ANAESTHESIA FOR MRI, ECT AND CARDIOVERSIN
ANAESTHESIA FOR MRI, ECT AND CARDIOVERSIONDR. ELDOANISH
Anesthesia outside the OTRadiology CT , MRI , Interventional
Cardiology Cardioversion , Catheterization
Gastro Colonoscopy , ERCP
Urology - ESWL
ASA guidelines for non OR anesthesia locationsReliable oxygen source with back up
Waste gas scavenging
Adequate monitoring equipment to meet basic standard anesthesia monitoring
A self inflating hand resuscitating bag
Adequate illumination of patient and machine
Emergency cart with drugs and equipment
A reliable means of communication for assistance
Problems faced in outside locationsAwkward layout for an anesthetist
Older machine models
Remoteness of the location and unavailability of assistance
Personnel less familiar with aspects of anesthesia than the OR staff
Diagnostic equipment hamper access to the patient
Pipped gases and suction might not be available
Anesthesia for MRI
Principle of MRIAtoms with odd number of protons when subjected to magnetic field will align themselves to the field
The magnetic field for an MRI is measured in terms of Tesla
1 T = 10, 000 guass
MRI machines have strengths varying from 0.15 2 T
HAZARDS OF MRI
Long imaging time ( > 20 minutes)
Any patient movement even physiological ( cardiac and vascular flow , peristalsis ) produces artifacts
Loud noices (> 90 db ) . So mandate noise protection
Intense magnetic field causes thermal injuries especially at sites of ECG electrodes , pulse oximeter probes
Avoid loops in monitoring wires and contact with conductors
Dislodgement of ferrous substances ( vascular clips , sharpnel , shunts , pacemakers , icd, mechanical heart valves , wired ETT)
Iron containing materials like scissors , pens , keys , gas cylinders can be attracted into it at extremely high velocities resulting in fatal injuries.
In MRI Brain, the airway will not be assessable during the procedure. So airway should be well maintained
MRI SuiteZone 1 : Public zone , free access
Zone 2 : interface b/n public area and mri suite . All movement by non mri personnel is supervised
Zone 3 : Area within which introduction of ferro magnetism is prohibited
Zone 4 : Scanning room
Monitoring in MRI Suite Central o2 / N2o / air
Electrical power sources.
ECG : ST and T abnormalities are seen because static magnetic field can induce voltage changes to the blood flow in the aorta.
ECG can be ridden with multiple artifacts.
Thermal injuries through the elctrodes. MRI compatible ecg electrodes made of carbon graphite are available. Avoid coiling.
NIBP : Usually no interference. connections b/n BP cuff and hoses should be plastic.
Pulse oximetry Thermal injuries
Capnography : MRI compatible capnogram machine should be used. If not available the machine should be placed as far away from the magnetic field as possible . So a long sampling line can result in delay in signal transduction.
Quench monitoring : The magnet superconductors are kept cool within liquid N2. If this coolant evaporates the ambient o2 falls rapidly. A quench monitor measures the ambient o2 levels.
Anesthetic EquipmentMachines to be made of stainless steel / brass / aluminium
Cylinders made of aluminum
ET Tube : Spring valve within the cuff distort the image . Avoid reinforced tubes.
Anesthesia Technique 1 . Verbal assurance : Explain to the patient regarding the procedure and assure the patient .
2 . Sedation : useful in children , anxious adults , those with language barrier.
Drugs commonly used for sedationTrichlofos sodium 50 -75 mg /kg hour prior to procedure
Oral chloral hydrate 80 100 mg /kg
Midazolam , orally ( 0.25 0.75 mg /kg) iv ( 0.03-0.08 mg /kg
Ketamine , orally 5-10 mg /kg im 2-3 mg/kg.
Chloral hydrateSedative and hypnotic drug with barbiturate-like features. Onset time if applied orally is 1530min, and duration is 60120min. If given in therapeutic doses it has only a slight effect on ventilation and blood pressure, but its therapeutic index is small. Dosing is between 80 and 100mg/kg.Side effects: nausea and vomiting, long recovery times and postoperative agitation have to be considered.
Oral or rectal dosing is 36mg/kg.
