Original ArticleMercury vs. Water: An Analysis of Multi-Institutional Survey Data Assessing Intracranial Pressure Unit of Measure Awareness
Introduction
Intracranial pressure (ICP) is a pervasive concept in neurosurgery. Along with this numerical value comes a unit of measure. Manometric measurements of pressure are commonly used in physiology and accordingly are used to report ICP. Unlike blood pressure, for which millimeters of mercury (mmHg) is universally used, both mmHg and centimeters of water (cmH2O) are used to report ICP in clinical practice and throughout the literature.
Precise quantitative assessments of ICP can be made through a variety of invasive techniques, including external ventricular drains (EVDs) and intracranial transducers. Intracranial hypertension is generally defined as sustained elevation of ICP. If left untreated, it can portend devastating clinical consequences. Thus, the accurate recording and reporting of this variable to direct precise interventions is a universally accepted tenet of managing intracranial hypertension.1 Proper management of this numerically quantifiable pathology requires the use of a common unit of measure among clinicians to communicate it consistently from initial measurement through ICP-based intervention and until the decision is made to discontinue invasive ICP monitoring.
The unit of measure for ICP transitions between mercury and water at every stage of ICP assessment and management, however. The fourth edition of the Brain Trauma Foundation guidelines recommends “treating ICP above 22 mmHg.”2 As an EVD is placed, the operator may take an opening pressure measurement by attaching a manometer or raising the level of the catheter until flow ceases. Subsequently, the height of the aqueous fluid can be measured by centimeter marks on a catheter, manometer, or ruler, and a cmH2O measurement is obtained. Most commercially available EVD bedside drainage systems have both cmH2O and mmHg unit graduations, whereas digital transducers project a numerical ICP on a bedside display monitor with an inconspicuously accompanied unit of measure. Given the foreseeable opportunity for confusion, in this study we investigated the awareness of ICP units of measure at academic institutions, with the aim of improving the communication of ICP in both clinical practice and the literature.
Section snippets
Methods
A verbal survey was conducted by a senior neurosurgery resident at each of four US academic institutions during 2018 that included all Neurosurgery Department residents and attendings. The following questions were administered in the same order, and answers could not be changed once reported:
Q1. What is your threshold for a concerning ICP?
Q2. How many minutes is that ICP sustained for you to be concerned?
Q3. What unit are you implying when you state that ICP?
Q4. What unit of measure is an ICP
Results
Sixty-seven of total 72 members (93%) of the four institutions participated in the survey, including 15 of 17 junior residents (88%), 23 of 23 senior residents (100%), and 29 of 32 attendings (91%).
Discussion
In 1999, the National Aeronautics and Space Administration (NASA) lost the $125 million Mars Climate Orbiter due to a miscommunication between Lockheed Martin, which was using the English units of measure, and NASA's Jet Propulsion Laboratory, which had used the metric system since 1990.3 During the orbital entry phase of the then 286-day mission, probe engines fired 60 km from the planet, which was approximately 100 km closer than planned and roughly 25 km past the point of no return.3 Similar
Conclusions
Variability and discrepancies in intended communication and reporting of ICP unit of measurement exist within and across academic neurosurgery departments. Clinicians should familiarize themselves with the unit of measurement used in the ICU in which they practice and the routines for setting EVD drainage bags. Institutions may consider standardizing the ICP unit of measurement used to align with evidence-based clinical guidelines. Given that guidelines communicate ICP goals in mmHg and most
References (11)
Errors based on units of measure
Lancet
(2004)- et al.
Intracranial pressure: to monitor or not to monitor? A review of our experience with severe head injury
J Neurosurg
(1982) - et al.
Guidelines for the management of severe traumatic brain injury, fourth edition
Neurosurgery
(2017) Metric mishap caused loss of NASA orbiter. CNN
(1999)- et al.
Liquid medication errors and dosing tools: a randomized controlled experiment
Pediatrics
(2016)
Cited by (1)
Invasive Pressure Monitors: Leveling the Playing Field
2023, Journal of Cardiothoracic and Vascular Anesthesia
Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.