OBJECTIVE To assess the accuracy of room-temperature thermodilution cardiac output measurements from the side port (SP) of the introducer catheter through which the pulmonary artery flotation catheter is inserted. METHODS Central venous port (CVP) cardiac output measurements were compared with SP cardiac output measurements using the same right-heart catheter. METHODS The general intensive care unit of the Hospital for Joint Diseases, Orthopedic Institute, New York. METHODS Thirty one orthopedic patients with 31 different right-heart catheters placed in the right internal jugular or right subclavian position were evaluated. METHODS Four injections of 10 ml of normal saline solution at room temperature were made through each port; the results of the last three injections were averaged. Cardiac output determinations from both ports were completed in less than 10 min. The order of port injection was random. RESULTS A significant difference was noted between cardiac output determinations from the two ports (p < 0.001, paired Student's t test) with the SP slightly overestimating thermodilution cardiac outputs by 6.34 +/- 8.38 percent compared to the CVP. A significant correlation was noted between both ports (r = 0.97, p < 0.001) with the linear regression line starting at the origin (y intercept not significantly different from zero) with a slope (1.07) significantly greater than 1. CONCLUSIONS If the proximal CVP becomes nonfunctional, room-temperature thermodilution cardiac outputs from the SP can be used with the understanding that a slight overestimation of cardiac output will occur.