Trauma is the leading cause of death among younger adults (1). Approximately 40% of these deaths in the US civilian population are due to traumatic brain injury (TBI), and as many as 33% to 50% of all traumatic deaths occur before hospital arrival (2). Traumatic brain injury is often accompanied by significant blood loss and resulting hypotension from extracranial injuries. Also of significance, uncontrolled hemorrhage accounts for the preponderance (∼60%) of deaths in patients with potentially salvageable injuries (3). These factors support the development of improved strategies for the early initial fluid resuscitation of patients with TBI and hemorrhagic shock (HS).
That TBI and HS often occur concomitantly because of multiple organ injuries (4–6) is of critical importance because even brief episodes of hypotension and hypoxemia can double TBI mortality (5). This is thought to be due to loss of cerebral autoregulation, resulting in secondary ischemic insult to the already vulnerable brain (6, 7). Experimental data also suggest that TBI impairs physiological compensatory mechanisms to HS (8–10). Treatment of patients experiencing both TBI and HS emphasizes aggressive fluid resuscitation through infusion of large volumes of crystalloid with goals of rapid volume expansion with blood pressure (BP) and cerebral perfusion pressure (CPP) restoration in the prehospital and hospital settings, respectively (11). However, there is considerable evidence that large-volume crystalloid resuscitation is detrimental in the setting of uncontrolled hemorrhage, and titrating fluids to achieve normal BPs in this setting is not beneficial (12, 13). Most recently, studies of combined TBI and uncontrolled hemorrhage demonstrate that aggressive resuscitation, as is currently recommended, may result in failure to optimize cerebrovascular hemodynamics and greater short-term mortality (14, 15). Consequently, artificial oxygen-carrying blood alternatives, such as hemoglobin-based oxygen carriers (HBOCs), given in low volumes, are currently under investigation as potential alternative resuscitation agents for the treatment of trauma and brain injury. These agents might facilitate a resuscitation strategy that minimizes hemorrhage volume while maximizing tissue oxygenation.
Given the promise of the HBOCs for the battlefield and prehospital settings, there has been extensive research in this area, and several agents are in varying stages of development and preclinical and clinical investigation. Several preclinical studies suggest that the oxygen-carrying resuscitative fluid, hemoglobin-based oxygen carrier 201 (HBOC-201 [Hemopure]; OPK Biotech, Cambridge, Mass), may be of benefit in the setting of HS either with or without concomitant TBI based on its ability to expand the intravascular volume, to stabilize hemodynamics with low-volume resuscitation, to transport and unload O2 with kinetics similar to that of adult human hemoglobin, and to increase tissue oxygenation (16–22). However, vasoactivity, manifested as vasoconstriction from local scavenging of nitric oxide (NO), has hampered development for trauma use (23). Consequently, much effort to attenuate HBOC-201’s vasoactivity is underway, mainly by modifying the molecular weight distribution and admixing NO donors such as sodium nitroglycerine (NTG). However, there is limited knowledge of the interaction between TBI-related physiological dysfunction and HBOC-based resuscitation in the prehospital setting. This is an important knowledge gap given the distinct challenges associated with treatment of combined TBI and HS in the prehospital setting. Therefore, the goal of this study was to identify the impact of TBI on a strategy of limited resuscitation of HS using HBOC and vasoattenuated HBOC in a model of large arterial vessel injury with free internal bleeding. We first hypothesize that TBI will impair resuscitation and decrease survival time during low-volume resuscitation of HS using multiple resuscitation fluids. Secondarily, we hypothesize that early limited resuscitation with HBOC-201 in the setting of a high-pressure vascular injury inflicted via aortic tear will improve survival over limited resuscitation with standard lactated Ringer’s (LR) solution. The knowledge gained from this study informs further research into the correct choice of resuscitation fluid strategy for use in the complex and challenging polytrauma patient with TBI and HS.
