Introduction

Anesthesia for neuroradiology forms an important part of neuroanesthesia services. This entails adequate understanding of the 4 P s; patient, pathology, procedure, and periprocedural environment. Both the number and complexity of diagnostic and interventional cases performed in neuroradiology under anesthesia are increasing. In our hospital, neuroradiological procedures performed in the past year constituted a significant 34% of operation theater (OT) procedures under anesthesia (Annual report 2013, NIMHANS, Bangalore). The areas of work include computed tomographic (CT) imaging and CT-guided stereotactic biopsy, stereotactic radiosurgery (SRS), magnetic resonance imaging (MRI), and diagnostic and interventional neuroradiology. Also, as compared to a decade ago, the volume within neuroradiology department where anesthesia services are provided has increased by a whopping 50% (Annual reports 2003 and 2013, NIMHANS, Bangalore). The pattern of procedures where anesthesia services are provided has also changed, with sedation for CT imaging decreasing and anesthesia for interventional procedures increasing.

Issues Relating to Anesthesia Care in Neuroradiology

Small crowded area with bulky equipment results in limited access to the patient for monitoring and interventions. Poor lighting, inadequate suction system, lack of medical air supply and single O ₂ pipeline, limited electrical points (for patient warming system, anesthesia workstation, monitors, infusion pumps, depth of anesthesia monitors, etc.) are other issues faced by anesthesiologists working in these areas. Breathing circuits, intravenous tubing systems, and monitoring cables should have sufficient length to prevent disconnections as patients are moved back and forth frequently during image acquisition/intervention ( Fig. 30.1 ). Separate intravenous access should be used to administer anesthetic drugs and fluids and to administer heparin/contrast agents. Manpower availability is also an issue with residents/trainees generally providing anesthesia service in neuroradiology units without additional trained medical or technical help and one nursing staff being shared between the anesthesiologist and the radiologist. Standard anesthesia monitoring like neuromuscular function and temperature monitoring and special monitors like bispectral index (BIS) or equipment like fiber-optic intubating scope and intubating laryngeal mask airway (LMA) are generally not available. Last, more flexibility and availability is expected from the elective team for emergency diagnostic/interventional procedures beyond the routine work hours. As a result, it is expected that more complications are possible compared to the OT including death, pulmonary aspiration, hypothermia, hypovolemia, airway complications, anaphylaxis, and radiation/electromagnetic exposure risks. One in every eighty-nine procedures had an event that was considered potentially harmful during sedation/anesthesia in 30,000 pediatric procedures outside the OT.

Magnetic resonance imaging suite with long intravenous and breathing circuit tubing systems, and monitoring cables.

Anesthesia for Computed Tomographic Study

Various sedative drugs alone or in combination have been used for CT imaging in children and uncooperative adults. The drugs used have changed over the past several decades. Monitoring has also improved during the same period. However, it is imperative to adhere to fasting guidelines, follow minimum monitoring standards during sedation for CT imaging, and have the anesthesiologist available to cater to an emergency. While minimal and moderate sedation can be performed by trained nurses, deep sedation and anesthesia require an anesthesiologist to administer them. The most commonly used sedatives in our hospital are triclofos (pedicloryl) and promethazine (phenergan) in infants, oral midazolam or intramuscular ketamine in children, and intravenous propofol or thiopentone in low doses in adults. Aerosolized nasal midazolam has also been found to be safe and effective for CT study in infants and children.

Anesthesia for Magnetic Resonance Imaging Study

MRI is increasingly performed due to its better anatomical resolution and lack of radiation exposure. However, prolonged duration for image acquisition and claustrophobic environment necessitates sedation in children, uncooperative adults, claustrophobic patients, and those who cannot lie motionless due to pain, involuntary movement, or cognitive impairment. MRI incorporates the use of static and gradient magnetic fields with radiofrequency pulses to produce precise images of the body. Magnetic field strengths in current clinical MRI systems range from 0.5 to 3.0 T.

Safety Issues

Projectile injuries and device malfunction are two issues that can put patient/staff at risk in an MRI environment. Ferromagnetic objects or equipment like pens, needles, coins, laryngoscopes, and stethoscopes due to their projectile capabilities pose a significant risk in patients undergoing MRI. Patients with implanted ferromagnetic devices or objects like pacemakers, aneurysm clips, implantable defibrillators/cardioverters, implantable infusion pumps, cochlear implants, and intraorbital/orthopedic metallic bodies are generally contraindicated for MRI studies as magnetic fields of the MRI scanner can potentially affect the function and safety of these devices ( Fig. 30.2 ). The concerned doctor or manufacturer should be consulted regarding the impact of such electromagnetic exposure on functioning of these devices. Other objects like credit/debit cards, pen drives, wrist watches, and mobile phones can malfunction when exposed to high magnetic fields. Apart from safety concerns, these objects can result in poor-quality images and artifacts. Proper education of all those working in the MRI area and use of handheld magnet to detect if the object is safe reduces adverse events. The other safety issue is that from gadolinium, an MRI contrast agent, which can occasionally produce allergic reaction.

Display sign in magnetic resonance imaging (MRI) area showing unsafe (MRI incompatible) devices.

