Vibratory sensory testing in acute compartment syndromes: a clinical and experimental study

Invasive and noninvasive diagnostic testing was correlated in 11 patients with acute compartmental syndromes of the forearm. The excellent correlation between diminished perception of vibration and increasing compartmental pressure suggested that 256 cycle per second (cps) vibratory stimuli may be useful clinically in determining the appropriate time for surgical intervention in the acute compartmental syndrome. In 12 adult male volunteers, elevated compartment pressures were created in the anterior tibial compartment of the leg. A decrease in perception to 256 cycle per second (cps) vibratory stimulus was the earliest sensory abnormality to occur with elevated tissue compartment pressures. Analysis of variance showed significantly that 256-cps vibration was the most reliable and earliest sensory modality to change at pressures of 35 to 40 mmHg. These clinical and experimental findings support the use of the 256-cps tuning fork as a noninvasive diagnostic test in the evaluation of the patient with suspected acute compartment syndrome.     

Acute hand compartment syndromes after closed crush: a reappraisal

Severe crush to the hand is associated with a poor prognosis. The authors investigated the hypothesis that compartment syndrome complicates such injuries. From 1996 to 2000, the authors retrospectively identified 11 patients who, after sustaining a closed crush injury, developed acute hand compartment syndrome. Diagnosis was made on clinical grounds in two patients (the intracompartmental pressure was not measured) and after clinical examination plus measurement of intracompartmental pressure in nine patients. In all cases, the muscle burst out once the fascia was released from the affected compartment. Clinical clues to elicit the diagnoses were massive hand swelling and tenseness to palpation. Classic symptoms, such as excruciating pain, were absent or their intensity was attributed to the trauma event (in six patients). Classic signs such as intrinsic muscle minus position and pain on stretching were absent in six and three patients, respectively. In addition, the latter stretch test could not be properly judged in five more patients because of interference by the associated injuries. None of the patients developed contracture or sequela that could be attributed to compartment syndrome. On the basis of this experience, it was concluded that crush injury does not in itself carry a poor functional prognosis, provided that attention is paid to the often-concomitant compartment syndrome. Elevated subfascial pressure may be present despite the absence of classic signs and symptoms.     

Intracompartmental pressure of the anterior tibial compartment as a function of body and joint position

We measured the intracompartmental pressure in the anterior tibial compartment of 27 volunteers under different experimental conditions. First we recorded the pressure in the proximal, middle, and distal third of the compartment as well as at different depths of the muscle. Thereafter the pressure was measured in the lying, sitting and standing positions, and at different positions of the knee and ankle joint. There was no difference in pressure in the various parts of the muscle. However, there was a significant difference between the lying, sitting and standing positions. Dorsal flexion of the ankle resulted in an increase in the intracompartmental pressure in dorsiflexion and plantarflexion of the ankle.     

Significance of intraabdominal compartment pressures following TRAM flap breast reconstruction and the correlation of results

Abdominal wall closure after transverse rectus abdominis myocutaneous (TRAM) flap breast reconstruction is often performed under considerable tension and may theoretically cause a component of abdominal compartment syndrome. This prospective study examined intraabdominal pressure after TRAM reconstruction and correlated the findings with clinical course and outcome.All patients who underwent pedicled TRAM flap breast reconstruction from November of 1999 to December of 2000 (n = 77) were included and compared with nonoperative controls (n = 24). Intraabdominal pressures were measured indirectly using the urinary catheter in the postanesthesia care unit on postoperative days 1 and 2. Outcome measures included vital signs, urinary output, net 24-degree fluid balance, and complications. The preoperative variables were age, body mass index, parity, and presence of an epidural. For statistical analysis, the TRAM patients were divided into three groups on the basis of type of closure (bipedicle, unipedicle, and mesh), which were compared by analysis of variance. A multivariate logistic regression was performed to identify risk factors for patients with intraabdominal pressures > or =20 mmHg who were thought to have a component of abdominal compartment syndrome. The incidence of complications was compared by chi-square, with statistical significance determined for p < 0.05.Average intraabdominal pressures were significantly higher in the bipedicled TRAM (14.1 mmHg) and unipedicle TRAM (9.9 mmHg) groups when compared with the mesh group (5 mmHg) and controls (3.7 mmHg; p < 0.001). Increased intraabdominal pressure was transient and peaked on postoperative day 1. Elevated pressure was associated with decreased urinary output, decreased net fluid balance, and increased respiratory rate. Patients with intraabdominal pressures > or =20 mmHg (n = 10) had a higher incidence of complications (60 percent) compared with patients who had pressures <20 mmHg (18 percent; p < 0.05). Elevated intraabdominal pressures were strongly associated with donor-site and general complications. Positive predictive factors for elevated pressure included body mass index and type of closure (bipedicled or bilateral). Multiple pregnancies seemed to have a protective effect.A transient component of abdominal compartment syndrome does exist after TRAM flap breast reconstruction. Bipedicle closure, nulliparous women, and increased body mass index were risk factors for elevated intraabdominal pressures. Tension-free mesh closure seemed to have a protective effect. Symptomatic trends and certain complications were associated with, and possibly explained by, an elevated intraabdominal pressure.     

