Calf venous compliance measured with head-up tilt equals supine calf compliance

Elevated calf compliance may contribute to orthostatic intolerance following space flight and bed rest. Calf venous compliance is measured conventionally with venous occulusion plethysmography in supine subjects. With this well-established technique, subjects undergo inflation of a pressure cuff around the thigh just above the knee, which increases calf venous pressure. A plethysmograph simultaneously measures calf volume elevation. Compliance equals calf volume elevation per mm Hg thigh occlusion (calf venous) pressure in relaxed legs of the supine subjects. Compliance may also be measured during stepwise head-up tilt (HUT) as calf volume elevation per mm Hg gravitational venous pressure elevation produced by HUT. However, during HUT on a tilt table with a footplate, calf muscles activate to counteract gravity: this is an obvious and natural response to gravitational force. Such muscle activation conceivably could reduce calf compliance, yet relatively little calf muscle activation occurs during HUT and orthostasis (<10% of maximal voluntary levels). Also, this activation produces minimal calf volume change (<0.3%). Therefore, we hypothesized that calf compliance measured with HUT equals that measured with supine venous occlusion.     

Venous cuff pressures from 30 mmHg to diastolic pressure are recommended to measure arterial inflow by plethysmography.

Venous occlusion strain gauge plethysmography (VOP) is based on the assumption that the veins are occluded and arterial inflow is undisturbed by the venous cuff pressure. Literature is not clear concerning the pressure that should be used. The purpose of this study was to determine the optimal venous occlusion pressure at which the highest arterial inflow is achieved in the forearm, calf, and leg by using VOP. We hypothesized that, for each limb segment, an optimal (range of) venous cuff pressure can be determined. Arterial inflow in each limb segment was measured in nine healthy individuals by VOP by using pressures ranging from 10 mmHg up to diastolic blood pressure. Arterial inflows were similar at cuff pressures between 30 and 60 mmHg for the forearm, leg, and calf. Arterial inflow in the forearm was significantly lower at 10 mmHg compared with the other cuff pressures. In addition, arterial inflows at 20 mmHg tended to be lower in each limb segment than flow at higher cuff pressures. In conclusion, no single optimum venous cuff pressure, at which a highest arterial inflow is achieved, exists, but rather a range of optimum cuff pressures leading to a similar arterial inflow. Venous cuff pressures ranging from 30 mmHg up to diastolic blood pressure are recommended to measure arterial inflow by VOP.     

Venous volume displacement plethysmography: Its diagnostic value in deep venous thrombosis as determined by receiver operator characteristic curves

The pitfall of several reviews of noninvasive venous assessment has been the expression of the test data solely in terms of diagnostic accuracy (the number of correct tests in ratio to all tests performed), where results of a test will vary according to disease prevalence. The advantages of receiver operator characteristic curve analysis are twofold: (1) it describes the dynamic relationship between sensitivity (the ratio of the number of true positive tests to the patients with deep venous thrombosis) and specificity (the ratio of true negative tests to the number of patients with no deep venous thrombosis) independent of disease prevalence; and (2) the threshold criteria that defines a positive test can be set by the best balance between sensitivity and specificity and then applied to a given patient population for its diagnostic accuracy. Venous volume plethysmography is a widely used, simple and rapid method. It was compared to the "gold standard" of phlebography in a prospective blind study of 70 limbs that were clinically suspect of having deep venous thrombosis (DVT). Venous volume displacement plethysmography was defined objectively by three quantitative parameters: (1) maximum venous outflow, (2) integer ratio, and (3) segmental venous capacitance ratio. The DVT (22 to 70 positive phlebograms) was divided by anatomic location into either calf vein DVT or proximal DVT (popliteal vein or above). By combining these three parameters, a balance between sensitivity and specificity was obtained to provide a rapid, objective method for screening patients with suspected DVT.     

Effects of a 6-mo endurance-training program on venous compliance and maximal lower body negative pressure in older men and women.

