I was pretty excited when I first read the Pratt et al1 abstract for "Zero-Calibrating External Ventricular Drains: Exploring Practice," particularly when the abstract results section stated, "Bedside trial showed equivalence when comparing intracranial pressure (ICP) tidal, ICP after EVD zero with NVC removal, and ICP after EVD zero without NVC removal." Upon further reading the manuscript, I was less enthusiastic as the bedside trial listed in the methods section was a single-subject observation with minimal details. I applaud the authors for formally starting the conversation by publishing the reality that there is variance in practice when zeroing an external transducer on an external ventricular drain (EVD). Given the data, I believe a proper clinical trial needs to be completed and repeated to state which method for zeroing the transducer on an EVD supports evidence-based practice. The differences in manufactured medical equipment are a confounder in a study of this type.

Zeroing a transducer is a function of a transducer, not a function of an EVD. Pressure transducers must be zeroed to atmospheric pressure no matter what pressure they measure. The question at hand is, "What is the best way to access air when zeroing a transducer attached to an EVD?" When zeroing through the filter on the burette, hypothetically, many factors can affect the accuracy of a zeroed atmospheric pressure. These factors include the density of the filter over time, the size of the opening to air on the burette, and the accuracy of level from the filter at the top of the burette to the zero-pressure reference point. Although research does correlate infection with EVD manipulation, I ponder whether there is evidence that opening a transducer to air causes increased infection. Furthermore, I wonder, are nurses making decisions on zeroing a transducer through the filter and potentially creating an inaccuracy at the baseline zero in response to an assumed potential for infection?

Contrary to the way this letter may sound, I do not have a strong opinion about zeroing the transducer one way over the other. My frustration lies with the extent of variability around what is arguably the most common diagnostic tool used with the brain-injured patient. I appreciate that the authors chose a zeroing technique as a clinical practice that best fits their facility. I believe that nurses ask about zeroing because they want the best outcomes for their patients. In this situation, as the authors indicate, when research evidence does not exist, I believe, until there is evidence to base practice, nurses are obligated to have this conversation and choose the best standard for their facility.

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