Monitoring of pulmonary fluid volume and stroke volume by impedance cardiography in patients on hemodialysis

Effect of hemodialysis on intra-abdominal pressure

OBJECTIVE

To study the effect of hemodialysis on intra-abdominal pressure.

METHODS

Five patients admitted between July and November of 2003 were evaluated in the intensive care unit. Intra-abdominal pressure was measured before and after hemodialysis, maintaining the ventilatory parameters except for PEEP (positive-end expiratory pressure).

RESULTS

Intra-abdominal pressure was significantly reduced by hemodialysis in all the 5 patients.

CONCLUSION

Hemodialysis significantly reduced intra-abdominal pressure in the 5 patients, an effect which could have influence over other organic systems. This reduction is related to the weight variation before and after hemodialysis, as well as to the loss of volume caused by this procedure.

Thoracic impedance measurements during orthostatic change test and during hemodialysis in hemodialyzed patients

Measurements were performed before and after hemodialysis (HD) in the supine and upright positions (orthostatic change test) and during HD session every 30 minutes in recumbent position on 11 HD patients. Two hydration states were compared: hyperhydration and normal hydration. Each patient served as his own control. Blood pressure and total body bioimpedance were obtained simultaneously. Thoracic impedance values (Zo) obtained during HD were significantly greater in the normal hydration state. There was strong correlation between Zo gain and total ultrafiltration; however, Zo gain divided through total ultrafiltration (calculated for every 100 ml) was also higher in this state. The ratio Zo/R (where R is resistance of total body bioimpedance) was stable during HD but was significantly higher in the normal hydration state. Zo gain during the orthostatic change test was significantly higher after HD than before HD in both hydration states. The anticipated difference in Zo gains between both hydration states was not significant. After echocardiographic analysis of patients, we determined that cardiac dysfunction or valvular defects in four patients were likely responsible for opposite reaction on orthostatic change test. During our measurements, we observed the influence of Zo changes on episodes such as intradialytic hypotension or acute atrial fibrillation. Thoracic impedance is an intriguing method for controlling pathophysiology of fluid distribution, but it requires the accurate definition of a patient's hemodynamics and strong conditions during measurement. During our measurements, we observed the influence of such episodes as intradialytic hypotension or acute atrial fibrillation on Zo changes.

Proportional changes in body fluid with hemodialysis evaluated by dual-energy X-ray absorptiometry and transthoracic bioimpedance with particular emphasis on the thoracic region

Alterations in body composition during extracorporeal hemodialysis (HD) were investigated in 12 hemodialysis patients (9 males and 3 females, mean age 50 +/- 15 years) with a mean ultrafiltration of 2.6 +/- 1.0 L. Analysis was performed using a dual-energy x-ray absorptiometry technique (DXA), which measures 3 principal components of the body: fat mass (FM): lean body mass (LBM), i.e., all soft tissues excluding fat; and bone mineral content (BMC). These 3 components were calculated for the whole body and for different body regions (namely, the thorax, trunk, lower limbs, and upper limbs). The thoracic cage region could be defined manually, separately from the trunk, and its tissue composition was calculated. DXA analysis was performed concomitant with a measurement of the basal thoracic impedance (TFI) by bioimpedance cardiography prior to and 1 h after dialysis. We found a significant decrease in the total LBM, from 55.8 +/- 8.8 to 53.3 +/- 9.3 kg (p < 0.05), but no change in either the FM or BMC. Moreover, there was a disproportional reduction in the LBM in different regions, being significantly greater in the thorax (7.47 +/- 3.7%) than in the other body regions (trunk 4.3 +/- 2.0%, lower limbs 5.4 +/- 2.1%, and upper limbs 4.7 +/- 1.5%). Regarding bioimpedance, a stronger significant correlation was detected between the percentage changes in the TFI and the changes in thoracic fluid (r = 0.80, p < 0.01) than between the changes in the TFI and the changes in the total body fluid (r = 0.63, p < 0.01). The absolute values of the TFI were also significantly and negatively correlated to the thoracic lean mass to fat mass ratio, both before and after HD (r = 0.82, p < 0.001 and r = 0.86, p < 0.001, respectively). In conclusion, DXA is a very sensitive technique to detect fluid changes during HD in the thorax when the thoracic cavity is defined as a region of interest as well as for the whole body. The data also indicate an extracellular compartmental imbalance between different regions with a significantly greater change in the thoracic region. Transthoracic bioimpedance is a useful technique for evaluating the HD induced changes in the thoracic fluid, rather than total body fluid.