MATERIALS AND METHODS
All protocols were approved by the Animal Care and Use Committee of the University of Washington. Animal housing and all experiments were conducted in compliance with the Animal Welfare Act and in accordance with the principles set forth in the Guide for the Care and Use of Laboratory Animals, National Academy of Science, 2011. Animals were maintained at the University of Washington, Department of Comparative Medicine facility, an Association for Assessment and Accreditation of Laboratory Animal Care–accredited facility. These experiments were conducted in two phases, with each phase lasting approximately 5 to 6 months. The model used in phase I was combined HS with TBI, whereas phase II was HS alone. During each phase, all animals were randomly assigned to receive one of the three resuscitation fluids (HBOC, HBOC + NTG 5 μg · kg−1 · min−1, or LR solution).
Instrumentation for monitoring and hemorrhage
Immature (4-month-old) female mixed-breed Yorkshire swine weighing 26.6 kg (95% confidence interval [CI], 25.7–27.5 kg) purchased from the Washington State University Swine Center were fasted overnight before surgery with free access to water. The animals were sedated with an intramuscular injection of ketamine (30 mg · kg−1) (BionichePharma, Galway, Ireland) followed by 3% isoflurane (VetOne, Boise, Idaho) by nasal cone to facilitate endotracheal intubation. A one-time intramuscular injection of buprenorphine (0.01 mg · kg−1) (Ben Venue Laboratories Inc, Bedford, Ohio) was given to reduce the concentration of isoflurane needed to achieve and maintain a surgical plane of anesthesia. Animals were allowed to breathe spontaneously; if they became bradypneic or apneic, they were placed on a volume-cycled ventilator and ventilated (ANESCO SAV 2500 Anesthesia Ventilator; ANESCO, Inc, Georgetown, Ky) with a tidal volume of 5 to 10 mL · kg−1 at a rate of 12 to 15 breaths/min adjusted to maintain ETCO2 between 35 and 40 mmHg (Capnomac Ultima; Datex, Madison, Wis). Bilateral femoral arteries and femoral veins were isolated and cannulated with PE240 polyethylene catheters for BP monitoring, blood sampling, and controlled hemorrhage and for intravascular infusion. An 8F introducer sheath was placed into the right external jugular vein, and a 7F Swan-Ganz thermodilution catheter (Edwards Life Sciences, Irvine, Calif) was advanced to the pulmonary artery for measurement of cardiac output (CO), pulmonary artery pressure, central venous pressure (CVP), and sampling for central venous blood gas. Splenectomy and bladder cannulation were then performed via midline laparotomy. The infrarenal aorta was then isolated, and a 5-0 monofilament stainless-steel surgical wire was inserted traversing 4 mm longitudinally for the induction of an aortic tear. The aortotomy wires were externalized through the midline abdominal incision, and the abdominal cavity was closed. This and similar aortic tear models of uncontrolled hemorrhage have been utilized in several previous studies (12–14, 24).
Brain instrumentation for fluid percussion/TBI
The scalp was widely incised and reflected posteriorly so that the cranium was exposed. A 16-mm-diameter craniotomy was performed in the right parietal region adjacent to the sagittal suture and anterior to the coronal suture. A three-way bolt was placed into the craniotomy to abut the intact dura. This bolt was then connected to a fluid percussion injury device as well as a pressure transducer (SenSym, Sunnyvale, Calif) to quantify brain injury. A second craniotomy was performed in the left posterior parietal region, and a neonatal intraventricular catheter (Phoenix Biomedical Corp, Valley Forge, Pa) was inserted into the left ventricle and connected to a pressure transducer to monitor intracranial pressure (ICP). A third craniotomy was performed on the midline suture, just anterior to the inion, and the sagittal sinus was cannulated with a 16-gauge Angiocath catheter for sagittal sinus blood sampling. A fourth craniotomy was made in the right frontoparietal region for placement of a Licox tissue oxygen/temperature probe (Integra Neurosciences Inc, Plainsboro, NJ). Any cortical bleeding was controlled with bone wax and thrombin gel. All craniotomies were sealed with dental cement to secure catheters and probes in place, and the animal allowed to re-equilibrate for 30 min during which fraction of inspired oxygen (FIO2) was maintained at 28% to 30%, end-tidal CO2 was kept at 35 to 40 mmHg, and body temperature, monitored by Swan-Ganz catheter, was maintained at 37°C ± 1°C with a warming blanket. All animals underwent brain instrumentation, regardless of whether TBI was inflicted.
