Short-term cardiovascular responses to rapid whole-body tilting during exercise.

Our objective was to characterize the responses of heart rate (HR) and arterial blood pressure (BP) to changes in posture during concomitant dynamic leg exercise. Ten men performed dynamic leg exercise at 50, 100, and 150 W and were rapidly and repeatedly tilted between supine (0 degrees ) and upright (80 degrees ) positions at 2-min intervals. Continuous recordings of BP and HR were made, and changes in central blood volume were estimated from transthoracic impedance. Short-lasting increases in BP were observed immediately upon tilting from the upright to the supine position (down-tilt), averaging +18 mmHg (50 W) to +31 mmHg (150 W), and there were equally short-lasting decreases in BP, ranging from -26 to -38 mmHg upon tilting from supine to upright (up-tilt). These components occurred for all pressure parameters (systolic, mean, diastolic, and pulse pressures). We propose that these transients reflect mainly tilt-induced changes in total peripheral resistance resulting from decreases and increases of the efficiency of the venous muscle pump. After 3-4 s (down-tilt) and 7-11 s (up-tilt) there were large HR transients in a direction opposite to the pressure transients. These HR transients were larger during the down-tilt (-15 to -26 beats. min(-1)) than during the up-tilt (+13 to +17 beats. min(-1)), and increased in amplitude with work intensity during the down-tilt. The tilt-induced HR fluctuations could be modelled as a basically linear function of an arterial baroreflex input from a site half-way between the heart and the carotid sinus, and with varying contributions of fast vagal and slow sympathetic HR responses resulting in attenuated tachycardic responses to hypotensive stimuli during exercise.

Lower leg fluid displacement during a simulated space shuttle launch.

Reductions in leg fluid volume of about 1/per leg or 10% of total leg volume have been reported during space flight. We wanted to test the hypothesis that a significant portion of these changes occur during the prelaunch and launch periods. Fluid volume changes in the lower leg were estimated in six men during a simulated Space Shuttle launch. After 2 h in the launch position, i.e. supine with elevated legs, the subjects were exposed to 500 s of two to three times increased g force in the anterio-posterior direction in a human centrifuge. During the prelaunch period one lower leg lost a mean of (113 SD 53) ml of fluid and there was little or no additional fluid reduction during the period of increased g force. This compares with the 178-ml reduction of lower leg volume that has been reported during the 1st day of Shuttle missions. We concluded that a significant portion of the fluid reduction observed in the lower leg during the early stages of space flight had already occurred before the launch.

Blood pressure and heart rate responses to sudden changes of gravity during exercise.

Heart rate (HR) and blood pressure responses to sudden changes of gravity during 80- to 100-W leg exercise were studied. One group was exposed to sudden changes between 1.0 and 0 g in the head-to-foot direction (Gz+), starting upright and with repeated 30-s tilts to the supine position. Another group was exposed to sudden Gz+ changes between 1.8 and 0 g in an aircraft performing parabolic flight. Arterial blood pressure at the level of the carotid (carotid distending pressure, CDP) showed a large transient increase by 27-47 mmHg when Gz+ was suddenly decreased and a similar drop when Gz+ was suddenly increased. HR displayed a reverse pattern with larger transients (-22 to -26 min-1) in response to Gz+ decreases and more sluggish changes of lower amplitude in the other direction. Central blood volume, as estimated from the inverse of transthoracic impedance (1/TTI), varied in concert with Gz+. A model is proposed in which HR responses are described as a function of CDP and 1/TTI after a time delay of 2.3-3.0 s and including a low-pass filter function with time constants of 0.34-0.35 s for decreasing HR and time constants of 2.9-4.6 s for increasing HR. The sensitivity of the carotid component was around -0.8 to -1.0 min-1 . mmHg-1 (4-7 ms/mmHg). The cardiopulmonary baroreceptor component was an additive input but was of modest relative importance during the initial HR responses. For steady-state HR responses, however, our model suggests that inputs from carotid and cardiopulmonary receptors are of equal importance.

Changes in lower limb muscle cross-sectional area and tissue fluid volume after transition from standing to supine.

