The effect of obesity on bioimpedance cardiac index

Background: Cardiac performance may be assessed noninvasively at the patient's bedside by using thoracic bioimpedance. However, it is unclear if this technique can be used reliably in critically injured obese patients because of increased body habitus and chest wall mass. Methods: A prospectively maintained database was used to identify all trauma patients admitted to the intensive care unit who underwent simultaneous measurement of cardiac performance by using both thoracic bioimpedance and thermodilution. Patients were divided into 2 groups based on their body mass index (BMI). Patients with a BMI less than 30 kg/m² were classified as nonobese, and patients with a BMI of 30 kg/m² or greater were categorized as obese. Results: There were 285 patients who underwent 1,138 simultaneous measurements of cardiac index by using both bioimpedance and thermodilution. There were 211 nonobese patients (BMI = 25 ± 3 kg/m²) and 74 obese patients (BMI = 34 ± 4 kg/m²). Bioimpedance correlated well with thermodilution for the entire population (r = .84, P < .0001), and was reliable equally in obese (r = .85, P < .0001) and nonobese (r = .82, P < .0001) patients. There actually was less test bias in the obese group (-.06 ± .69) than in the nonobese group (-.16 ± .75, P = .04). Conclusions: Thoracic bioimpedance technology may be used reliably as a noninvasive alternative to pulmonary artery catheterization for assessment of cardiac performance in critically injured obese patients.