Transtracheal Doppler Cardiac Output Monitoring: Comparison to Thermodilution During Noncardiac Surgery

The validity of transtracheal Doppler (TTD) cardiac output (CO) monitoring during noncardiac surgery has not been established. A prospective evaluation was undertaken in 30 patients undergoing noncardiac surgery to assess the agreement between TTD and thermodilution measurements of CO. Linear regression, Bland-Altman analysis, and receiver operator characteristic (ROC) techniques were employed to evaluate the accuracy, reliability, and trending capability of TTD monitoring. A total of 250 simultaneous TTD and thermodilution CO values were compared. TTD and thermodilution CO measurements were highly correlated (P < 0.005, r = 0.84) and Bland-Altman analysis revealed a small systematic underestimation of thermodilution CO (mean bias = −0.25 L/min) with a SD of the bias of 0.88 L/min and a mean percent error of 12.4%. TTD performed particularly well in patients in whom the Doppler signal was stable throughout surgery and required minimal manipulation. In these patients, linear regression yielded the relation TTD CO = 0.96 thermodilution CO + 0.15 with a correlation coefficient r = 0.92. Mean percent error was 10.0% with a mean bias of −0.02 L/min and a SD of the bias of 0.58 L/min. The ability of TTD to track directional changes in thermodilution CO was evaluated by regression analysis and a ROC plot. Changes in TTD CO were highly correlated to changes in thermodilution CO (r = 0.81). ROC plots showed that changes in TTD CO reliably identified large (greater than 15%) changes in thermodilution CO with a sensitivity of 92% and a specificity of 87%. Clinical experience with the TTD device is needed to obtain accurate measurements. A total of 10 cases was required for our research team to obtain consistent performance with TTD monitoring. Our results demonstrate that TTD can accurately track directional changes in thermodilution CO and, particularly in those patients in whom a stable signal is obtained, provides accurate estimates of thermodilution CO. Limitations of the TTD technique were a failure rate of 13% and the requirement of repeated probe manipulation in 27% of patients. Continuous CO monitoring during noncardiac surgery can be achieved with TTD, but shortcomings in the current technology, including a difficult user interface, warrant trained personnel for operation.