Time until onset of sedation is 1560min, and duration is 60240min.
Potential relevant cardiovascular and respiratory depression and the contraindications in patients with porphyria have to be considered.
KetamineCommonly ignored as a sedative for MRI as it has an analgesic component which is not necessary for MRI.
Dosing is 11.5mg/kg when applied intravenously or 45mg/kg when injected intramuscularly.
Onset time is 13min, and duration is 1530min.
Ketamine used alone may be useful for sedation in patients with respiratory risk factors.
MidazolamUsed alone is not suitable for MRI sedation as its duration is too short for a successful procedure of 2030min.
It has to be either re-injected or used in combination with fentanyl or pentobarbital or ketamine.
The combination of sedatives is a risk factor for respiratory complications.
Combined sedation drug use in children is not acceptable because the effects are hardly predictable and therefore risky.
PropofolPropofol seems to be a perfect drug for sedation because it is effective, has a short recovery time and can easily be titrated to the required sedation level.Dosing is normally 25mg/kg/h intravenousShort induction and a recovery time of 8min are convincing advantages of propofol use When using propofol only for sedation purposes the low therapeutic tolerance has to be stressed.Consequently the physician must monitor the respiratory rate and manage the paediatric
DexmedetomidineSelective alpha-2 agonist which can be used by non anaesthesiologists. No relevant respiratory effects of this drug are known. Haemodynamic side-effects such as low blood pressure and low heart rate are common. A loading dose of 1 mcg/kg over 10min followed by 0.5 mcg/kg/h as an infusion for sedation maintenance is recommended. Life-threatening complications have to be expected if dexmedetomidine is used in combination with digoxin. Because of these side-effects the drug is not suitable for patients with cardiac compromise.
Several studies investigating dexmedetomidine for sedation have been published recently.
Mason and colleagues  reported MRI procedures for 747 children and showed successful imaging in 97.6%. Cardiovascular side-effects (bradycardia never exceeding a 20% range from standard values) were seen in 16%. Oxygen saturation was always above 95%.
In children with obstructive sleep apnoea syndrome a comparison between dexmedetomidine and propofol for MRI sleep induction revealed effective sedation without the need for additional airway equipment in 88.5 versus 70% of scans .
Some other investigations found no difference in successful scanning between dexmedetomidine and propofol in 60 children between 1 and 7 years old but propofol showed advantages in induction, recovery and discharge time.
No oxygen desaturation was seen in the dexmetedomidine-sedated children.
Similar results were reported by Heard and collegues, who compared a midazolamdexmedetomidine combination with propofol for sedation
Lubisch et al. published a retrospective study of children with autism and other neuro behavioural disorders. Three hundred and fteen patients with a mean age of 3.9 years were sedated with dexmedetomidine, most commonly for MRI, while 90% of patients received concomitant midazolam. Seven patients required intervention for cardiac events and one for a respiratory event. There were two episodes of recovery-related agitation; 98.7% of sedations were successfully completed .Dexmedetomidine could, if one takes account of the contraindication of cardiovascular comorbidity, be a favourable sedative drug for MRI scanning.
Contraindications for sedationPotential for airway obstruction
h/o apnoeic spells
Resp diseases with a saturation of < 94 % on RA
Recent food intake
General anesthesia critically ill and uncoperative individuals
Airway is secured either with LMA/ ETT in an induction room adjacent to scan room with all standard monitors.
Post induction transfer patient to scan room and resume ventilation.
Maintain anesthesia with volatile agents / propofol
At the end of procedure patient is returned back to induction room and awakened.
Anaesthesia for ECT
ProcedureProgrammed electrical stimulation of cns to trigger seizure activity
After induction of anesthesia 2 electrodes are attached to patients scalp
Seizure is monitored by observing the patient as well as EEG
The minimum seizure duration needed for therapy to be effective is 25 secs.
Physiolocal effectsCNS increased icp , cbf
Initial exagerrated PNS activity bradycardia , asystole , premature ventricular contractions.
This is followed by a symp surge tachycardia , hypertension . ST depressions and t inversions
Secondary to sympathetic overactivity the sympathetic surge peaks 2 minutes following stimulation and is usually self limiting