P. M. J. M. de Vries

Ventilation and perfusion imaging by electrical impedance tomography: a comparison with radionuclide scanning

Electrical impedance tomography (EIT) is a technique that makes it possible to measure ventilation and pulmonary perfusion in a volume that approximates to a 2D plane. The possibility of using EIT for measuring the left-right division of ventilation and perfusion was compared with that of radionuclide imaging. Following routine ventilation (81mKr) and perfusion scanning (99mTc-MAA), EIT measurements were performed at the third and the sixth intercostal level in 14 patients with lung cancer. A correlation (r = 0.98, p < 0.005) between the left-right division for the ventilation measured with EIT and that with 81mKr was found. For the left-right division of pulmonary perfusion a correlation of 0.95 (p < 0.005) was found between the two methods. The reliability coefficient (RC) was calculated for estimating the left-right division with EIT. The RC for the ventilation measurements was 94% and 96% for the perfusion measurements. The correlation analysis for reproducibility of the EIT measurements was 0.95 (p < 0.001) for the ventilation and 0.93 (p < 0.001) for the perfusion measurements. In conclusion, EIT can be regarded as a promising technique to estimate the left-right division of pulmonary perfusion and ventilation.

Measurement of transcellular fluid shift during haemodialysis

A method is presented to measure transcellular fluid shifts during haemodialysis based on a simplified model of the electrical admittance of biological tissues. It allows for the measurement of intracellular and extracellular conductivities and their ratios. The method is noninvasive, clean and harmless, and can be easily computerised in order to be performed continuously. A typical example is given of a recording during haemodialysis.

Optimalisation of the spot electrode array in impedance cardiography

THE TECHNIQUE of impedance cardiography has been suggested as a low-cost, simple, safe and non-invasive technique for monitoring cardiac function. As a means of measurement of cardiac systolic time intervals, its validity has received strong support (PETROVICK e t al., 1980; SHEPS et al., 1982; BALASUBRAMAMIAN et al., 1978; GOLLAN et al., 1978). However, as a method for monitoring stroke volume, for which it has attracted the most interest, impedance cardiography remains a controversial methodology (DONOVAN et al., 1986; GOLDSTEIN et al., 1986; MILLER and HORVATH, 1977; MUZZI et al., 1985; MOHAPATRA, 1981; PORTER and SWAIN, 1983; LAMBERTS et al., 1984). The method was introduced in clinical practice by Kubicek et al., together with his originally proposed equation and band electrode array (KUBICEK et al., 1966). However, the band electrodes were not practical for use; they are difficult to apply correctly and uncomfortable for the patient. New electrode arrays have been introduced using disposable spot electrodes. Currently, the most frequently used spot electrode array is the 8-spot electrode array used by Bernstein (BERNSTEIN, 1986). Unfortunately, Bernstein also introduced a new equation, based on Srameks findings (SRAMEK et al., 1983), to calculate stroke volume from the impedance cardiogram at that same time. This resulted in several studies comparing the Kubicek and SramekBemstein equation to other methods to estimate stroke volume. However, apart from incidental notes, no reports have been published in which the replacement of the band electrodes by the 8-spot electrodes was evaluated. Other spot electrode arrays using only 4-spot electrodes, however, have been tested in comparison with the band electrodes (Qu et al., 1986), but the results have never appeared to be reproducible (SHERWOOD et al., 1992).

Microprocessor-based system for measurement of electrical impedances during haemodialysis and in postoperative care

The paper describes an instrument designed for accurate measurement of the mean specific electrical impedance of tissue. Two basic measurements are possible. One implementation called tetrapolar impedance plethysmography is employed for investigation with respect to transcellular fluid shifts during haemodialysis. The other measuring configuration called differential tetrapolar impedance plethysmography, deals with measurement of intrathoracic fluid content, which is important in monitoring treatment and prevention of lung oedema. An alternating current of 1 mA is passed through the tissue at different frequencies. Two measuring channels are available for differential tetrapolar measurement. Only one channel is required for straightforward tetrapolar measurement. Recovery of the impedance signals is achieved by synchronous demodulation. The impedance signal is passed through an analogue-to-digital convertor for digital processing to estimate the specific impedance in case of a differential tetrapolar measurement. Output display is possible via an xy recorder. The design of the instrument is discussed and some results shown.

