Accuracy of Intracranial Pressure Monitoring-Single Centre Observational Study and Literature Review

Abstract

Intracranial hypertension and adequacy of brain blood flow are primary concerns following traumatic brain injury. Intracranial pressure (ICP) monitoring is a critical diagnostic tool in neurocritical care. However, all ICP sensors, irrespective of design, are subject to systematic and random measurement inaccuracies that can affect patient care if overlooked or disregarded. The wide choice of sensors available to surgeons raises questions about performance and suitability for treatment. This observational study offers a critical review of the clinical and experimental assessment of ICP sensor accuracy and comments on the relationship between actual clinical performance, bench testing, and manufacturer specifications. Critically, on this basis, the study offers guidelines for the selection of ICP monitoring technologies, an important clinical decision. To complement this, a literature review on important ICP monitoring considerations was included. This study utilises illustrative clinical and laboratory material from 1200 TBI patients (collected from 1992 to 2019) to present several important points regarding the accuracy of in vivo implementation of contemporary ICP transducers. In addition, a thorough literature search was performed, with sources dating from 1960 to 2021. Sources considered to be relevant matched the keywords: "intraparenchymal ICP sensors", "fiberoptic ICP sensors", "piezoelectric strain gauge sensors", "external ventricular drains", "CSF reference pressure", "ICP zero drift", and "ICP measurement accuracy". Based on single centre observations and the 76 sources reviewed in this paper, this material reports an overall anticipated measurement accuracy for intraparenchymal transducers of around ± 6.0 mm Hg with an average zero drift of <2.0 mm Hg. Precise ICP monitoring is a key tenet of neurocritical care, and accounting for zero drift is vital. Intraparenchymal piezoelectric strain gauge sensors are commonly implanted to monitor ICP. Laboratory bench testing results can differ from in vivo observations, revealing the shortcomings of current ICP sensors.

Keywords: ICP sensor; brain injury; head trauma; intracranial pressure; intraparenchymal sensor; zero drift.

Figure 1

General shape of the pressure-volume curve (upper panel) and related brain compliance (change in intracranial volume over change in intracranial pressure, lower panel). There are three distinct zones. ICP first increases linearly with extra volume (zone of good compensatory reserve). Upon further volume load, the curve becomes exponential, indicating poor compensatory reserve. Past this zone, with further volume load, ICP is critically high, leading to arterial bed compression, decreased blood flow, and a high threat of brain ischaemia. This graph is a compilation of many previous works, starting from Lofgren and Zwetnow through Marmarou et al., and many more contemporary authors [21].

Figure 2

An ICP recording in one patient after TBI. ICP was recorded using two intraparenchymal microsensors (ICP- left hemisphere, ICP2- right hemisphere). In the upper panel, the two pressures are very well correlated in time, even though around 6 mm Hg of constant difference between the two readings is observed. In the lower panel, in contrast, the difference is seen to have increased to 20 mm Hg three days later. This patient suffered from diffuse brain injury without midline shift. The reason for the difference in readings was unknown. The true value of the ICP cannot be determined from these sensors.

Figure 3

The ‘dead brain’ in a jar (pressurised externally). The microsensor in the brain tissue(cam1)shows a pressure measured at nearly 20 mm Hg lower than that of the water (cam2).

Figure 4

A recording of arterial blood pressure (ABP) and intraparenchymal pressure (IPP- bottom panel, grey line) together with EVD pressure (ICP- bottom panel, black line) using an external transducer in a patient after a poor-grade subarachnoid haemorrhage. The (left panel) demonstrates the results with the drain opened, whereas the right panel demonstrates the results with the drain closed. With an open EVD, the two pressure readings failed to correlate. EVD pressure is held constant at a value representing the calibrated level of the drain above the heart. With a closed EVD (right panel), the two measured pressure values correlate over time.

Figure 5

Bench Test Procedure: A bottle is filled with deionised water, leaving 20 mL of air to be removed during dynamic catheter testing. The bottle is then submerged horizontally in a water bath at a constant temperature of 35 °C. Static pressure on the bottle (representing pressure detected by ICP catheters) and reference static pressure (representing true ventricular pressure) are compared by changing the height of a water column in a 1.5 m graded vertical tube. Static pressure is released at intervals by allowing the water to flow out of an opened stopcock; conversely, pressure is increased by infusing fluid into the tubing [34,41].