Anaesthesia for MRI, ECT, Cardioversion

ANAESTHESIA FOR

MRI, ECT AND CARDIOVERSION

DR. ELDOANISH

Anesthesia outside the OT• Radiology – CT , MRI , Interventional

• Cardiology – Cardioversion , Catheterization

• Psychiatry – ECT

• Gastro – Colonoscopy , ERCP

• Urology - ESWL

ASA guidelines for non OR anesthesia locations• Reliable 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 locations

• Awkward 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 MRI• Atoms 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 Suite• Zone 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 Equipment• Machines 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 sedation• Trichlofos sodium 50 -75 mg /kg ½ 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 hydrate• Sedative and hypnotic drug with barbiturate-like

features. • Onset time if applied orally is 15–30min, and duration is

60–120min.• 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.

Pentobarbital• short-acting barbiturate.

• Oral or rectal dosing is 3–6mg/kg.

• Time until onset of sedation is 15–60min, and duration is 60–240min.

• Potential relevant cardiovascular and respiratory depression and the contraindications in patients with porphyria have to be considered.

Ketamine• Commonly ignored as a sedative for MRI as it has an

analgesic component which is not necessary for MRI.

• Dosing is 1–1.5mg/kg when applied intravenously or 4–5mg/kg when injected intramuscularly.

• Onset time is 1–3min, and duration is 15–30min.

• Ketamine used alone may be useful for sedation in patients with respiratory risk factors.

Midazolam• Used alone is not suitable for MRI sedation as its

duration is too short for a successful procedure of 20–30min.

• 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.

Propofol• Propofol 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 2–5mg/kg/h intravenous• Short 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

Dexmedetomidine• Selective 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 midazolam–dexmedetomidine combination with propofol for sedation

• Lubisch et al. published a retrospective study of children with autism and other neuro behavioural disorders. Three hundred and fifteen 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 sedation• Potential 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

Procedure• Programmed 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 effects• CNS 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

• Nor adrenaline and adrenaline levels increase following ect

• Glucose haemostasis is affected . NIDDM have a favorable response , but worsening of IDDM.

Interactions• TCA : • Blocks reuptake of NA , 5 HT and DA

• increases central sympathetic tone .

• anticholinergic , antihistaminic & sedative effects .

• A combination of TCA + Atropine can increase post op delirium .

• MAOI : Blocks metabolism of NA , 5 HT and DA

• The use of indirectly acting sympathomimetics can lead to hypertensive crisis.

• Reduce dose of direct acting sympathomimetics to treat hypotension .

• They are hepatic microsomal inhibitors , so can prolong duration of opiods .

• Meperidine to be avoided as it causes fatal excitatory response

• Lithium : • prolongs NMB . • Prolongs action of BZD , barbiturates.

Need for anesthesia

• To reduce psychological / physical trauma

PAC• Co existing conditions : neurological , cardiac ,

osteoporosis

• Concomitant medications with special emphasis on anti psychotics

• Involve bystander too in history taking since patient might be a poor historian.

Induction• IV induction is usually preferred .

• Standard pre induction monitors

• Glycopyrollate 5 mcg/kg to prevent bradycardia as well as for antisialagogue effect

• Adequate pre oxygenation

• Methohexital : 0.75-1 mg/kg most commonly used.

• Propofol : 0.75 mg / kg can also be used , but decreases the seizure duration

• TPS and BZDS are avoided anticonvulsive action

• Etomidate can prolong seizure duration , so is also an alternative

Neuromuscular blockade• Prevents physical trauma.

• Only partial block is needed as peripheral seizure visualization shouldn’t be hampered.

• A BP cuff can be inflated and kept in the limb intended for seizure visualization prior to administering NMB

• SCH : 0.5 mg/kg most commonly used

• Once relaxation is adequate and mask ventilation proper a soft bite block is kept .

• If additional stimuli are needed repeat iv anesthetics / sevoflurane can be used.

• Intubation might be warranted in those with GERD , hiatal hernia , pregnancy

• Post procedure ventilation is done until patient awakens

• Esmolol / Labetolol can be used to control episodes of tachycardia and hypertension . Labetolol preferred .

• Accurate documentation of drugs used and any outward events like arrhythmias , hypertension and post op agitation .

Contra indications for ECT• Phaeochromocytoma

• Increased ICP

• Recent CVA

• Cardiovascular conduction defects

• High risk pregnancy

• Aortic / cerebral aneurysm

Cardioversion

• To convert supraventricular and ventricular arrhythmias to sinus rhythm by delivery of a DC shock

• In case of a long standing arrhythmia with no associated hemodynamic instability cardi0version is done on op basis.

• In case of a chronic AF , rule out presence of atrial thrombi prior to cardioversion.

• Standard monitoring and all emergency equipment needs to be available

• The patient is pre oxygenated and given a small dose of iv anesthetic until he / she is un responsive.

• Immediately prior to counter shock , remove the mask and ensure no person is touching the person / the cart• • After cardioversion is completed the patient is ventilated

with 100% oxygen until consciousness is regained.

• If done on an emergency basis , adequate fasting might not be done . Intubation is a good option .

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