Pressure measurement beneath retractors for protection of delicate tissues.

Neurosurgeons have always worried lest damage be done to delicate neurologic tissues during retraction for exposure of intracranial lesions. An apparatus capable of measuring pressure beneath a retractor has been developed in our laboratory to investigate this danger. (Figures 1 through 4.) The apparatus consists of a Numoto [I-31 pressure switch embedded in a Silastic@ envelope together with an electroencephalographic cortical surface recording electrode. The envelope fits over the end of a brain spatula and is interchangeable from spatula to spatula. The platinum electrodes from the inner leaves of the Numoto switch connect by copper wires to an onoff relay switch, which in turn causes an infusionwithdrawal pump to infuse or withdraw from a reservoir, which in turn connects to a manometer and the pressure switch. Thus, when pressure inside the switch is exceeded by outside pressure, the platinum electrodes are in contact and the relay switch “tells” the infusionwithdrawal pump to infuse. Air entering the reservoir causes displacement of the fluid within it, and the small quantity of locked air beyond the fluid column enters the switch. Pressure meanwhile is simultaneously recorded on the manometer. When pressure inside the switch exceeds pressure outside, the platinum electrodes are separated. The infusion-with-