Aging and chronic exercise training influence leg venous compliance. Venous compliance affects responses to an orthostatic stress. The extent to which exercise training in a previously sedentary older population will affect venous compliance and tolerance to the simulated orthostatic stress of maximal lower body negative pressure (LBNP) is unknown. The purpose of this investigation is to determine the influence of a 6-mo endurance-training program on calf venous compliance and responses and tolerance to maximal LBNP in older men and women. Twenty participants (exercise group: n = 10, 5 men, 5 women; control group: n = 10, 6 men, 4 women; all >60 yr) underwent graded LBNP to presyncope or 4 min at -100 mmHg before and after a 6-mo endurance-training program. Utilizing venous occlusion plethysmography, calf venous compliance was determined in both groups using the first derivative of the pressure-volume relation during cuff pressure reduction before training, at 3 mo, and at the end of the training program. The exercise group improved their fitness with the 6-mo endurance-training program, whereas the control group did not change (14 +/- 3 vs. <1 +/- 2%; P < 0.05). LBNP tolerance did not differ between groups or across trials (P = 0.47). Venous compliance was not different between groups or trials, either initially or after 3 mo of endurance training, but tended to be greater in the exercise group after 6 mo of training (P = 0.08). These data suggest that a 6-mo endurance-training program may improve venous compliance without affecting tolerance to maximal LBNP in older participants.     

Venous occlusion plethysmography reduces arterial diameter and flow velocity

The measurement of peripheral blood flow by plethysmography assumes that the cuff pressure required for venous occlusion does not decrease arterial inflow. However, studies in five normal subjects suggested that calf blood flow measured with a plethysmograph was less than arterial inflow calculated from Doppler velocity measurements. We hypothesized that the pressure required for venous occlusion may have decreased arterial velocity. Further studies revealed that systolic diameter of the superficial femoral artery under a thigh cuff decreased from 7.7 +/- 0.4 to 5.6 +/- 0.7 mm (P less than 0.05) when the inflation pressure was increased from 0 to 40 mmHg. Cuff inflation to 40 mmHg also reduced mean velocity 38% in the common femoral artery and 47% in the popliteal artery. Inflation of a cuff on the arm reduced mean velocity in the radial artery 22% at 20 mmHg, 26% at 40 mmHg, and 33% at 60 mmHg. We conclude that inflation of a cuff on an extremity to low pressures for venous occlusion also caused a reduction in arterial diameter and flow velocity.     

Ventilation studied with circulatory occlusion during two intensities of exercise

Our purpose was to study the possible role of a pulmonary chemoreceptor in the control of ventilation during exercise. Respiratory gas exchange was measured breath-by-breath at two intensities of exercise with circulatory occlusion of the legs. Eight male subjects exercised on a cycle ergometer at 49 and 98 W for 12 min; circulation to the legs was occluded by thigh cuffs (26.7 kPa) for two min after six min of unoccluded exercise. PETCO2 and VO2 decreased and PETO2 increased significantly during occlusion at both workloads. Occlusion elicited marked hyperventilation, as evidenced by sharp increases in VE, VE/VCO2, and VE/VO2. A sudden sharp increase in PETCO2 was seen 12.3 +/- 0.5 and 6.5 +/- 1.2s after cuff release in all subjects during exercise at 49 and 98 W, respectively. At 49 W a post-occlusion inflection in VE was seen in 7 subjects 21.1 +/- 5.8s after the PETCO2 inflection. Three subjects showed an inflection in VE at 98 W 23.3 +/- 7.5 s after the PETCO2 inflection. There were significant increases in PETCO2, VO2, VCO2 and VE after cuff release. VE mirrored VCO2 better than VO2, post occlusion. On the basis of a significant lag time between inflections in PETCO2 and VE following cuff release, it is concluded that the influences of a pulmonary CO2 receptor were not seen.     

Prevention of Deep Vein Thrombosis by Intermittent Pneumatic Compression of Calf

A consecutive, randomly allocated, controlled clinical trial of the prophylactic effect of intermittent pneumatic compression of the calf on the incidence of postoperative deep vein thrombosis showed that in patients without malignant disease there was a highly significant reduction in the incidence of thrombosis. In patients with malignant disease the incidence of thrombosis was higher than in those without, and there was no reduction in incidence by the application of intermittent compression. In the absence of malignant disease, severity of operation and the age of the patient were the most significant aetiological factors. We found no relation between the incidence of deep venous thrombosis and obesity, length of preoperative stay, location of hospital, or duration of anaesthesia. We suggest that intermittent pneumatic compression as used in this trial is a safe, effective, and extremely practical method of preventing postoperative deep vein thrombosis in patients not suffering from malignant disease.     

