The cerebral microcirculation is protected during experimental hemorrhagic shock*

OBJECTIVE: Decreases in buccal microcirculation are indicative of the severity of hemorrhage, but incidental observations suggest that this may not apply to the cerebral microcirculation. We therefore hypothesized that the cerebral microcirculation may be preserved in hemorrhagic shock in which systemic and buccal microcirculatory flow are reduced. We propose to relate changes in the macrocirculation to the buccal and cerebral microcirculations during hemorrhage and after fluid resuscitation. DESIGN: Prospective, randomized, controlled animal study. SETTING: University-affiliated research laboratory. SUBJECTS: Sprague-Dawley rats. INTERVENTIONS: Fifteen male Sprague-Dawley rats were anesthetized and endotracheally intubated. Craniotomy exposed the parietal cortex for orthogonal polarization spectral imaging. Mean arterial pressure, cardiac output, arterial blood gases, and lactate were measured concurrently with determination of microcirculatory indices in buccal and cerebral areas. Animals were randomly assigned to bleeding either 35% or 25% of estimated total blood volume and compared with sham bled animals. Hypovolemia was maintained for 60 mins in test animals, after which saline in amounts to 2 times the blood loss, was administered over 30 mins. Cerebral and buccal microvascular indices were measured in vessels smaller than 20 mum, representing capillaries. MEASUREMENTS AND MAIN RESULTS: Mean arterial pressure and cardiac output were reduced and arterial blood lactate was increased in relationship to the magnitude of blood loss. Saline infusion increased mean arterial pressure and cardiac output. Buccal microcirculation decreased after bleeding but was restored after saline infusion. However, the cerebral microcirculation was essentially unaffected by hemorrhage and saline infusion. CONCLUSION: In contrast to the systemic decreases in pressure and flow characteristics of hemorrhagic shock, including decreases in microcirculations of buccal mucosa, cerebral microvascular flow was preserved during moderate and severe blood losses.