Sleep complaints and sleep disordered breathing in hemodialysis patients

BACKGROUND The objective of the study was to determine the prevalence of sleep complaints and of sleep disordered breathing (SDB) in hemodialysis patients not selected for sleep complaints and to determine the effect of hemodialysis on SDB. The feasibility of home recording of sleep related respiration in these patients was also studied. METHODS The patients completed a questionnaire and parameters of SDB were examined in the home setting on nights following dialysis and nights following no dialysis with the Edentrace II Recording System. RESULTS Six (46%) of 13 patients had sleep complaints. Symptoms suggestive for sleep apnea syndrome were found in four (31%) of these 13 patients. In three (75%) of these four patients SDB was found. Sleep related respiration was monitored in 15 patients. Registrations satisfactory for interpretation were obtained in all patients. SDB was observed in five (33%) of these 15 patients. There were no significant differences in parameters of SDB between nights following dialysis and nights following no dialysis. CONCLUSIONS Home recording of sleep related respiration in hemodialysis patients is feasible. Sleep complaints and SDB are common in these patients. No clinically significant differences in SDB were found between nights following dialysis and nights following no dialysis.

Systemic Vascular Resistance in Intradialytic Hypotension Determined by Means of Impedance Cardiography

Background: Recurrent intradialytic hypotension still is a major source of discomfort in hemodialysis patients today, its origin being subject to extensive research. Different hypotheses have been raised to unravel this problem, without forming one coherent point of view. The aim of this study was to gain more insight into the mechanisms causing intradialytic hypotension by determining cardiovascular performance noninvasively during hemodialysis in a large group of patients. Methods: In the present study the variations in blood volume, stroke volume, cardiac output and systemic vascular resistance were investigated in 68 patients on chronic intermittent hemodialysis utilizing bioelectrical impedance cardiography. In addition, blood volume was monitored continuously with an on-line optical device. Results: Twenty-four patients experienced symptomatic hypotension during dialysis treatment. Compared to the hemodynamically stable patients, the hypotensive patients manifested a slightly greater decline in blood volume (mean ± SEM; –9.4 ± 1.2 vs. –6.5 ± 0.8%, p = 0.04) and cardiac output (–11.8 ± 4.2 vs. –7.3 ±2.7%, p = NS). The main difference, however, was a highly significant decrease in systemic vascular resistance (–17.9 ± 4.4%) in the hypotensive group compared to a rise in the stable group (+6.2 ± 3.5%, p < 0.001). Conclusion: Intradialytic hypotension seems the consequence of an inadequate compensatory response to ultrafiltration-induced blood volume reduction, resulting in a fall in systemic vascular resistance. The degree of hypovolemia itself appears to be less important in the origin of acute, intradialytic hypotensive episodes. Noninvasive monitoring during hemodialysis provides an opportunity to gain more insight into the pathophysiology of intradialytic hypotension and offers the possibility for controlled intervention and possible prevention of this complication.

Thoracic geometry and its relation to electrical current distribution: consequences for electrode placement in electrical impedance cardiography

In thoracic impedance cardiography (TIC) measurements the neck electrodes are often positioned at the basis of the neck, close to the neck-thorax transition. Theoretically, this neck-thorax transition will cause inhomogeneities in the current density and potential distribution. This was simulated using a 3D finite element method, solely representing the geometrical neck-thorax transition. The specific conductivity was 7 10−3 (Ωcm)−1 and the injected current was 1 mA. As expected, the model generated inhomogeneities in the current distribution at the neck-thorax transition, which reached as far as 5 cm into the neck and 20 cm into the thorax. These results are supported by in vivo measurements performed in 10 young male subjects, in which the position of the neck electrodes was varied. A two-way ANOVA revealed that the stroke volume of the lowest neck position was significantly different from the other positions. Small shifts in the position of the neck electrode resulted in large changes in impedance and stroke volume (127 to 82 ml for the Kubicek equation). To standardise the electrode position, the authors strongly recommend placement of the neck electrodes at least 6 cm above the clavicula.

The Effect of Profiled Hemodialysis on Intradialytic Hemodynamics when a Proper Sodium Balance Is Applied

