Figure 2
From: Hydrodynamics of the Certas™ programmable valve for the treatment of hydrocephalus
Typical pressure-flow curves for settings 1 – 8 in order from left to right. For each shunt setting, the valve opening pressure is determined during the rising phase of the inlet pressure. When the pressure enters the decreasing phase of the inlet pressure, the flow reduces, but with some hysteresis so that the closing pressure of the valve is lower than the opening pressure. The hysteresis was more pronounced for setting 8. The calculation for estimation of Rout and opening pressure is exemplified by the dotted line fitted against the opening phase of data from setting 3. The slope of the curve gives 1/Rout and the intersection of the line with the pressure axis is the opening pressure.