This study shows that the baseline pressures (zero point) of Codman and Raumedic ICP sensors can be altered by ESD's at magnitudes that are clinically relevant. The observations indicate severe limitations with currently used ICP sensors.
ICP sensors used for clinical monitoring of ICP
The ICP sensors tested in this study are widely used ICP sensors. Both the Codman [10, 14, 16, 21, 22, 25, 27] and Raumedic [12, 19, 29, 30] sensors have undergone extensive bench and clinical testing. In general, the assessment of ICP sensors has previously focused on long-term-drift of the sensors, sensitivity to temperature changes, and inter-sensor accuracy comparisons [10, 11, 13–19, 22, 25].
All the ICP sensors measure pressure relative to atmospheric pressure, which means that they have to be zeroed before measuring ICP. Hence, their zero point equates the atmospheric pressure, and the ICP level displayed on the monitor represents the difference between pressure level within the intracranial compartment and the sensor zero point. It should be noted that in daily clinical practice, various notations are used to refer to the zero point, such as set point, reference pressure, or baseline pressure. In this paper and previous publications [30–33], we have preferred the term baseline pressure, when referring to the zero point of the ICP sensor.
Depending on clinical state, the upper normal threshold of ICP varies between 15 and 25 mmHg [1–4]. Obviously, if the baseline pressure (zero point) spontaneously shifts > 10-20 mmHg, the ICP presented to the physician becomes wrong. Since the continuous monitoring of ICP is done for surveillance of patients with brain injury, e.g. due to traumatic brain injury, stroke complications to brain surgery [1–3], false ICP values represent a likely hazard to the patient. For example, when ICP increases, efforts may be done to reduce the ICP; such efforts include medication, artificial ventilation and surgical procedures.
When the impact of ESD's on ICP sensors previously has not been considered, the reason may be that the issues of non-physiological changes in baseline pressure have not been regarded as a problem in ICP monitoring.
Electrostatic discharges in the hospital environment
There are different ways to test ESD's; it has also been addressed that there is a need for more standardized methods [40]. The rational for our experimental setup was to best possible test ESD's of clinically relevant magnitudes. Therefore, the ESD's were delivered from a test person, and caused pulse peak discharges to the sensor typically in the range 0.5 - 5 kV. Such ESD's may not be unpleasant to the test person, and are below the levels that can be seen clinically [39, 41]. ESD's of magnitudes < 2-3 kV may not even be appreciated by the personnel taking care of the patients. It was previously demonstrated that potentials > 30 kV could be induced on the bed framework when the bedding is pulled from the bed; the degree of charging being dependent on the material of hospital bedding [39]. In comparison, previous tests in our hospital showed that ESD's of 20-40 kV could be seen, depending on the textiles used in clothing (Jensen, Grimnes, unpublished data). Using the test approach described here, we avoided ESD's of magnitudes that are not clinically relevant. Only in a few instances, we managed to deliver 7 kV potential changes to the sensor (10 kV in one sensor that first responded markedly to 5 kV). Accordingly, the voltages referred to here are quite low.
Different characteristics of Codman and Raumedic ICP sensors
There were some differences between the Codman and Raumedic sensors in their responses to ESD's. The Codman sensors consistently responded to electrostatic changes of 2-3 kV, with sudden shifts in baseline pressure. Gradual drift was only seen in 2 of 25 Codman sensors (8%). These findings compare with our clinical observations of spontaneous alterations in baseline ICP despite unchanged ICP waveform. The observation that baseline pressure was changed maximally > 10 mmHg in 13 of 25 (52%) sensors (and > 20 mmHg in 3 (12%) sensors), indicate that effects of ESD's are of a magnitude that likely would affect patient management.
The Raumedic sensors responded differently depending on their design. Two types of responses were seen, namely gradual drifts and sudden shifts in baseline pressure. While the NeuroVent P-C was completely unstable to ESD's, even at levels of 0.5 kV, the NeuroVent P was less affected. The P-C type incorporates a ceramic coating on the sensor tip while the P type uses titanium. Also the NeuroDur sensor using titanium was more stable, where the tip seemed connected to the sensor with a 5 MΩ resistance. Nevertheless, the observation of alterations in baseline pressure > 10 mmHg in 10 of 32 (31%) Raumedic sensors (> 20 mmHg in 4 of 32 (12.5%) indicate that the effects of ESD's would affect patient management also when using these sensors. In a recent study comparing simultaneous ICP signals from Raumedic NeuroVent and NeuroDur sensors, we encountered average differences between sensors during over-night monitoring > 10 mmHg in 4 of 12 (33%) patients [30].
While leakage current was seen in only one Codman sensor, and no Raumedic NeuroVent P-C sensors, current leakage was seen in 2 of 3 Raumedic NeuroVent P sensors that responded to ESD's, and in all three Raumedic NeuroDur sensors responding to ESD's. The testing of leakage current indicated that in Raumedic titanium sensors (NeuroVent P and NeuroDur) there is an internal 5 MΩ resistance between the metal shell and the connector. Hence, the sensors with resistance different from 5 MΩ (Table 4) might have a broken protection resistor. We found, however, no evidence of sensor damage using microscopy, though damage to ICP sensors may happen both during the implantation and explanation.
Control of risk associated with ESD's
A major issue with both the Codman and Raumedic ICP sensors is that the health care personnel get no warning about sudden shifts in baseline pressure (zero point) of ICP sensors, or even damage to the ICP sensor during/after implantation. Thereby it is impossible for the physician or nurse to know whether changes in ICP are related to ESD's or not. The Codman sensor cannot be re-zeroed because this is done within the operating room before sensor implantation. The Raumedic sensors, on the other hand, can be re-zeroed after implantation; however, this procedure is not necessarily done by the nurse/physician when ICP is changing.
While the present study focused on effects of ESD's on ICP sensors, the baseline pressure can also be affected by user-related wrong zeroing or even damage to the sensor during implantation, which may not be recognized. Therefore, it can be questioned why modern monitoring systems include no warning. Such warning should be incorporated as part of risk control.
We suggest that a robust way of incorporating risk control is by determining the ICP from the ICP waveform itself. Thereby quality control is accomplished and the issue of baseline pressure alterations is eliminated. The first author previously described a procedure for automatic identification of the cardiac-induced waves in the ICP waveform [42]. Using this approach, the ICP parameters such as the mean ICP wave amplitude (MWA), can be determined from the cardiac induced ICP waves [42]. Since such determination of single wave pressure parameters is done within the ICP signal itself, the analysis results is not affected by changes in baseline pressure. The automatic identification of verified cardiac induced ICP waves also recognizes other ICP sensor-related issues. For example, if an ICP sensor is placed wrong by mistake, artificial waves and no cardiac induced ICP waves will be identified, providing feedback to the user that the ICP signal is erroneous.