Muscle soreness and intramuscular fluid pressure: comparison between eccentric and concentric load

This study investigates the dynamic and resting intramuscular pressures associated with eccentric and concentric exercise of muscles in a low-compliance compartment. The left and righ leg anterior compartments of eight healthy males (ages 22-32 yr) were exercised by either concentric or eccentric contractions of the same load (400 submaximal contractions at constant rate, 20/min for 20 min at a load corresponding to 15% of individual maximal dorsiflexion torque). Tissue fluid pressures were measured with the slit-catheter technique before, during, and after the exercise. Average peak intramuscular pressure generated during eccentric exercise (236 mmHg) was significantly greater than during concentric exercise (157 mmHg, P less than 0.001). Peak isometric contraction pressure in the eccentrically exercised compartment was significantly higher both within 20 min postexercise and on the second postexercise day (P less than 0.001). Resting pressure 2 days postexercise was significantly higher on the eccentrically exercised side (10.5 mmHg) compared with the concentrically exercised (4.4 mmHg, P less than 0.05). The ability to sustain tension during postexercise isometric contractions was impaired on the "eccentric" side. Soreness was exclusively experienced in the eccentrically exercised muscles. We conclude that eccentric exercise causes significant intramuscular pressure elevation in the anterior compartment, not seen following concentric exercise, and that this may be one of the factors associated with development of delayed muscle soreness in a tight compartment.     

Design and evaluation of a device for measurement of interface pressure

An electropneumatic device has been designed to measure the interface pressure profile under compression bandages. The device uses commercially available pneumatic sensors (Talley Group Ltd, SJ235/3) and measures interface pressure at up to eight sites simultaneously along the lower limb, with an accuracy of ± 3 mmHg. Measurements can be made in one of two modes: continuously at a rate of up to three samples per second with the results output to a suitable display device, or single measurements of interface pressure can be made and displayed on a digital display incorporated in the device. This enables the monitoring and recording of interface pressure under compression bandages during either ambulation or passive recumbency. The electropneumatic system is described together with its following characteristics: the hysteresis of the Talley pneumatic sensors, the accuracy of pressure measurement and the maximum achievable sample rate. Dynamic measurements in a single normal volunteer are shown.     

Noninvasive in vivo glucose detection in human finger interstitial fluid using wavelength‐modulated differential photothermal radiometry

We present a noninvasive and noncontacting biosensor using Wavelength Modulated Differential Photothermal Radiometry (WM‐DPTR) to monitor blood glucose concentration (BGC) through interstitial fluid (ISF) probing in human middle fingers. WM‐DPTR works in the interference‐free mid‐infrared range with differential wavelengths at the peak and baseline of the fundamental glucose molecule absorption band, giving rise to high glucose sensitivity and specificity. In vivo WM‐DPTR measurements and simultaneous finger pricking BGC reference measurements were performed on diabetic and nondiabetic volunteers during oral glucose tolerance testing. The measurement results demonstrated high resolution and large dynamic range (~80 deg) change in phase signal in the normal‐to‐hyperglycemia BGC range (5 mmol/L to higher than 33.2 mmol/L), which were supported by negative control measurements. The immunity to temperature variation of WM‐DPTR yields precise and accurate noninvasive glucose measurements in the ISF.      

