ANAESTHESIA FOR NEUROIMAGING CECT, MRI, ANGIOGRAPHY

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ANAESTHESIA FOR NEUROIMAGINGCECT, MRI,
ANGIOGRAPHY
www.anaesthesia.co.in anaesthesia.co.in_at_gmail.co
m
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Guidelines for non-operating room anesthetizing
location

• Reliable oxygen source with a backup
• Suction source
• Waste gas scavenging
• Minimum mandatory monitoring equipment
• Sufficient safe electrical outlets
• Patient and anesthesia machine illumination with
  battery powered backup.

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Guidelines for non-operating room anesthetizing
location (contd)

• Sufficient space for anesthesia care team.
• Emergency cart with defibrillator
• Emergency drugs and equipments.
• Reliable two-way communication to report
  assistance

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Facilities

• Location
• Unfamiliar anesthetic equipments
• Anesthetic implication of procedure performed
• Remoteness of available assistance
• Personnel less familiar than usual in operating
  suites.
• Space for anaesthesia equipment and drugs may be
  limited
• Piped gases, suction, isolates a power not always
  available

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Recovery

• Medically stable before discharge/ transfer
• Recovery facilities and staff
• Provision of O2 delivery and monitoring on
  transport cart
• Availability of personnel trained in ACLS
• Specific discharge criteria

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Radiation safety

• Dosimeter
• Maximal permissible radiation dose
• ? 50 MSV (milliseverts) annually
• ? life time cumulative dose of 10 msv x age
• Monthly exposure of 0.5 MSV for pregnant women
• Lead aprons antithyroid shields, using movable
  leaded glass screens,
• Innovative techniques Video monitoring and
  remote mirroring of monitor data

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Computed Tomography
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Principles of Computed Tomography

• Two dimensional, cross sectional image
• Typical scan comprises 20 sections
• Absorption value ? Hounsfield unit
• Isodense ? normal brain parenchyma
• Hyperdense ? bone, fresh haemorrhage
• Hypodense ? Edema, Necrosis

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C T Scan (Indications)

• Intracranial pathology
• Intraspinal and paraspinal pathology
• Enhancement of
• intracranial neoplasms,
• infarcts,
• vascular lesions,
• abscesses by
• intravenous administration of water-soluble
  iodinated contrast material

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CT Scan (Indications) contd

• Modality of choice for detection of skull
  fractures and acute SAH in emergency setting
• Spiral acquisition CT scan is also popular
  because larger anatomic regions can be scanned
  quickly

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CT scan (Limitations)

• Insensitive for posterior fossa Structures view
  as Image degradation by the artifact is produced
  by interface of bone and brain parenchyma.
• Contrast media are to be introduced in GI system
  in patients who are sedated or anaesthetized.

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Anaesthetic Considerations

• Risk of aspiration if Airways unprotected
• Risk of adverse sequelae associated with contrast
  media is higher in patient undergoing CECT versus
  other types of radiological studies

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Contrast considerations

• Goldberg divided these reactions into five
  classes
• Vasomotor, peripheral vasodilation from direct
  action of the Intravenous Contrast Medium
• Vasovagal from CNS effects of the media
• Dermal, erythematous and urticarial response to
  histamine release
• Osmotic
• Anaphylactoid which involves release of histamine
  and other mediators

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Prevention of Contrast reactions

• Fluid administration
• Before, during, and after procedure
• Low osmolality contrast media
• Acetyl cysteine

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Treatment of contrast media reactions
Adrenergic agonists Epinephrine 3-5µg/kg IV bolus Epinephrine 1-4µg/min IV infusion
Methylaxanthines Aminophylline 5-6mg/kg/20 min, initial dose Aminophylline 0.5-0.9mg/kg/hr, maintenance dose
Anticholinergics Atropine 0.5-2.0 mg IV
Antihistamines Diphenhydramine 25-50 mg IV
Steroids Methylprednisolone 100-1,000 mg IV Dexamethasone 4-20 mg IV
Intravenous fluids Normal saline (infuse to maintain normal blood pressure)
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Anesthetic Considerations

• IV Sedation/General Anesthesia is required to
• reduce patient movement
• children/mentally challenged/confused
• provide airway protection
• unconscious
• provide controlled ventilation
• reduce anxiety and claustrophobia

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MRI AIIMS
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Principles of MRI

• Atomic nuclei with odd number of protons or
  neutrons.
• H1, C13, F19, Na23, P31
• Biological tissues have high water content.
• H acts as Dipole
• Magnetic Dipole movement
• Gyromagnetic property

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Magnetic resonance imaging

• Anaesthesia in the MRI suite poses unique
  problems
• Limited patient access and visibility
• Absolute need to exclude ferromagnetic components
• Magnetic field interference /malfunction of
  monitoring equipment
• Nil movement of anesthetic and monitoring
  equipments
• Limited access to emergency personnel

