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

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.

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.

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).

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.