Evaluation of shunt valve flow dynamics in series

Dysregulation of cerebrospinal fluid (CSF) volume results in hydrocephalus, a disease that leads to over 30,000 surgical shunt-valve implantations annually in the US. These shunt-valves require a trial-and-error process to determine optimal settings for each individual, sometimes resulting in implantation of multiple valves in series. This work sought to evaluate two mathematical models of the relationship between valve opening pressure settings in series and resultant drainage using a benchtop system to aid clinicians in determination of optimal shunt-valve settings.

A gravity-driven in-vitro flow system at 37 °C with a simulated ICP of 22 cmH₂O + 60 cmH₂O from valve to simulated peritoneal cavity was built. Differential pressure and gravitational valves were tested in isolation and series at various settings. The relationship between flow rate and the pressure drop across a valve is expressed with a valve coefficient. Results of isolated valve trials were used to calculated valve coefficients for each valve, which were then used to calculate combined valve coefficients to predict flowrate of valves in series. Flowrate predictions were compared to experimental results to evaluate each mathematical model presented here.

In isolation, differential pressure and gravitational valves had low intra- and inter-valve variability (p > 0.05). Valves in series had highly variable flowrates across trials and sets of valves in both supine and upright positions (p < 0.05). Using calculated combined valve coefficients to predict flowrates of valves in series, the average percent error was 15 ± 7% and 23 ± 18% in the supine and upright positions, respectively.

In all, neither of the two models outperformed the other and both were insufficient to properly characterize the relationship between drainage and opening pressures of valves in series. These results indicate low flowrate variability of isolated valves but high variability of valves placed in series. Without a consistent model from which opening pressure setting of valves in series can be determined, physicians must rely on a trial-and-error method in optimal opening pressure determination which directly impacts patient outcomes. These findings underscore the difficulties faced by physicians in determination of optimal valve settings for shunted patients.