Transthoracic echocardiography: an accurate and precise method for estimating cardiac output in the critically ill patient

Abstract

Background: Cardiac output (CO) monitoring is a valuable tool for the diagnosis and management of critically ill patients. In the critical care setting, few studies have evaluated the level of agreement between CO estimated by transthoracic echocardiography (CO-TTE) and that measured by the reference method, pulmonary artery catheter (CO-PAC). The objective of the present study was to evaluate the precision and accuracy of CO-TTE relative to CO-PAC and the ability of transthoracic echocardiography to track variations in CO, in critically ill mechanically ventilated patients.

Methods: Thirty-eight mechanically ventilated patients fitted with a PAC were included in a prospective observational study performed in a 16-bed university hospital ICU. CO-PAC was measured via intermittent thermodilution. Simultaneously, a second investigator used standard-view TTE to estimate CO-TTE as the product of stroke volume and the heart rate obtained during the measurement of the subaortic velocity time integral.

Results: Sixty-four pairs of CO-PAC and CO-TTE measurements were compared. The two measurements were significantly correlated (r = 0.95; p < 0.0001). The median bias was 0.2 L/min, the limits of agreement (LOAs) were -1.3 and 1.8 L/min, and the percentage error was 25%. The precision was 8% for CO-PAC and 9% for CO-TTE. Twenty-six pairs of ΔCO measurements were compared. There was a significant correlation between ΔCO-PAC and ΔCO-TTE (r = 0.92; p < 0.0001). The median bias was -0.1 L/min and the LOAs were -1.3 and +1.2 L/min. With a 15% exclusion zone, the four-quadrant plot had a concordance rate of 94%. With a 0.5 L/min exclusion zone, the polar plot had a mean polar angle of 1.0° and a percentage error LOAs of -26.8 to 28.8°. The concordance rate was 100% between 30 and -30°. When using CO-TTE to detect an increase in ΔCO-PAC of more than 10%, the area under the receiving operating characteristic curve (95% CI) was 0.82 (0.62-0.94) (p < 0.001). A ΔCO-TTE of more than 8% yielded a sensitivity of 88% and specificity of 66% for detecting a ΔCO-PAC of more than 10%.

Conclusion: In critically ill mechanically ventilated patients, CO-TTE is an accurate and precise method for estimating CO. Furthermore, CO-TTE can accurately track variations in CO.

Keywords: Cardiac output monitoring; Intensive care; Pulmonary artery catheter; Transthoracic echocardiography.

Fig. 1

Study flow chart. PAC pulmonary artery catheter

Fig. 2

a Correlation between CO-PAC and CO-TTE (r = 0.95; p < 0.0001). b Bland–Altman plot for CO-PAC and CO-TTE (n = 64 pairs of measurements). Solid line: bias; dashed line: LOA. CO-PAC cardiac output measured by pulmonary artery catheter, CO-TTE cardiac output estimated by transthoracic echocardiography

Fig. 3

a Correlation between absolute values of ΔCO-PAC and ΔCO-TTE (r = 0.92; p < 0.0001). b Bland–Altman plot for ΔCO-PAC and ΔCO-TTE (n = 26 pairs of measurements). Solid line: bias; dashed line: LOA. CO-PAC cardiac output measured by pulmonary artery catheter, CO-TTE cardiac output estimated by transthoracic echocardiography

Fig. 4

Four-quadrant plot of %ΔCO-TTE vs %ΔCO-PAC. A central exclusion zone of 15% (square) was applied. Concordance rate was 94%. CO-PAC cardiac output measured by pulmonary artery catheter, CO-TTE cardiac output estimated by transthoracic echocardiography

Fig. 5

Polar plot showing changes in ΔCO-TTE in comparison with ΔCO-PAC. A central exclusion zone of 0.5 L/min (half circle) and ± 30° axes (solid lines) are indicated. Mean polar angle: 1.0°; 95% radial LOAs: –26.8 to 28.8° (dotted lines). Concordance rate (between –30 and 30°) was 100%. CO-PAC cardiac output measured by pulmonary artery catheter, CO-TTE cardiac output estimated by transthoracic echocardiography