Modified separation of parts as an intervention for intraabdominal hypertension and the abdominal compartment syndrome in a swine model

Standard therapy for abdominal compartment syndrome is laparotomy. In many patients, laparotomy involves a recent incision; for others, volume of resuscitation may be the cause. The components separation technique allows difficult abdominal closure. The authors studied the effect of a modified separation of parts on abdominal compartment syndrome in an animal model. Eight pigs were instrumented for measurement of central venous pressure, mean arterial pressure, peak airway pressure, and intraabdominal pressure. Intraabdominal hypertension to 25 mmHg was established with intraperitoneal fluid infusion. Modified separation of parts was performed by sequential release of the abdominal wall layers. With increased intraabdominal pressure, mean arterial pressure (55.3 +/- 12.0 to 65.3 +/- 11.0), central venous pressure (7.7 +/- 2.4 to 13.3 +/- 6.9), and peak airway pressure (20.2 +/- 2.4 to 25.3 +/- 4.1; p < 0.05) also increased. Maximum intraabdominal pressure was 26.0 +/- 1.2 mmHg. Skin incision resulted in a decrease in intraabdominal pressure to 21.7 +/- 4.5, external oblique release to 18.3 +/- 3.9, internal oblique release to 13.2 +/- 4.0, and transversus muscle incision to 7.0 +/- 2.5 mmHg (p < 0.05). With completion of components separation, mean arterial pressure remained increased (63.2 +/- 16.9), central venous pressure decreased (6.8 +/- 3.6; p < 0.05), and peak airway pressure decreased (22.7 +/- 3.9; p < 0.05). Modified separation of parts technique effectively releases intraabdominal hypertension and reverses the physiologic derangements associated with abdominal compartment syndrome in the animal model.     

The effect of GSM and TETRA mobile handset signals on blood pressure, catechol levels and heart rate variability

An acute rise in blood pressure has been reported in normal volunteers during exposure to signals from a mobile phone handset. To investigate this finding further we carried out a double blind study in 120 healthy volunteers (43 men, 77 women) in whom we measured mean arterial pressure (MAP) during each of six exposure sessions. At each session subjects were exposed to one of six different radio frequency signals simulating both GSM and TETRA handsets in different transmission modes. Blood catechols before and after exposure, heart rate variability during exposure, and post exposure 24 h ambulatory blood pressure were also studied. Despite having the power to detect changes in MAP of less than 1 mmHg none of our measurements showed any effect which we could attribute to radio frequency exposure. We found a single statistically significant decrease of 0.7 mmHg (95% CI 0.3-1.2 mmHg, P = .04) with exposure to GSM handsets in sham mode. This may be due to a slight increase in operating temperature of the handsets when in this mode. Hence our results have not confirmed the original findings of an acute rise in blood pressure due to exposure to mobile phone handset signals. In light of this negative finding from a large study, coupled with two smaller GSM studies which have also proved negative, we are of the view that further studies of acute changes in blood pressure due to GSM and TETRA handsets are not required.     

Changes in the extents of viable and necrotic tissue, interstitial fluid pressure, and proliferation kinetics in clone A human colon tumour xenografts as a function of tumour size

Abstract. Total, viable and necrotic tumour tissue, tumour cell yields, and colony forming efficiencies were measured in clone A human colon tumour xenografts as neoplasms grew from about 100 mm³ to about 6000 mm³. The volumes of the total, viable and necrotic compartments were fit using the Verhulst equation to obtain estimates of growth rates and maximal sizes of the various compartments (carrying capacities). Additionally, at four discrete tumour volumes (250, 850, 2500 and 5500 mm³), hypoxic percentages, proportions of parenchymal tumour and host cells, interstitial fluid pressures, and proliferation kinetics including measurements of apoptosis were determined. There were interesting relationships between the shapes of the curves for total, viable and necrotic tissue to some of the other endpoints measured. Specifically, the volumetric growth curves for the total and viable tumour tissue compartments were identical to a volume of approximately 1000 mm³, but diverged at larger sizes, with the viable cell compartment exhibiting a smaller carrying capacity. The shape of the growth curve for the necrotic compartment exactly mimicked that for the total volume compartment, but was delayed in time by about 21 days. Similarity in shape to that of the overall tumour volume/necrotic volume curves was also seen for the curve for the increase in interstitial fluid pressure, and for the increase in the size of the host cell compartment. In contrast, the growth of the hypoxic compartment and of the parenchymal tumour cell compartment were similar in shape to that of the viable compartment. These data indicate that these compartments are functionally linked. Marked changes in cell kinetic parameters occurred as tumour size increased from 250–5500 mm³. The labelling index and growth fractions decreased from 0.256–0.125, and 0.77–0.40 respectively, and the cell loss factor increased from 0.52–0.74. The volumetric and potential doubling times increased from 4.3–17.6 and 2.1–4.6 days respectively. The cell kinetic changes could not be clearly related to the changes in shape of either the overall tumour volume or the viable tumour volume.     

