Some physical principles of intracranial pressure measurement.

The general formula for the pressure difference across a curved flexible membrane has been applied to a number of intracranial pressure (I.C.P.) measuring systems and certain criteria deduced for the accurate measurement of I.C.P. The Numoto bag, the implanted extradural transducer and the intracerebral bag have been shown to be acceptable providing these criteria have been met. Some deductions about the magnitude sign and constancy of the zero error have been made. Some other I.C.P. measuring systems have been shown to have serious and unknown zero errors which vary from hour to hour. Because of the curved surface of the ventricle itas been shown that ventricular fluid pressure cannot equal the tissue pressure (though it may closely approximate it) and the difference between the two increases as hydrocephalus progresses.