Prevention of Early Postoperative Deep Vein Thrombosis by Intermittent Compression of the Leg during Surgery

A clinical trial is described in which the effect of intermittent compression of the lower limb during surgery on the incidence of early postoperative deep vein thrombosis was assessed. Deep vein thromboses were diagnosed by the ¹²⁵I-fibrinogen uptake test. Peroperative intermittent compression was achieved by means of an inflatable plastic splint coupled to a pneumatic controller. By compressing only one leg of each patient, each patient acted as his own control.With a sequential statistical analysis, 39 patients were required to pass the 5% level of significance. Eleven thrombi were detected in the control (uncompressed) legs and two occurred in the compressed legs; one of the latter was bilateral. The investigation shows that increasing the pulsatility of the venous flow in the leg is a potent prophylactic against postoperative deep vein thrombosis.     

Age- and fitness-related differences in limb venous compliance do not affect tolerance to maximal lower body negative pressure in men and women.

Aging and chronic exercise training influence leg venous compliance. Venous compliance affects responses to an orthostatic stress; its effect on tolerance to maximal lower body negative pressure (LBNP) in the elderly is unknown. The purpose of this study was to determine the influence of age and fitness, a surrogate measure of exercise training, on calf venous compliance and tolerance to maximal LBNP in men and women. Forty participants, 10 young fit (YF; age = 22.6 +/- 0.5 yr, peak oxygen uptake = 57.1 +/- 2.0 ml.kg(-1).min(-1)), 10 young unfit (YU; 23.1 +/- 1.0 yr, 41.1 +/- 2.0 ml.kg(-1).min(-1)), 10 older fit (OF; 73.9 +/- 2.0 yr, 39.0 +/- 2.0 ml.kg(-1).min(-1)), and 10 older unfit (OU; 70.9 +/- 1.6 yr, 27.1 +/- 2.0 ml.kg(-1).min(-1)), underwent graded LBNP to presyncope or 4 min at -100 mmHg. By utilizing venous occlusion plethysmography, calf venous compliance was determined by using the first derivative of the pressure-volume relation during cuff pressure reduction. We found that the more fit groups had greater venous compliance than their unfit peers (P < 0.05) as did the young groups compared with their older peers (P < 0.05) such that OU < YU = OF < YF. LBNP tolerance did not differ between groups. In conclusion, these data suggest that aging reduces, and chronic exercise increases, venous compliance. However, these data do not support a significant influence of venous compliance on LBNP tolerance.     

Differential control of forearm and calf vascular resistance during one-leg exercise

The purpose of this study was to determine whether blood flow (BF) and vascular resistance (VR) are controlled differently in the nonactive arm and leg during submaximal rhythmic exercise. In eight healthy men we simultaneously measured BF to the forearm and calf (venous occlusion plethysmography) and arterial blood pressure (sphygmomanometry) and calculated whole limb VR before (control) and during 3 min of cycling with the contralateral leg at 38, 56, and 75% of peak one-leg O2 uptake (VO2). During the initial phase of exercise (0-1.5 min) at all work loads, BF increased and VR decreased in the forearm (P less than 0.05), whereas calf BF and VR remained at control levels. Thereafter, BF decreased and VR increased in parallel and progressive fashion in both limbs. At end exercise, forearm BF and VR were not different from control values (P greater than 0.05); however, in the calf, BF tended to be lower (P less than 0.05 at 75% peak VO2 only) and VR was higher (23 +/- 9, 44 +/- 14, and 88 +/- 23% above control at 38, 56, and 75% of peak VO2, respectively, all P less than 0.05). In a second series of studies, forearm and calf skin blood flow (laser-Doppler velocimetry) and arterial pressure were measured during the same levels of exercise in six of the subjects. Compared with control, skin BF was unchanged and VR was increased (P less than 0.05) in the forearm by end exercise at all work loads, whereas calf skin BF increased (P less than 0.05) and VR decreased (P less than 0.05). The present findings indicate that skeletal muscle and skin VR are controlled differently in the nonactive forearm and calf during the initial phase of rhythmic exercise with the contralateral leg. Skeletal muscle vasodilation occurs in the forearm but not in the calf; forearm skin vasoconstricts, whereas calf skin vasodilates. Finally, during exercise a time-dependent vasoconstriction occurs in the skeletal muscle of both limbs.     

