K5i / 20B07: Compare and contrast external ventricular drains and intraparenchymal fibreoptic pressure monitors.

20B07: Exam Report

Compare and contrast external ventricular drains and intraparenchymal fibreoptic pressure monitors.

22% of candidates passed this question.

This question is ideally suited to a tabular format, where candidates are expected to highlight the significant similarities and differences as well as why a certain monitor may be chosen in preference to another rather than compile two lists written next to each other. To score well in this question, a statement of what could be measured (ICP: global vs local), a description of the measurement principles, along with other measurement related information like calibration and sources of error was required. Also sought was information regarding anatomical placement (e.g., lateral ventricle for EVD) and method of placement.

 

Furthermore, a comparison with each other (e.g., higher infection/bleeding risk with EVD, greater risk of trauma due to size and insertion, expertise to insert, cost, therapeutic benefit, risk of blocking) was required for completion. Candidates who structured these elements into advantages and disadvantages were generally able to elucidate this information and score better.

K5i / 20B07: Compare and contrast external ventricular drains and intraparenchymal fibreoptic pressure monitors

Intraparenchymal

EVD

Intraparenchymal Fiberoptic

Description

EVD

A flexible plastic catheter placed inside the lateral ventricles.

Can be either unilateral or bilateral placement

Gold standard for ICP monitoring 

Intraparenchymal Fiberoptic

A think catheter inserted into parenchyma of the brain

Indication

EVD

Intraparenchymal Fiberoptic

Hydrocephalus 

Post surgical/trauma intracranial haemorrhage 

Meningitis 

Location of Tip

EVD

Lateral ventricles

Intraparenchymal Fiberoptic

Below the dura – usually a few cms

Component

EVD

Plastic catheter 

Wheatstone bridge

Fluid filled non compressible tubing 

Pressurised fluid bag 

Drainage bag 

Monitor 

Intraparenchymal Fiberoptic

Microsensor – intracranial

Fibre Optic cable 

Monitor 

Principle

EVD

Pressure is transmitted to a wheatstone bridge transducer via fluid filled non-compressible tubing 

Intraparenchymal Fiberoptic

Changes in ICP move a displaceable mirror at the tip of the sensor to alter the intensity of light reflected back along the fiber optic cable 

Calibration

EVD

Able to be zero’d post insertion

Intraparenchymal Fiberoptic

Cannot be zero’d post insertion

Sources of Error

EVD

Migration of catheter 

Blockage 

Incorrect leveling to tragus 

Damping and resonance 

Intraparenchymal Fiberoptic

“Drift” 

Increased inaccuracy >72hrs 

Only measures ICP locally 

Advantage

EVD

Has diagnostic values (CSF sampling, elevated ICP, new haemorrhage) as well as therapeutic values (drainage of excessive CSF, administration of medication) 

Global measurement of ICP

Can be converted to cerebral shunt – long term treatment 

Cheap 

Intraparenchymal Fiberoptic

Less expertise required for insertion 

Less traumatic 

Able to be placed in patients with small collapsed ventricles 

Less infection risk

No risk of blockage 

Disadvantage

EVD

More skill required for insertion 

More traumatic compared to bolt 

Increased risk of ventriculitis 

May be blocked 

Complications: bleeding, malplacement, obstruction, migration, infection

Intraparenchymal Fiberoptic

Only local ICP is measured 

No therapeutic value: CSF cannot be drained 

Less accurate due to drift and unable to be calibrated post insertion

Expensive 

Author: Zoe Guo