P. M. J. M. De Vries

Use of bioelectrical impedance analysis measurements in the clinical management of patients undergoing dialysis.

The use of bioelectrical impedance analysis (BIA) in patients with end-stage renal disease who are receiving dialysis provides researchers with two important applications: 1) a biological model in which the underlying assumptions of BIA can be tested, and 2) if valid, a tool that can be used to improve the clinical management of patients receiving dialysis. We review the rationale of and purpose for using BIA in the dialysis population, the physiologic changes that occur during dialysis that influence BIA measurements, and last, conclusions reached from the current scientific literature.

Improvement of cardiac imaging in electrical impedance tomography by means of a new electrode configuration

Until now, electrical impedance tomography (EIT) has been used for cardiac imaging with the electrodes attached transversally at the level of the fourth intercostal space at the anterior side. However, the results obtained with this electrode configuration have been disappointing. The aim of the present study was to improve the measurement design of EIT for cardiac imaging. Therefore, magnetic resonance imaging (MRI) scans were analysed in two healthy subjects to determine the optimum anatomical plane in which atria and ventricles are clearly visually separated. From these findings, we proposed a new oblique plane at the level of the ictus cordis anteriorly and 10 cm higher posteriorly. EIT pictures obtained in the oblique plane revealed a better visual separation between the ventricles and atria than with the electrodes attached in the transverse plane. Comparison between volume changes measured by means of MRI and impedance changes in different regions of interest measured with EIT were performed with the electrodes in the proposed oblique plane. Ventricular and atrial volume changes measured by MRI show the same pattern as do impedance changes measured by EIT. Furthermore, we assessed the reproducibility and validity of the oblique electrode configuration in ten healthy mate volunteers during rest and during exercise compared with the currently used transverse electrode configuration. The reproducibility coefficient assessed from repeated measurements with the electrodes attached in the oblique plane was 0.98 at rest and 0.85 during exercise. For the transverse plane the reproducibility coefficient was 0.96 at rest and 0.66 during exercise. The well-known increase in stroke volume during exercise is 40% in healthy subjects. The increase in impedance change during exercise compared with rest was 34 +/- 13% (20-59%) for the oblique plane and 68 +/- 57% (13-140%) for the transverse plane. From these results we infer that the stroke volume is assessed more accurately by using the oblique plane. From these findings, we conclude that the oblique plane improved the cardiac measurements, because (i) a better spatial separation of the heart compartments is obtained, (ii) the results are more reliable and (iii) measurements during exercise are more accurate with the electrodes attached in an oblique plane.

Assessment of the haemodynamic response to exercise by means of electrical impedance cardiography: method, validation and clinical applications

Over the past three decades, the technique of electrical impedance cardiography (EIC) has developed into a valid and reliable instrument for the assessment of stroke volume. Recent developments have made EIC suitable for routine use during exercise testing, too. However, standardization of electrode positioning, stroke volume calculation, and data processing is lacking. In our opinion the most reliable options are, respectively, a modified semicircular electrode array, the Kubicek equation including a haematocrit-based resistivity value, and computerized signal averaging. Although EIC derived stroke volume calculation is based on several debated assumptions, numerous validation studies have shown good accuracy and reproducibility, also during exercise. Addition of EIC measurements during standard clinical exercise testing might be of benefit in occupational medicine, cardiology and pulmonary medicine. Although in the latter setting no validation studies have been performed, major methodological problems are not expected.

Impedance cardiography. Importance of the equation and the electrode configuration.

ObjectiveElectrical impedance cardiography (EIC) has been suggested as a non-invasive method to measure cardiac output. In several studies it proved to be a reliable method, although there were some restrictions. In 1966 Kubicek et al. developed an impedance cardiac output system based upon electrodes and a specific stroke volume formula. In 1983 Sramek et al. developed a new electrode configuration, and a new equation to calculate stroke volume, an equation that was adjusted by Bernstein in 1986. Since then these two methods have been used in clinical medicine. The purpose of the present study was to compare both electrode configurations and both stroke volume calculation equations with each other. The cardiac output (CO) values obtained by means of EIC are compared with CO values obtained by means of thermodilution.DesignProspective study.SettingSurgical intensive care unit of a university hospital.Patients20 mechanically ventilated patients after cardiac surgery.Measurements and resultsSimultaneous measurement of CO by means of electrical impedance cardiography (COEIC) and thermodilution (COTD) was performed. COEIC was obtained using the lateral spot electrode configuration (LS) and an adjusted circular electrode configuration (SC). The formulas of Sramed (S), Sramek-Bernstein (SB), Kubicek (K) and an adjusted Kubicek formula (aK) were employed. Using the LS electrode configuration, significant differences were found between COEIC and COTD with the S formula (p<0.005), the K formula (p<0.001), and the aK formula (p<0.05). Using the SC electrode configuration, significant differences between COEIC and COTD were found with the K formula (p<0.005), the S formula (p<0.01), and the SB formula (p<0.05). No significant difference was found between EIC and TD using the LS electrode configuration together with the SB formula or using the SC electrode configuration with the aK formula. In both cases a good correlation was found between COEIC and COTD (r=0.86,p<0.001 andr=0.79,p<0.001, respectively). The mean difference between EIC and TD was 0.15±0.96 l/min and 0.19±1.19 l/min, respectively.