Hemorrhage and TBI
Fluid percussion TBI was inflicted in the standard fashion as follows. The three-way bolt in the right frontoparietal craniotomy position was connected to a saline-filled column, which is connected on the opposite end to a pendulum arm with a weight at its distal end. The pendulum is pulled back a standard distance and allowed to fall, striking a Plexiglas piston, which in turn strikes a rubber seal at the end of the fluid-filled cylinder. The resulting fluid wave that is generated in this closed system transmits a 15-millisecond pressure pulse to the intact dura, which is quantified via the high-pressure transducer, also connected to the three-way craniotomy bolt.
Immediately following TBI, the animals were rapidly placed in supine position, and controlled hemorrhage was initiated from the right femoral artery catheter with a MasterFlex roller pump (Cole-Parmer, Vernon Hills, Ill) to bleed 35 mL · kg−1 over 30 min, the rate preprogrammed to decrease exponentially over time simulating the natural course of arterial hemorrhage. Once the mean arterial pressure (MAP) reached 50 mmHg, the infrarenal aortotomy wire was pulled, creating a 4-mm aortic tear for uncontrolled hemorrhage in the closed abdomen. When MAP reached 30 mmHg, the roller pump was switched to a pause/restart mode to keep the MAP at 25 to 30 mmHg for 15 min. The shed blood from the femoral artery was collected into a blood collection bag containing citrate anticoagulant for blood transfusion. This period represents simulated injury before medic arrival. This method of inducing uncontrolled hemorrhage has been utilized in several previous studies
Following the 15-min shock period, animals were randomly assigned to one of three resuscitation strategies based on fluids infused: (a) LR solution control, (b) HBOC, and (c) HBOC + NTG 5 μg · kg−1 · min−1. Resuscitation was divided into two phases: (I) prehospital resuscitation (105 min) and (II) in-hospital surgical repair of aortic tear at 105 min and further care up to 360 min (Fig. 1).
During the prehospital phase, the animals were resuscitated with either LR solution (20 mL · kg−1), HBOC (10 mL · kg−1), or HBOC + NTG (10 mL · kg−1 HBOC + 5 μg · kg−1 · min−1 NTG) given at seven time points: resuscitation time 0, 15, 30, 45, 60, 75, and 90 min as needed if MAP was less than 60 mmHg. Each of these fluids was infused over 10 min. The NTG (American Regent Inc, Shirley, NY) was infused concomitantly with HBOC for an equal period. At simulated hospital arrival (105 min), the abdomen was opened, and intraperitoneal hemorrhage was measured by soaking preweighed gauze. A hemostatic primary repair of the aortic injury was performed using 6-0 Prolene suture. The peritoneum and midline fascia were then closed with 0 silk, and the skin closed with surgical staples. The animals were then resuscitated to restore normal physiological parameters using a goal-directed protocol using normal saline to achieve MAP of 70 mmHg or greater and shed blood to achieve hemoglobin of 7 g · dL−1 or greater. If both ICP is greater than 20 mmHg and MAP is greater than 40 mmHg, one bolus of mannitol (Hospira Inc, Lake Forest, Ill) was given, 1 g · kg−1 intravenously administered, up to a maximum of 3 g · kg−1. Mechanical ventilation was titrated to achieve arterial blood oxygen saturation greater than 92% and end-tidal CO2 between 35 and 40 mmHg. Resuscitation was continued in this manner up to 6 h after the initial injury or until death. Surviving animals were killed humanely under anesthesia using intravenous pentobarbital (Euthanasia III solution; Med-Pharmex, Inc, Pomona, Calif) overdose. Animals in the HS-only group were instrumented and treated similarly, except that no fluid percussion TBI was inflicted.
