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Impedance cardiography. Importance of the equation and the electrode configuration
B J van der Meer1, H H Woltjer, A M Sousman
1Department of Internal Medicine, Academic Hospital Vrije Universiteit, Amsterdam, The Netherlands.
Objective:
Electrical 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.
Design:
Prospective study.
Setting:
Surgical intensive care unit of a university hospital.
Patients:
20 mechanically ventilated patients after cardiac surgery.
Measurements And Results:
Simultaneous 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 Sramek (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 differences 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 and r = 0.79, p < 0.001, respectively). The mean difference between EIC and TD was 0.15 +/- 0.96 1/min and 0.19 +/- 1.19 1/min, respectively.