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Whether measurement of cardiac output using the thermal dilution technique (TDCO) is valid in the presence of tricuspid regurgitation (TR) is controversial. We assessed the accuracy and precision of the technique in pigs by comparison with data from an electromagnetic flowmeter on the aorta (EMCO). TR was created with sutures that immobilized the free-wall leaflets of the tricuspid valve, and cardiac output was adjusted with dobutamine to give values comparable to control measurements. TR reduced forward stroke volume from 17.2 to 12.6 mL/beat and caused the right atrium to dilate and pulse in synchrony with the right ventricle. Acute TR did not affect the linear regression relation between TDCO and EMCO and did not alter the correlation coefficient (r = 0.94 during both control and TR). These data demonstrate that acute TR does not affect the accuracy or precision of TDCO in pigs.