Anesthetics play a crucial role in ensuring patient comfort and managing pain during medical procedures, tests, and surgeries. Each type of anesthetic has unique characteristics in terms of its mechanism of action, indications, duration of effect, and potential side effects. Creating a suitable anesthesia regimen depends heavily on the patient’s medical history and the nature of the procedure, including the expected discomfort and duration of the surgery.

The American Society of Anesthesiologists (ASA) describes four main types of anesthesia: local anesthesia, regional anesthesia, IV sedation, and general anesthesia. Local anesthesia, such as lidocaine, is used to numb a specific area without affecting the patient’s consciousness. It is commonly employed in minor procedures like mole removals. Regional anesthesia, such as an epidural, numbs a larger area without causing unconsciousness and is frequently used in childbirth. IV sedation, involving drugs like midazolam, induces varying degrees of decreased alertness, ranging from minimal to deep, and is suitable for procedures like colonoscopies. General anesthesia induces complete unconsciousness and is typically used for major surgeries of longer duration.

Understanding the pharmacokinetics of each agent, particularly their duration of action, is crucial for selecting the appropriate anesthesia for each surgery. Local anesthetics, for instance, block sodium channels, preventing impulse transmission and leading to numbness for a relatively short period. Local anesthetics can be categorized based on where they are metabolized, with amino amides like bupivacaine and ropivacaine being broken down in the liver and amino esters like procaine being broken down by plasma cholinesterases. The half-life of local anesthetics varies, with lidocaine having a half-life of around 90 minutes, and bupivacaine nearly 300 minutes. Selection of a particular agent must take into account the procedure’s duration. For lengthy procedures, bupivacaine is the logical choice, but studies have found it to be associated with relatively high injection pain. One strategy is to provide the initial 60 minutes of anesthesia with a less irritating agent (lidocaine or prilocaine) and then reinject the anesthetized tissue with bupivacaine to provide analgesia well into the postoperative period. Common clinical uses of local anesthetics are minor outpatient procedures, such as wisdom teeth extraction or skin biopsies, which may take 30 minutes to an hour.

When the surgery requires powerful pain relief and a controlled state for a longer duration (i.e. several hours), general anesthesia is often the best option, provided there are no contraindications. It is typically initiated with IV sedatives and analgesics, followed by the maintenance of anesthesia with inhalational agents. Intravenous induction is often preferred by patients, but inhalational induction is sometimes necessary, especially in children or when IV access is challenging. Propofol, a rapid-acting agent, is commonly used for induction, although it can lead to respiratory depression and requires close monitoring. Following the induction of general anesthesia, additional agents are required to sustain the anesthetic state for the remainder of the surgery due to the brief duration of action of most induction agents.

Sevoflurane is frequently chosen as an inhalational agent for maintaining anesthesia during surgical procedures lasting less than two hours because of its low blood and tissue solubilities. When compared to older, potent inhalation agents with higher blood and tissue solubilities (such as isoflurane), sevoflurane offers more rapid uptake during induction, changes in anesthetic depth during maintenance, and emergence. However, a drawback of using sevoflurane as the primary maintenance agent for longer surgical procedures is its cost. Isoflurane is commonly used to maintain general anesthesia during longer surgical cases. It is more potent than sevoflurane or desflurane, cost-effective, and widely available. However, isoflurane has a slower onset during induction and slower recovery due to its higher blood and tissue solubility, which can lead to prolonged emergence, especially after long periods of administration.

The selection of anesthetic agents is a critical aspect of ensuring patient comfort and safety during medical procedures. To make informed decisions on their usage, healthcare practitioners should understand the characteristics and pharmacokinetics of each type of anesthetic.

References

1. Aldecoa C, Bettelli G, Bilotta F, Sanders RD, Audisio R, Borozdina A, Cherubini A, Jones C, Kehlet H, MacLullich A, Radtke F, Riese F, Slooter AJ, Veyckemans F, Kramer S, Neuner B, Weiss B, Spies CD. European Society of Anaesthesiology evidence-based and consensus-based guideline on postoperative delirium. Eur J Anaesthesiol. 2017 Apr;34(4):192-214. doi: 10.1097/EJA.0000000000000594.
2. Berger M, Schenning KJ, Brown CH 4th, Deiner SG, Whittington RA, Eckenhoff RG, Angst MS, Avramescu S, Bekker A, Brzezinski M, Crosby G, Culley DJ, Eckenhoff M, Eriksson LI, Evered L, Ibinson J, Kline RP, Kofke A, Ma D, Mathew JP, Maze M, Orser BA, Price CC, Scott DA, Silbert B, Su D, Terrando N, Wang DS, Wei H, Xie Z, Zuo Z; Perioperative Neurotoxicity Working Group. Best Practices for Postoperative Brain Health: Recommendations From the Fifth International Perioperative Neurotoxicity Working Group. Anesth Analg. 2018 Dec;127(6):1406-1413. doi: 10.1213/ANE.0000000000003841. PMCID: PMC6309612.
3. American Society of Anesthesiologists Task Force on Intraoperative Awareness. Practice advisory for intraoperative awareness and brain function monitoring: a report by the american society of anesthesiologists task force on intraoperative awareness. Anesthesiology. 2006 Apr;104(4):847-64. doi: 10.1097/00000542-200604000-00031. PMID: 16571982.
4. Zorrilla-Vaca A, Healy RJ, Wu CL, Grant MC. Relation between bispectral index measurements of anesthetic depth and postoperative mortality: a meta-analysis of observational studies. Can J Anaesth. 2017 Jun;64(6):597-607. doi: 10.1007/s12630-017-0872-6. Epub 2017 Mar 30. PMID: 28361391.
5. Kertai MD, White WD, Gan TJ. Cumulative duration of “triple low” state of low blood pressure, low bispectral index, and low minimum alveolar concentration of volatile anesthesia is not associated with increased mortality. Anesthesiology. 2014 Jul;121(1):18-28. doi: 10.1097/ALN.0000000000000281. PMID: 24936920.
6. Sessler DI, Sigl JC, Kelley SD, Chamoun NG, Manberg PJ, Saager L, Kurz A, Greenwald S. Hospital stay and mortality are increased in patients having a “triple low” of low blood pressure, low bispectral index, and low minimum alveolar concentration of volatile anesthesia. Anesthesiology. 2012 Jun;116(6):1195-203. doi: 10.1097/ALN.0b013e31825683dc. PMID: 22546967.
7. Short TG, Campbell D, Frampton C, Chan MTV, Myles PS, Corcoran TB, Sessler DI, Mills GH, Cata JP, Painter T, Byrne K, Han R, Chu MHM, McAllister DJ, Leslie K; Australian and New Zealand College of Anaesthetists Clinical Trials Network; Balanced Anaesthesia Study Group. Anaesthetic depth and complications after major surgery: an international, randomised controlled trial. Lancet. 2019 Nov 23;394(10212):1907-1914. doi: 10.1016/S0140-6736(19)32315-3. Epub 2019 Oct 20. PMID: 31645286.