Hydrocephalus shunts and waves of intracranial pressure.

The majority of contemporary hydrocephalus valves are designed to introduce a low resistance to flow into the cerebrospinal fluid (CSF) drainage pathway, and an therefore intended to stabilise intracranial pressure (ICP) at a level close to the shunt's operating pressure. However, this goal cannot always be attained. Accelerated CSF drainage with vertical body posture in ventriculo-peritoneal shunts is one reason for the ICP decreasing below the shunt's operating pressure. Another possible factor has been studied: the impact of the pulsating pattern in the ICP on the operating pressure. Six popular constructions of medium-pressure valves were studied (Radionics Low-profile, Delta, Hakim Precision, Holter, Integra In-line and Hakim NMT). Valves were mounted in the testing rig in the UK. Shunt Evaluation Laboratory and perfused with de-ionised water at a rate of 0.3 ml min(-1), and proximal pulsating pressures of different amplitudes (from 2 to 30mmHg peak-to-peak) and frequencies (70-10 cycles min(-1)) were superimposed. Laboratory findings were compared with clinical material containing recordings of ICP made in patients to diagnose reasons for ventriculomegaly. The mean operating pressure decreased in all valves when the simulated amplitude of heart pulsations increased. The rate of this decrease was dependent on the type of valve (variable from 2.5 to 5 mm Hg per increase in peak-to-peak amplitude by 10 mm Hg). The decrease was not related to the frequency of the wave. The relationship between pulse amplitude and ICP in 35 patients with blocked shunts was strong (R = 0.48; p < 0.03; slope 0.14) and in 25 patients with properly functioning shunts was non-significant (R = 0.057; p = 0.765). Two examples of decrease in mean ICP in the presence of increased vasogenic ICP waves in shunted patients are presented. The shunt operating pressure, which 'sets' the ICP in shunted patients may be influenced by the dynamics of a patient's ICP waveform.