Issues Related to Monitoring and Anesthetic Delivery

Monitoring standards similar to the one in OT must be maintained for MRI procedures under sedation/anesthesia. However, monitors, infusion pumps, and anesthesia machine need to be MRI compatible ( Fig. 30.3 ). Those units that do not have these facilities can use conventional systems with extralong cables, lines, and circuits channeled from outside the MRI scanner room. A slave monitor outside the MRI scanner room can overcome poor visibility and distance from the patient ( Fig. 30.4 ). Additionally, a video camera permits visual contact with the patient especially with respect to respiration or patient movement ( Fig. 30.5 ). Infusion pumps placed outside the scanner can also help in titration of anesthetic depth and infusion rate of fluid/vasopressor from outside. Alternatively, if it is MRI compatible, the anesthesiologist can enter the scanner room between the sequences to manipulate the pump.

Magnetic resonance imaging (MRI) room with MRI compatible anesthesia and related equipments.

​A multi-parameter slave monitor outside the magnetic resonance imaging room.

Patient viewing camera based monitor outside the magnetic resonance imaging room.

Patient Comfort

Children including infants can occasionally cooperate for MRI study without sedation. Presence of a parent inside the scanner room, feeding and wrapping an infant, reduction in noise from the scanner using music through head phones/ear plugs, maintaining the patient warm, positioning patients as per their comfort (patient with back pain), and proper explanation of the procedure, on some occasions, obviates the need for sedation/anesthesia. This in turn avoids the need for fasting and risks associated with sedation/anesthesia, facilitates faster discharge, and reduces cost to the patient.

Management of Complications

Desaturation with/without respiratory arrest and hemodynamic compromise (hypotension/bradycardia) are the two most common potentially life-threatening complications seen during sedation in the MRI room. Airway obstruction from deep sedation and apnea during bolus administration of the anesthetic drug are the most common reasons for oxygen desaturation. Reduction in infusion rate, head and neck manipulation, and artificial airway placement (Guedel or nasopharyngeal airway, LMA/tracheal tube) are required in such situations. Inadvertent continuation of high concentration of inhalational agents in intubated patients or high infusion rate of intravenous drugs, discontinuation of vasopressors in patients who were receiving it, or inadequate monitoring of blood pressure (BP)/heart rate are some of the reasons for cardiovascular collapse during MRI study. Most complications in MRI room are avoidable if vigilant monitoring is performed. Occasionally, oxygen saturation (SpO ₂ ) reading may not be obtainable due to poor circulation, cold temperature, ill-fitting probe, or patient movement. Electrocardiographic (ECG) recording might interfere during some MRI sequences and produce artifact ( Fig. 30.6 ). End-tidal carbon dioxide (ETCO ₂ ) waveforms and value may not be reliable because of dilution from room air or poor contact within the oxygen mask. BP is generally not monitored in small children mostly because inflation of the cuff can awaken these children. However, it is advisable to ensure that all the monitors [ECG, SpO ₂ , ETCO ₂ , and non-invasive BP (NIBP)] be continuously functional to avoid complications. When monitoring fails, the imaging sequence should be terminated and the possible reason for malfunction should be verified physically. The periprocedural risks can be reduced by good presedation assessment, close physiological monitoring, using appropriate equipment, having a safe environment, and the presence of well-trained personnel.

Multi-parameter monitor showing electrocardiogram artifact during a magnetic resonance imaging sequence.

Anesthesia for Diagnostic Angiography

Anesthetic service is sought during angiography in children (Moyamoya disease), in uncooperative adults [poor-grade subarachnoid hemorrhage (SAH)], or during spinal angiography (to produce apnea during contrast injections). Various drugs like fentanyl, midazolam, droperidol, and diazepam have been used alone or in combination to provide sedation during digital subtraction angiography with varying success and adverse effects. Propofol or dexmedetomidine as infusion is used because of easy titration of sedation depth, hemodynamic stability, and rapid offset of effects for neurological assessment. Dexmedetomidine is associated with less airway compromise and patient movement than propofol with similar recovery profile and hence may be preferred in patients with SAH. General anesthesia (GA) with either LMA or tracheal intubation is an alternative but a less commonly used technique for diagnostic angiography, and is administered when intermittent apnea is needed during spinal angiography. Zhang et al. in their study in 20 patients demonstrated that ETCO ₂ measurements sampled from the nose and the pharynx were accurate and reliable as compared to partial pressure of arterial CO ₂ in nonintubated patients during diagnostic angiography.

For assessing lateralization or shift of neurological function (language/speech/memory), prior to surgery or before embolization of arteriovenous malformation (AVM), Wada or superselective anesthesia for functional evaluation (SAFE) test is performed, respectively. Patient should remain awake during the procedure and cooperate for this testing. An intra-arterial (carotid or selective vessel) propofol in 20 mg increments is used in our center. For assessing neurological function before permanent sacrifice of the vessel, balloon occlusion test (BOT) is performed with the patient awake. Monitoring for neurological deficits and providing induced hypotension (up to 30% of baseline) with intravenous drugs (nitroglycerine/nitroprusside or labetalol) during carotid occlusion to assess ischemic tolerability and adequacy of collateral circulation are the important functions performed by the anesthesiologist. Placement of an arterial line helps in titrating BP during this procedure. Occasionally, it may not be possible to perform this procedure when awake; in those conditions, neurophysiological monitoring will be useful. Li et al. reported use of motor evoked potential (MEP) in monitoring motor pathway integrity and predicting cerebral blood flow changes under intravenous anesthesia for selective methohexital testing prior to AVM embolization.