A simplified echocardiographic formula to estimate cardiac index in the intensive care unit

Abstract

Background and aim: Measurement of cardiac index (CI) is crucial in the hemodynamic assessment of critically ill patients in the intensive care unit (ICU). The most reliable trans-thoracic echocardiography (TTE) technique for CI estimation is the left ventricular outflow tract (LVOT) Doppler method that requires, among other parameters, the LVOT cross-sectional area (CSA) measurement. However, inherent and practical disadvantages, mostly related to the ICU setting, hamper LVOT-CSA assessment. In this study, we aimed to validate a simplified formula, leveraging on LVOT-velocity time integral (VTI) and heart rate (HR) only, for non-invasive estimation of CI in ICU patients.

Methods and results: We prospectively enrolled 50 consecutive patients admitted to our ICU requiring pulmonary artery catheterization (PAC) over a one-year period. For each patient we measured the CI by PAC (CI_PAC) and TTE. The latter was obtained both with the "traditional formula" (traditional CI_TTE), requiring LVOT-CSA assessment, and our new "simplified formula" (simplified CI_TTE). The correlation between the simplified CI_TTE and CI_PAC was strong (r = 0.81) and resulted significantly greater than the traditional CI_TTE and CI_PAC correlation (r = 0.70; p < 0.05 for Pearson r coefficients comparison). Both TTE-based CI showed an acceptable agreement (+0.19 ± 0.48 L/min/m² for simplified CI_TTE and - 0.18 ± 0.58 L/min/m² for traditional CI_TTE) with the reference CI_PAC.

Conclusion: In this study, we validated a practical approach, leveraging on TTE LVOT-VTI and HR only, for non-invasive estimation of CI in ICU patients.

Keywords: Cardiac index; Echocardiography; Intensive care unit; Left ventricular outflow tract; Pulmonary artery catheterization; Simplified formula.