The importance of valve alignment in determining the pressure/flow characteristics of differential pressure shunt valves with anti-gravity devices

Object: The proper functioning of shunt valves in vivo is dependent on many factors, including the valve itself, the anti-siphon device or ASD (if inc luded), patency of inlet and outlet tubing, and location of the valve. One important, but sometimes overlooked, consideration in valve function is the valve location relative to the tip of the ventricular inlet catheter. As w ith any pressure measurement, the zero or reference position is an importan t concept. In the case of shunt valves, the position of the proximal inlet catheter tip is fixed and therefore serves as the reference point for all p ressure measurements. This study was conducted to document the importance o f this relationship for the pressure/flow characteristics of the shunt valv e. Methods: We bench-tested differential pressure valves (with integral ant i-gravity devices; AGDs) were connected to an "infinite" reservoir, and the starting head pressure for each was determined from product inserts. The i nlet catheter tip was fixed at this position, and the valve body was moved in relation to the inlet catheter tip. Outflow rates were determined gravim etrically for positions varying between 4 cm above and 8 cm below the inlet catheter tip. Conclusions: All differential pressure valves utilized in th is study that contained AGDs showed significant increases in outflow rate a s the valve body was moved incrementally below the level of the inlet cathe ter tip. To allow functioning as a zero-hydrostatic pressure differential p ressure valve, the AGD and the inlet catheter tip should be aligned at the same horizontal level.