The pulse pressure/heart rate ratio as a marker of stroke volume changes during hemorrhagic shock and resuscitation in anesthetized swine

Abstract

Emergency physicians and anesthesiologists need accurate estimates of stroke volume when massive unexpected hemorrhage occurs. Using an animal model of hemorrhagic shock under general anesthesia, we hypothesized that the pulse pressure-heart rate ratio (PP/HR) would be an accurate marker of stroke volume changes during hemorrhage and resuscitation. In 16 swine under bispectral index-controlled, intravenous propofol-remifentanil anesthesia, pressure-controlled hemorrhagic shock was induced to achieve 30 mm Hg of mean arterial pressure, after which treatment was randomized to fluid (HES, n = 4), norepinephrine (NE, n = 4), both (HES + NE, n = 4), or neither (control, n = 4). Pulmonary artery thermodilution continuous cardiac output, stroke volume, and central arterial pressures were recorded at baseline (T0), after 30 minutes (T30) and 60 (T60) minutes of hemorrhage, during treatment (T90 and T120) and after blood retransfusion (T180). At T60, blood withdrawal was 995 (301) mL (38 [8] mL/kg), resulting in a 70% decrease in stroke volume and a 3.3-fold decrease in PP/HR (each p < 0.01). When stroke volume data pointed at T0, T30 and T60 were plotted against the various hemodynamic variables under study, the PP/HR ratio exhibited the strongest relationship to stroke volume (r = 0.72). The area under the receiver operating characteristic curve set to detect a 15% stroke volume decrease was larger for PP/HR (0.95 [0.94-0.97]) than for mean arterial pressure (0.91 [0.89-0.93]) (p < 0.013). During resuscitation in the HES and NE groups, correlation coefficients were significantly higher between stroke volume and PP/HR (0.75 [0.63-0.84] and 0.79 [0.67-0.86]) than between stroke volume and mean arterial pressure (0.52 [0.32-0.67], p = 0.042, and 0.49 [0.28-0.65], p = 0.0018, respectively). The PP/HR ratio was strongly related to stroke volume during hemorrhagic shock and resuscitation in anesthetized swine.