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:

 

Risk of physical harm:

Magnet disasters

Risk of lethal injury from projectiles

Risk of lethal injury if ferromagnetic prosthetic material

Need formal check of patient

Need formal check of self and assistants

5 Gauss line at the door: cannot pass without formal check

Everything should be labelled: safe vs conditional vs unsafe

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

 

Equipment issues:

Things that are unsafe

Standard trolleys

Standard anaesthesia machines (note also need long tubing)

Standard syringe pumps

Standard laryngoscopes

Standard monitoring displays

Standard monitoring equipment (BP cuff/cable, sats probe, ECG dots)

Monitoring

Usually basic: sats probe + BP cuff + gas analysis (O2/CO2/volatile)

Interference: ECG spikes and ST/T abnormalities, i.e. for rhythm only

Pacemakers

Interference: if 5 Gauss line

-Inactivated or reprogrammed or converted to asynchronous

Catastrophic injury

-Older devices are ferromagnetic

 

Remote environment issues

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 e.g. cardiac arrest

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

 

Other considerations

Limited access to patient

1)Inside closed door 2) Head often inside scanner 3) Window often opaque

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)

Need for 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