Elsevier

World Neurosurgery

Volume 129, September 2019, Pages e657-e662
World Neurosurgery

Original Article
Mercury vs. Water: An Analysis of Multi-Institutional Survey Data Assessing Intracranial Pressure Unit of Measure Awareness

https://doi.org/10.1016/j.wneu.2019.05.238Get rights and content

Background

Intracranial pressure (ICP) reporting impacts neurosurgical care. Millimeters of mercury (mmHg) and centimeters of water (cmH2O) are both used to report ICP in clinical practice and the literature. In this study, we investigated ICP unit of measure awareness in the neurosurgical community.

Methods

A survey was conducted at four US academic neurosurgery departments asking the following questions: What is your threshold for a concerning ICP? How many minutes is that ICP sustained for you to be concerned? What unit are you implying when you state that ICP? What unit of measure is an ICP reported on the monitor when transduced? When setting an external ventricular drain Becker bag level, what unit of measure do you set it to? Do you ever manually check an ICP based on a column of cerebrospinal fluid? How many cmH2O is 20 mmHg?

Results

An ICP of 20 and sustained for five minutes were the two most common answers. Some 71% of residents and 34% of attendings reported using cmH2O as the unit of measure; 18% of residents and 24% of attendings implied different units when discussing ICP than the unit they thought was transduced; and 53% of residents and 34% of attendings did not know the transduced ICP unit of measure reported in their intensive care unit.

Conclusions

Variability and discrepancies regarding the ICP unit of measure exist in academic neurosurgery departments. Clinicians should familiarize themselves with their hospital's practices. Institutions and all of medicine may consider standardizing the ICP unit of measure, using mmHg as a universal nomenclature.

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)

  • S. Benvenga

    Errors based on units of measure

    Lancet

    (2004)
  • R.K. Narayan et al.

    Intracranial pressure: to monitor or not to monitor? A review of our experience with severe head injury

    J Neurosurg

    (1982)
  • N. Carney et al.

    Guidelines for the management of severe traumatic brain injury, fourth edition

    Neurosurgery

    (2017)
  • R. Lloyd

    Metric mishap caused loss of NASA orbiter. CNN

    (1999)
  • H.S. Yin et al.

    Liquid medication errors and dosing tools: a randomized controlled experiment

    Pediatrics

    (2016)
There are more references available in the full text version of this article.

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.

View full text