Hemorrhagic shock is a common and life-threatening complication of severe trauma in humans and dogs alike. Massive blood loss leads to inadequate oxygen delivery to tissues and decreased oxygen carrying capacity. Compensatory mechanisms can fail if shock goes untreated, resulting in circulatory collapse and death.
Until recently, determining the severity of hemorrhagic shock as well as the response to treatment could only be performed by evaluating macrovascular variables, venous blood gas analysis, or invasive tissue monitoring. While these parameters are helpful in gauging response to resuscitation, they do not necessarily reflect microcirculatory perfusion. Microvascular compromise can persist despite normalization of systemic parameters. Sidestream dark field microscopy (SDF) allows for direct imaging and real-time assessment of the microcirculation. This device is a handheld instrument that directly images and records videos of the microcirculation, allowing off-line analysis of microvascular flow parameters.
Although numerous studies have been performed in search of the ideal resuscitation fluid for hemorrhagic shock, no clear benefit of any fluid has been identified. Additionally, administration of fluids following hemorrhage results in decreased plasma viscosity, which may cause microvascular vasoconstriction secondary to decreased nitric oxide production. Evidence suggests that hyperviscous fluids may provide an advantage over traditional shock therapy, by restoring blood viscosity and enhancing nitric oxide production; thereby improving microvascular perfusion.
Hemoglobin-based oxygen carrying (HBOC) fluids have been developed to promote oxygen delivery to the tissues while avoiding the potential complications of blood component transfusions. HBOCs have been associated with a number of adverse effects in humans, owing primarily to their ability to scavenge nitric oxide and promote vasoconstriction We hypothesized that resuscitation from hemorrhagic shock with a viscosity-enhanced HBOC solution would restore macrovascular parameters to baseline, as well as preserve microvascular flow (as seen with SDF) as compared to a standard HBOC solution.
The test fluid (hyperHBOC) was made by addition of 0.3% alginate to a standard HBOC solution with a resultant average viscosity of 4.93 cP. Twelve dogs were randomly assigned to either the hyperHBOC group (n=6) or the sHBOC group (n=6). The dogs were placed under general anesthesia and instrumented. They were and allowed a 30 minute equilibration period prior to induction of hemorrhage. Controlled hemorrhage was induced until a mean arterial pressure of 35-40 mmHg was achieved, then shock was maintained for 60 minutes. At the end of this period each dog received a 30 ml/kg bolus of either hyperHBOC or sHBOC for resuscitation.
Data were collected at baseline, after the end of the shock period and 30, 60, 120, and 180 minutes post-resuscitation. At each time point, macrovascular parameters as well as videos of the microcirculation of the buccal mucosa and jejunal serosa were collected and analyzed.
There were no significant differences between groups at baseline and after the shock period. Following resuscitation, both groups showed return to baseline for all variables. Dogs receiving hyperHBOC had higher oxygen extraction ratios and decreased central venous saturation compared to those receiving sHBOC.
Based on these results, there is no apparent benefit to administration of hyperHBOC over a sHBOC solution for the resuscitation of hemorrhagic shock in dogs.