Vascular adaptation to 4 wk of deconditioning by unilateral lower limb suspension

Physical inactivity or deconditioning is an independent risk factor for atherosclerosis and cardiovascular disease. In contrast to exercise, the vascular changes that occur as a result of deconditioning have not been characterized. We used 4 wk of unilateral lower limb suspension (ULLS) to study arterial and venous adaptations to deconditioning. In contrast to previous studies, this model is not confounded by denervation or microgravity. Seven healthy subjects participated in the study. Arterial and venous characteristics of the legs were assessed by echo Doppler ultrasound and venous occlusion plethysmography. The diameter of the common and superficial femoral artery decreased by 12% after 4 wk of ULLS. Baseline calf blood flow, as measured by plethysmography, decreased from 2.1 +/- 0.2 to 1.6 +/- 0.2 ml.min(-1).dl tissue(-1). Both arterial diameter and calf blood flow returned to baseline values after 4 wk of recovery. There was no indication of a decrease in flow-mediated dilation of the superficial femoral artery after ULLS deconditioning. This means that functional adaptations to inactivity are not simply the inverse of adaptations to exercise. The venous pressure-volume curve is shifted downward after ULLS, without any effect on compliance. In conclusion, deconditioning by 4 wk of ULLS causes significant changes in both the arterial and the venous system.     

Pulseless electrical activity during electroconvulsive therapy: a case report

Background

Arrhythmias resulting in cardiac arrest during electroconvulsive therapy have been reported. Most reported cases of cardiac arrest had asystole as the initial rhythm. Pulseless electrical activity as an initial rhythm of cardiac arrest during electroconvulsive therapy has never been reported. Also, thromboembolism after inflation of pneumatic tourniquet during lower limb surgery has been reported but never following tourniquet inflation during an electroconvulsive therapy.

Case presentation

We report a case involving an 81- year- old female who presented to us for an electroconvulsive therapy for severe depression and developed pulseless electrical activity immediately after electroconvulsive therapy. She was successfully resuscitated and was later found to have bilateral pulmonary emboli with a complete occlusion of the right lower lobe pulmonary artery. The source of embolus was from her left lower extremity deep venous thrombus, which we believe, got dislodged intraoperatively after inflation of pneumatic tourniquet. Our patient not only survived the massive pulmonary embolus, but also showed significant improvement in her mental status compared to her pre-admission level at the time of discharge to a sub-acute rehabilitation centre.

Conclusion

We recommend that patients who are elderly and at high risk of thromboembolism should selectively undergo a preoperative doppler ultrasound for deep venous thrombosis. Also, selective application of tourniquet in the upper limb, to monitor for seizure activity, would reduce the incidence of pulmonary thrombo-embolism as embolic events are significantly less from deep venous thromboses of upper extremities when compared to lower extremities.     

Value of Doppler ultrasound in diagnosis of clinically suspected deep vein thrombosis.

Doppler ultrasound was used to study 120 legs of 106 patients with suspected deep vein thrombosis (DVT) or pulmonary embolism. Venography was subsequently performed in all. DVT was confirmed by venography in 44 legs and was confined to the calf in 10 of these. Ultrasound detected three calf thromboses and 29 out of 34 more extensive thromboses. Of five undetected thrombi that were proximal to the calf one was associated with partial occlusion and four with extensive collateral circulation. Of the 76 limbs without venographic evidence of thrombosis 21 were thought to have DVT by ultrasound; 18 of these false-positive results could be attributed to external compression of veins, two to excessive tenderness precluding adequate examination; and in one no explanation was found. This test gives more accurate results than judging by clinical signs alone, but users must be aware of its limitations and, particularly, the causes of false-positive and false-negative results.     

Regional blood volume and peripheral blood flow in postural tachycardia syndrome

Variants of postural tachycardia syndrome (POTS) are associated with increased ["high-flow" POTS (HFP)], decreased ["low-flow" POTS (LFP)], and normal ["normal-flow" POTS (NFP)] blood flow measured in the lower extremities while subjects were in the supine position. We propose that postural tachycardia is related to thoracic hypovolemia during orthostasis but that the patterns of peripheral blood flow relate to different mechanisms for thoracic hypovolemia. We studied 37 POTS patients aged 14-21 yr: 14 LFP, 15 NFP, and 8 HFP patients and 12 healthy control subjects. Peripheral blood flow was measured in the supine position by venous occlusion strain-gauge plethysmography of the forearm and calf to subgroup patients. Using indocyanine green techniques, we showed decreased cardiac index (CI) and increased total peripheral resistance (TPR) in LFP, increased CI and decreased TPR in HFP, and unchanged CI and TPR in NFP while subjects were supine compared with control subjects. Blood volume tended to be decreased in LFP compared with control subjects. We used impedance plethysmography to assess regional blood volume redistribution during upright tilt. Thoracic blood volume decreased, whereas splanchnic, pelvic, and leg blood volumes increased, for all subjects during orthostasis but were markedly lower than control for all POTS groups. Splanchnic volume was increased in NFP and LFP. Pelvic blood volume was increased in HFP only. Calf volume was increased above control in HFP and LFP. The results support the hypothesis of (at least) three pathophysiologic variants of POTS distinguished by peripheral blood flow related to characteristic changes in regional circulations. The data demonstrate enhanced thoracic hypovolemia during upright tilt and confirm that POTS is related to inadequate cardiac venous return during orthostasis.     