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Fig. 1A. Photographs show aftermath of incident
2, which occurred at first institution. Portable
anesthesia tank of nitrous oxide (arrows) lies in
bore of 1.5-T MR unit. T table.
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Safety considerations

• Effect of magnet or ferrous objects- dislodgement
  and malfunction of implanted biologic devices
• Vascular clips, shunts, wired spiral ET
  parameter, ICD, mechanical heart values,
  implanted biological pumps

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Safety considerations (Contd)

• Problems with ICDs and cardiac pacemaker include
• Heating by induced current
• Inhibition of pacemaker output
• Reed switch malfunctioning
• Torque on pacemaker
• Death from torque of vascular clip with MRI
  magnetic field has been reported
• Clips with low ferromagnetic properties safe lt1.5
  Tesla

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Anaesthesic considerations

• Cerebrovascular disease and CAD
• Intracranial mass lesion
• Hyperkalemic response
• Upper motor Lower Motor Leisons
• Encephalitis
• CVA
• Closed head injury
• severe burns
• Acute trauma
• Patients after prolonged bed rest

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General considerations (contd)

• Prolonged patient immobility increased CNS
  depressant effects of anesthesia.
• Inadequate history due to a decreased level of
  consciousness
• Full stomach, cervical spine injury and multiple
  organ involvement

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Location of anaesthetic equipment

• Outside the magnetic field (5-30 G) line 8-9 feet
  from head
• Requires long monitor leads ventilation tubings
• Large compressive volume in circuit
• Delay in changes of volatile gas concentrations
• Increased risk of disconnection
• Close to magnetic field
• Must be non-ferromagnetic substance
• Electrical equipment require appropriate fibres
• Do not move gas machine once scan has begun

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Anaesthetic goals

• Maintenance of patient immobility and
  physiologically stability
• Manipulating systemic and regional blood flow
• Treating sudden unexpected complications during
  the procedure

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Patient immobility

• Images are composed of multiple data acquisitions
  to give the final image
• One scan can take up to 20 minutes
• Movement at any point can affect image quality
• Movement from original position affects the
  homogeneity and maximal strength of the field

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Assessment

• Detailed history and patient evaluation
  (including neurological)
• H/o previous surgery, allergy and contrast
  reactions
• Any neck, back or joint pathology
• Advised to continue their usual prescribed
  medications
• In female pregnancy should be ruled out

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Premedication

• Anxiolytic
• Patient Anxiety (4 of scans are aborted)
• confined space
• loud noises (greater than 95 db)
• temperature of the magnetic bore
• length of procedure
• education and counseling
• gradual familiarization with enclosed space
• prone positioning

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Premedication (contd)

• In patients with h/o allergies steroids and
  antihistamines
• Antihypertensives to be continued in hypertensives

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Radiation safety

• Radiation safety direct adverse biologic effect
  of MRI magnetic field are not believed to exist

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MRI consent form
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Anaesthetic Management

• Choice of anesthetic technique
• Modalities of management
• Intravenous sedation
• General anaesthesia

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Goals of sedation

• To improve patient comfort
• Help allay burning pain associated with injection
  of dye
• Allows rapid decrease in level of sedation for
  neurological testing

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Sedation

• Conscious sedation is a medically induced CNS
  depression in which communication is maintained
  so that the patient can respond to verbal
  commands
• Deep sedation is defined as medically induced
  CNS depression in which patient is essentially
  unconscious and does not respond to verbal
  commands
• In such a state patient breaths spontaneously
  however protective reflexes may be lost,
  maintenance of airway is not assured

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Sedation

• Spectrum
• Light to deep
• Conscious sedation deep sedation (without
  recognition)
• Deep sedation general anaesthesia

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Contraindication to sedation

• Increased risk of pulmonary aspiration
• Possibility of airway obstruction or respiratory
  irregularities
• Raised ICP- raised PaCO2 could be dangerous
• Respiratory centre is desensitized to CO2
• Renal or hepatic dysfunction which alters drug
  kinetics
• Uppredictable drug effect, as sedative may
  increase restlessness

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Advantages Disadvantages
Allow neurological examination Avoidance of hemodynamic changes associated with endotracheal intubation Sudden patient movement Unprotected airway Hypoxia Hypercapnia
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Characteristics of an Ideal Agent for Intravenous
Conscious Sedation

• Rapid onset (lipid solubility)
• No pain on injection
• Short duration
• Inactive metabolites
• Minimal Cardiopulmonary depression
• Wide therapeutic index
• Specific antagonist available

