PP according to Vt and HR:RR ratio. Beyond chest wall compliance, ΔRESPPP is influenced by Vt , HR:RR ratio  and fluid responsiveness status. This is confirmed in our study population by using a composite index including these respiratory settings: Vt × HR:RR ratio. Two-way analysis of variance disclosed that the product of Vt × HR:RR ratio and the responder versus nonresponder status independently influenced the value of ΔRESPPP (P = 0.0013 and P = 0.0014, respectively). The results of post hoc tests (Fisher's procedure of least significant difference) between quartiles of (Vt × HR:RR ratio) are shown. With regard to the need for this physiological analysis, patients with ultrasonographic evidence of acute cor pulmonale (n = 4) were excluded. Vt, tidal volume; HR, heart rate. RR, respiratory rate; ΔRESPPP, respiratory changes in pulse pressure. Responders are defined as those patients with a 10% increase in cardiac output after 500-ml volume expansion. The central boxes represent the values from the lower to the upper quartile (25th to 75th percentile). The middle line represents the median value.