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

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1 Article ID: WMC Pilot Of Spontaneous Breathing Vs. Ventilated Model For Hemorrhage And Resuscitation In The Rabbit Author(s):Dr. Jonathan S. Jahr, MD, Dr. Robert A. Gunther, PhD, Dr. Bernd Driessen, DVM, PhD, Mr. Randall Holtby, Dr. Patricia L. To, PhD, Dr. Anthony T.W. Cheung, PhD Corresponding Author: Dr. Jonathan S. Jahr, MD, Professor of Clinical Anesthesiology, David Geffen School of Medicine at UCLA, Department of Anesthesiology - United States Submitting Author: Mr. Randall Holtby, B.S., Department of Anesthesiology, David Geffen School of Medicine at UCLA - United States Article ID: WMC Article Type: Research articles Submitted on:30-nov-2010, 08:33:44 PM GMT Article URL: Subject Categories:ANAESTHESIA Published on: 03-Dec-2010, 05:35:23 PM GMT Keywords:Hemorrhage, shock, animal model, spontaneous breathing, ventilation, critical care How to cite the article:jahr, MD J, Gunther, PhD R, Driessen, DVM, PhD B, Holtby R, To, PhD P, Cheung, PhD A. Pilot Of Spontaneous Breathing Vs. Ventilated Model For Hemorrhage And Resuscitation In The Rabbit. WebmedCentral ANAESTHESIA 2010;1(12):WMC Webmedcentral > Research articles Page 1 of 6

2 Pilot Of Spontaneous Breathing Vs. Ventilated Model For Hemorrhage And Resuscitation In The Rabbit Abstract 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 mg IV after placement of 22-gauge catheter in an ear vein. Endotracheal intubation was performed with a mm ID cuffed tube. Group A rabbits (n=6) were allowed to breath room air spontaneously (FiO ) and I.V. anesthesia was maintained with either: diazepam mgkg -1 hr -1 and propofol mgkg -1 min -1 or ketamine 0.4mgkg -1 hr -1 and propofol mgkg -1 min -1. Group B (n=5) received controlled ventilation and anesthesia was maintained with 1.5% isoflurane and IV diazepam (12.5 gkg -1 min -1 ). FiO 2 was and ventilatory volume was 10 ml/kg with a rate adequate to maintain endtidal CO 2 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 O 2 saturation (SvO 2 ). 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 mmhg and maintained for 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 arterial pressure (MAP)(mmHg). To compare the response to hemorrhage between the two groups, baseline values were subtracted from the corresponding post-hemorrhage values in order to adjust for baseline differences with p < 0.05 significant. Results: Hemorrhage volume ( vs ml [+SEM]) was significant between groups A and B (p=0.0236). Change from baseline in MAP ( vs mmhg) (p=0.0844) and Sv02 % change from baseline ( vs ) (p=0.0671) between the two groups was not significant. Conclusion and summary: This pilot 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. Introduction Numerous models exist for the study of hemorrhagic shock and resuscitation; however, most models do not involve an anesthetic regimen that allows for spontaneous ventilation on room air [1,2]. These models include controlled ventilation, and in doing so introduce a confounding variable that is not present in real life trauma events. A spontaneous breathing model may be a better simulation for studying trauma-related hypovolemia in humans because it eliminates the effects of ventilation in the model. In the current study, the rabbit was chosen as the Webmedcentral > Research articles Page 2 of 6

3 model for study. Rabbits represent species in the 3-5 kg body weight range and are potentially useful for studying new resuscitation agents when quantities are limited [3]. We compared two anesthetics in a rabbit hemorrhagic shock model to determine if one would optimize the hemorrhage response. Methods After UC Davis IACUC approval, eleven New Zealand white rabbits were studied in two groups. Both groups were allowed water up to two hours before procedure, and received ketamine 50 mg/kg IM for premedication. Both groups received induction with diazepam 0.5 mg/kg IV and propofol mg IV after placement of 22-gauge catheter in an ear vein. Endotracheal intubation was performed with a mm ID cuffed tube. Group A rabbits (n= 6) were allowed to breathe room air spontaneously (FiO ) and I.V. anesthesia was maintained with any one of -1 the following regimens: 1.) diazepam 0.75mg kg-1 hr and propofol mg kg -1 min-1 2.) diazepam 0.5mg kg -1 hr-1 and propofol mg kg -1 min-1 3.) ketamine 0.4mg kg -1 hr-1 propofol mg kg -1 min -1. Group B rabbits (n= 5) were attached to a ventilator and anesthesia was maintained with 1.5% isoflurane and IV diazepam (12.5 µg kg -1 min-1 ). FiO 2 was and ventilatory volume was 10 ml/kg with a rate adequate to maintain endtidal CO 2 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 subsequent venous 0 2 saturation (SvO 2 ) measurements. Following placement of vascular lines, 45 minutes were allowed for equilibration of physiological parameters after instrumentation, prior to collecting baseline values (Arterial ph, Hct, SvO2, lactate, hemorrhage volume, CO, temperature, heart rate, CVP, and MAP). Animals were bled over 30 min to reach a mean arterial pressure of mmhg. This pressure was then maintained for min. Additional blood was removed if necessary. 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 for 11 different variables between the two groups: arterial ph, Hct, SvO2 (%), lactate (mmol/l), hemorrhage volume (%), hemorrhage volume (ml), CO average (ml/min), temperature (degree C), heart rate (HR) (beats/min), CVP (mmhg) and MAP (mmhg). To compare the response to hemorrhage between the two groups, baseline values were subtracted from the corresponding post-hemorrhage values in order to adjust for baseline differences. Results Out of the 11 absolute post-hemorrhage change from baseline values between the two groups, only hemorrhage volume was considered statistically significant (p=0.0236, parametric) (Table 1). Differences in MAP (mmhg) change from baseline (p=0.0844, parametric) and Sv02 (%) change from baseline (p=0.0671, parametric) between the two groups approximated significance (Table 1). Discussion According to extensive overviews which summarize current hemorrhagic shock models, many researchers are not using spontaneous ventilation in room air to simulate hemorrhage [1, 4]. A small amount of studies do incorporate a spontaneous breathing model in both rabbits and rats, but their focus is usually resuscitation as opposed to the actual model for hemorrhage [5, 6]. These studies are based on the assumption that spontaneous models better simulate hemorrhage in the field, for which there is marginal evidence. The most recent overview (January 2009) fails to even mention room air ventilation as an alternative to the ventilated model [4]. This indicates that our model appears to be unique. What the overviews do offer, however, is an indication that our model is valid. First, fixed-pressure models like the one used in the current study are favored over fixed-volume models because they have higher reproducibility and standardization [1]. Fixed-volume techniques may vary in response to a particular hemorrhage volume since different individuals have unequal total blood volumes [1]. In addition, the spontaneous ventilation model provides a good compromise when weighing issues of clinical relevance as well as ethics. The use of anesthetic regimens in hemorrhagic shock models has been shown to confound results, since anesthesia may alter the animal s normal physiology [4]. The only other alternative to models which employ anesthesia is the conscious animal hemorrhagic shock model, in which individuals are hemorrhaged without anesthesia [4]. The spontaneous ventilation model mimics real life Webmedcentral > Research articles Page 3 of 6

