Volumetric assessment of preload in trauma patients: addressing the problem of mathematical coupling

The availability of the volumetric thermodilution pulmonary artery catheter allows preload assessment based on ventricular volume rather than pressure. This technique has been shown clinically to be a better measure of preload than the pulmonary artery occlusion pressure (PAOP). Critics of the technique argue that the use of thermodilution to measure cardiac output (CO) accounts for the better correlation between right ventricular end-diastolic volume (RVEDV) and CO than PAOP and CO, since stroke volume derived from the CO is a common term to both RVEDV and CO. Previous studies have attempted mathematical corrections for this coupling effect, but direct comparisons using a nonthermodilution measure of CO have not been reported. Our objective was to evaluate the importance of mathematical coupling between RVEDV and CO by assessing the ability of RVEDV to predict CO measured by thermodilution (COTH) compared with CO simultaneously determined by the Fick principle (COFICK). We performed a prospective study of 53 consecutive trauma patients admitted to a Level I trauma center between 10/1/94 and 6/1/95 who received a volumetric pulmonary artery catheter. Using linear regression analysis, RVEDV and PAOP were correlated with simultaneous measurements of both COFICK determined via indirect calorimetry and COTH. Fisher's z-transformation was used to evaluate the correlation coefficients for significant differences (p < .05). The correlation coefficients for RVEDV vs. COTH and RVEDV vs. COFICK were similar (.48 vs. 0.45, p = .76). There was a significant correlation between COTH and COFICK (r = .74, p < .001). RVEDV was significantly better than PAOP at predicting both COTH (p < .001) and COFICK (p = .04). Multivariate regression analysis confirmed that RVEDV was the only estimate of preload which was significantly related to CO. We conclude that mathematical coupling does not have a significant clinical effect on the relationship between RVEDV and CO.