Blood pressure changes validate phase related external suction, a controlled method for stimulation of human baroreceptors

Phase related external suction (PRES), a new controlled method for manipulating activity in human baroreceptors, applies precisely timed bursts of suction and pressure within the cardiac cycle through an external neck cuff. Seven healthy adult men participated in 32 pseudo-random trials of baroreceptor stimulation and inhibition. Blood pressure was assessed both intra-arterially and with a noninvasive device. In the present study, PRES baroreceptor stimulation elicited invasively measured blood pressure decreases of about 2.5 mmHg (0.33 kPa) and heart rate decreases of about 5 beats · min^–1, while baroreceptor inhibition increased invasively measured blood pressure by about 1.5 mmHg (0.20 kPa) and heart rate about 2.5 beats · min^–1. It was concluded that PRES is an effective method for baroreceptor manipulation with weaker size effect but better control of nonspecific factors in human subjects than other baroreceptor manipulation techniques. The noninvasive blood pressure measurement device was less sensitive to experimental variation than was the invasive device.     

Operation Everest II: elevated high-altitude pulmonary resistance unresponsive to oxygen

High altitude increases pulmonary arterial pressure (PAP), but no measurements have been made in humans above 4,500 m. Eight male athletic volunteers were decompressed in a hypobaric chamber for 40 days to a barometric pressure (PB) of 240 Torr, equivalent to the summit of Mt. Everest. Serial hemodynamic measurements were made at PB 760 (sea level), 347 (6,100 m), and 282/240 Torr (7,620/8,840 m). Resting PAP and pulmonary vascular resistance (PVR) increased from sea level to maximal values at PB 282 Torr from 15 +/- 0.9 to 34 +/- 3.0 mmHg and from 1.2 +/- 0.1 to 4.3 +/- 0.3 mmHg.l-1 X min, respectively. During near maximal exercise PAP increased from 33 +/- 1 mmHg at sea level to 54 +/- 2 mmHg at PB 282 Torr. Right atrial and wedge pressures were not increased with altitude. Acute 100% O2 breathing lowered cardiac output and PAP but not PVR. Systemic arterial pressure and resistance did not rise with altitude but did increase with O2 breathing, indicating systemic control differed from the lung circulation. We concluded that severe chronic hypoxia caused elevated pulmonary resistance not accompanied by right heart failure nor immediately reversed by O2 breathing.     

Compartmental vascular changes in dogs after nephrectomy, DOCA, and saline: effect of nifedipine

Hypertension (mean arterial pressure, (MAP) 131 +/- 3 mmHg) developed in 18 dogs 4 weeks after left nephrectomy, deoxycorticosterone acetate (DOCA), 5 mg/kg sc twice weekly), and 0.5% NaCl drinking solution. This can be compared with MAP (95 +/- 7 mmHg) of 13 dogs with nephrectomy alone and MAP (86 +/- 4 mmHg) of dogs without nephrectomy. The two-compartment model of the circulation revealed no differences in systemic vascular compliance, compartmental compliance, or flow distribution to the compartments. However, the time constant for venous return for the compartment with the rapid time constant was increased from 0.05 +/- 0.004 min in control animals to 0.07 +/- 0.006 min in the nephrectomy alone group and 0.09 +/- 0.008 min in the hypertensive group (p less than 0.001), as a result of an increase in venous resistance. Arteriolar resistance in this compartment was also increased in the hypertensive animals, as was the mean circulatory filling pressure and overall resistance to venous return. Nifedipine (0.025-0.05 mg/kg) reduced MAP by 15% in the nephrectomy alone group and by 22% in the hypertensive group, with reduction in arteriolar resistance only in the fast time constant compartment. In the slow time constant compartment, arteriolar resistance was increased by more than 100% and flow decreased by more than 50% after nifedipine. Unilateral nephrectomy, DOCA, plus NaCl resulted in hypertension by increasing arteriolar resistance in a vascular compartment with a fast time constant for venous return. Nifedipine countered this effect by inducing arteriolar vasodilation in this compartment. In addition, nifedipine reduced the mean circulatory filling pressure and overall resistance to venous return.     