Non-invasive assessment of cardiac output during exercise in chronic obstructive pulmonary disease: comparison of the -rebreathing method and electrical impedance cardiography

In exercise testing of patients with chronic obstructive pulmonary disease (COPD), non-invasive assessment of stroke volume (SV) and cardiac output (CO) would be valuable. Electrical impedance cardiography (EIC) has proved to be a valid and reliable instrument in healthy subjects. In this study it is investigated whether this also applies to patients with COPD. In 19 COPD patients simultaneous SV measurements were performed during steady-state exercise using the CO2-rebreathing method and EIC (using a fixed blood resistivity value (rho = 135 or 150 omega cm: EIC-135 and EIC-150) or a haematocrit based rho (EIC-ht)). Although close correlations were found (overall correlation between CO2-rebreathing and EIC-ht: R = 0.92 for CO, R = 0.79 for SV), SV and CO measured by means of EIC were significantly higher at low-intensity exercise and lower at high-intensity exercise. The mean differences between the CO2-rebreathing method and EIC-ht were 0.55 ml for SV and 0.01 l min-1 for CO (overall exercise data). The limits of agreement (2SD of the mean difference) were 24.7 ml for SV and 2.56 l min-1 for CO. These figures are comparable to what is found when healthy subjects are studied. CO was closely correlated to oxygen uptake using the CO2-rebreathing as well as the EIC method; the slope of the regression line was closer to what has been reported in the literature with EIC. Results were better with the EIC-ht than with the EIC-135 and EIC-150 methods. It is concluded that EIC is a reliable and valid method for measurements of SV and CO in COPD during exercise.

Impedance cardiography in cardiac surgery patients: abnormal body weight gives unreliable cardiac output measurements

Background: To study the accuracy of cardiac output measurement by means of Electrical Impedance Cardiography (EIC) in post‐cardiac surgery patients.

INTRADIALYTIC HYPOTENSION IN RELATION TO PRE-EXISTENT AUTONOMIC DYSFUNCTION IN HEMODIALYSIS PATIENTS

After having monitored haemodynamics during haemodialysis, we examined autonomic nervous function in rest prior to a next dialysis session in 28 patients on chronic intermittent haemodialysis. The aim was to compare intradialytically hypotensive with stable patients to assess whether blood pressure regulating mechanisms were related to basal autonomic function, assessed as heart rate variability (HRV) tested by means of the deep breathing test, the lying-to-standing test, and the Valsalva maneuver. Impedance cardiography was used to determine stroke volume and cardiac output during dialysis. In addition, blood pressure was registered automatically and systemic vascular resistance calculated. Blood volume variation was monitored by an on-line optical device. Intradialytic hypotension was observed in 10 patients (36%). Systemic vascular resistance in hypotensive patients decreased considerably (-14.0 ± 5.9%), while it increased in stable patients (+9.9 ± 4.6%, p = 0.004). Heart rate rose significantly in hypotensive patients (11.5 ± 3.8%>) in comparison to stable patients (-0.2 ± 2.8%, p = 0.02). However, no significant differences in autonomic function were observed between hypotensive and stable patients. Although both groups showed impaired autonomic function, no significant correlation between changes in haemodynamics during dialysis and autonomic function at rest could be ascertained. In conclusions, hypotension during haemodialysis is not related to a patients autonomic function at rest. This suggests that structural neuronal differences are not responsible for the severe decrease in systemic vascular resistance in intradialytic hypotension.

Non-invasive evaluation of left ventricular function by means of impedance cardiography

Background: Simple, accurate, continuous non‐invasive cardiac monitoring during the peri‐ and postoperative periods for patients at risk of cardiac failure would be very useful. Electrical impedance cardiography (EIC) has been proposed as an accurate method for non‐invasive measurement of cardiac function. However, in recent years the accuracy of EIC in stroke volume (SV) measurement has been questioned and this prevented global acceptance of the method. Beside SV, EIC is capable of measuring several other left ventricular contractility indices, which are measured directly from the impedance signal. The aim of this study was to compare these variables with the echocardiographically derived left ventricular wall motion score (WMS) as the reference method.

DETECTION OF HYDRATION STATUS BY TOTAL BODY BIOELECTRICAL IMPEDANCE ANALYSIS (BIA) IN PATIENTS ON HEMODIALYSIS

The purpose of the present study was to investigate whether total body bioelectrical impedance analysis (BIA) could be appropriate to assess normohydration (i.e. dry weight) in hemodialysis patients. This study is warranted, because the simultaneous assessment of both hydration and nutritional status by BIA requires the presence of a situation of normohydration in order to guarantee valid conclusions about the nutritional analysis. Segmental bioelectrical impedance was performed to classify patients according to their hydration status. BIA measurements revealed significant differences in TBW, ECW and ICW/ECW between three hydration subgroups (under-, normo-, and overhydration), whereas ICW was similar. Therefore, TBW, ECW and ICW/ECW appear appropriate variables to assess hydration status in patients on hemodialysis. Hemodialysis diminished ECW significantly, whereas ICW did not change, suggesting that a decrease of ECW explains the fluid loss during hemodialysis.

The influence of pulsatile flow on blood resistivity in impedance cardiography

The purpose of the study was to investigate the resistivity change over the cardiac cycle. This is important for the correct application of thoracic impedance cardiography (TIC). The ratio of spatial mean velocity over the vessel radius of the ascending and descending aorta of two female and eight male subjects (age ranging from 23 to 69 years) were measured in supine position using MRI. Based on Vissers (1989) equation the relative resistivity change was calculated. In all subjects the authors found a change of less than 15%, which is smaller than rigid tube experiments predicted. However, the peak resistivity change occurs at the same time as the peak in the impedance signal. Thus, the effects of resistivity changes on stroke volume calculation in TIC needs further investigation.