In the current state of technology, the ventricular catheter connected to an external strain gauge is the most accurate, low-cost, and reliable method of monitoring intracranial pressure (ICP). It also can be recalibrated in situ. ICP transduction via fiberoptic or micro strain gauge devices placed in ventricular catheters provide similar benefits, but at a higher cost.

In patients for whom ICP monitoring is indicated, a decision must be made about what type of monitoring device to use. The optimal ICP monitoring device is one that is accurate, reliable, cost effective, and causes minimal patient morbidity.

The Association for the Advancement of Medical Instrumentation (AAMI) has developed the American National Standard for Intracranial Pressure Monitoring Devices in association with a Neurosurgery committee. The purpose of this standard is to provide labeling, safety, and performance requirements, and to test methods that will help assure a reasonable level of safety and effectiveness of devices intended for use in the measurement of ICP. According to the AAMI standard, an ICP device should have the following specifications:

• Pressure range 0-100 mm Hg.
• Accuracy ± 2 mm Hg in range of 0-20 mm Hg.
• Maximum error 10% in range of 20-100 mm Hg.

Current ICP monitors allow pressure transduction by external strain, catheter tip strain gauge, or catheter tip fiberoptic technology. External strain gauge transducers are coupled to the patient's intracranial space via fluidfilled lines whereas catheter tip transducer technologies are placed intracranially. There is evidence that external strain gauge transducers are accurate.1 They can be recalibrated, but obstruction of the fluid couple can cause inaccuracy. In addition, the external transducer must be consistently maintained at a fixed reference point relative to the patient's head to avoid measurement error.

Micro strain gauge or fiberoptic devices are calibrated prior to intracranial insertion and cannot be recalibrated once inserted, without an associated ventricular catheter. Consequently, if the device measurement drifts and is not recalibrated, there is potential for an inaccurate measurement.

For this update, Medline was searched from 1996 through April of 2006 (see Appendix B for search strategy), and results were supplemented with literature recommended by peers or identified from reference lists. Of 39 potentially relevant studies, 7 were added to the existing tables and used as evidence for this question (see Evidence Tables I and II).

The scientific discussion of ICP monitoring technology is divided into the following sections:

As specified in the Methods section of this document, the strongest evidence for the accuracy and reliability of ICP monitors would be derived from well designed studies that compare simultaneous readings from the monitor being tested to those of an established reference standard and that, among other things, would include large samples of broad-spectrum patients. The ventricular fluid coupled ICP monitor is the established reference standard for measuring ICP. Fourteen publications were identified that simultaneously compared the ventricular monitor to other monitors in a total of 273 patients with TBI (see Evidence Table I). Location of pressure transduction devices varied across studies. Sample sizes for the individual studies ranged from five to 51 patients. Due to changes in technology, only more current publications were considered relevant.

Four studies compared readings from the reference monitor to those of parenchymal strain gauge catheter tip pressure transducer device. Of those, two were published since 1995 one of which indicated that readings from the parenchymal strain gauge device varied within 2 mm Hg from those of the reference standard.

In four studies that compared readings from the reference monitor to those of parenchymal fiberoptic catheter tip pressure transduction devices only one was published since 1995, and reported a strong correlation between initial parenchymal and ventricular measurement.

Precision of parenchymal ICP monitors has also been assessed by comparing the measurement value at the time of ICP monitor removal with zero atmosphere (degree of difference = drift). Data from eight studies published since 1995 are presented in Evidence Table II. Of these, two publications report accuracy for the micro strain gauge transducer and six for the fiberoptic. However, the literature on fiberoptic transducers is outdated, as there were significant improvements for the fiberoptic transducer in the manufacturing and testing processes in 1999 (manufacturer correspondence), and studies were conducted with data collection from populations treated before the improvements were made. In 153 separate parenchymal ICP probe measurements there were less than 1% of readings above or below 5 mm Hg, when compared to zero atmosphere, at the time of the ICP device removal.

A pressure transduction device for ICP monitoring can be placed in the epidural, subdural, subarachnoid, parenchymal, or ventricular location. Historically, ventricular ICP is used as the reference standard in comparing the accuracy of ICP monitors in other intracranial compartments. The potential risks of catheter misplacement, infection, hemorrhage and obstruction have led to alternative intracranial sites for ICP monitoring.