Hemodynamic parameters, heart rate (HR), BP, MAP, pulmonary arterial pressure, left ventricle pressure, CVP, ICP, CPP (MAP − ICP = CPP), and electrocardiogram were continuously monitored and recorded with a Biopac multichannel digital data acquisition system (Biopac Systems, Inc, Goleta, Calif) throughout the experiment. ETCO2, FIO2, isoflurane concentration, respiration rate, and tidal volume were monitored continuously with Datex Ultima Capnometer. Cardiac output, core body blood temperature (via Swan-Ganz catheter), brain tissue oxygen, transcutaneous oxygen were recorded every 5 min to R180 and every 15 min thereafter to R360. Arterial, mixed venous, and sagittal sinus blood samples were collected at baseline, time T = 0 and every 15 min until 2 h, then at 30-min intervals until 4 h and then every 1 h thereafter. Standard metabolic markers including lactate, base excess, blood gas, and glucose were analyzed with a blood gas analyzer ABL 805 (Radiometer, Copenhagen, Denmark).
Continuous data were normally distributed and expressed as mean with 95% CIs. The primary outcome of interest was survival. Cox proportional hazard regression was used as time-to-event–based analyses to compare survival rates for each covariate (type of resuscitation fluid and presence of TBI), thus allowing for adjustment for one another. A Cox proportional hazard was chosen for this analysis because it allows survival time to be adjusted for multiple covariates and gives a conditional risk ratio (or hazard ratio) for each covariate rather than making assumptions based on an overall percentage of survival, which can be problematic, given that there were only seven to eight animals in each fluid resuscitation group for each condition. This statistical approach rather allows for scaling of a baseline hazard function by the model’s covariates to give a general hazard function and specific hazard function for each covariate.
Secondary normally distributed continuous outcome variables that were measured at a single point in time (e.g., blood loss, fluid requirement) were compared using two-way analysis of variance (ANOVA), with the type of fluid resuscitation and presence of TBI included as the primary effects. A mixed model was used to compare the effects of resuscitation fluid type and TBI on continuous data measured longitudinally over time (e.g., MAP, HR). In these instances, mixed-model repeated-measures two-way ANOVA was used to identify parameter estimates for time, treatment groups, and their interaction. For all analyses, two-sided P ≤ 0.05 was considered statistically significant for each overall effect. Individual comparisons were made after adjusting for multiple comparisons by the method of Tukey-Kramer.
Based on a previously demonstrated absolute survival benefit of 60% with HBOC-201 versus LR solution in a similar model, seven to eight animals per group will provide at least 80% power to detect a survival benefit of greater than 20%. This would also allow us to detect a change in lactate concentration of at least 4 mmol · L−1 and a change in intraperitoneal blood loss of at least 20 mL · kg−1 between groups with 80% power to detect differences. All analyses were performed using JMP-9 statistical package (SAS Inc, Cary, NC).
Using a Cox proportional hazards model with the covariates of resuscitation fluid type and presence of TBI, we found no independent effect of TBI on survival time after adjusting for resuscitation fluid (TBI effect likelihood ratio, χ2 1 = 1.45, P = 0.22). The calculated relative risk ratio was elevated above the baseline hazard ratio for TBI but not significantly (relative risk, 1.5; 95% CI, 0.8–2.8). However, when examining the effect of resuscitation fluid on survival time in the same model, we found a significant independent effect of resuscitation fluid on survival time (resuscitation fluid effect likelihood ratio, χ2 1 = 6.7, P = 0.03). This difference was attributed only to a significantly increased risk ratio for death for HBOC + NTG versus LR solution (relative risk, 2.8; 95% CI, 1.3–6.1) (Table 1).