4 trauma by allowing animals to breathe room air during hemorrhage, without subjecting individuals to potentially unethical procedures such as those employed in the conscious model. The results of our experiment indicate that the spontaneous ventilation model more effectively mimics normal physiology during hemorrhage. The fact that a significantly higher blood volume could be hemorrhaged from the ventilated condition supports this claim. On average, ml of blood could be removed from the isoflurane group, while only 75.7 ml was hemorrhaged in the propofol group. The ventilated model provides more hemodynamic stability allowing for more blood volume to be drawn off. In contrast, the spontaneous ventilation model is more fragile without oxygenation, demonstrating its more relevant simulation of hemorrhagic shock. This reconfirms early strategies currently used in the clinical field to improve survival following hemorrhage: increasing the fraction of inspired oxygen by controlling ventilation. Although hemorrhage volume was the only variable considered statistically significant, potential differences may exist between the two models for the ten additional variables, and these differences could have clinical implications. Lack of statistical significance is most likely due to inadequate sample size, not absence of a real difference between the two models. Elucidation of these differences will require greater focus of the research community on the importance of a spontaneous breathing model for hemorrhagic shock. Additionally, it is important to note that although spontaneous breathing appears to better replicate trauma in the field, experimental hemorrhage is inevitably somewhat controlled [5]. Uncontrolled hemorrhage would be ideal for simulation, but blood must be withdrawn systematically to allow for standardization and replication. Lamb SV, Vincent SE, Boston RA, Jahr JS, Cheung ATW. Effects of low-volume hemoglobin glutamer-200 versus normal saline and arginine vasopressin resuscitation on systemic and skeletal muscle blood flow and oxygenation in a canine hemorrhagic shock model. Critical Care Medicine 2007; 35 (9): Jahr JS, Lurie F, Xi S, Golkaryeh M, Kuznetsova O, Kullar R, Driessen B. A novel approach to measuring circulating blood volume: the use of a hemoglobin-based oxygen carrier in a rabbit model. Anesthesia and Analgesia 2001; 92: Moochhala S, Wu J, Lu, J. Hemorrhagic shock: an overview of animal models. Frontiers in Bioscience 2009; 14: Yu Y, Gong S, Chao S, Zhao K, Lodato R, Wang C. Increased survival with hypotensive resuscitation in a rabbit model of uncontrolled hemorrhagic shock in pregnancy. Resuscitation 2009; 80: Takasu A, Minagawa Y, Ando S, Yamamoto Y, Sakamoto T. Improved survival time with combined early blood transfusion and fluid administration in uncontrolled hemorrhagic shock in rats. Journal of Trauma- Injury Infection & Critical Care 2009; 68 (2): Since this study originated from pilot data, similar experiments should be performed to replicate and improve the methods of our own. Improvements should include larger and more consistent sample sizes across variables and possibly additional physiologic variables which would indicate clinical importance. References 1. Lomas-Niera JL, Perl M, Chung CS, Ayala A. Shock and hemorrhage: an overview of animal models. Shock 2005; 1: Driessen B, Zarucco L, Gunther RA, Burns PM, Webmedcentral > Research articles Page 4 of 6

5 Illustrations Illustration 1 Summary of the 11 change from baseline variables, accompanied by the median, mean and standard error (SE) for each group (propofol and isoflurane). Corresponding Variable (Change parametric from p-values are also shown. Parametric Group N Q1 Median Mean Q3 SE Baseline) p-values Arterial PH Hct SvO2 (%) Lactate (mmol/l) Hemorrhage Volume (% est. blood vol.) Hemorrhage Volume (ml) CO Average (ml/min) Temperature ( C) HR (beats/min) CVP (mmhg) MAP (mmhg) Isoflurane Propofol Isoflurane Propofol Isoflurane Propofol Isoflurane Propofol Isoflurane Propofol Isoflurane Propofol Isoflurane Propofol Isoflurane Propofol Isoflurane Propofol Isoflurane Propofol Isoflurane Propofol Webmedcentral > Research articles Page 5 of 6

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