Anaesthesia for MRI, ECT, Cardioversion

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ANAESTHESIA FOR MRI, ECT AND CARDIOVERSIN

ANAESTHESIA FOR MRI, ECT AND CARDIOVERSIONDR. ELDOANISH

Anesthesia outside the OTRadiology CT , MRI , Interventional

Cardiology Cardioversion , Catheterization

Psychiatry ECT

Gastro Colonoscopy , ERCP

Urology - ESWL

ASA guidelines for non OR anesthesia locationsReliable oxygen source with back up

Suction source

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

Unfamiliar equipment

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

Plastic laryngoscopes

Copper stylet

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.

Pentobarbitalshort-acting barbiturate.

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 [30] 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 [31].

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 [34].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

Raised ict

Epilepsy

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