Impact of TBI
The mean weight of the TBI group (n = 21, 7 per resuscitation fluid) was 27.5 kg (95% CI, 26.3–28.5 kg), which was statistically greater than the HS-only group (n = 24, eight per resuscitation fluid; 25.9 kg; 95% CI, 24.9–27.0 kg), but more likely clinically irrelevant. Traumatic brain injury–specific outcomes are summarized in Table 2. Mean TBI pressure applied to the dura in the TBI group averaged 3.4 atm (95% CI, 3.2–3.5 atm). Total catheter hemorrhage volume required to reach goal hypotensive MAP and maintain it for 15 min was less with TBI compared with those with HS alone, although a similar degree of shock was attained as demonstrated by mean lactate concentrations at the end of the standard hemorrhage period that were similar between groups. There was no significant independent effect of TBI on volume of fluid resuscitation given, intraperitoneal blood loss, or total hemorrhage volume. Of the TBI animals, two animals in the LR solution, two in the HBOC, and one animal in the HBOC/NTG group required mannitol for an elevated ICP. Each of the animals in the LR solution and HBOC groups received a total of three infusions of mannitol (3 g · kg−1 or 15 mL · kg−1), whereas the animal in the HBOC/NTG group received only one infusion (1 g · kg−1 or 5 mL · kg−1).
There was a significant independent effect of TBI on MAP (P = 0.01), HR (P < 0.0001), and systemic vascular resistance (SVR) (P = 0.0002), after adjusting for resuscitation fluid and phase of resuscitation (prehospital vs. hospital) (Fig. 2). Overall, MAP, HR, and SVR were higher in the HS-only group. Central venous pressure tended to be higher in the TBI + HS group (P = 0.068), and CO was not different but tended to be higher in the TBI + HS group. Pulmonary artery pressure was also not different (P = 0.11) (Fig. 3).
There was no significant independent effect of TBI on ICP (P = 0.16), a trend toward overall higher CPP in the HS-only group (P = 0.09), and no difference in brain tissue oxygenation by Licox probe (Integra Neurosciences Inc) (P = 0.7). There was a significant independent effect of TBI on cerebrovascular venous oxygen saturation, which was higher in the HS-only group (P = 0.0027) (Fig. 4). There were no differences in metabolic markers of resuscitation including overall levels of arterial lactate (P = 0.4), arterial pH (P = 0.21), or arterial base excess (P = 0.49) comparing TBI and HS-only groups during hemorrhage and resuscitation (Fig. 5).
Impact of resuscitation fluid
Outcomes stratified by resuscitation fluid are summarized in Table 3. End-of-hemorrhage lactate was higher in the HBOC + NTG group compared with the HBOC group. As expected, the LR solution group received significantly more resuscitation fluid volume (20 mL · kg−1 per bolus) compared with the HBOC-containing fluids (10 mL · kg−1 per bolus) during prehospital resuscitation, but the number of boluses required to maintain MAP of greater than 60 mmHg was not different. Total NS required during hospital treatment was less in the HBOC + NTG group compared with HBOC.
There was a significant independent effect of resuscitation fluid type after adjusting for presence of TBI on multiple hemodynamic and metabolic variables measured during the experimental protocol as outlined in Table 4. In general, MAP was highest in the HBOC + NTG group, and HR was lowest in the HBOC group. Intracranial pressure was highest with LR solution with a correspondingly lower CPP compared with the HBOC groups. However, brain tissue oxygenation was lowest with HBOC, and cerebral lactate was elevated with HBOC-containing fluids compared with LR solution, suggesting a detrimental effect of HBOC on cerebral oxygen delivery and metabolism, even though CPP was greater. The same was true with central mixed venous oxygen saturation, which was lower with HBOC-containing fluids relative to LR solution. Pulmonary arterial pressure was highest in the HBOC group and was significantly greater than HBOC + NTG and LR solution, suggesting an attenuation of pulmonary vasoactivity with the addition of NTG to HBOC; however, SVR was actually highest in the HBOC + NTG relative to HBOC and LR solution. In summary, these data demonstrate an overall better hemodynamic and metabolic resuscitation with LR solution compared with HBOC-containing fluids. The addition of NTG may be associated with locally reduced pulmonary vasoreactivity by the noted decrease in pulmonary arterial pressure, but did not significantly reduce SVR in this model.
