2.7. Study protocol

When a patient met the inclusion criteria, the investigating physicians collected a first set of demographic, ventilatory, and hemodynamic data (Base 1) before fluid expansion. After the RM had been performed, a second dataset (RM) was recorded. A third dataset was recorded 5 minutes after all the hemodynamic variables had returned to baseline values (Base 2). Next, fluid expansion (500 mL of Ringer lactate over 10 minutes via a pressure bag) was performed. On the basis of very sparse literature data on the maximum possible effect of fluid expansion on hemodynamic variables, we decided to acquire a fourth and last set of hemodynamic data (FE) 5 minutes after the fluid expansion.^[9,11,13]

ΔrespPP was automatically calculated by the Philips monitoring system; this method has already been validated.^[14] ΔrespSV was calculated as described previously: ΔrespSV = ([SVmax − SVmin]/[SVmax + SVmin])/2 × 100, where SVmin and SVmax are the minimum and maximum SV values over a single respiratory cycle, respectively.^[13] All values correspond to the mean of 3 measurements. ΔrecPP was calculated as follows: ΔrecPP = (PP_base1 – PP_RM)/(PP_base1) × 100. ΔrecSV was calculated as follows: ΔrecSV = (SV_base1 – SV_RM)/(SV_base1) × 100, where SV_RM is the mean of the last 3 SVs at the end of the RM. ΔrecSV and ΔrecPP are expressed as absolute values.

Nonresponders and responders were defined with regard to the change in SV (expressed as a percentage) after fluid expansion.^[15] A positive response (fluid responder) was defined as an SV increase of at least 15% between Base 2 and fluid expansion.^[15] This cut-off was chosen in accordance with the literature data on fluid expansion and because it is twice the value of the interobserver/intraobserver reproducibility of SV measurements using EDM.