Pilot Of Spontaneous Breathing Vs. Ventilated Model For Hemorrhage And Resuscitation In The Rabbit

Introduction: Most models of hemorrhage and resuscitation do not use an anesthetic with spontaneous ventilation on room air; this may be a better human trauma simulation [1,2]. We compared two anesthetics in a rabbit shock model to determine if one would optimize the hemorrhage response. Methods: After UC Davis IACUC approval, 11 New Zealand white rabbits were studied. Two groups were allowed water up to 2 hours before procedure, and received ketamine 50 mg/kg IM for premedication and received induction with diazepam 0.5 mg/kg IV and propofol 2.0-3.0 mg IV after placement of 22-gauge catheter in an ear vein. Endotracheal intubation was performed with a 2.5-3.0 mm ID cuffed tube. Group A rabbits (n=6) were allowed to breath room air spontaneously (FiO2 0.21) and I.V. anesthesia was maintained with either: diazepam 0.5-0.75mgkghr and propofol 0.075-0.6 mgkgmin or ketamine 0.4mgkghr and propofol 0.075-0.6 mgkg min. Group B (n=5) received controlled ventilation and anesthesia was maintained with 1.5% isoflurane and IV diazepam (12.5 gkgmin). FiO2 was 0.21-0.25 and ventilatory volume was 10 ml/kg with a rate adequate to maintain endtidal CO2 at 40 mmHg. Bilateral femoral artery and vein cut-downs were performed to allow placement of arterial catheters for systemic hemodynamic monitoring including cardiac output, arterial pressures, and pulse rate, as well as venous access for venous O2 saturation (SvO2). Equilibration of physiological parameters occurred for 45 min prior to collecting baseline values. Animals were bled over 30 min to decrease the mean arterial pressure to 30-41 mmHg and maintained for 30-60 min. Post-hemorrhage values were recorded 1-2 hrs after initiation of hemorrhage. Comparisons were carried out parametrically using the t-test (comparing means) for absolute post-hemorrhage change from baseline values between the two groups: arterial pH, Hct, SvO2 (%), lactate (mmol/L), hemorrhage volume (%), hemorrhage volume (ml), cardiac output (CO) average (ml/min), temperature (degree C), heart rate (HR)(beats/min), central venous pressure (CVP)(mmHg) and mean arter ial pressure (MAP)(mmHg). To compare the response to hemorrhage between the two groups, baseline values were subtracted f rom the corresponding post-hemorrhage values in order to adjust for baseline differences with p < 0.05 significant. Results: Hemorrhage volume (75.7+7.8 vs. 108.8+9.5 ml [+SEM]) was significant between groups A and B (p=0.0236). Change from baseline in MAP (-47.0+5.3 vs. -34.6+2.9 mmHg) (p=0.0844) and Sv02 % change from baseline (-56.0+3.3 vs. -46.4+3.1) (p=0.0671) between the two groups was not significant. Conclusion and summary: This pi lot study demonstrates that significantly more blood could be hemorrhaged with isoflurane compared to propofol, allowing for better compensation to hemorrhage than propofol. However, the propofol group represents more accurate out of hospital trauma conditions due to a higher baseline MAP. Although differences in SvO2 were not significant, values were elevated at baseline for isoflurane (85%) compared to (79%) with propofol, suggesting that spontaneous ventilation in room air more closely approximates normal physiology (75%). Rabbit models of hemorrhagic shock and resuscitation were compared to assess a simulated trauma with spontaneous ventilation and breathing room air. Compared to a ventilated model with higher FiO2, this model was more sensitive to blood loss.