Hemodynamic effects of lipids in humans

Evidence suggests lipid abnormalities may contribute to elevated blood pressure, increased vascular resistance, and reduced arterial compliance among insulin-resistant subjects. In a study of 11 normal volunteers undergoing 4-h-long infusions of Intralipid and heparin to raise plasma nonesterified fatty acids (NEFAs), we observed increases of blood pressure. In contrast, blood pressure did not change in these same volunteers during a 4-h infusion of saline and heparin. To better characterize the hemodynamic responses to Intralipid and heparin, another group of 21 individuals, including both lean and obese volunteers, was studied after 3 wk on a controlled diet with 180 mmol sodium/day. Two and four hours after starting the infusions, plasma NEFAs increased by 134 and 111% in those receiving Intralipid and heparin, P < 0.01, whereas plasma NEFAs did not change in the first group of normal volunteers who received saline and heparin. The hemodynamic changes in lean and obese subjects in the second study were similar, and the results were combined. The infusion of Intralipid and heparin induced a significant increase in systolic (13.5 +/- 2.1 mmHg) and diastolic (8.0 +/- 1.5 mmHg) blood pressure as well as heart rate (9.4 +/- 1.4 beats/min). Small and large artery compliance decreased, and systemic vascular resistance rose. These data raise the possibility that lipid abnormalities associated with insulin resistance contribute to the elevated blood pressure and heart rate as well as the reduced vascular compliance observed in subjects with the cardiovascular risk factor cluster.     

Effect of proinflammatory cytokines, tumor necrosis factor-alpha and interferon-gamma on epithelial barrier function and matrix metalloproteinase-9 in Madin Darby canine kidney cells.

BACKGROUND: Elevated matrix metalloproteinase-9 production during inflammation may be deleterious to epithelial barrier function. Therefore we examined the effect of proinflammatory cytokines on the expression and regulation of matrix metalloproteinase-9 in a model renal epithelial cell system. Tight junctions limit diffusion between compartments and permit directional transport of solutes. Impairment of these junctional complexes by proteolysis may contribute to renal failure through loss of barrier function. METHODS: The renal epithelial cell model, MDCK cells were employed to examine metalloproteinase activity and mRNA expression. Epithelial barrier function was determined using paracellular flux studies. RESULTS: We found that matrix metalloproteinase-9 expression (MMP-9) and activity is markedly elevated in response to tumor necrosis factor-alpha exposure through a mitogen-activated protein kinase dependent pathway. The MMP-9 is predominately secreted into the apical compartment and elevated MMP-9 expression correlates with impaired cell barrier function that was restored using a specific inhibitor of MMP activity. Addition of recombinant MMP-9 to the apical compartment of MDCK cultures significantly elevated paracellular flux rate. CONCLUSIONS: We provide direct evidence for a MMP-9-mediated mechanism that produces junctional disruption. Collectively, these findings support the hypothesis that impaired epithelial barrier function due to activation of tissue/matrix degrading mechanisms occurs in response to specific inflammatory cues.     

Validation of the Tekscan system for statistic and dynamic pressure measurements of the human femorotibial joint]

In vitro dynamic pressure measurements in the healthy and pathologically altered knee joint help to improve our understanding of the loading pattern on femorotibial surfaces. The aim of the study was to evaluate a piezoresistive pressure measuring system. A human cadaveric knee was mounted in a material-testing machine (Bionix 858) using a specially designed knee-holding device. Axial loading of the knee, flexed at 20o, at 500 N, 1000N and 1500 N was then carried out. For the static investigations, the piezoresistive measuring system (Tekscan), was compared with the FUJI measuring system. In addition, dynamic measurements were also performed with the Tekscan System. With the exception of the lateral compartment at a load of 1500 N, no differences in maximum pressures were observed between the two systems. Nor were there any differences with regard to contact surfaces, either in the medial or lateral compartment (p > 0.05). However, the reproducibility of the data was significantly higher with the Tekscan System (p < 0.01). Dynamic pressure measurements obtained with the knee flexed 20 to 90o showed that the lateral contact area shifted from anterior to posterior, while the medial contact area remained virtually unchanged. The Tekscan System proved to be more reliable than the FUJI System, and permits simultaneous measurements in both compartments. The Tekscan System is suitable for dynamic measurement of the femorotibial joint, and permits measurements to be made under more physiological conditions.     

