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. Methods: In a prospective study, we compared cardiac output measurements by means of thermodilution (CO(TD)) with impedance cardiographic-derived values (CO(EIC)) in 37 mechanically ventilated patients after cardiac surgery. Both methods were used simultaneously. Results: CO(EIC) values were weakly correlated with CO(TD) in the total group when the equation of Sramek- Bernstein was employed to calculate CO(EIC) (r=0.60, P<0.001, mean difference and standard deviation: -0.06±1.25 1 · min-1). After exclusion of the 12 patients whose body weight differed >15% from their ideal body weight, no significant difference was found between the mean values (5.40±1.80 1 · min-1 (CO(EIC)) vs 5.31±1.69 1 · min-1, n=25) while the correlation coefficient increased substantially (r=0.85, P<0.001, mean difference and standard deviation: 0.09±0.96 1 · min-1). Conclusions: The results of this study indicate that weight is a very important factor in unreliable measurement of CO by impedance cardiography in cardiac surgery patients. The calculation equation as proposed by Sramek and Bernstein is not accurate enough in patients with more than 15% of weight deviation. Therefore, the use of impedance cardiography in these patients is of limited value until an accurate correction factor has been developed.