Estimation of the flow rates in a cerebrospinal fluid shunt by measurement of the radioactivity clearance from the reservoir (author's transl)

Abstract

The estimation of flow rate in cerebrospinal fluid shunt was evaluated in vitro using radionuclide and a gamma camera. Approximately 50-100 μCi of 99mTc-pertechnetate in a volume of 0.05 ml was injected into a Pudenz 12 mm or a standard Rickham reservoir through a 26-gauge needle, and a time-activity curve at the region of the reservoir was recorded for a duration of 10 minutes with a gamma camera and data processor (Picker Dyna Camera 2C). The flow rate of the perfusion fluid in the shunt system was controlled by a hydrostatic pressure with a siphon or an infusion pump. The flow rates were changed from 0.01 ml/min to 1.0 ml/min. This range covered the rate of cerebrospinal fluid production. The perfusion fluid was distilled water or diluted alubumin solution. The time-activity curves represented the radioactivity clearance from the reservoir, and they showed two exponentials (the first and the second compartments) on the experiments using an infusion pump. The flow rates (F ml/min) determined from the radioactivity clearance half-time (T1/2 min) of the first compartment were identical with those calculated from the classical clearance formula, F = 0.693 V/T1/2 where V is the physical volume (V ml) of the reservoir. On the other hand, the relationships between F and T1/2 of the second compartment were -1.3 log10(T1/2) - 0.016 ≤ log10(F) ≤ -1.3 log10(T1/2) + 0.16 in the Pudenz reservoir and -1.9 log10(T1/2) 0.12 ≤ log10(F) %le; -1.8 log10(T1/2) + 0.076 in the Rickham reservoir, which coincided with the results of the experiments using a siphon in each reservoir. The regression equation was log10(F) = -1.3 log10(T1/2) + 0.087 or log10(F) = -1.8 log10(T1/2) + 0.019 in the Pudenz reservoir or the Rickham one, respectively. The concentration of the albumin in the perfusion fluid, ranging between 25 mg/dl and 5, 000 mg/dl, showed no significant effect on the T1/2 of the second compartment. In the Rickham reservoir, however, it was difficult to record the T1/2 of the second compartment when the albumin was more than 1, 000 mg/dl, because the radioactivity in the reservoir was quickly cleared during the first compartment in these conditions. The duration of the first compartment observed on the experiment using an infusion pump was within only a few minutes at each flow rate, which is too short to record in clinical practice. Thus, the cerebrospinal fluid flow rate in the shunt system can be determined by using the above equations derived from the second compartment rather than the classical clearance formula.