BioMEMS: forging new collaborations between biologists and engineers

This video describes the fabrication and use of a microfluidic device to culture central nervous system (CNS) neurons. This device is compatible with live-cell optical microscopy (DIC and phase contrast), as well as confocal and two photon microscopy approaches. This method uses precision-molded polymer parts to create miniature multi-compartment cell culture with fluidic isolation. The compartments are made of tiny channels with dimensions that are large enough to culture neurons in well-controlled fluidic microenvironments. Neurons can be cultured for 2-3 weeks within the device, after which they can be fixed and stained for immunocytochemistry. Axonal and somal compartments can be maintained fluidically isolated from each other by using a small hydrostatic pressure difference; this feature can be used to localize soluble insults to one compartment for up to 20 h after each medium change. Fluidic isolation enables collection of pure axonal fraction and biochemical analysis by PCR. The microfluidic device provides a highly adaptable platform for neuroscience research and may find applications in modeling CNS injury and neurodegeneration.     

Transcapillary fluid shifts in tissues of the head and neck during and after simulated microgravity.

To understand the mechanism, magnitude, and time course of facial puffiness that occurs in microgravity, seven male subjects were tilted 6 degrees head-down for 8 h, and all four Starling transcapillary pressures were directly measured before, during, and after tilt. Head-down tilt (HDT) caused facial edema and a significant elevation of microvascular pressures measured in the lower lip: capillary pressures increased from 27.7 +/- 1.5 mmHg (mean +/- SE) pre-HDT to 33.9 +/- 1.7 mmHg by the end of tilt. Subcutaneous and intramuscular interstitial fluid pressures in the neck also increased as a result of HDT, whereas interstitial fluid colloid osmotic pressures remained unchanged. Plasma colloid osmotic pressure dropped significantly by 4 h of HDT (21.5 +/- 1.5 mmHg pre-HDT to 18.2 +/- 1.9 mmHg), suggesting a transition from fluid filtration to absorption in capillary beds between the heart and feet during HDT. After 4 h of seated recovery from HDT, microvascular pressures in the lip (capillary and venule pressures) remained significantly elevated by 5-8 mmHg above baseline values. During HDT, urine output was 126.5 ml/h compared with 46.7 ml/h during the control baseline period. These results suggest that facial edema resulting from HDT is caused primarily by elevated capillary pressures and decreased plasma colloid osmotic pressures. The negativity of interstitial fluid pressures above heart level also has implications for maintenance of tissue fluid balance in upright posture.     

Inside Front Cover: In vivo noninvasive visualization of retinal perfusion dysfunction in murine cerebral malaria by camera‐phone laser speckle imaging (J. Biophotonics 1/2019)

A novel, camera phone‐based laser speckle imager creates new possibilities for quantitative and noninvasive investigations into diagnosis and pathogenesis of cerebral malaria through the eye. In a longitudinal study, a camera‐phone imager detected decreased retinal blood flow speed as experimental cerebral malaria developed in a murine model. The device may ultimately permit recognition of the syndrome prior to the onset of clinical symptoms which is not currently possible. Further details can be found in the article by Itay Remer, Lorraine F. Pierre‐Destine, David Tay, Linnie M. Golightly, and Alberto Bilenca ( e201800098 ).

     

The interstitial fluid content in working muscle modifies the cardiovascular response to exercise

The volume of interstitial fluid in the limbs varies considerably, due to hydrostatic effects. As signals from working muscle, responsible for much of the cardiovascular drive, are assumed to be transmitted in this compartment, blood pressure and heart rate could be affected by local or systemic variations in interstitial hydration. Using a special calf ergometer, eight male subjects performed rhythmic aerobic plantar flexions in a supine position with dependent calves for periods of 7 min. During exercise heart rate, blood pressure, oxygen uptake (VO2) and blood lactate concentrations were measured in two different tests, one before and after interstitial calf dehydration through limb elevation for 25 min, compared to the other, a control with unaltered fluid volume in a maintained working position. Impedance plethysmography showed calf volume to be stabilized in the control position. Leg elevation by passive hip flexion to 90 degrees resulted in a fast (vascular) volume decrease lasting less than 2 min, followed by a slow linear fluid loss from the interstitial compartment. Then, when returned to the control position, adjustment of vascular volume was completed within 2 min and exercise could be performed with dehydration remaining in the interstitium only. Cardiovascular response was identical at the start of both tests. However, exercising with dehydrated calves elicited a significantly larger increase in heart rate compared to the control, whereas VO2 was identical. The blood pressure response was shown to be only slightly enhanced. Structural interstitial features varying with hydration, most likely chemical or mechanical ones, may have been responsible for this amplification of signals.