Background: Profiled hemodialysis (HD) has been claimed to ameliorate intradialytic complications such as hypotension. Frequently, these profiles are based on providing the patient with an accumulating sodium load. This increases the risk of interdialytic complications, such as hypertension and increased weight gain. The present study investigated the effect of profiled HD, without an accompanying sodium loading, on intradialytic hemodynamics in stable HD patients. Methods: In eight stable HD patients a standard hemodialysis (S-HD) was compared to a decreasing Na+-profiled hemodialysis (Na-HD), and an ultrafiltration profiled hemodialysis (UF-HD). Care was taken to have the sodium balances similar during these sessions. The patients were monitored non-invasively during dialysis with respect to their cardiac performance by means of electrical impedance cardiography, their variation in blood volume by means of an on-line optical measurement, and their hydration state by means of body impedance analysis. Results: Sodium balance and mean arterial sodium concentrations were similar in the three treatments. Intradialytic hemodynamics during UF-HD were similar to those of S-HD. However, Na-HD improved blood pressure preservation, remarkably without significant blood volume preservation, due to a better stroke volume preservation in the first hour of dialysis. Conclusion: Sodium-balanced, Na-profiled HD improves blood pressure preservation in stable HD patients without providing the patients with a sodium load. This effect is due to a better stroke volume preservation early in dialysis, without a significant reduction in blood volume decrease. UF-HD, as mono-therapy, has no beneficial effect on intradialytic hemodynamics in stable patients.

Influences of lung parenchyma density and thoracic fluid on ventilatory EIT measurements

Ventilatory impedance changes can be measured by electrical impedance tomography (EIT). Several studies have pointed out that the ventilatory-induced impedance change measured over the lungs shows a linear relationship with tidal volume. However, EIT measures the ventilatory impedance changes relative to a reference. Therefore, changes in the reference due to lung parenchyma destruction (increase of thoracic impedance) or lung water (decrease of thoracic impedance) might influence ventilatory EIT measurements. A study was designed to evaluate the influence of the density of lung parenchyma and the thoracic fluid content on ventilatory EIT measurements. Eleven emphysema patients with a variable degree of lung parenchyma destruction, nine haemodialysis patients with general fluid overload and ten healthy subjects were measured. The impedance changes were measured with the subject in the supine position breathing a constant tidal volume of 1 litre starting at the maximum end-expiratory level. In the emphysema group a significantly lower impedance change between ins- and expiration was found in comparison with the healthy subjects (11.6 +/- 6.4 AU l-1 versus 18.6 +/- 4.2 AU l-1, p < 0.05), whereas the haemodialysis group showed a significantly larger impedance change between ins- and expiration before haemodialysis (30.5 +/- 13.1 AU l-1, p < 0.05). A significant decrease in ventilation-induced impedance change during dialysis was found (30.5 +/- 13.1 AU l-1 versus 21.4 +/- 8.6 AU l-1, p < 0.01). Furthermore, a significant correlation between lung function parameters, which indicate the severity of lung parenchyma destruction, and the measured impedance change was found in emphysema patients. From these results it can be concluded that the density of lung parenchyma and the thoracic fluid content have a serious impact on the ventilation-induced impedance change.

The influence of weight on stroke volume determination by means of impedance cardiography in cardiac surgery patients

ObjectivesObesity is thought to be one of the conditions in which the impedance cardiographic method is less reliable for estimating stroke volume (SV). This led to the introduction of a weight correction factor, σ, into the equation according to Sramek and Bernstein. However, no scientific evidence has been published to support the use of this factor. The objectives of the present study are to evaluate the influence of body weight on the accuracy of impedance cardiography and to validate Bernsteins weight correction factor by comparison with thermodilution in patients after coronary bypass surgery.DesignProspective clinical study.SettingA surgical intensive care unit in a university hospital.Patients37 consecutive patients 24–36 h after coronary bypass surgery, sub-divided into a normalweight group (n=24), patients whose weight deviated less than 15% from their ideal weight, and an obese group (n=13), patients whose weight deviated more than 15% from their ideal weight.MeasurementsKubiceks impedance cardiographic method and Sramek and Bernsteins method to assess SV are applied and compared to thermodilution. In order to study the validity of σ, the results are compared between 24 patients with normal weight and 13 obese patients.ResultsA significant correlation between miscalculation of SV by impedance cardiography and the degree of obesity for Sramek and Bernsteins method is found when σ is not included in the equation (r=−0.55,p<0.05). This relation, however, remained significant when σ was included in the equation (r=−0.40,p<0.05). Kubiceks method shows no significant correlation for this relation (r=−0.30). Besides this, Sramek and Bernsteins method underestimates SV significantly in the obese group, independent of the use of σ in the equation. These results are explained as being intrinsic to the equation, according to Sramek and Bernstein. In the whole group the impedance-derived SV did not significantly differ from SV as measured by means of thermodilution, independent of the method used to calculate SV. However, a considerably better correlation and agreement (mean difference ±2 standard deviations is found when Kubiceks method is applied (r=0.90, 0.5±17.1 ml vs 0.64, −4.9±31.8 ml for Sramek and Bernsteins method).ConclusionsWeight significantly influences Sramek and Bernsteins method of impedance cardiography, whereas Kubiceks method is not biased by this factor.