To explore the short-term hemodynamic change of fluid challenge (FC) with crystalloid or colloid and define fluid responsiveness at the optimal time in patients with septic shock.

A prospective observational study was conducted. Septic shock patients monitored with pulmonary catheters admitted to medical intensive care unit (ICU) of the Peking Union Medical College Hospital from July 2016 to December 2018 were enrolled. All included patients received FC and were divided into two groups according to the type of fluid used, i.e. crystalloid group (normal saline for 500 mL) and colloid group (4% succinyl gelatin for 500 mL). The choice of fluid type was decided by the attending physician. Hemodynamic variables were measured at baseline, and 0 (immediately), 10, 30, 45, 60, 90, 120 minutes after FC, included cardiac index (CI), heart rate (HR), mean artery pressure (MAP), central venous pressure (CVP) and pulmonary arterial wedge pressure (PAWP). Fluid responsiveness was defined as CI increased by more than 10% after FC. The data were analyzed by repeated measurements of variance between the two groups as well as responders and nonresponders.

Forty patients were included, 20 cases each in colloid group and crystalloid group; of whom 26 were fluid responders with 12 of colloid group and 14 of crystalloid group. Of the 14 nonresponders, 8 were of colloid group and 6 of crystalloid group. ① Compared with before FC, CI (mL·s^-1·m^-2) was significantly increased in crystalloid and colloid groups after FC (71.7±16.7 vs. 65.0±16.7, 68.3±25.0 vs. 63.3±23.3, both P < 0.05). In the colloid group, volume expansion increased the CI to maximum (76.7±18.3) at 30 minutes after FC, at 120 minutes after FC, a significantly higher CI (70.0±16.7) was also observed (P < 0.05), an increased in CI≥10% was observed at 60 minutes after FC. In the crystalloid group, CI was increased to maximum at 10 minutes (73.3±28.3) and decreased to baseline at 60 minutes, an increased in CI≥10% was also observed at 10 minutes after FC. In addition, there was no significant difference in CI changes between colloidal group and crystalloid group at different time points after FC. ② CI did not change over time in nonresponders groups, whereas in responders CI increased parallelly to that in both crystalloid and colloid groups over time. However, an increased in CI≥10% was observed through the 120 minutes after FC in responders of colloid group compared with that of at 30 minutes after FC in crystalloid group. There was significant difference in CI changes between colloidal group and crystalloid group at 30, 45, 60, 90 minutes after FC (mL·s^-1·m^-2: 18.3±3.3 vs. 8.3±1.7, 18.3±3.3 vs. 5.0±1.7, 13.3±1.7 vs. 3.3±1.7, 11.7±3.3 vs. 3.3±1.7, all P < 0.05). ③ The maximal values of CVP and PAWP were observed at the end of FC. In colloid group, both the two variables were notably higher than that before FC over 120 minutes compared with that of only at 10 minutes in crystalloid group. The MAP in colloid increased to maximum immediately at the end of FC and decreased to baseline at 45 minutes, however, the MAP in crystalloid group and HR of both groups showed no differences over 120 minutes.

Hemodynamic changes were significantly different between crystalloid and colloid after FC in patients with septic shock. Therefore, the timing of fluid responsiveness assessment should be different individually. The assessment time of colloid group may be prolonged to 30 minutes after FC while that of crystal group can be at 10 minute after FC.

Fluid challenge; Crystal fluid; Colloidal fluid; Septic shock; Hemodynamics