Changes in cerebral blood flow modalities during hemorrhage in rats

The optimal life sign monitor in the practice of emergency medicine would be a technique to detect the earliest pathological change. Since the brain is the most sensitive organ in response to hypoxia/ischemia, "cardio-pulmonary resuscitation should be brain-oriented" (Peter Safar, 1998). Nevertheless, monitoring of cerebral blood flow (CBF) is not performed in the practice of emergency medicine in either civilian or military environments. CBF reactivity monitoring is an appropriate primary parameter to evaluate cerebral resuscitation due to a systemic or regional cerebral injury leading to possible irreversible brain injury. Unlike CBF monitoring this technique of CBF reactivity is being proposed as a non-invasive, mobile, and non-operator dependant means of evaluating an unconscious patient. The ideal life sign monitor would be one that reflects CBF non-invasively, continuously and globally. Because the most common cause of death following serious military and civilian injuries is hemorrhage, the goal of this study was to describe CBF changes during hemorrhage and to compare various CBF modalities in order to identify the optimal CBF vital sign monitor. A computer controlled rat hemorrhage model was introduced for resuscitation purposes. In anesthetized rats blood was removed at a rate required to achieve a mean arterial blood pressure (MABP) of 40 mmHg over 15 min. Shed blood was measured and used as an indicator of compensation/decompensation phase. Continuous recording was taken during 30 minutes of compensatory phase with intracranial rheoencephalogram – (iREG; n = 14), laser Doppler flowmetry (LDF; n = 3), and carotid flow by ultrasound (n = 11). Data were stored and processed off-line. During the initial phase of hemorrhage, when MABP was close to 40 mmHg, intracranial REG amplitude transiently increased (81 %), while LDF (78 %) and carotid flow (52 %) decreased and changed with systemic arterial pressure. Intracranial REG amplitude change suggests classical CBF autoregulation, indicating its close relationship to arteriolar changes 1. The studies indicated that 1) iREG may reflect cerebrovascular responses more accurately than changes in local CBF measured by LDF and carotid flow; CBF autoregulation was present at the beginning of SAP 40 mmHg. The physiological background or resolution of this virtual conflict is: "Flow through large arteries may be constant despite changes of regional flow in tissue" 2 REG may indicate promise as a continuous, non-invasive life sign monitoring tool with potential advantages over EKG, ultrasound, and other measurement techniques normally applied in clinical practice. Additional quantitative studies are in progress; the following studies are needed: 1) to test CBF autoregulation during the entire period of hemorrhage (with CO2 inhalation); 2) to estimate REG during graded increases in intracranial pressure; 3) to estimate REG during graded decreases in SAP; and 4) to test various data processing methods of the REG signal.