Minimal volume for a fluid challenge in postoperative patients

Introduction

An effective fluid challenge should increase the mean systemic filling pressure (Pmsf ) in order to increase the venous return. The objective of this study was to determine the minimum volume of intravenous fluid required to significantly increase the Pmsf.

Methods

Patients following cardiac surgery were randomly allocated to receive 1, 2, 3 or 4 ml/kg (body weight) of crystalloid over 5 minutes using a 60 ml syringe. Pmsf was measured using the arterial pressure after stopping blood flow in the arm with a pneumatic tourniquet inflated for 1 minute. Cardiac output (CO) was also recorded at baseline and immediately after the fluid infusion. CO was measured with LiDCO or pulmonary artery catheter, and a positive response was considered an increase of 10% from baseline. From previous data, the least significant change for Pmsf was 15%. Medians were compared using the independent samples media test, and proportions were compared using a chi-square test. Statistical significance was considered when P < 0.05.

Results

Fifty patients were included, 40.8% of them were responders. The proportion of responders increases with the increase of dose of fluids (Table 1). The regression equation was: change of Pmsf (%) = 4.4 (dose of fluids ml/kg, 95% CI 2.3 to 6.5) - 1.6 (95% CI 7.4 to 4.3, R² = 0.28, F(1.47) = 17.8, P < 0.001). The predicted dose of fluids required to achieve a change in Pmsf of 15% is 3.7 ml/kg crystalloids.

Table 1 Change of Pmsf-arm and CO.

Conclusion

The minimum volume required to perform an effective fluid challenge is 4 ml/kg infused in 5 minutes. However, only 30% of the variation of change in Pmsf can be explained by the dose of i.v. fluid given. The proportion of responders increases with the volume of fluids.