Volume status and volume responsiveness in postoperative cardiac surgical patients: An observational, multicentre cohort study.

BACKGROUND: The best strategy to identify patients in whom fluid loading increases cardiac output (CO) following cardiac surgery remains debated. This study examined the utility of a calculated mean systemic filling pressure analogue (Pmsa ) and derived variables to explain the response to a fluid bolus. METHODS: The Pmsa was calculated using retrospective, observational cohort data in the early postoperative period between admission to the intensive care unit and extubation within 6 hours. The venous return pressure gradient (VRdP) was calculated as Pmsa  - central venous pressure. Concurrent changes induced by a fluid bolus in the ratio of the VRdP over Pmsa , the volume efficiency (Evol ), were studied to assess fluid responsiveness. Changes between Pmsa and derived variables and CO were analysed by Wilcoxon rank-sum test, hierarchial clustering and multiple linear regression. RESULTS: Data were analysed for 235 patients who received 489 fluid boluses. The Pmsa increased with consecutive fluid boluses (median difference [range] 1.3 [0.5-2.4] mm Hg, P = .03) with a corresponding increase in VRdP (median difference 0.4 [0.2-0.6] mm Hg, P = .04). Hierarchical cluster analysis only identified Evol and the change in CO within one cluster. The multiple linear regression between Pmsa and its derived variables and the change in CO (overall r2  = .48, P < .001) demonstrated the best partial regression between the continuous change in CO and the concurrent Evol (r = .55, P < .001). CONCLUSION: The mean systemic filling Pmsa enabled a comprehensive interpretation of fluid responsiveness with volume efficiency useful to explain the change in CO as a continuous phenomenon.