There are two major important observations from these data. First, in this multiphased treatment model, there was no significant impact of TBI on survival during low-volume resuscitation of uncontrolled HS, although presence of TBI was associated with a decreased compensatory response to acute hemorrhage. Second, in this high-pressure aortic bleeding model, limited resuscitation with HBOC did not improve survival, which is in contrast to previous studies using solid organ injury as the source of free hemorrhage.
Impact of TBI
The lack of effect of TBI on survival time may be partially explained by inequalities in the volume of blood withdrawn during hemorrhage. In this pressure-guided model, HS-only animals required on average 150% more blood to be withdrawn to meet the prespecified MAP = 30 mmHg. Yet, lactate levels were similar at the end of the shock period, indicating a similar degree of shock and suggesting an exaggerated hypotensive response to blood removal in the TBI + HS group. Therefore, TBI + HS animals likely maintained significantly more blood volume during resuscitation, which is supported by the strong trend toward higher CVP with TBI. However, this logically advantageous retained blood volume did not translate to improved MAP, CO, perfusion, or survival time during low-volume resuscitation due to a significantly lower SVR that also accompanied TBI. Taken together, these data suggest that there was TBI-specific impairment of global vascular responsiveness during hemorrhage and resuscitation, thus negating the advantage of increased circulating blood volume. This may also account for the worse outcome in the TBI + HS animals treated with HBOC + NTG. One might hypothesize that, in animals with an already compromised vascular response, the administration of an agent known to cause vasodilatation, such as NTG, might easily further impair the animal’s already compromised physiological response to HS.
Historical study of the cardiovascular responses to progressive isolated hemorrhage reports the immediate response as an increase in HR, maintaining BP for a period as hemorrhage progresses, until a vagally mediated reduction of sympathetic tone and bradycardia predominate, resulting in fainting and hypotension (25). Tissue damage can modify this response by prolonging the initial period of tachycardia and maintaining BP for longer periods, which has been associated with reduced survival experimentally (26, 27). McMahon et al. (28) also found a similar association between brain-specific injury and cardiovascular response to hemorrhage in rats where HR remained elevated longer, and there was a delayed hypotensive response to graded fixed-volume hemorrhage with TBI. However, other investigators have found impaired vascular responses similar to our witnessed response during hemorrhage and resuscitation in a rat model, where vascular compensation was blunted in the setting of TBI (29). Our data agree with an immediate attenuation of initial tachycardia and impaired peripheral vasoactivity during hemorrhage and fluid resuscitation in the setting of brain injury. The differences noted between ours and previous models may also be due to uncontrolled hemorrhage or tissue injury, which can also contribute to altered cardiovascular responses during hemorrhage (26).
Even brief episodes of hypotension are known to contribute to secondary brain injury during resuscitation of HS. So the exaggerated hypotensive response to hemorrhage seen with TBI is likely a clinically relevant phenomenon that deserves further study. Our data also suggest that estimating blood loss according to vital signs may be disadvantageous in the presence of concomitant TBI. Estimating blood loss and degree of shock by vital signs, including HR and BP, are currently accepted standard methods of classifying shock severity in trauma patients and drive many resuscitative algorithms (30). However, tools such as shock index perform poorly in the setting of combined TBI and hemorrhage (31). Given the data from this study suggesting a TBI-specific exaggerated bradycardic and hypotensive response to hemorrhage, estimation of blood loss and resuscitation based solely on these parameters may lead to less-effective fluid resuscitation.