A practical, cost-effective, noninvasive system for cardiac output and hemodynamic analysis

Impedance cardiography is a relatively inexpensive, noninvasive technique for measuring cardiac output on the basis of resistive changes in the thorax to electrical current flow. In conjunction with blood pressure monitoring and physiologic maneuvers, the technique may be used to monitor thoracic and total body fluid volume and express a variety of contractility indexes as well as relative and absolute measurements of stroke volume. We have tested hemodynamics in our laboratory by using a cost-effective, powerful microcomputer-based portable noninvasive technique, which makes possible the ensemble averaging of impedance cardiographic waveforms. In conjunction with physiologic maneuvers, the technique has been implemented at our institution and has provided helpful information in our experience in evaluating volume overload, hypertension, hypotension, shock, and heart failure. It is hoped that this noninvasive, relatively cost-effective approach will be more widely appreciated in the future, given the economic realities of medicine today.

Clinical evaluation of impedance cardiography

We tested the validity of thoracic impedance cardiography for measuring cardiac output in man by comparing absolute values obtained using the non-invasive impedance method with values obtained using the invasive thermodilution method. We also compared per cent changes in cardiac output using impedance cardiography and thermodilution in response to environmental manipulations including cardiac pacing and intravenous administration of ergonovine, dipyridamole, or isoproterenol. Among 19 patients, absolute values for cardiac output, using the impedance and the thermodilution techniques, agreed well (r = 0.85, P less than 0.001). The per cent change in cardiac output by impedance cardiography was positively correlated with the per cent change by thermodilution for the several manipulations (overall r = 0.87, P less than 0.001). Impedance cardiography does appear, in general, to measure cardiac output and stroke volume validly in man, even in situations where heart rate and stroke volume change in opposite directions.

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.

Plasma volumes, noradrenaline levels and renin activity during posture changes in end-stage renal failure

In nine normal subjects and nine patients with end-stage chronic renal failure (CRF) we studied the effect of prolonged (110 min) postural changes on the plasma volume, intrathoracic volume, plasma noradrenaline concentration, haemodynamic variables, and plasma renin activity (PRA). Upon standing, plasma volume decreased rapidly by about 11% in both groups as measured from the control volume and changes in haematocrit. This was accompanied by identical increments of plasma colloid osmotic pressure. The changes in intrathoracic volume (monitored by measurement of the electrical impedance of the thorax), as well as the alterations in plasma noradrenaline, blood pressure, and heart rate, were also comparable in the two groups. These similarities were in contrast with divergent responses of PRA. The increase in PRA on standing was significantly blunted in the CRF patients compared with the normal response (to 1.4 +/- 0.4 fold of the supine value in CRF v. 6.0 +/- 3.2 fold in the normals, P less than 0.0004). It is concluded that the attenuated increase in PRA upon standing in patients with CRF is not a consequence of diminished sympathetic stimulation or an altered response of the intravascular volume. Second, the unabated decrease in plasma volume upon standing pleads against a decrease of tissue compliance in CRF.

Application of impedance cardiography in critical care medicine

In spite of good correlations between cardiac output measurements by impedance and established invasive procedures (dye- and thermo-dilution) reported by numerous authors it is doubtful uptil now whether calculations of stroke volume according to the formula of Kubicek et al. (1974) can provide absolutely reliable results. The origin of the dz/dt curve and influencing factors of impedance wave have to be cleared up prior to the total acception of impedance cardiography as a reliable method for determining non-invasive stroke volume. This is true in spite of the agreement of all authors we know, that the reproducibility of the impedance cardiography values is as good as in dye or thermo-dilution measurements. However, for patient monitoring it is sometimes more important to assess the relative changes in stroke volume than to measure its absolute value. For long-term non-invasive monitoring of myocardial contractility in critically ill patients or after pharmacological interventions impedance cardiography may be recommended. Besides systolic time intervals, such as pre-ejection time and ventricular ejection time, three more reliable parameters can be derived from the first derivate of impedance wave. Impedance plethysmography has been shown as a reliable method to diagnose deep vein thrombosis and good correlations between impedance and strain-gauge plethysmography and phlebographic findings are reported. In addition fluid volume changes in the leg, venous capacity, venous outflow and arterial inflow may be determined by impedance plethysmography in a simple way. There is no doubt that alterations in the fluid content of biological tissue may measured by impedance technique. However, correlations between changes in the transthoracic impedance and fluid content of the thorax can be quantified only in a single subject which serves as its own control. Overall standardization is not possible. The reason for interindividual differences in the thoracic impedance at a given reduction of body water are due to anatomical differences, intrapulmonary air volume and pressure, location of the electrodes, electrical conductivity of the tissue and, above all, due to the position of the body. Therefore if transthoracic impedance is determined sequentially measurements must be performed with special attention to the position of the body to get reproducible results. Rapid infusion of colloids or blood transfusion may decrease transthoracic impedance due to intravascular volume expansion even at a net fluid lost during forced furosemide-induced diuresis or extracorporal hemodialysis.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Non-invasive monitoring of cardiac function during haemodialysis

