MRI anaesthesia

 

Summary:

Situation

Magnet very strong: e.g 1.5T = 15,000 Gauss (c.f. planet Earth 0.5 Gauss)

Magnet is always on

Implications

1.Many hazards to self, staff and patient

2.Many constraints on equipment allowed

3.Remote environment

4.Limited access to patient

 

Zones:

Zone 1

Public access (e.g. reception, waiting area)

Zone 2

In between (e.g. store bag and steth here)

Zone 3

Control room (i.e. access restricted, people must be checked)

Zone 4 (5 Gauss)

Magnet room (i.e. consequential magnetic field; only approved people and equipment can enter)

Danger (30 Gauss)

Projectile hazards become significant

 

Equipment designation:

MRI safe

Non-ferromagnetic, non-electrically conductive

e.g. IV cannula needle (stainless steel)

MRI conditional

Ferromagnetic or electrically conductive

Conditions for safe use must be defined and observed

e.g. pilot tube of ETT/LMA must be taped still

MRI unsafe

Significantly ferromagnetic

e.g. standard laryngoscope, prostheses, surgical clips etc

 

MRI anaesthesia considerations:

 

Magnet issues:

Risks

·      Physical injury: external projectiles, internal prostheses

·      Monitoring interference: ECG spikes, ST/T abnormalities

·      Pacemaker interference: asynchronous, inactivation, reprgramming

Approach

·      Formal check of patient

·      Formal check of self and assistants

·      Labelling of all equipment

·      Barricade the 5 Gauss line

·      MRI safe equipment

·      Basic monitoring only

 

Other physical threats:

Burns

·      No conducting material on patient

·      e.g. no normal ECG dots, no normal sats probe

Hearing loss

·      Very noisy 110-130dB (max safe ~90dB)

·      Putty or earmuffs for the patient

Traction

·      Table moves in and out a long way

·      Need secure airway / IV line

 

Remote environment:

Bring your stuff

·      Helpers if complex patient

·      Airway equipment if anticipating difficulty

·      Emergency drugs present

Case planning

·      Consider GA (sevo) rather than sedation (midazolam, dexmed)

·      Consider IV rather than inhalational induction

·      Consider ETT rather than LMA

·      Consider awake rather than deep extubation

·      Low threshold for transfer patient to main PACU

Disaster planning

·      Get patient out ASAP and shut the door

·      Call for help early (since slow to arrive)

·      Need designated resus area

·      Defibrillator cannot enter MRI scanner room

 

Remote patient:

Problems

·      Closed room

·      Opaque window

·      Patient in tunnel

Approach

·      Ensure patient stable, attachments secured before leaving the room

·      Consider ETT rather than LMA (in case of laryngospasm or aspiration)

·      Consider inhaled rather than IV anaesthesia (in case of disconnection)

 

Scanning requirements:

Immobility

·      Awake: hard to keep still if young, intellectually disabled, claustrophobic

·      Sedation: difficult - movement if not enough, airway obstruction if too much

·      GA: need adequate hypnotic concentration; not painful, hence opioid and hypnotic usually unnecessary

Contrast anaphylaxis

·      e.g. gadolinium

·      Note again remote environment

 

 

Feedback welcome at ketaminenightmares@gmail.com