Ventriculoperitoneal Shunt Drainage Increases With Gravity and Cerebrospinal Fluid Pressure Pulsations: Benchtop Model

Abstract

Background: There have been few improvements in cerebrospinal fluid (CSF) shunt technology since John Holter introduced the silicon valve, with overdrainage remaining a major source of complications.

Objective: To better understand why valves are afflicted by supra-normal CSF flow rates. We present in Vitro benchtop analyses of flow through a differential pressure valve under simulated physiological conditions.

Methods: The pseudo-ventricle benchtop valve testing platform that comprises a rigid pseudo-ventricle, compliance chamber, pulsation generator, and pressure sensors was used to measure flow rates through a differential pressure shunt valve under the following simulated physiological conditions: orientation (horizontal/vertical), compliance (low/medium/high), and pulsation generator force (low/medium/high).

Results: Our data show that pulse pressures are faithfully transmitted from the ventricle to the valve, that lower compliance and higher pulse generator forces lead to higher pulse pressures in the pseudo-ventricle, and that both gravity and higher pulse pressure lead to higher flow rates. The presence of a valve mitigates but does not eliminate these higher flow rates.

Conclusion: Shunt valves are prone to gravity-dependent overdrainage, which has motivated the development of gravitational valves and antisiphon devices. This study shows that overdrainage is not limited to the vertical position but that pulse pressures that simulate rhythmic (eg, cardiac) and provoked (eg, Valsalva) physiological CSF pulsations increase outflow in both the horizontal and vertical positions and are dependent on compliance. A deeper understanding of the physiological parameters that affect intracranial pressure and flow through shunt systems is prerequisite to the development of novel valves.

Keywords: CSF overdrainage; CSF pulsations; Cerebrospinal fluid (CSF) shunts; Hydrocephalus; Shunt malfunction; Shunt overdrainage; Shunt valve testing.

FIGURE 1.

PV fluidic system. Fluid system schematic showing a reservoir providing constant pressure head (h_in), flow through a needle valve, PV, shunt valve, compliance chamber, pressure pulse generator, pressure transducers (1, 2, and 3), and scale. Control electronics, data acquisition hardware, and priming and bleeding ports are not shown.

FIGURE 2.

Ventricle-to-valve transmission coefficient. Sample runs at the lowest compliance (compliance chamber 0 ml) and highest pulse generator force (3V) showing ventricular (P1), pre-valve (P2), and post-valve (P3) pressures vs time; note the overlap between the P1 and P2 curves, which indicates a high transmission coefficient (0.98 +/− 0.06). Because the tip of the distal catheter is positioned 40 cm below the PV to simulate the vertical posture, P3 is lower in the vertical compared to the horizontal orientation by approximately 40 cmH₂0 secondary to gravity-driven siphoning. A, Horizontal orientation. B, Vertical orientation.

FIGURE 3.

A and B, Pulse pressure vs pulse generator force (voltage). Values in square brackets are linear regression estimates of intercept and slope from 3 observations made per combination of compliance, orientation, and valve presence levels. Increases in voltage are associated with increases in pulse pressure within each grouping, with the highest rate of change in pressure per volt occurring at the lowest compliance volume (0 ml). C and D, Outflow vs pulse pressure. Values in square brackets are linear regression estimates of intercept and slope from 3 observations made per combination of compliance, orientation, and valve presence levels. Increases in pulse pressure are associated with increases in outflow within each grouping; horizontal orientation and valve were associated with lower outflow overall.