Dynamic changes of cerebrospinal fluid in upright and recumbent shunted experimental animals.

The effect of positional change on intracranial CSF pressure, valve flow rates and atrial pressures were studied in 40 experimental dogs. The flow rate with a 30 to 60 mm H2O Holter valve changed from the recumbent to the head-up position: during the first minute it increased 2 1/2 times and then progressively decreased to become stabilized 20-30 min later at a value slightly lower than the head-up value. At the same time pressure dropped rapidly to -23 mm H2O and then slowly, during 30 min to -120 mm H2O. In dogs without a shunt the pressure drop was initially similar to that of shunted dogs but recovered to almost the initial positive pressure in 30 min. The flow rate from a reservoir to the atrium through a shunt system when changing to the head-up position showed a siphon effect. Atrial pressure does not change with positioning. The findings were similar with cisternal-peritoneal shunts. Flow rate, therefore, increase in the head-up position. Negative intracranial CSF pressure was due to a siphon effect through the shunt system.