The cardiac function during haemodialysis has been studied using a non-invasive method in a prospective investigation of 7 elderly patients, all of whom had an enlarged heart on the chest X-ray and in all of whom experience had shown that a fall in blood pressure developed during haemodialysis. The cardiac output was determined by impedance cardiography and showed no significant changes during dialysis. A significant rise was observed in the heart rate and this was found to reach a maximum during the periods in which the blood pressure was lowest. Left ventricular function was evaluated from the systolic time intervals, PEP/LVET (PEP = pre-ejection period and LVET = left ventricular ejection time) and the noninvasive contraction index BP diastolic/PEP. The electromechanical systole (QS2), LVET and the non-invasive contraction index all decreased significantly during dialysis, while PEP and PEP/LVET were significantly increased. All the changes were most pronounced at the time when the blood pressure was lowest. It is concluded from the investigation that the cause of the fall in blood pressure during haemodialysis of elderly patients with impaired cardiac function may be ascribed to transient reduction in left ventricular performance.

Considerations on impedance cardiography

Impedance cardiography is an electronic plethysmographic technique which provides information on cardiac stroke index, myocardial performance, thoracic fluid content and peripheral circulation. The method has gained popularity in recent years because it is not invasive. While less precise in absolute terms than invasive methods the results are reproducible, and the technique accurately assesses variations in measurements. It may be used in most anaesthetized patients without the possibility of any of the complications which sometime accompany the use of more precise invasive methods. This paper describes impedance cardiography and methods for its use.

Transthoracic electrical impedance during extracorporeal hemodialysis in acute respiratory failure ("Shocked Lung syndrome")

The alteration (delta Z0) of transthoracic electrical impedance (TEI) during extracorporeal hemodialysis (EHD) was investigated in two Groups of patients with acute renal and acute respiratory failure, that differed with respect to the severity of respiratory insufficiency. Group I had moderate respiratory failure (FiO2 0.31 +/- 0.10, PaO2 84 +/- 14 mmHg), and Group II had severe respiratory failure (FiO2 0.75 +/- 0.17, PaO2 77 +/- 14 mmHg). There was a significant correlation between increase in TEI (delta Z0) and decrease in body weight (delta BW) in each individual patient, but the slope of regression lines was remarkably flattened in Group II. In Group I, delta TEI was 1.9 +/- 0.9 omega, the calculated TEI for 500 gr decrease in BW (delta Z0-500 gr) was 0.59 +/- 0.21 omega, and a significant correlation existed between pooled data of delta Z0 and delta BW. In Group II TEI increased less significantly, delta TEI was 0.6 +/- 0.3 omega (P less than 0.001), delta Z0-500 gr was 0.26 +/- 0.27 omega (P less than 0.01), and there was no correlation between pooled data of delta Z0 and delta BW. Increase of TEI in Group II could be completely attributed to increase in hematocrit. It is concluded that patients of Group I with acute renal failure and moderate respiratory failure lost intrathoracic fluid during EHD, whereas patients of Group II with severe respiratory failure did not. TEI during EHD may serve as a test for detection of fixed fluid within the pulmonary interstitium indicating a poor prognosis of the acute respiratory failure.

Evaluation of transthoracic electrical impedance in the diagnosis of pulmonary edema

To evaluate the clinical usefulness of measuring transthoracic electrical impedance in patients with pulmonary edema, we studied 27 normal subjects and 33 patients. In normal subjects, impedance increased when body position changed from supine to standing (p less than 0.01) and when lung volume increased from residual volume to total lung capacity (p less than 0.01). Impedance values vari7 omega) when measured hourly without removing the electrodes. In only five of 11 patients with clinically severe pulmonary edema and five of eight with radiologically severe pulmonary edema were impedance values outside the normal range. Changes in impedance values correlated with changes in clinical and radiographic indexes of pulmonary edema. We conclude that (1) single measurements of impedance are useless in diagnosing pulmonary edema, and (2) impedance changes as predicted as pulmonary edema increases and decreases, and these changes are readily detected by simpler methods of evaluation.