Cerebral perfusion pressure tended to be higher and cerebral venous O2 saturation was significantly greater in the HS-only group, suggesting a detrimental effect of TBI on cerebral physiology. Cerebral venous O2 saturation remained depressed from baseline levels (<60 mmHg) in both groups until late in the hospital phase, when the HS-only group recovered to near-baseline levels. This is likely a reflection of both the low-volume resuscitation strategy and increased MAP in the HS-only group because ICP did not differ significantly. Local changes in cerebrovascular resistance and autoregulation with TBI, known to shift the cerebral autoregulatory curve rightward, have been used to explain increased cerebral injury with TBI during resuscitation (32). Our results support an important role for global cardiovascular dysfunction in addition to cerebral deregulation as drivers of cerebral injury during TBI with HS. These findings argue for proper systemic resuscitation, with an emphasis on restoring vascular tone, as a priority for the initial resuscitation of the polytrauma casualty with TBI and HS. Other polytrauma models of combined TBI and HS support the importance of systemic vascular tone during fluid resuscitation by demonstrating that goal CPP could be achieved only with recovery of SVR (33). Of course, restoration of vascular resistance and BP must be balanced with risk for rebleeding from wounds. As is the current standard of care in the intensive care unit, we suggest that there may be a role for early invasive hemodynamic and metabolic monitoring in the emergency department to better guide the initial fluid resuscitation of polytrauma patients with HS and TBI.
Impact of resuscitation fluid
A second important observation of these data is the significant impact that type of resuscitation fluid had on outcomes. Limited resuscitation with LR solution was associated with significantly better survival compared with HBOC + NTG–treated animals and a trend toward better survival compared with the HBOC group. These differences were most pronounced in the TBI group. Given the evidence for impaired vasomotor response to hemorrhage in the TBI group, we suspect that perhaps the additional vasorelaxation afforded by the addition of NTG led to a further impairment in physiological response to hemorrhage. Although the differences did not reach statistical significance, there was a clear trend toward greater intraperitoneal hemorrhage in the HBOC-treated animals, which may have also contributed to the shorter survival time.
Our results are contrary to previous studies using similar polytrauma models in which low-dose HBOC-201 appeared to be advantageous by improving global hemodynamics, cerebral oxygenation, and perfusion and reducing contralateral neuronal injury compared with LR solution (16, 33). However, much of the benefit of HBOC-201 on cerebral resuscitation in these previous studies appears to be due to its vasopressor effect, increasing central BP and thus improving CPP, and much of this previous work was performed without free bleeding. A study by Stern et al. (34), using a model of combined TBI and uncontrolled hemorrhage inflicted via liver laceration and a simulated prehospital resuscitation time of 75 min, demonstrated no difference in hemorrhage volume but improved outcome with limited resuscitation with HBOC as compared with LR solution. In that study, cardiac indices, CPPs, and brain tissue oxygen tension were significantly better, and lactate and base deficit lower in the HBOC-treated animals as compared with the animals treated with LR solution.
In the current study, animals subjected to combined TBI + HS with aortic tear and provided initially limited resuscitated with HBOC experienced a trend toward greater free bleeding as compared with those resuscitated with LR solution; total hemorrhage volumes in the HBOC- and LR solution–resuscitated animals were 46.5 mL · kg−1 (95% CI, 35.8–57.2 mL · kg−1) and 29.4 mL · kg−1 (95% CI, 18.7–40.1 mL · kg−1), respectively. In contrast, in the previous study by Stern et al. (34) that utilized a TBI/liver injury model, there was no difference in free bleeding between animals resuscitated with HBOC versus LR solution; total hemorrhage volumes in the HBOC- and LR solution–resuscitated animals were 72.4 ± 14.2 and 74.4 ± 15.1 mL · kg−1 respectively. Yet, in both the current and the previous study, HBOC infusion resulted in a similar initial hemodynamic response, that is, an increase in SVR. It is noteworthy that the current study and the previous study by Stern et al. (34) were similar in all regards with the exception of injury pattern (aortic tear vs. liver laceration) and a slightly longer simulated prehospital period in the current model. Therefore, it is reasonable to hypothesize that the cause of the disparate findings in these two studies may be related to the differing injury patterns. Specifically, the data suggest that, in the setting of a localized high-pressure (large artery) vascular injury, the pressor effect of HBOC may cause an increase in hemorrhage volume as compared with limited resuscitation with an agent that has no vasoactive properties (i.e., LR solution), whereas this response is not observed in a low-pressure injury such as a liver laceration. An additional cause may be related to the severity of shock at the start of resuscitation. Mean arterial lactate levels in the current study were 4.7 mmol/L compared with 2.0 mmol/L in the previous study by Stern et al. (34), suggesting greater oxygen debt at onset of resuscitation. It is conceivable that the infusion of a vasoactive agent under these circumstances without additional volume resuscitation may further compromise perfusion to vital organs, including myocardium. That survival was poorest in the TBI + HS group that received NTG infusion in addition to HBOC may also suggest a role for NO production in TBI-induced vascular disturbances.
