Cardiac output measurement using Trans-esophageal echocardiography versus Thermodilution in patients undergoing off-pump CABG

Perioperative cardiac function monitoring plays a crucial role in the assessment, treatment, and prognosis of patients undergoing cardiac surgery of which Cardiac output (CO) measurement is one of the most important parameters.

Pulmonary artery catheter (PAC) is used to measure CO by thermodilution (TD) technique and is still considered as gold standard method since 1970s when it was first introduced by Dexter et al and further modified by William Ganz et al.

PAC is invasive and is associated with numerous complications like pneumothorax, arrhythmia, infection, pulmonary artery rupture, valve injury, knotting and thrombo-embolism. It is prone to technical errors due to loss of injectate, temperature variability, thermistor malfunction, coiling of catheter or variability in timing of injectate. Intracardiac shunts, mechanical ventilation and valvular dysfunction may lead to incorrect interpretation. This led to the development of less invasive methods of CO monitoring such as Echocardiography, thoracic electrical bioimpedance, and magnetic resonance imaging, arterial wave contour analysis, PiCCO, transpulmonary thermodilution, transpulmonary lithium dilution which have questionable benefits during open-heart cardiovascular surgery.

Trans-esophageal echocardiography (TEE) allows both cardiac structure and functions evaluation. Valvular pathology can also be detected by it. However, it requires training and skills by the operator and has a higher cost. Certain requirements must be fulfilled: (a) the absence of turbulence (b) parallel orientation of the Doppler beam to the direction of the blood flow in the left ventricle outflow tract (LVOT) and aortic valve. Various routes for CO measurements are: at the level of pulmonary artery (PA), mitral valve (MV) or aortic valve (AV) and at left ventricular outflow tract (LVOT) with pulsed wave doppler (PWD) and continuous wave doppler (CWD).

It has been observed that use of PAC in elective coronary artery bypass graft (CABG) surgery increases patients risk adjusted mortality and morbidity. In addition, TEE provides cause of low CO, hemodynamic instability and other associated lesions such as aortic pathology. TEE has been shown to improve mortality. Hence, if CO measurement by PAC and TEE are comparable or if TEE method is superior, use of PAC can be avoided in CABG patients. With lesser complications than PA catheter based thermodilution techniques, we would like to assess if TEE based CO measurement can be interchanged with the latter in patients undergoing off pump coronary artery bypass graft (OPCAB) as use of TEE is recommended (Class I) for intra-operative monitoring of CABG patients.

Lacunae in existing knowledge:

Whether echocardiography can replace thermodilution method in CO measurement still remains controversial. Certain studies including systematic reviews and meta-analysis show good correlation between TEE and TDwhile others have concluded TEE cannot be interchanged with TD and have suggested further evaluation. The accuracy of CO measurement using TEE has not been adequately validated, especially in CABG surgeries.