Venous smooth muscle tone and responsiveness in older adults.

Venous compliance is lower in older adults compared with younger adults. It is possible that alterations in venous smooth muscle tone and responsiveness may contribute to the age-related differences in venous compliance. To determine the effects of sympathetic activation [cold pressor test (cold pressor test); rhythmic ischemic handgrip (rhythmic ischemic handgrip)] and endothelium-independent decreases in smooth muscle tone [sublingual nitroglycerin (nitroglycerin)] on venous compliance in young and older adults, forearm and calf venous compliance was measured in 12 young (22 +/- 1 yr) and 12 old (65 +/- 1 yr) supine subjects using venous occlusion plethysmography. Venous compliance was assessed at baseline, during the cold pressor test and rhythmic ischemic handgrip tests, and after nitroglycerin administration. All pressure-volume relationships were modeled with a quadratic regression equation, and beta1 and beta2 were used as indexes of venous compliance. A repeated-measures ANOVA was used to determine the effect of the age and trial on venous compliance. Calf regression parameters beta1 (0.0639 +/- 0.0126 vs. 0.0503 +/- 0.0059, young vs. older; P < 0.05) and beta2 (-0.00054 +/- 0.00011 vs. -0.00041 +/- 0.00005, young vs. older; P < 0.05) were significantly less in older adults at baseline. Similarly, forearm regression parameters, beta1 and beta2 were lower in older adults at baseline. Venous compliance was not effected by the cold pressor test test, rhythmic ischemic handgrip, or sublingual nitroglycerin in either group. Data suggest that forearm and calf venous compliance is lower in older adults compared with young. However, this difference probably cannot be explained by alterations in smooth muscle tone or responsiveness.     

Effects of menstrual cycle and oral contraceptive use on calf venous compliance

Numerous studies have shown that the female sex hormones estrogen and progesterone have multiple effects on the vasculature. Thus our goal was to investigate the effects of estrogen and progesterone on calf venous compliance by looking for cyclic changes during the early follicular, ovulatory, and midluteal phases of the menstrual cycle and during high and low hormone phases of oral contraceptive use. Additionally, we wanted to compare the venous compliance of normally menstruating women, oral contraceptive users, and men. We studied eight normally menstruating women (23 +/- 1 yr of age) during the early follicular, ovulatory, and midluteal phases of the menstrual cycle. Nine triphasic oral contraceptive users (21 +/- 1 yr of age) were studied during weeks of high and low hormone concentrations. Eight men (23 +/- 1 yr of age) were studied twice within 2-4 wk. With the use of venous occlusion plethysmography with mercury in-Silastic strain gauges, lower limb venous compliance was measured by inflating a venous collection cuff that was placed on the thigh to 60 mmHg for 8 min and then reducing the pressure to 0 mmHg at a rate of 1 mmHg/s. Venous compliance was calculated as the derivative of the pressure-volume curves. There were no differences between early follicular, ovulatory, and midluteal phases of the menstrual cycle or between high and low hormone phases of oral contraceptive use (P > 0.05). Male venous compliance was significantly greater than in normally menstruating women (P < 0.001) and oral contraceptive users (P < 0.002). These data support a sex difference but also suggest that venous compliance does not change with menstrual cycle phase or during the course of oral contraceptive use.     

Venous return from distal regions affects heat loss from the arms and legs during exercise-induced thermal loads

