Hemorrhagic shock in rats

Rats are commonly used as models of hemorrhagic shock. Both constant-pressure and constant-volume protocols have been used, although the latter is considered by some to be a better model of human shock. However, differences in survival have been reported when the same experiment is done by different investigators. Changes in oxygen consumption have been reported to correlate with mortality after shock, and these changes may be implicated in the variability seen in different experimental studies. Using the constant-volume model, in which a given percentage of the animal's estimated blood volume is withdrawn, we have determined oxygen consumption rates before and after hemorrhage in Sprague Dawley rats. Our results indicate that specific oxygen consumption rates decrease once rats reach a certain weight, but removing a given percentage of blood reduces the rate by a constant amount regardless of the weight of the animal. In addition, there appears to be a "critical" level of oxygen consumption needed for survival. Thus use of a constant-volume protocol could result in survival for smaller but not larger rats. It is suggested that rat weights must be the same to compare results from different investigators.