III. SEDATION AND ANESTHESIA

A. Definition of Sedation and Anesthesia. Many NOR procedures are performed under “sedation’ or “monitored anesthesia care” (MAC). The Centers for Medicare and Medicaid Services defines Anesthesia as “the administration of a medication to produce a blunting or loss of, pain perception (analgesia); voluntary and involuntary movements; autonomic function; and memory and/or consciousness.”

B. The Continuum of Anesthesia. Anesthesia exists along a continuum and the transition from minimal sedation to general anesthesia is not clear cut (Table 32-4).

IV. environmental considerations FOR nora

A. Radiography and Fluoroscopy. Fluoroscopy is used to obtain real-time moving images of the internal structures that are widely used in many NOR locations (interventional radiology, cardiac catheterization, electrophysiologic procedures, gastroenterology suite).

B. Hazards of Ionizing Radiation

1. The use of fluoroscopy for both diagnostic and interventional procedures exposes patients and staff to the effects of ionizing radiation (Table 32-5)

2. Staff, including anesthesiologists, must be aware of the hazards of occupational exposure to ionizing radiation (limit time of exposure, increase distance from the source of radiation, use protective shielding).

3. Dosimeters. Anesthesiologists who are involved in NOR cases that involve exposure to ionizing radiation must wear dosimeters to ensure dose limits are not exceeded.

V. INTRAVENOUS CONTRAST AGENTS. Intravenous contrast agents are commonly used in radiologic and MRI to enhance vascular imaging. Radiologic contrast media are iodinated compounds classified according to their osmolarity (high, low or iso-osmolar) and their ionicity (ionic or nonionic). Nonionic contrast agents cause less discomfort on injection and have a lower incidence of adverse reactions.

A. Renal Adverse Reactions

1. Contrast agents are eliminated via the kidneys, and contrast-induced nephropathy (CIN) occurs with an incidence of 7% to 15% (making it the third leading cause of hospital-acquired acute renal failure).

2. CIN is defined as an increase in serum creatinine of 0.5 mg/dL or a 25% increase from the baseline within the first 24 hours, peaking at 5 days.

3. Preventive measures to avoid CIN include adequate hydration, maintaining a good urine output, and using sodium bicarbonate infusions to improve elimination of the contrast agent.

B. Hypersensitivity reactions to contrast media are divided into immediate (<1 hour) and nonimmediate (>1 hour) reactions (Table 32-6).

1. Although widely used, the effectiveness of corticosteroids and antihistamines in preventing hypersensitivity reactions to contrast agents in unselected patients is doubtful.

2. Treatment of severe hypersensitivity reactions includes discontinuing the causative agent; providing supportive therapy, including oxygen, airway securement, cardiovascular support with fluids, vasopressors, and inotropes; and if required, administering bronchodilators.

VI. SPECIFIC PROCEDURES: DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

A. Angiography causes minimal discomfort and may be performed under local anesthesia with or without light sedation. (The patient must remain completely motionless.)

1. Angiography is usually performed via the femoral artery, and liberal use of local anesthetic at the puncture site precludes the need for intravenous (IV) analgesia.

2. The injection of contrast media into the cerebral arteries may cause discomfort, burning, or pruritus around the face and eyes. Hypotension and bradycardia may also occur.

B. Interventional Neuroradiology (see Table 32-2)

1. Cerebral aneurysms and arteriovenous malformations (AVMs) are particularly amenable to occlusive endovascular treatments.

2. Interventional neuroradiology is the standard treatment for posterior circulation aneurysms, especially basilar artery tip aneurysms.

3. Procedural and Anesthetic Technique Considerations in Interventional Neuroradiology

a. For most interventional neuroradiologic procedures, arterial access is gained using a 6- or 7-French gauge sheath via the femoral or, rarely, the carotid or axillary artery.

b. Anticoagulation (heparin) is required during and up to 24 hours after interventional radiologic procedures to prevent thromboembolism.

c. General anesthesia and conscious sedation (propofol infusions, dexmedetomidine) are both suitable techniques for interventional neuroradiology depending on the complexity of the procedure, the need for blood pressure manipulation, and the need for intraprocedural assessment of neurologic function.

d. Controlled hypotension is often requested to facilitate embolization of AVMs; beta-blockers and hydralazine are commonly used.

e. The superselective anesthesia functional examination (SAFE) is an extension of the Wada test performed before therapeutic embolization to ensure the catheter tip has not been placed in a vessel that supplies an eloquent area of the brain or spinal cord.

C. Computed tomography (CT) is painless, and most adults do not require sedation or anesthesia (absolute requirement for the patient to remain motionless) (see Table 32-1). Contrast agents for CT imaging may be administered orally (risk of full stomach).

D. Radiofrequency Ablation

1. The majority of these procedures are tolerated without sedation.

2. High-frequency jet ventilation has been used for patients undergoing anesthesia for radiofrequency ablation of liver (minimizes liver motion)

E. Transjugular intrahepatic portosystemic shunt is created via a catheter inserted in the internal jugular vein and directed into the liver (functions to decompress the portal circulation in patients with portal hypertension) (Table 32-7).

F. Magnetic Resonance Imaging (MRI)

1. Hazards of MRI. MRI is devoid of the risks related to ionizing radiation. However, ferromagnetic equipment such as IV poles, gas cylinders, laryngoscopes, and pens become potentially lethal projectiles. Considerable noise is generated by the rapidly alternating currents of the MRI scanner. (Patients and staff should wear ear protection.)

a. Patient monitors, ventilator equipment, and electrical infusion pumps may all malfunction when they come too close to the magnetic field.

b. The electrocardiogram is sensitive to the changing magnetic signals, and it is nearly impossible to eliminate all artifacts.

2. Anesthetic Technique. Sedation for MRI may be provided as either oral sedation with benzodiazepines or minimal IV sedation.

3. Pediatric Sedation and Anesthesia for MRI and CT Scan

a. Most children (especially those younger than 5 years) require sedation (chloral hydrate, midazolam, ketamine, dexmedetomidine) or general anesthesia for successful acquisition of MRI or CT images.

b. General anesthesia with formal airway management should be considered when patency of the airway during sedation cannot be guaranteed (those with obstructive sleep apnea or symptomatic gastroesophageal reflux disease).

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