Inline Shunt Flow Monitor for Hydrocephalus

In hydrocephalus, cerebrospinal fluid (CSF) builds up in the cranial cavity causing swelling of the head and potentially brain damage. A shunt to drain the fluid into a body cavity is now universally used, but failure is all too common. Techniques for ascertaining shunt failure are time-consuming, expertise-dependent, and often inconclusive. We report here an inline system that reliably and quantitatively measures the CSF flow rate. The system uses a single thermistor to both heat the surrounding and to sense the temperature. In the heating mode, the thermistor is subjected to a 5 s voltage pulse. In the sensing mode, it is part of a Wheatstone's bridge, the output being proportional to temperature. The signal, V_i - V_f, which is the net change ΔV in the bridge output immediately before and after the heat pulse, depends both on the flow rate and the surrounding temperature. In vitro, a single equation, flow rate = 3.75 × 10^-6 × ΔV^{(-9.568+1.088 V} provided good prediction for the flow rate, with 6.3% RMS relative error. The sensor behavior is reported for flow rates between 0-52.5 mL/h at 32-39 °C, adequately covering the range of interest.