Physiological changes after fluid bolus therapy in cardiac surgery patients: A propensity score matched case-control study

Abstract

Objective: Fluid bolus therapy (FBT) is ubiquitous in intensive care units (ICUs) after cardiac surgery. However, its physiological effects remain unclear.

Design: : We performed an electronic health record-based quasi-experimental ICU study after cardiac surgery. We applied propensity score matching and compared the physiological changes after FBT episodes to matched control episodes where despite equivalent physiology no fluid bolus was given.

Setting: The study was conducted in a multidisciplinary ICU of a tertiary-level academic hospital.

Participants: The study included 2,736 patients who underwent Coronary Artery Bypass Grafting and/or heart valve surgery.

Main outcome measures: Changes in cardiac output (CO) and mean arterial pressure (MAP) during the 60 minutes following FBT.

Results: We analysed 3572 matched fluid bolus (FB) episodes. After FBT, but not in control episodes, CO increased within 10 min, with a maximum increase of 0.2 l/min (95%CI 0.1 to 0.2) or 4% above baseline at 40 min (p < 0.0001 vs. controls). CO increased by > 10% from baseline in 60.6% of FBT and 49.1% of control episodes (p < 0.0001). MAP increased by > 10% in 51.7% of FB episodes compared to 53.4% of controls. Finally, FBT was not associated with changes in acid-base status or oxygen delivery.

Conclusion: In this quasi-experimental comparative ICU study in cardiac surgery patients, FBT was associated with statistically significant but numerically small increases in CO. Nearly half of FBT failed to induce a positive CO or MAP response.

Keywords: Cardiac surgery; Hypotension; Intensive care; Resuscitation.

Fig. 1

Patient and FBT flow chart. a) Patient's admissions with reasons for exclusion. b) FB counts with reasons for exclusion and effects of propensity score matching on numbers included. c) Control episodes counts with reasons for exclusion.

Fig. 2

Propensity score matching quality. The average absolute standardized difference (SDiff) of all non-binary covariates considered in propensity score matching (PSM). Comparing the values before and after PSM. An SDiff <10% is commonly regarded as good matching quality.

Fig. 3

Hemodynamic effects of FBT. a–j: For each variable, the top plot shows the average value of 30 min before the fluid bolus therapy (before gray line) and the average values of 5 min intervals from 0 to 60 min after the FB end. In the lower plot the values are the mean of the changes from baseline calculated for each time point individually. In both plots, this is shown for the FBT and control episodes separately. The 95% confidence intervals are shown as error bars. The red and blue numbers represent the number of measurements at each time point for FB (red) and control episodes (blue).