Methods of experimental and clinical assessment of the relative measurement accuracy of an intracranial pressure transducer. Technical note.

The assessment of the actual measurement accuracy of an intracranial pressure (ICP) transducer is imperative but still very difficult in practice. The authors tested the Codman MicroSensor ICP transducer experimentally. Additionally, a bedside test for assessment of measurement accuracy was used before and after clinical monitoring. For laboratory testing, seven new transducers were examined for measurement accuracy at increasing pressures ranging from 0 to 75 mm Hg. Drift was evaluated for 10 days at six different pressure levels ranging from 0 to 50 mm Hg. Temperature drift was assessed over a temperature range from 20 to 45 degrees C. The percentage absolute difference was used in the assessment of measurement accuracy. For clinical testing, measurement accuracy was assessed intraoperatively just before the insertion of the transducer and in an open water bath after its explantation, at 10 cm H2O both times. The maximum percentage absolute difference was 9% at a pressure of 10 mm Hg, and declined toward 2.3% at a pressure of 75 mm Hg. The maximum drift over 10 days was -4 mm Hg. Within the range of 30 to 40 degrees C, temperature drift was negligible. Over a period of 2 years, between June 1997 and June 1999, 40 ICP transducers were implanted in 35 patients by one surgeon. Of these devices, a malfunction was detected in two of them by testing them in a water bath before insertion. Experimental and clinical results indicate that this miniature strain-gauge transducer measures accurately; however, control readings for the probe by means of measurement in an open water bath just before insertion are strongly recommended.

ICP measurement control: laboratory test of 7 types of intracranial pressure transducers.

Intracranial pressure (ICP) monitoring has become an important parameter in the assessment of comatose patients, with raised intracranial pressure. The transducers in use have to fulfill the criteria of measurement accuracy, practicability and cost-effectiveness. However, these requirements are not always met in clinical practice. The need for ongoing quality control through independent laboratories remains. We have developed a laboratory set-up for the evaluation of intracranial pressure probes. Seven different types of currently used transducers have been tested for measurement accuracy. Under in vitro conditions 3 parameters were assessed: measurement accuracy, a 24 h drift and 10 day drifts. Tests for measurement accuracy were performed at increasing pressure levels of up to 80 mmHg. They were repeated 10 times per probe. This test allowed the simultaneous assessment of 5 different ICP probes. Drift was evaluated for 24 h and 10 days, at 6 pressure levels between 0 and 50 mmHg. Seven different types of ICP probes were tested (HanniSet, Camino, Codman, Spiegelberg, Medex, Epidyn and Gaeltec). Measurement accuracy was best with HanniSet probes. The maximum errors with this transducer were 3 mmHg. Camino and Codman showed similar results. Spiegelberg had slightly larger deviations. With Epidyn and Gaeltec the highest error were noted, up to 10 mmHg in the high pressure range. The 24 h drift was lowest with HanniSet (0.2 mmHg) and Camino (0.8 mmHg). The largest drifts were seen with Medex, Spiegelberg and Gaeltec (1.8 mmHg). Ten day drift was lowest with HanniSet (0.1 mmHg/day) and Codman (0.2 mmHg/day). The highest long-term drifts were found with Epidyn and Gaeltec (1.5 mmHg/day). Drift did not exhibit a linear pattern. After an initial rise in drift during the first 24-72 h, it decreased slowly during the next 7 days. Most ICP probes revealed measurement inaccuracy and drift. These results emphasize the necessity for ongoing evaluations of ICP probes. Therefore, tests for quality assurance are essential to establish a consistent standard of proficiency of ICP transducers.

ICP measurement accuracy: the effect of temperature drift. Design of a laboratory test for assessment of ICP transducers.

Intracranial pressure (ICP) monitoring has become the mainstay of multimodal neuromonitoring of comatous patients after head injury. In the presence of rising ICP and faced with pressures, difficult to control, aggressive measures, such as hypothermia may be used. The ICP readings should not be influenced by temperature changes. A laboratory test was designed to simulate temperature variations between 20 degrees C and 45 degrees C at different pressure levels under physiological conditions. Five types of transducers were examined: Epidyn Braun Melsungen, ICT/B-Titan Gaeltec, Camino-OLM-110-4B, Codman MicroSensor ICP-Transducer, Neurovent ICP transducer Rehau Ag+Co. Tests were performed at 6 different pressure levels between 0 mmHg and 50 mmHg. The results show very low drifts of less than 0.15 mmHg degree C-1 for Codman, Epidyn and Neurovent. Gaeltec and Camino exhibited higher drifts of 0.18 mmHg and 0.2 mmHg degree C-1 respectively. Within the temperature range from 35 degrees C to 42 degrees C all probes tested show insignificant temperature drift. Whether these results also apply to other types of transducers needs further evaluation. Problems and requirements related to the design of a laboratory test for the in vitro assessment of ICP transducers are discussed in detail.

ICP monitoring with a re-usable transducer: experimental and clinical evaluation of the Gaeltec ICT/b pressure probe.

Intracranial pressure monitoring requires reliable transducers at a justifiable price. At present, transducers for single or repeated use are available. We examined the Gaeltec model ITC/b solid state miniature transducer experimentally and clinically. Measurement accuracy was assessed in vitro at increasing steps of 5 mmHg from 0 to 80 mmHg. While new and recently serviced probes revealed minimal deviations from the preset values, frequently used transducers differed up to 7 mmHg. This occurred especially in the high pressure range above 50 mmHg. Additionally the drift was investigated at different pressure levels. After 24 hours we already found drifts of 2 mmHg with new and serviced probes and up to 4 mmHg with used ones. In clinical practice we implanted 150 transducers in 121 patients from 1983 until 1995. The probes were re-used up to twelve times, the average time being 7 times. 32.7% of all measurements were regarded as not reliable. Dislocation (16.7%), inability to calibrate (3.3%) and defect pressure probes (3.3%) were the most common complications. Repeated use of the Gaeltec ICT/b probe also seemed to result in an additional decay of measurement quality. The strain of frequent cleaning and sterilizing may have caused changes of the physical properties of the probes with time. Whether these results also apply to other types of ICP probes for repeated use needs further evaluation.