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 (P_msa) and derived variables to explain the response to a fluid bolus.

Methods: The P_msa 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 P_msa - central venous pressure. Concurrent changes induced by a fluid bolus in the ratio of the VRdP over P_msa, the volume efficiency (E_vol), were studied to assess fluid responsiveness. Changes between P_msa 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 P_msa 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 E_vol and the change in CO within one cluster. The multiple linear regression between P_msa and its derived variables and the change in CO (overall r(2) = .48, P <.001) demonstrated the best partial regression between the continuous change in CO and the concurrent E_vol (r = .55, P <.001).

Conclusion: The mean systemic filling P_msa enabled a comprehensive interpretation of fluid responsiveness with volume efficiency useful to explain the change in CO as a continuous phenomenon.