Pulse pressure respiratory variation as an early marker of cardiac output fall in experimental hemorrhagic shock

Pulse pressure (DeltaPp) and systolic pressure (DeltaPs) variations have been recommended as predictors of fluid responsiveness in critically ill patients. We hypothesized that changes in DeltaPp and DeltaPs parallel alterations in stroke volume (SV) and cardiac output (CO) during hemorrhage, shock, and resuscitation. In anesthetized and mechanically ventilated mongrel dogs, a graded hemorrhage (20 mL/min) was induced to a target mean arterial pressure (MAP) of 40 mm Hg, which was maintained for additional 30 min. Total shed-blood volume was then retransfused at a 40 mL/min rate. CO, SV, right atrial pressure (RAP), pulmonary artery occlusion pressure (PAOP), and continuous mixed venous oxygen saturation (SvO(2)) were assessed. Both DeltaPp and DeltaPs were calculated from direct arterial pressure waveform. Removal of about 9% of estimated blood volume promoted a reduction in SV (14.8 +/- 2.2 to 10.6 +/- 1.3 mL, P < 0.05). At approximately 18% blood volume removal, significant changes in CO (2.4 +/- 0.2 to 1.5 +/- 0.2 mL/min, P < 0.05), DeltaPp (12.6 +/- 1.4 to 15.8 +/- 2.0%, P < 0.05), and SvO(2) (82 +/- 1.4 to 73 +/- 1.7%, P < 0.05) were observed. Alterations in MAP, RAP, PAOP, and DeltaPs could be detected only after each animal had lost over 36% of estimated initial blood volume. There was correlation between blood volume loss and SV, CO, and SvO(2), as well as between blood loss and MAP, DeltaPp, and DeltaPs. Blood volume loss showed no correlation with cardiac filling pressures. DeltaPp is a useful, early marker of SV and CO for the assessment of cardiac preload changes in hemorrhagic shock, while cardiac filling pressures are not.