Microvascular adjustments during irreversible hemorrhagic shock in rat skeletal muscle

A study was made of microvessel response to hemorrhagic hypotension and the subsequent restoration of blood volume. The experiments were conducted on anesthetized rats in which the cremaster muscle was exteriorized for intravital microscopy. Variables measured during hypotension (40 mm Hg for 60 min) and after blood restoration (120 min observation) included systemic blood pressure, heart rate, arteriolar and venular diameter, sensitivity to epinephrine, velocity and volumetric flow rate. These findings were correlated with 24-hr survival statistics. The response to hemorrhagic hypotension is a reflection of two separate adjustments, cardiac output and peripheral vascular behavior. In survivors, the microvascular sequelae following blood replacement was one of continuous improvement of muscle perfusion, whereas in nonsurvivors the picture was one of progressive deterioration. The consistent hallmark of irreversibility, arteriolar hyposensitivity, was associated with a continuous falling off in mean arterial pressure despite restoration of normal blood volume.