppler probe (7.5-MHz linear array transducer) was adjusted manually over the radial artery by one of the investigators. Mean vessel internal diameter (Dm) was calculated as systolic internal diameter/3 + diastolic internal diameter×2/3, as previously reported (4,10). Radial blood flow (RBF; ml/min) was calculated every beat as the product of mean VEL and vessel cross sectional area (πDm 2 /4) multiplied by the corresponding heart 5 rate (HR).…”