Discrepancy Between Internal and External Intracranial Pressure Transducers: Quantification of an Old Source of Error in EVDs?

Background: Intracranial pressure monitoring remains the foundation for prevention of secondary injury after traumatic brain injury and is most commonly performed using an external ventricular drain or intraparenchymal pressure monitor. The Integra Flex ventricular catheter combines an external ventricular catheter with a pressure transducer embedded in the tip of the catheter to allow continuous pressure readings while simultaneously draining cerebrospinal fluid. Discrepancies between measurements from the continuously reported internal pressure transducer and intermittently assessed and externally transduced ventricular drain prompted an analysis and characterization of pressures transduced from the same ventricular source. Methods: More than 500 hours of high-resolution (125 Hz) continuous recordings were manually reviewed to identify 73 hours of simultaneous measurements (clamped external ventricular drain) from internal and external transducers in patients with traumatic brain injury. Results: A significant positive bias was found in pressure readings obtained from external relative to internal measurements. The 2 methods of measurement generally correlated poorly with each other and variably. Although proportional bias was found with Bland-Altman analysis, coherence revealed rare shifts in the external transducer as a major source of discrepancy. Infrequent changes in the 0-level of the external transducer were found to be the primary source of discrepancy. Relative to the observed differences, no significant trend was observed over time between the 2 modalities. Conclusions: This study suggests that the internal pressure transducer may be a more reliable estimate of intracranial pressure relative to bedside external transducers due to the inherent behavioral requirement of leveling.