Predictors of between-method differences in cardiac output measurement using thoracic electrical bioimpedance and thermodilution

Abstract

Objectives: To evaluate the usefulness of transthoracic electrical bioimpedance in trending changes in cardiac output after cardiac surgery, and to identify predictors of differences between cardiac output measured by thermodilution and transthoracic electrical bioimpedance methods.

Design: Prospective repeated-measures study.

Setting: University-affiliated tertiary care center.

Patients: Thirty-four adult patients undergoing elective cardiac surgery with routine pulmonary artery catheter placement.

Interventions: Simultaneous paired cardiac output measurements by transthoracic electrical bioimpedance and thermodilution were made at four time points: within 2 hrs of intensive care unit (ICU) admission; when the patient reached a normothermic temperature; after extubation; and 24 hrs after ICU admission.

Measurements and main results: Mean measurements by each method over time did not differ, except at ICU admission, when compared by repeated-measures analysis of variance. For each time point, bias and precision between methods were calculated. Bias calculations ranged from 0.02 to 0.21 L/min/m2. Precision calculations ranged from 1.06 to 1.52 L/min/m2. Predictors of between-method differences identified by a multiple regression model of hemodynamic variables were: increased systemic vascular resistance index, decreased mean arterial pressure (MAP), and the presence of atrial or ventricular pacing.

Conclusions: While mean postoperative cardiac output measurements did not differ by method over time, agreement between transthoracic electrical bioimpedance and thermodilution methods was poor in the immediate postoperative period, with precision calculations indicative of clinically significant differences. Increased systemic vascular resistance index and decreased MAP were predictive of larger between-method differences.