Pulse Pressure Waveform in Hydrocephalus: What It is and What It Isn't

Marek Czosnyka, Ph.D.; Zofia Czosnyka, Ph.D.; Nicole Keong, M.B., B.S., M.R.C.S.; Andreas Lavinio, M.D.; Piotr Smielewski, Ph.D.; Shahan Momjian, M.D.; Eric A. Schmidt, Ph.D.; Gianpaolo Petrella, M.D.; Brian Owler, Ph.D.; John D. Pickard, F.MED.SCI.

Disclosures

Neurosurg Focus. 2007;22(4):E1 

In This Article

Material and Methods

Approximately 2100 clinical infusion studies were performed in 980 patients suffering from hydrocephalus of various types and origins: idiopathic NPH 47%, post–subarachnoid hemorrhage NPH 12%, noncommunicating hydrocephalus 22%, and other 19%. The mean age among patients was 65 years (range 24 –94 years), and the male/ female ratio was 2:1. All of the patients had attended the hydrocephalus clinic on the referral of the treating neurosurgeon, with evidence of ventricular dilation on brain imaging (computed tomography or MR imaging) and clinical symptoms belonging to the Hakim triad. Given the nature of the patient selection, many had complex clinical problems. The group was evaluated using a constant-rate infusion study (lumbar 20%, preimplanted Ommaya reservoir 38%, shunt prechamber [G1] 40%, or open external ventricular drainage 2%) and/or overnight ICP monitoring in addition to the normal clinical and imaging assessments. Forty-four percent of the tests were performed in patients with the shunt in situ, to check its performance.

The infusion studies were performed via two lumbar needles, a shunt prechamber proximal to the valve, or a preimplanted ventricular access device. If lumbar access was required, lumbar needles (usually 21-gauge Whitacre) were used. For intraventricular access (reservoir or shunt prechamber), two needles (25-gauge butterfly) were inserted. One needle was connected via a stiff saline-filled tube to a pressure transducer and the other to an infusion pump mounted on a purpose-built trolley containing a pressure amplifier (Simonsen & Weel) and an IBM-compatible personal computer running ICM+ software (www.neurosurg.cam.ac.uk/icmplus). A strict aseptic technique was used to keep all the prefilled tubing and the transducer sterile. The skin was very carefully prepared with antiseptic solution.

After 10 minutes of baseline measurements, the infusion of normal saline or Hartmann solution was started at a rate of 1.5 ml/minute (or 1 ml/minute if the baseline pressure was higher than 15 mm Hg) and continued until a steady state ICP plateau was reached. If the mean ICP increased to more than 40 mm Hg, the infusion was stopped immediately. After ceasing the saline infusion, ICP was recorded until it returned to the previously recorded baseline level. All compensatory parameters were calculated using computer-supported methods based on physiological models of CSF circulation.[7] Baseline ICP and CSF Rout (the latter calculated as the plateau ICP reached during the test minus the baseline ICP, divided by the infusion rate) characterize static conditions of CSF circulation. The elastance coefficient[1] characterizes the ability of the system to store an extra volume of fluid—a greater coefficient indicates that a smaller volume can be stored under the same incremental pressure conditions. During the infusion study, the ICP waveform was processed through a Fourier transform analysis[5,6] to determine the pulse amplitude of ICP (AMP) as the magnitude (peak to peak) of the first harmonic component related to the heart rate. This method is an alternative to time-domain analysis,[1,11] and in our experience, both methods (Fig. 1) are generally equivalent. During the study, ICP pulse amplitude or peak-to-peak pulse pressure amplitude (PPamp) increases with mean ICP.[1] The rate of increasing amplitude per rise in ICP is called AMP/P slope, where P represents pressure.[1,6]

Left: Tracings demonstrating the principles used in calculating the pulse amplitude from time-domain (PPamp) and spectral-domain (AMP) analysis. Right: Graph demonstrating the results of comparing both methods in nearly 80 patients (ICP recorded after head injury) and showing an excellent linear relationship between amplitudes detected with both methods. b/m = beats per minute.

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