Continuous monitoring of cerebral perfusion by thermal clearance

Currently, no commercially available system exists to continuously monitor the effective tissue perfusion within the parenchyma of the brain. While several methods exist for accurately measuring cerebral perfusion; among them: ¹³³xeonon clearance, hydrogen clearance and radiolabeled microsphere injection; none of these methods provides continuous monitoring. The Cook Incorporated VH8500 Volumetric Hyperthermia Treatment System (Bloomington, IN, USA) was initially developed to treat brain tumours by maintaining constant, moderate hyperthermia within a defined tissue volume over an extended duration. The system continuously adjusts the power applied to heating elements in order to maintain a constant temperature within the treatment volume. Because tissue perfusion is a primary factor responsible for removing heat from tissue, monitoring the amount of power applied to the heating elements allows one to continuously estimate tissue perfusion in the vicinity of the heating elements. In the current study, regional blood flow in the vicinity of heater/sensor catheters implanted in the brain parenchyma of three dogs was estimated by the VH8500 tissue perfusion algorithm and directly measured with radioactive labeled microspheres. The accuracy of the perfusion estimate (Thermal Perfusion Index) was evaluated by comparing these values. A range of blood flow was achieved in each animal by infusing nitroprusside. It was found that with the perfusion estimation algorithm of the Cook Incorporated VH8500 Volumetric Hyperthermia Treatment System as it is currently implemented, the Thermal Perfusion Index tended to underestimate regional perfusion as measured with radioactive microspheres, but the relationship was nearly linear. Thus, the system currently tracks changes in regional blood flow. Such continuous measurement of parenchymal brain tissue perfusion would have both clinical and research applications for a variety of head trauma recovery and neurosurgical monitoring situations. Especially if research demonstrates that changes in regional brain perfusion predict detrimental changes in neurologic function before they become irreversible.