[A device to measure CSF flow in a shunt tube and its clinical application]

An implantable device for measurement of cerebrospinal fluid (CSF) flow in a shunt tube has been developed. The unit is energized by an extracorporeal high frequency generator (200 kHz), and electrolysis creates bubbles in the shunt tube. Bubble flow velocity is detected as reflected sound using a pair of ultrasonic Doppler probes (Saneisokkuki Doppler Flowmeter Type 1935) placed apart on the skin surface and in parallel with the tube. CSF flow is expressed in ml/min. by calculating velocity and tube diameter. The unit consists of a coil with a 200 kHz capacitor, a silicon diode to rectify the high frequency, and a Zener diode to regulate maximum output voltage of 20 V. The output is fed to a pair of platinum electrodes inside the unit's tunnel through which the CSF flows. The unit is moulded in epoxy resin and coated with medical grade silicon rubber. In vitro, CSF flow rates ranging from 0.033 ml/min to 1.0 ml/min. could be measured by this flowmeter model. In vivo, however, it was difficult to detect a flow rate of less than 0.006 ml/min. To measure the slower flow rate, a so-called bubble-detecting-tube made from an 11 cm stainless steel tube coated with silicon rubber is centrally inserted between the two ends of the separated shunt tube. The bubble flow velocity is detected by a tissue impedance detector's pair of probes placed apart on the skin surface. Clinically, CSF flow was measured in three cases of hydrocephalus (two cases of normal pressure hydrocephalus and one case of pineal tumor with non-cummunicating hydrocephalus). The flow rates were found to be, respectively, 0.10 ml/min., 0.063 ml/min., and 0.20 ml/min. The merits of the unit include its ability to repeatedly measure CSF flow at short intervals, and also to measure dynamic CSF flow under various conditions.