Re-Zeroing an External Ventricular Drain After Patient Position Change
When a patient's position changes, re-zero the EVD transducer by leveling it to the anatomical reference point (typically the tragus of the ear or external auditory meatus, which approximates the foramen of Monro), then opening the transducer to atmospheric pressure by removing the nonvented cap.
Step-by-Step Re-Zeroing Procedure
1. Reposition the Transducer to the Correct Anatomical Level
- Move the transducer to align with the tragus of the ear or external auditory meatus
- This anatomical landmark approximates the foramen of Monro regardless of patient position (supine, head of bed elevated, etc.)
- Use a carpenter's level or laser level to ensure precise alignment
2. Zero the Transducer to Atmospheric Pressure
- Remove the nonvented cap from the transducer to open it to air 1, 2
- Press the "zero" button on the monitoring system
- Replace the nonvented cap after zeroing is complete
3. Resume Monitoring or Drainage
- Return the system to the prescribed drainage level or monitoring mode
- Document the re-zeroing in the patient record
Evidence-Based Rationale
The literature strongly supports the open method (removing the nonvented cap) for zeroing:
- Manufacturer instructions explicitly direct removal of the nonvented cap to open the transducer to air 1
- Clinical practice guidelines from neuroscience professional organizations reference the open method 1
- A 2023 critical appraisal found no evidence that zeroing through the burette filter provides equivalent accuracy to the open method 1
While one 2022 study showed equivalence in a bedside trial comparing methods 2, this represents lower-quality evidence from a single institution. The consensus from manufacturers and professional guidelines supports the open method for ensuring the most accurate physiological values for treatment decisions 1.
Critical Considerations During Patient Transfer
When transferring patients with EVDs between locations:
- For short transfers within a facility: The neurosurgical team may recommend clamping the EVD temporarily to prevent overdrainage if the drain is kept too low during movement 3
- Always seek neurosurgical team guidance before transfer 3
- Re-zero immediately upon arrival at the new location before resuming drainage or monitoring
Common Pitfalls to Avoid
- Failing to re-zero after position changes: This is the most common error and leads to inaccurate ICP readings that can result in inappropriate treatment decisions
- Zeroing through the filter instead of opening to air: This may not provide accurate atmospheric pressure calibration 1
- Incorrect anatomical reference point: Always use the tragus/external auditory meatus, not arbitrary points on the bed or patient
- Forgetting to re-level the drainage system: Both the transducer AND the drip chamber must be repositioned to the correct height relative to the anatomical landmark