THE INFLUENCE OF WEIGHT ON STROKE VOLUME DETERMINATION BY MEANS OF IMPEDANCE CARDIOGRAPHY IN CARDIAC-SURGERY PATIENTS

Objectives: Obesity is thought to be one of the conditions in which th e impedance cardiographic method is less reliable for estimating strok e volume (SV). This to the introduction of a weight correction factor, sigma, into the equation according to Sramek and Bernstein. However, no scientific evidence has been published to support the use of this f actor. The objectives of the present study are to evaluate the influen ce of body weight on the accuracy of impedance cardiography and to val idate Bernstein's weight correction factor by comparison with thermodi lution in patients after coronary bypass surgery. Design: Prospective clinical study. Setting: A surgical intensive care unit in a universit y hospital. Patients: 37 consecutive patients 24-36 h after coronary b ypass surgery, sub-divided into a normal-weight group (n = 24), patien ts whose weight deviated less than 15% from their ideal weight, and an obese group (n = 13), patients whose weight deviated more than 15% fr om their ideal weight. Measurements: Kubicek's impedance cardiographic method and Sramek and Bernstein's method to assess SV are applied and compared to thermodilution. In order to study the validity of sigma, the results are compared between 24 patients with normal weight and 13 obese patients. Results: A significant correlation between miscalcula tion of SV by impedance cardiography and the degree of obesity for Sra mek and Bernstein's method is found when a is not included in the equa tion (r = -0.55, p < 0.05), This relation, however, remained significa nt when a was included in the equation (r = -0.40, p < 0.05). Kubicek' s method shows no significant correlation for this relation (r = -0.30 ). Besides this, Sramek and Bernstein's method underestimates SV signi ficantly in the obese group, independent of the use of sigma in the eq uation. 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 o f thermodilution, independent of the method used to calculate SV. Howe ver, a considerably better correlation and agreement (mean difference +/- 2 standard deviations is found when Kubicek's method is applied (r = 0.90, 0.5 +/- 17.1 ml vs 0.64, -4.9 +/- 31.8 ml for Sramek and Bern stein's method). Conclusions: Weight significantly influences Sramek a nd Bernstein's method of impedance cardiography, whereas Kubicek's met hod is not biased by this factor.