Techniques for Providing Analgesia and Sedation

There is considerable overlap among the types of analgesia and/or sedation currently available. The determination of which is best is based on the type of procedure being performed, patient clinical status, patient preference, interventionalist preference, and availability of an anesthesiologist. The American Society of Anesthesiologists (ASA) Task Force on Sedation and Analgesia by Non-Anesthesiologists has developed practice guidelines⁵ that define sedation and analgesia as a continuum that ranges from minimal sedation through general anesthesia (Table 19-1). Anesthesiologists recognize this continuum and thus are generally able to develop an anesthetic plan that meets the changing needs of the interventionalist. There are four main options of locoregional anesthesia and sedation available to the interventional patient:

All patients vary in terms of their tolerance to discomfort and their ability to cooperate with the IR team. The main indication is to decrease the pain that is experienced during a planned procedure. In nonemergent settings, the patient’s needs can be addressed ahead of time at the consultation/informed consent visit, where the patient’s anxiety can be reduced by a thorough explanation of the analgesia technique. This generally reduces the need for heavy preprocedural sedation. This also has a major impact on the type of analgesia and/or sedation offered; for example, such a discussion before a nontunneled central venous line placement can modify a patient’s initial preference for moderate sedation to a more easily administered local anesthesia. In other instances, the discussion with the patient may result in a request for anesthesia support for a transjugular intrahepatic portosystemic shunt (TIPS) or endovascular aneurysm repair (EVAR). The final component in developing such a plan is how to make the patient most comfortable during the planned procedure.

Contraindications to Analgesia and Sedation

Because of advances in monitoring and pharmaceutical agents, anesthesiologists rarely hear the statement “the patient is too sick for anesthesia.” There are no defined contraindications for the patient to undergo some form of anesthesia. We are, however, occasionally presented with patients who are at such great risk for anesthetic morbidity and mortality that a careful discussion of the risk/benefit ratio of the proposed procedure should be undertaken before proceeding. Additionally, unless the procedure is emergent, all patients should be optimized medically before proceeding with elective procedures, including nothing-by-mouth status.⁵

Equipment

Please visit www.expertconsult.com for a full discussion of this topic.

Immediate access to resuscitation equipment and drugs should be available anywhere sedative or analgesic drugs are administered. There is a thorough summary of equipment and pharmaceutical agents that should be immediately accessible in the ASA Practice Guidelines for Sedation and Analgesia by Non-Anesthesiologists.⁵ If radiology is involved in the purchase of anesthesia equipment (e.g., anesthesia machines, monitoring equipment, infusion pumps), it is recommended that equipment be magnetic resonance imaging (MRI) compatible for maximum flexibility. Strong consideration should be given to the purchase and constant use of capnography to assess patient ventilation. Continuous capnography is an ASA standard for all patients undergoing general anesthesia and is generally used by anesthesiologists to assess ventilation in all MAC cases. Capnography has been shown to improve outcome when used for moderate sedation cases.⁷ In contrast, chest plethysmography, commonly used in interventional suites as a means of assessing respiration, can artifactually indicate ventilation when in fact the patient’s airway is obstructed and no gas exchange is occurring.

Precautions for Providing Analgesia and Sedation

Please visit www.expertconsult.com for a full discussion of this topic.

Medical conditions consistently shown to increase perioperative risk include advanced age, emergent procedures,⁸ history of ischemic heart disease or congestive heart failure, renal insufficiency, and diabetes mellitus.⁹ One of the most consistent systems for predicting perioperative risk⁹ is the ASA Physical Status Classification System (Table e19-1). Not only is the ASA classification predictive of outcome, it is also straightforward, allows physicians and other team members to use a common terminology, and can be helpful in the preoperative categorization and preparation of patients. There are four important concepts regarding sedation and analgesia for interventional patients:

Subcutaneous or intradermal infiltration with a local anesthetic is frequently used in IR to achieve anesthesia of the skin. Local infiltration techniques typically involve extravascular injection of the drug at the procedure site. The discomfort due to the injection of a local anesthetic is often the worst aspect of commonly undertaken minor procedures; this is related to the acidic pH of the infiltrated solution.

Local anesthetics reversibly interrupt neural conduction by blocking sodium channels located on internal neuronal membranes. This results in inhibition of sodium permeability necessary for action potential propagation and pain signal formation.¹¹

All local anesthetics are subdivided clinically into two classes of drugs, the aminoesters and the aminoamides. The aminoesters have an ester linkage between the aromatic end and the intermediate chain. Ester-type local anesthetics undergo rapid metabolism through plasma pseudocholinesterase. These drugs include cocaine, procaine, benzocaine, and tetracaine. The aminoamides contain an amide linkage between the aromatic end and the intermediate chain. These drugs include lidocaine, bupivacaine, mepivacaine, prilocaine, and etidocaine. Amide-type local anesthetics undergo oxidative dealkylation via the hepatic cytochrome P450 enzyme system, as well as conjugation. As such, the clearance of lidocaine and bupivacaine is highly dependent on hepatic blood flow, extraction, and enzyme function. Caution with the use of large volumes is advised in patients with liver dysfunction.¹²

Adverse reactions associated with local anesthetics may arise from direct toxicity, reaction to an added vasoconstrictor or preservative, or allergic reaction. Toxicities result from high blood levels of local anesthetic, usually as a consequence of accidental intravascular injection, increased uptake from perivascular areas, or overdose. Common adverse reactions associated with local anesthetics include central nervous system toxicity, cardiovascular toxicity, neuronal toxicity, vasoconstrictor reactions, and allergic reactions.¹³ Prevention of such reactions is contingent on both appropriate dosage administration, clinical vigilance for early detection of toxic reactions, and prevention methods such as syringe aspiration for blood prior to injection of the anesthetic.

The clinical action of a local anesthetic is often described by its potency, speed of onset, and duration of action. Lidocaine and bupivacaine are two of the most commonly used local anesthetics for skin infiltration in IR patients. Vasoconstrictors such as epinephrine are often added to local anesthetics to prolong the duration of action and improve the quality of the local anesthetic block.

For more information on the primary pharmacology of local anesthetics, please visit www.expertconsult.com.

Lidocaine

Lidocaine (Lidocaine HCL [Hospira Inc., Lake Forest, Ill.]) was the first clinically used aminoamide local anesthetic, introduced into clinical practice by Nils Löfgren, a Swedish chemist. Lidocaine remains the most widely used agent because of its inherent potency, short latency, tissue penetration, and intermediate duration of action. Lidocaine concentration is expressed in percentage as grams per 100 mL (e.g., lidocaine 1% is 1 g/100 mL, which is 10 mg/mL). Available concentrations range from 0.5% to 5%. At concentrations of 0.5% to 1%, lidocaine provides local anesthesia with a duration of 1 to 2 hours. Lidocaine’s versatility is demonstrated by clinical application for almost any regional anesthetic application. It is effective in infiltration, providing topical anesthesia, a peripheral nerve block, and both epidural and spinal blocks. There is a spectrum of toxicities directly related to the lidocaine serum concentration (Fig. 19-1).^1,14