The following statements regarding ICP monitor location are derived from the primarily Class III evidence included in this review:

• Ventricular pressure measurement is the reference standard for ICP monitoring.
• ICP measurement by parenchymal micro strain gauge pressure transduction is similar to ventricular ICP. Some investigators have found that subdural and parenchymal fiberoptic catheter tip pressure monitoring did not always correlate well with ventricular ICP (note that currently available fiberoptic transducers have not been the subject of a clinical publication)
• Fluid coupled epidural devices or subarachnoid bolts and pneumatic epidural devices are less accurate than ventricular ICP monitors. Significant differences in readings have been demonstrated between ICP devices placed in the parenchyma versus the subdural space.

ICP monitoring complications include infection (see Infection Prophylaxis topic), hemorrhage, malfunction, obstruction, or malposition. While the current literature suggests these complications generally do not produce long term morbidity in patients, they can cause inaccurate ICP readings, and they can increase costs by requiring replacement of the monitor.

i. Hemorrhage. Hemorrhage associated with an ICP device is not defined in the majority of reports reviewed in terms of volume of hematoma on head CT, or in terms of morbidity. There were eight publications on ventriculostomy associated hematomas reporting an average incidence of 1.1% versus an article on subarachnoid bolts (no hematomas), subdural catheters (no hematomas), and micro strain gauge devices (three hematomas in 28 patients, 11%). There have been no publications on the complication rate of an improved fiberoptic transducer in populations studied since 1999. Significant hematomas receiving surgical evacuation occurred in 0.5% of patients in published reports with more than 200 patients receiving ICP monitoring.

ii. Malfunction. Malfunction or obstruction in fluid coupled ventricular catheters, subarachnoid bolts, or subdural catheters has been reported as 6.3%, 16%, and 10.5% respectively. In reports of ventricular catheter malposition, 3% of patients needed operative revision. There have been no publications on the complication rate of an improved fiberoptic transducer in populations studied since 1999. Malfunctions of micro strain gauge devices are reported as 0%.

As delineated above, each type of pressure transduction system and intracranial location of the monitor has a profile of potential complications. Calibration, monitoring for infection, and checking fluid coupled devices for obstruction are necessary tasks in maintaining an optimal ICP monitoring system. Table 2 below summarizes each type of ICP monitor by the parameters discussed above.

Estimated costs of the various ICP devices are presented in Tables 1 and 2. The non-disposable hardware that need to be purchased with fiberoptic and strain gauge catheter tip ICP devices range in cost from $6,000 to $10,000 per bed. ICP transduction with an external strain gauge costs $208 versus an average of $545 for micro strain gauge or fiberoptic transducers.

In patients who receive ICP monitoring, a ventricular catheter connected to an external strain gauge transducer is the most accurate and cost effective method of monitoring ICP. Clinically significant infections or hemorrhage associated with ICP devices causing patient morbidity are rare and should not deter the decision to monitor ICP.

Parenchymal transducer devices measure ICP similar to ventricular ICP pressure but have the potential for measurement differences due to the inability to recalibrate. These devices are advantageous when ventricular ICP is not obtained or if there is obstruction in the fluid couple. Subarachnoid or subdural fluid coupled devices and epidural ICP devices are currently less accurate.

• The specifications standard for ICP monitoring should include in vivo clinical ICP drift measurement. In vitro testing of devices does not necessarily reflect clinical performance. Specifications for ICP devices should be reviewed in the context of what data is useful in the management of patients that receive ICP monitoring.
• It is unclear if a difference in pressure between ventricular and parenchymal ICP is normal. Studies measuring ventricular and parenchymal ICP simultaneously report both positive and negative differences. However, these studies are difficult to interpret if the ICP device was inaccurate. A study of parenchymal and ventricular ICP measurements using an accurate transducer device is needed.
• Research is needed to answer the question, does parenchymal monitoring in or near a contusion site provide ICP data that improves ICP management, and subsequent outcome, compared to other sites of ICP monitoring?
• Further improvement in ICP monitoring technology should focus on developing multiparametric ICP devices that can provide simultaneous measurement of ventricular CSF drainage, parenchymal ICP, and other advanced monitoring parameters. This would allow in situ recalibration and give accurate ICP measurements in case of transient fluid obstruction.