Background: Traditional models of shock classify severity based on the volume of hemorrhage. Clinically, hemorrhage occurs at a variable rate, usually slowing as blood pressure drops; however most animal experimental models use a constant rate of hemorrhage. Our hypothesis was that rapid bleeding followed by slower bleeding using a fixed total volume would result in a greater physiologic insult.
Materials and methods: Yorkshire pigs (S and S Farms, Ranchita, CA) underwent placement of jugular and femoral catheters after anesthesia. All animals were hemorrhaged a total of 30 mL/kg. The animals were divided into constant rate hemorrhage over 10 min (Constant-10) (3 mL/kg/min), constant rate hemorrhage over 20 min (Constant-20) (1.5 mL/kg/min), or a varying rate of hemorrhage of 2.15 mL/kg/min over 7 min, and then 1.15 mL/kg/min over the remaining 13 min (Physiologic-20). Shock, mean arterial pressure (MAP) < or = 20 mmHg, was maintained for 60 min. Resuscitation was performed with Ringer's lactate (RL) and shed blood (2:1 ratio), until shed blood was exhausted and then only RL to maintain a MAP > or =60 mmHg for 3 h.
Results: Physiologic-20 shock resulted in significantly increased maximal heart rate, peak serum lactate, and volume of required RL resuscitation. Adequacy of resuscitation was ensured by MAP, urine output, and clearance of serum lactate.
Conclusions: A more physiologic method of fixed volume hemorrhagic shock results in a significantly increased physiologic response as demonstrated by increased volume of fluid resuscitation. This differential physiologic response may represent an improved hemorrhagic shock model, and could have implications for future hemorrhagic shock studies.