To study the role of venous return from distal parts of the extremities in influencing heat loss from the more proximal parts, changes in mean skin temperature (Tsk) of the non-exercising extremities were measured by color thermography during leg and arm exercise in eight healthy subjects. Thirty minutes of either leg or arm exercise at an ambient temperature (Ta) of 20 degrees C or 30 degrees C produced a greatly increased blood flow in the hand or foot and a great increase in venous return through the superficial skin veins of the extremities. During the first 10 min of recovery from the exercise, blood flow to and venous return from the hand or foot on the tested side was occluded with a wrist or ankle cuff at a pressure of 33.3 kPa (250 mm Hg), while blood flow to the control hand or foot remained undisturbed. During the 10-min wrist occlusion, Tsk increased significantly from 28.3 degrees +/- 0.41 degrees C to 30.1 degrees +/- 0.29 degrees C in the control forearm, but remained at nearly the same level (28.0 degrees +/- 0.34 degrees C to 28.2 degrees +/- 0.25 degrees C) in the occluded forearm. In the legs, although Tsk on both sides was virtually identical (32.0 degrees +/- 0.31 degrees C, control vs 32.0 degrees +/- 0.36 degrees C, tested) before occlusion, Tsk on the control side (32.6 degrees +/- 0.27 degrees C) was significantly higher than that on the tested side (32.2 degrees +/- 0.21 degrees C) after ankle occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prediction of human orthostatic tolerance by changes in arterial and venous hemodynamics in the microgravity environment

In this article, we intentionally present exclusively the results of our recent studies of arterial and venous hemodynamics as predictors of human orthostatic tolerance during space flight and after the return to Earth. The possibility of in-flight orthostatic tolerance prediction by arterial hemodynamic responses to the lower body negative pressure (LBNP) and venous hemodynamic changes in response to occlusion of the lower extremities is demonstrated. For the first time, three levels of cerebral blood flow deficits during the determination of orthostatic tolerance in the course of the LBNP test performed in microgravity. We offer quantitative arguments for the dependence of the cerebral blood flow deficit on the degree of tolerance of the LBNP test. Patterns of arterial hemodynamics during LBNP were successfully used to diagnose the actual orthostatic tolerance and to follow its trend during flight, which testifies to the possibility of predicting orthostatic tolerance changes in an individual cosmonaut during space flight. Occlusion plethysmography of the legs revealed three levels of response of the most informative venous parameters (capacity, distensibility, and rate of filling) of the lower extremities correlated to the severity of decrease in orthostatic tolerance.     

Radionuclide plethysmography for noninvasive evaluation of peripheral arterial blood flow

We validated a noninvasive radionuclide plethysmography technique to evaluate peripheral arterial blood flow during reactive hyperemia. This method, based on the measurement of blood volume variations during repetitive venous occlusions, was compared with strain-gauge venous impedance plethysmography. The technique uses 99mTc-labeled autologous red blood cells scintigraphy to determine the rate of change of forearm scintigraphic counts during venous occlusion. Thirteen subjects were simultaneously evaluated with radionuclide and impedance plethysmography. Six baseline flow measurements were performed to evaluate the reproducibility of each method. Twenty-seven serial measurements were then made to evaluate flow variation during forearm reactive hyperemia. After 30 min of recovery, resting forearm blood flows were again evaluated. Impedance and radionuclide methods showed excellent reproducibility with intraclass correlation coefficients of 0.96 and 0.93, respectively. There was also good correlation of flows between both methods during reactive hyperemia (r = 0.87). Resting flows at 30 min after reactive hyperemia were slightly lower than at baseline with both methods. We conclude that radionuclide plethysmography could be used for the noninvasive evaluation of forearm blood flow and its dynamic variations during reactive hyperemia.     

Effects of spaceflight on human calf hemodynamics.

Chronic microgravity may modify adaptations of the leg circulation to gravitational pressures. We measured resting calf compliance and blood flow with venous occlusion plethysmography, and arterial blood pressure with sphygmomanometry, in seven subjects before, during, and after spaceflight. Calf vascular resistance equaled mean arterial pressure divided by calf flow. Compliance equaled the slope of the calf volume change and venous occlusion pressure relationship for thigh cuff pressures of 20, 40, 60, and 80 mmHg held for 1, 2, 3, and 4 min, respectively, with 1-min breaks between occlusions. Calf blood flow decreased 41% in microgravity (to 1.15 +/- 0.16 ml x 100 ml(-1) x min(-1)) relative to 1-G supine conditions (1.94 +/- 0.19 ml x 100 ml(-1) x min(-1), P = 0.01), and arterial pressure tended to increase (P = 0.05), such that calf vascular resistance doubled in microgravity (preflight: 43 +/- 4 units; in-flight: 83 +/- 13 units; P < 0.001) yet returned to preflight levels after flight. Calf compliance remained unchanged in microgravity but tended to increase during the first week postflight (P > 0.2). Calf vasoconstriction in microgravity qualitatively agrees with the "upright set-point" hypothesis: the circulation seeks conditions approximating upright posture on Earth. No calf hemodynamic result exhibited obvious mechanistic implications for postflight orthostatic intolerance.