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Sedation techniques for small children
Agent Dose Comment
Chloral hydrate 75-100mg/kg Allow 15-20 min onset
Pentobarbital 5mg/kg PO Mix with concentrated Kool-Aid
Ketamine 4-5mg/kg IM Analgesia, secretions, asthmatics
Ketamine 1-2mg/kg TV Analgesia, secretions, asthmatics
Methohexital 20-30 mg/kg PR Onset 8-10 minutes, messy
Methohexital 1-2 mg/kg IV Give slowly watch for apnea
Propofol 2-3mg/kg IV, dilute with lidocaine, 50-125 µg?kg/min Painful injection may cause apnea repeat bolus or give infusion
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PROPOFOL

• Ultra-short acting
• Fast onset and short duration of action,
  antiemetic effects
• Unconsciousness within 1 minute
• Loss of protective reflexes, apnea
• Intermittent boluses or a continuous infusion of
  50 to 150 µg/kg/min.
• No analgesic properties
• Recovery within 10 to 15 minutes

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Ketamine

• Excellent analgesia and sedation but not
  immobility.
• No respiratory depression
• Best used in sedation for painful procedures
• IV Ketamine dose (0.5-2.0mg/kg)
• Intramuscular injection (3-4mg/kg)
• Increases ICP, best avoided in intracranial
  pathology
• Non-purposeful motion associated with ketamine
  makes its use in MRI limited

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Indications for GA

• Immobiliy
• Pediatrics
• Extreme pain on lying
• Ventilator dependency
• Claustrophobia

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Anaesthetic Considerations
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Anaesthetic Considerations

• Anaesthetic Induction
• outside the magnet easier
• ideally all monitors are portable and can be
  brought into the MRI suite
• attention to transport gurney (trolley)
  (ferromagnetic)

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Anaesthetic Considerations

• Airway Management
• Anaesthetic Techniques
• volatile agents (only certain gases may be
  available)
• IV (infusion pumps may pose problem)

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Anaesthetic Considerations

• either ETT (consider Rae) or LMA (no reinforced
  ETT)
• Limited access to patient/airway

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General Anaesthesia
Advantages Disadvantages
Immobile patient (improved image quality) Patient comfort Control over airway Hemodynamic control Inability to perform intraoperative neurological assessment Hypertension and raised ICT during intubation and extubation
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Various choices and combinations advocated include

• Neurolept anaesthesia
• Droperidol / fentanyl / midazolam
• Propofol bolus and infusion
• Propofol bolus and opioid infusion
• Opioid bolus and infusion

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EKG monitoring

• Attention to burns
• transmit by shielded cables, telemetry,
  fiberoptics
• use fiber carbon leads although some magnetic
  leads do not cause interference
• EKG within static field does show change
• Faraday's Law
• Changes are greatest at leads I, II, V1 and V2.
• Superimposed potentials are greatest in the early
  T waves, and late ST segments
• Mimic the EKG changes of conditions such as
  hyperkalemia and pericarditis.
• rapidly changing magnetic field induce potential
  differences across the loops formed by the EKG
  leads
• appear as spikes and may be confused as R waves

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Monitoring (contd)
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Monitoring (contd)

• EKG recommendations
• braid or twist the leads
• Place leads as close to the center of the
  magnetic field
• Keep electrodes close together and in the same
  plane
• V5, V6 are close to the QRS axis and far from the
  voltage changes induced by blood flow through the
  transverse aorta.

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Monitoring (contd)

• Pulse oximetry
• originally difficult to measure due to filters
  reducing the signal
• Capnography
• time delay in long circuits (up to 10 seconds)
• prolonged upslope due to length
• NIBP
• insure plastic connectors
• Invasive pressures
• possible as long as transducer is kept close to
  patient to reduce damping by long columns of
  water
• appropriate filtering of signal
• Precordial/Esophageal Stethoscope
• unreliable due to noise

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Monitoring (contd)

• CNS monitoring
• To check integrity of CNS
• Awake or sedated patient neurological
  examination
• Patients under anaesthesia
• SSEP, MEP
• EEG
• BIS

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• Resuscitation
• impossible to manage inside the magnet
• Emergence
• outside the magnet
• recover as per routine

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Efficacy of MRI relative to CT for the initial
diagnosis of central nervous system diseases
__________________________________________________
_____________________ Disease MRI CT ____________
__________________________________________________
_________ Cerebrovascular TIA-RIND Emboli
Ischemic infarction Intracerebral
hemorrhage Trauma Craniocerebral
Spinal Tumours Glioma Low grade
supratentorial High grade
supratentorial Meningioma
supratentorial Pituitary Sinuses
and orbits Brain stem Cervical
spine Cervical disc disease Lumbar
disc disease Regional cerebral bloow
flow Research phase __________________________
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www.anaesthesia.co.in anaesthesia.co.in_at_gmail.co
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