Lower limbs show acute fluid shift in response to transition from upright to supine body position. It is hypothesized that this would affect tomographic estimations of muscle mass and composition. Seven healthy subjects were investigated during the initial 120 min of bed rest, using repeated computerized tomography (CT) and continuous bioelectrical impedance analysis (BIA). Thigh and calf muscle cross-sectional area (CSA) decreased (P < 0.05) by 1.9 and 5.5% whereas fat CSA decreased (P < 0.05) by 4.1 and 4.4%, respectively. Radiological density (RD) of muscle showed a simultaneous increase (P < 0.05) by 4.8% in calf but not (P > 0.05) in thigh. No changes occurred (P > 0.05) in muscle or fat CSA or muscle RD in either thigh or calf between the first and second hour of bed rest. Fluid shift, as estimated by BIA, showed an exponential decay in thigh (tau th = 30 min) and calf (tau c2 = 37 min) by 2.5 and 8.7%, respectively, from first to 120 min of bed rest. Moreover, the calf showed an initial rapid (tau c1 = 8 s) 2.2% decrease. The demonstrated short-term changes in leg CSA were more pronounced in the calf than in the thigh. They were similar in muscle and subcutaneous fat. These fluid shifts merit consideration when tomographic imaging techniques are used to estimate muscle mass and composition.

Blood pressure reduction during hemodialysis correlates to intradialytic changes in plasma volume.

Blood pressure alterations during hemodialysis were related to changes in body fluid in 14 patients with chronic renal failure. Changes in plasma volume (PV) and extracellular volume (ECV) were calculated from determinations of fluid volumes before and after hemodialysis, using 125I-albumin and 51Cr EDTA respectively. Reduction in body water was estimated from body weight changes. Weight loss was 3.3 +/- 0.3 kg (range 1.8-6.0 kg). The relative reduction of fluid was greater in the ECV, 21.6 +/- 3.2%, compared to plasma volume, 6.9 +/- 1.8%. The reduction in systolic blood pressure was related to both absolute (r = 0.66, p less than 0.05) and relative PV reduction (r = 0.72, p less than 0.02). There was no correlation between blood pressure reduction and weight loss or ECV changes. Only minor alterations were found in diastolic blood pressure. Plasma volume maintenance relates to blood pressure changes. Plasma volume monitoring could be useful for improving intradialytic hemodynamic control.

Study of the electrical impedance of blood from house painters.

Using a four-electrode technique the electrical impedance was measured at 150 kHz (100 microA) on whole blood samples from 13 male house painters (age 19-49 years; median 35 years) exposed occupationally to vapors from alkyd paints containing organic solvents and from a control group of 7 healthy unexposed men (age 27-53 years; median 37 years). The blood impedance of the two groups was monitored prior to and 9 days after an initial administration of acetylsalicylic (600 mg). The resistivity calculated from the impedance of whole blood from the painters remained significantly higher than the corresponding values for the controls. The hematological values for the two groups were within normal ranges, although the painters had significantly higher platelet counts than the controls. Effects of long-term exposure of the painters to volatile solvents are discussed.

Whole body impedance measurements reflect total body water changes. A study in hemodialysis patients.

Fluid volume changes during hemodialysis was monitored by continuous whole body impedance measurements. The fluid changes recorded using this method was compared to fluid volume changes measured in plasma water (PV) using 125I-albumin, and extracellular volume (ECV) using 51Cr-EDTA before and after treatment, and total body water (TBW) changes reflected by continuous bed scale monitoring. Changes in impedance correlated to TBW changes, r = 0.80, p less than 0.001, while correlations to changes in ECV and PV were: r = 0.57 and r = 0.55, respectively, p less than 0.05. Alterations in body fluid volumes recorded with whole body impedance is best correlated to total body water changes. The use of continuous whole body impedance monitoring has been shown to offer a simple non-invasive method for recording total body water changes during hemodialysis.

Transthoracic electrical impedance at 1 and 100 kHz--a means for separating thoracic fluid compartments?

The electrical impedance of biological tissues varies with their water and electrolyte contents. Alternating current above 5-10 kHz passes both intra- and extracellular fluid, and lower frequency current preferentially extracellular fluid. In an attempt to evaluate thoracic fluid in different compartments, transthoracic electrical impedance (TEI) was measured at 1 and 100 kHz in 15 consecutive patients, without overt left heart failure and under haemodynamic surveillance, during the first two days of myocardial infarction. To achieve different states of hydration the patients were given i.v. furosemide, either 40 mg t.i.d. (high dose group--HDG) or 20 mg once daily (low dose group--LDG). Effects of altered body position and the respective furosemide injections, were evaluated on both days. Mean 24-h-diuresis was 3.9 l in the HDG and 2.5 l in the LDG. After 24 h mean pulmonary arterial diastolic pressure had decreased by 30% in the HDG but remained almost unchanged in the LDG. By then mean basal TEI was increased only in the HDG, by 17% at 1 kHz and 13% at 100 kHz. On Day 1, within one hour after furosemide, TEI increased temporarily by 3-5%, at both frequencies and in both groups. On Day 2, this short-term increase was similar in the LDG at both frequencies and in the HDG at 100 kHz, but not at 1 kHz, consistent with a major extracellular fluid loss in the HDG. TEI at 1 and 100 kHz may thus reflect extracellular and total thoracic fluid, respectively.

