Influence of Vasopressors on Brain Oxygenation and Microcirculation in Anesthetized Patients With Cerebral Tumors

Background:

During brain tumor surgery the main objective for the neuroanesthesiologist is to maintain a low intracranial pressure (ICP) and a sufficient cerebral perfusion pressure (CPP) (CPP = mean arterial blood pressure [MABP] - ICP) to ensure adequate cerebral oxygenation. Brain tumors are often associated with edema and increased ICP and a specific recommendation for CPP thresholds during brain tumor surgery does not exist. In daily practice vasopressors are used to increase MABP to maintain a CPP between 50-70 mmHg in accordance with current guidelines for management of patients with traumatic brain pathology.

However, recent studies show that arterial blood pressure correlates only poorly with microcirculatory flow. Elevation of blood pressure may result in reduced capillary perfusion and oxygen delivery despite reaching resuscitation end-points with CPP within 50-70 mmHg. This is supported by an experimental study demonstrating vasopressor-induced mismatch between cerebral perfusion and oxygenation, possibly due to microvascular heterogeneity, as suggested by the authors. In addition, recent studies in healthy anesthetized subjects demonstrate that, despite an increase in MABP, cerebral oxygenation simultaneously decreases after phenylephrine but remains unchanged after administration of ephedrine. Thus, increasing CPP with vasopressors may lead to paradoxical microcirculatory response with vasopressor-induced tissue hypo-perfusion and hindered tissue oxygenation.

Brain microcirculation is the primary site of oxygen exchange. The investigators have recently proposed that red blood cell capillary transit time heterogeneity (CTTH) may affect tissue oxygen tension in patients with ischemic stroke, subarachnoid hemorrhage and traumatic brain injury. According to this theory capillary compression due to edema and elevated ICP may cause redistribution of capillary flows into patterns with functional shunting of oxygenated blood through the cerebral capillary bed. This capillary dysfunction may further hinder oxygen diffusion into cerebral tissue and ultimately cause cerebral ischemic damage. The dissimilar effects of phenylephrine and ephedrine on cerebral oxygenation may be caused by a different influence on brain capillary perfusion or alternated CTTH. Currently there are no studies available on the effects of commonly used vasopressors on brain microcirculation and oxygenation in patients undergoing craniotomy for brain tumors.

Hypothesis: