Noninvasive versus invasive assessment of cardiac output after cardiac surgery: clinical validation

Abstract

The accuracy of noninvasive cardiac output (CO) measurement techniques, such as electrical bioimpedance (BIO), suprasternal continuous-wave Doppler (CWD), pulsed-wave Doppler (PWD), and transesophageal continuous-wave Doppler (TED) ultrasound has been variably judged in recent years. In addition, clinical comparisons are hampered by the fact that there is no generally accepted gold standard in CO measurement. After coronary artery bypass surgery in 25 patients, CO was simultaneously determined by invasive standard techniques (thermodilution [TD] and Fick methods) plus BID, CWD, PWD, and TED. There was an excellent agreement found between TD and the Fick method (CO F = 0.13 + 1.01 · CO TD; r = 0.96; n = 99). Thermodilution was thus chosen to be the reference method. Bioimpedance underestimated CO TD (CO BIO = 0.47 + 0.60 · CO TD; r = 0.78; n = 111). Allowing physiological ejection times only led to an improved agreement between BIO and TD (CO BIO = 0.05 + 0.69 · CO TD; r = 0.82; n = 79), but BID still significantly underestimated CO TD ( P < 0.0005). Using physiologic ejection times during COcwO determination reduced the scatter of data as compared with TD; however, CWD still considerably overestimated CO TD, when CO CWD computation was based on the echocardiographic aortic diameter (ECHO) (CO CWD ECHO = 0.79 + 1.40 · CO TD; r = 0.84; n = 52). With the surgical aortic diameter (SURG), the agreement improved (CO CWD SURG = 0.75 + 1.16 · CO TD; r = 0.89; n = 44), but overestimation of COTD remained significant ( P < 0.05). Irrespective of the aortic diameter, CO PWD values showed a considerable scatter of data compared with CO TD (CO PWD ECHO = 1.26 + 0.60 · CO TD; r = 0.62; n = 64 and CO PWD SURG = 1.42 + 0.41 CO TD; r = 0.47; n = 61). Correlation of absolute CO TED values to thermodilution depended on the method used for calibration. All investigated noninvasive CO measurement techniques unreliably measured relative CO changes. Despite its invasiveness, TD remains the method of choice for accurate CO determination in adult patients following cardiac surgery.