H. H. Woltjer

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).

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.

Haemodynamic response to exercise in healthy young and elderly subjects

Abstract Whereas with advancing age, peak heart rate (HR) and cardiac index (CI) are clearly reduced, peak stroke index (SI) may decrease, remain constant or even increase. The aim of this study was to describe the patterns of HR, SI, CI, arteriovenous difference in oxygen concentration (Ca-vO2), mean arterial pressure (MAP), systemic vascular resistance index (SVRI), stroke work index (SWI) and mean systolic ejection rate index (MSERI) in two age groups (A: 20–30 years, n = 20; B: 50–60 years n = 20. After determination of pulmonary function, an incremental bicycle exercise test was performed, with standard gas-exchange measurements and SI assessment using electrical impedance cardiography. The following age-related changes were found: similar submaximal HR response to exercise in both groups and a higher peak HR in A than in B[185 (SD 9) vs 167 (SD 14) beats · min−1, P < 0.0005]; increase in SI with exercise up to 60–90 W and subsequent stabilization in both groups. As SI decreased towards the end of exercise in B, a higher peak SI was found in A [57.5 (SD 14.0) vs 43.6 (SD 7.7) ml · m−2, P < 0.0005]; similar submaximal CI response to exercise, higher peak CI in A [10.6 (SD 2.5) vs 7.2 (SD 1.3) l · min−1 · m−2, P < 0.0005]; no differences in Ca-vO2 during exercise; higher MAP at all levels of exercise in B; higher SVRI at all levels of exercise in B; lower SWI in B after recovery; higher MSERI at all levels of exercise in A. The decrease in SI with advancing age would seem to be related to a decrease in myocardial contractility, which can no longer be compensated for by an increase in preload (as during submaximal exercise). Increases in systemic blood pressure may also compromise ventricular function but would seem to be of minor importance.

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.

The haemodynamic response to exercise in chronic obstructive pulmonary disease: assessment by impedance cardiography

This study aimed to determine the differences in haemodynamic responses to a standard incremental exercise test between outpatients with chronic obstructive pulmonary disease (COPD) and age-matched controls and to discover the relationship between severity of airflow obstruction and exercise haemodynamics in COPD. Twenty-two male patients with COPD (forced expiratory volume in one second (FEV1)/vital capacity (VC))<80% predicted) and 20 age-matched male controls performed an incremental exercise test (10 W x min(-1)) with ventilatory function and changes in stroke volume (deltaSV) and cardiac output (deltaCO) measured by means of electrical impedance cardiography (EIC). Submaximal deltaSV and deltaCO were lower in COPD patients. Peak exercise deltaSV were equal in patients and controls (128+/-33 versus 129+/-29%, p=0.98), whereas peak deltaCO was lower in patients (COPD versus controls: 232+/-71 versus 289+/-54%, p<0.005). In COPD patients, FEV1 (% pred) was significantly correlated to deltaSV at all submaximal exercise intensities, to peak exercise deltaSV and to peak exercise deltaCO. FEV1/VC (% pred) was significantly correlated to deltaSV at 30 and 60 W. In conclusion, in chronic obstructive pulmonary disease an aberrant haemodynamic response to exercise was found, especially in patients with severe airflow obstruction. This aberrant response is related to the degree of airflow obstruction and may limit exercise performance in patients with severe chronic obstructive pulmonary disease.

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.

The lowering of stroke volume measured by means of impedance cardiography during endexpiratory breath holding

Impedance cardiography is a reliable method for estimating stroke volume (SV). Breathing, however, causes artefacts, which can be avoided by measuring during breath holding. This study investigated whether SV determination is accurate during breath holding. Twelve healthy subjects were tested in the supine position at rest and during two levels of exercise: 100 and 200 W. Averaged SV values were monitored by means of impedance cardiography before and after endexpiratory breath holding. During breath holding, SV measurement was on a beat-to-beat basis. An obvious decrease in SV during breath holding was noticed, being significant only during exercise (mean decrease of 38% at 100 W and 58% at 200 W). The rest measurements were repeated with open and closed glottis, which yielded the same results. This indicates that the SV decrease was not caused by a Valsalva-like manoeuvre. The mean SV value calculated by means of impedance cardiography for the total breath hold period was significantly lower than the SV during breathing, both at rest (91.7 +/- 2.4%) and at 100 W (90.5 +/- 7.0%). From this study it can be concluded that averaging of the impedance signal, measured while the subject is breathing, is preferential to measuring during breath holding, because the latter condition systematically underestimates SV.