Effects of furosemide and slow-release furosemide on thoracic fluid volumes.

Transthoracic electrical impedance (TEI) was used to assess the relative effectiveness of a 60 mg sustained-release furosemide preparation (FR) and a 40 mg standard furosemide tablet (F), in reducing the fluid content in the thoracic cavity. A double-blind crossover study was performed, in which 12 men with a history of one or more myocardial infarctions and mild left heart failure treated with 40 mg furosemide once daily participated. The trial, lasting 28 days, was divided into two 14-day periods. Each participant received one active drug and one placebo preparation daily, the same regimen being maintained for 14 days, when the active substances were switched. TEI, body weight, serum potassium, sodium, creatinine, and urate were measured immediately prior to the start of the study, after 14 days, and at the end of the study. TEI was measured at frequencies of 1 and 100 kHz with a constant current of 100 microA, during a period of one hour following an intravenous injection of 40 mg furosemide, when the urine volumes were measured also. TEI and urine production after the furosemide injections were similar irrespective of the drug preparation. No evidence of treatment period interaction was seen. No significant differences were demonstrated in body weight and blood chemistry during the trial. These results suggest clinical equipotency of the two preparations in mild left heart failure.

Influence of furosemide and body posture on transthoracic electrical impedance in AMI.

Transthoracic electrical impedance (TEI) is related to thoracic fluid content. In order to analyze this relation, we studied the acute effects of changes in body posture and of intravenous administration of furosemide on TEI in patients with different diuretic regimens. The TEI was measured using a tetrapolar electrode system, and a 100 microA constant current at 100 kHz. The measurements were performed repeatedly during the first two days in 15 consecutive patients with acute myocardial infarction (AMI) and without overt left heart failure, as evaluated from clinical data and bedside catheterization. It was concluded that TEI appears to be a sensitive, noninvasive means of evaluating changes in thoracic fluid content in AMI.

Fluid volume changes during hemodialysis monitored with the impedance technique.

A fluid monitor based on the tetrapolar impedance technique was employed for continuous measurements of fluid volume change during hemodialysis treatment and compared to concomitant ultrafiltration and bed scale readings. Oral fluid intake before and during the dialysis immediately could be recorded with the bed scale as a weight change. In the readings of the fluid monitor, however, the response to fluid intake could be recorded only gradually, as the fluid was absorbed from the gastrointestinal tract. Taking this into account, together with a previously described artifact due to shifts in body position, an excellent agreement was obtained between impedance and bed scale fluid volume monitoring. It is concluded that impedance measurements give a better estimation of the amount of metabolically active fluid than does body weight monitoring.

Evaluation of impedance technique for fluid-volume monitoring during hemodialysis.

Fluid-volume changes during hemodialysis were studied in 30 patients at four separate treatments in each patient. Readings of fluid-volume changes obtained by a fluid-balance monitor with non-invasive tetrapolar impedance technique were compared to changes in weight caused by the treatment. A correlation coefficient of r = 0.90 (p less than 0.001) was found. A survey of papers describing impedance measurements during hemodialysis or diuresis is also presented.

Fluid volume monitoring with electrical impedance technique during hemodialysis.

A tetrapolar whole-body impedance technique has been developed for bedside monitoring of fluid volume changes during hemodialysis. A microprocessor is utilized for control and calculations. Recorded values of fluid volume changes for 10 patients were compared to concomitant changes in weight. A linear relationship with a correlation coefficient of r = 0.89 was obtained. Fluid volume changes as small as 0.1 L could be detected.

Automatic recording of biological impedances.

An automatic recording system for measurements of biological impedances at different frequencies is described. The system comprises a signal synthesizer and a network analyser controlled by a desk calculator. The impedance and phase angle are recorded at one hundred and sixty logarithmically-spaced frequencies between 100 Hz and 1 MHz. A pilot study was conducted to explore the usefulness of the system for investigating fluid balance problems.