Evaluating the additive properties of differential pressure valves in a novel benchtop shunt model.

OBJECTIVE

Cerebrospinal fluid (CSF) over-drainage is an unintended sequela of using a shunt valve to treat hydrocephalus, and a second valve may be added in series to better control drainage. In this study, we tested whether the opening pressures of two valves in series-Codman Hakim ball valve (HBV) and the Miethke M.blueⓇ adjustable gravitational valve (AGV)-can be treated as being additive using a novel in vitro benchtop model.

METHODS

Six dual-valve circuits were tested in triplicates for different combinations of HBV and AGV settings adding up to a summed opening pressure (OP) of either 20 or 40 cmH2O. This "theoretical" sum was based on the addition of the nominal OP settings as stated by the manufacturer. Kruskal-Wallis tests and linear mixed-effect models were employed to analyze the flow rates for HBV-AGV settings with the same theoretical OP.

RESULTS

Kruskal-Wallis analysis demonstrated that HBV-AGV settings for each circuit, at a theoretical OP of 20 and 40 cmH2O, showed statistically significant differences in flow rate. In linear mixed-effects models, as HBV OP was increased and composed a larger fraction of the theoretical OP, the flow rate decreased (effect size: -1.567 × 10- 3 mL/min/cmH2O, standard error: 1.663 × 10- 4 mL/min/cmH2O, p < 2 × 10- 16 at 20 cmH2O, and effect size: -3.735 × 10- 3 mL/min/cmH2O, standard error: 1.475 × 10- 4 mL/min/cmH2O, p < 2 × 10- 16 at 40 cmH2O, respectively).

CONCLUSION

An analysis of the flow data demonstrated statistically significant differences between combinations of HBV-AGV settings adding up to the same theoretical opening pressure. These findings suggest that in practice, the nominal OPs of Hakim and M.blueⓇ valves may not be additive. Clinicians should be aware that drainage patterns can change when adjusting OPs of HakimⓇ and M.blueⓇ valves under the assumption that HBV-AGV settings with the same summed theoretical OP behave alike.