This study has several important limitations. First, the use of the swine model limits extrapolation to the clinical setting. To address this, we have developed a relevant model that closely mimics the time course of events that occur in the prehospital period following injury as well as in a simulated in-hospital phase, including an operative intervention. Second, to test both of our hypotheses required that we utilize two differing injury profiles, a model of TBI with HS and a model of HS alone, and that we test our resuscitation strategies in each model. Although such a complex model better ensures clinical relevance, it may limit hypothesis testing by the introduction of many uncontrolled variables. We attempted to isolate the specific effects of TBI on physiology and outcomes by adjusting for treatment phase and resuscitation fluid during data analysis. Nevertheless, this study reports important effects of TBI on hemorrhage and low-volume resuscitation of HS with standard LR solution and HBOC that deserve further focused attention. Further study of cardiovascular responses would benefit from volume-controlled hemorrhage models.
A third limitation, also related to the use of two injury models, one with and one without TBI, is that animals in the TBI arm of the study received mannitol infusions as needed to treat elevations in ICP, as would be provided in an emergency department, intensive care unit, or operative setting. Therefore, this represented an asymmetry between animals that sustained HS and TBI and those that sustained HS alone. Given that these infusions were provided only after control of hemorrhage from, and operative repair of, the aortic injury had been accomplished and at a time when aggressive fluid resuscitation was being provided to normalize systemic hemodynamic and metabolic parameters, administration of mannitol in the volumes provided is unlikely to have affected hemodynamic response. In addition, as noted in the results section, no more than two animals from any group received mannitol. And perhaps most importantly, clinical data demonstrate that, in the setting of polytrauma, administration of mannitol has no effect on hemodynamic parameters (35).
A final limitation is the use of isoflurane anesthesia. Although an unanesthetized model may be more clinically relevant, this would not be humane or ethically responsible. To reduce the concentration of isoflurane required to achieve and maintain an adequate level of anesthesia, we provided a single dose of buprenorphine. This is consistent with care provided in the clinical setting, where trauma victims often receive opiates in the form of morphine sulfate to control pain.
Survival was not different with combined TBI and HS compared with HS alone during limited resuscitation in this uncontrolled hemorrhage model. However, less hemorrhage was required to produce an equivalent degree of shock in the presence of TBI, and cardiovascular response to fluid resuscitation was blunted during fluid resuscitation when TBI was present. These results suggest a distinct systemic physiology associated with combined TBI and HS that may require targeted resuscitation strategies to improve outcomes. In addition, data from the current study, which uses a high-pressure vascular injury model of uncontrolled hemorrhage, demonstrate decreased survival for animals provided limited resuscitation with HBOC as compared with LR solution. These findings are different than previously published data from models of uncontrolled hemorrhage inflicted via liver injury that demonstrate improved survival with limited resuscitation with HBOC. These data demonstrate the need to consider wound geometry and type, specifically high-pressure vascular versus low-pressure solid organ injuries, as well as shock severity when considering resuscitation strategies.
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Hemorrhage; traumatic brain injury; shock; fluid resuscitation; hemoglobin-based oxygen carrier