Investigation of sympathetic mechanism of inverse relationship between scalp and leg blood flow with increasing intracranial pressure in the rabbit

Intracranial pressure (ICP) is one of most important factors in determining health and function of cerebroscular system. However the methods of measuring ICP are usually invasive . In an attempt to find a noninvasive method to assess ICP, in previous study web found a reverse relationships between changes in ICP and blood flow in scalp (SBF) and leg skin (LSBF) . In this study the role of sympathetic nervous system in mediating this relationship was Investigated. Ten NZW rabbits were anaesthetized and their femoral artery and vein cannulated to record blood pressure, and inject the drugs. Another cannula was inserted into the subdural space following the skin incision and craniotomy in parietal region. Scalp and leg skin areas were shaved and the laser Doppler flowmeter probes were placed over them to record the flow, on two channels of recorder . Arterial pressure and ICP were recorded on the other two recorder channels through blood pressure transducers connected to related cannulae. After recording base line values, phentolamine ( a- blocker) was injected through venous  (1.2 mg/ kg); and 45 min later, ICP was increased in 9 steps from 1.2 +_ 0.2 mmHg to 20+_0.2 mmHg. (2 mmHg each step, 3 min apart) by augmentation of a saline reservoir connected to ICP cannula. Blood pressure (BP) . SBF and LSBF were recorded simultaneously.BP was reduced from 91.5+_3.7 to 72.5+_2.2 mmHg, SBF from 157+_28  to 37+_40 and LSBF from 169+_26 to 114+_20 arbitrary units by phentolamine. ICP increment had no effect on  SBF, but increased the LSBF to normal level ( 173+_43 arbitrary units), and increased BP to 100.6+_6.7 mmHg . The correlation coefficient between changes in ICP and SBF was zero ( compared to -0.84 in normal animals in previous study) and between ICP and LSBF was +0.19 ( compared to -0.39 in normal animals) . Overall the results showed that, sympathetic nervous system  is the mediator of changes in SBF and LSBF due to changes in ICP as reverse relationships between ICP and SBF and LSBF in normal animal was blocked and changed to direct relationship respectively in phentolamine treated animal.