New concept for the pressure setting of a programmable pressure valve and measurement of in vivo shunt flow performed using a microflowmeter. Technical note.

The aim of this study was to establish a standard method for determining the pressure setting of the Codman Hakim valve (CHV) in patients with hydrocephalus. The authors' investigation was twofold. It focused on: 1) the relationships among CHV setting, intracranial pressure (ICP), intraabdominal pressure (IAP), hydrostatic pressure (HP), and perfusion pressure (PP); and 2) the shunt flow in 18 patients with normal-pressure hydrocephalus. With the patient in a sitting position, the pressure environment around the ventriculoperitoneal shunt stabilized when PP became equal to the CHV setting. The lower the CHV setting, the lower the ICP obtained in patients in a sitting position (ICPsit) settled. This indicated the possibility of calculating the CHV setting by the equation CHV setting = HP + ideal ICPsit - IAP, where the ideal ICPsit was estimated to be between -70 and -140 mm H2O. The CHV setting was individually determined for 18 patients by using this method. The ICPsit, was controlled at a level equal to the estimated ICPsit in most cases, which supported the rationality of our concept. Shunt flow was intermittent or very low when the patient assumed a supine position and between 200 and 600 microl/minute when the patient was seated. Determining the CHV setting by using the equation CHV setting = HP + ideal ICPsit - IAP was found to be useful when directly measuring HP and IAP in patients and estimating the ideal ICPsit to be between -70 and -140 mm H2O. Postoperative shunt control performed using this method was satisfactory, and shunt complications and the number of CHV resettings were lower than in those published in previous reports. Shunt-flow measurement performed in vivo and in real time by using a microflowmeter should be useful not only in testing the functioning of shunt systems, but also in clarifying the pathophysiology of hydrocephalus.