Reasons for, and perceived versus actual efficacy of fluid boluses following cardiac surgery: Prospective observational study.

Background: Fluid bolus (FB) contribute to positive fluid balance and may not have a beneficial effect. Following cardiac surgery FB are common but often based on simple physiology (heart rate, blood pressure, urine output) as measures of fluid responsiveness are more complex and may not be valid in this cohort. Method(s): Post-operative cardiac surgical patients who received a FB in ICU in the first 24 hours were included. Using a questionnaire based tool, the reasons for a FB and the perceived efficacy by the prescriber were recorded. Physiological data were recorded before and one hour after each FB. A FB was said to be "mean arterial pressure (MAP) responsive" if the FB caused a sustained increase in MAP of at least 5 mmHg. Result(s): Over 5 months, 74 patients (95% on-pump surgery) were recruited. A total of 135 FBs were administered. Each patient received a median of 1(1-2) FB, with a median volume of 0.25 (0.25-0.5) liters. Postoperatively 47% of patients received one FB, 31% received two FBs and 22% received three or more FBs. The most common fluid used was 4% albumin (85%). The main reasons for giving a FB were low MAP (72%), low urine output (21%) and low filling pressure (11%). Overall, 1 hour after a FB, there was an increase in MAP and central venous pressure, and a decrease in urine output, hemoglobin and serum calcium level. However, there was no change in heart rate, vasopressor rate, central venous oxygen saturation or serum lactate levels (Table). Overall, FBs contributed to 45% of fluid balance and only 48% of FB were MAP responsive. Conclusion(s): Lower MAP was the most common reason for FB and sustained physiological efficacy of FB were small.