Anesthetic Agents: General and Local Anesthetics T IMOTHY J
Total Page:16
File Type:pdf, Size:1020Kb
Chapter 16 Anesthetic Agents: General and Local Anesthetics T IMOTHY J. MAHER Drugs Covered in This Chapter Inhaled general anesthetics • Propofol • Levobupivacaine • Ether • Fospropofol • Lidocaine • Halothane • Thiopental • Prilocaine • Desflurane Local anesthetics • Procaine • Ropivacaine • Enflurane • Articaine • Tetracaine • Isoflurane • Benzocaine • Methoxyflurane • Bupivacaine • Sevoflurane • Chloroprocaine • Nitrous oxide • Cocaine Intravenous general anesthetics • Dibucaine • Etomidate • Dyclonine • Ketamine • Mepivacaine Abbreviations BTX, batrachotoxin GABA, g-aminobutyric acid NO, nitric oxide CNS, central nervous system HBr, hydrobromic acid NMDA, N-methyl-D-aspartate COCl2, phosgene HCl, hydrochloric acid PABA, p-aminobenzoic acid EEG, electroencephalograph MAC, minimum alveolar concentration PCP, phencyclidine EMLA, Eutectic Mixture of a Local Na/K-ATPase, sodium-potassium STX, saxitoxin Anesthetic adenosine triphosphatase TTX, tetrodotoxin 508 LLemke_Chap16.inddemke_Chap16.indd 550808 112/9/20112/9/2011 44:14:08:14:08 AAMM CHAPTER 16 / ANESTHETIC AGENTS: GENERAL AND LOCAL ANESTHETICS 509 SCENARIO Paul Arpino, R.Ph. CDL is a 70-year-old obese man scheduled for carpal tunnel sur- During the preoperative assessment before the scheduled day of gery. A review of his medical file indicates a history of obstruc- surgery, the team discovers that CDL has an undefined allergy tive sleep apnea and benign prostatic hypertrophy (BPH). CDL to procaine (Novocain) and that he experienced severe blistering sleeps with a continuous positive pressure airway device and his after a dental procedure many years ago and was told he cannot BPH is treated with tamsulosin, 0.4 mg daily. Given that patients receive “drugs like Novocain again.” with sleep apnea are at high risk for respiratory depression, the clinical team decides that a peripheral nerve block would be a (The reader is directed to the clinical solution and chemical analy- better alternative to both neuraxial and general anesthesia. sis of this case at the end of the chapter). INTRODUCTION nitrous oxide at a public demonstration of “laughing gas.” One of the volunteers, a pharmacy clerk named Anesthesia, defi ned as a loss of sensation with or with- Samuel Cooley, injured his leg while under the infl uence out loss of consciousness, can be effectively achieved with of this gas and appeared to experience no pain. The next a wide range of drugs with very diverse chemical struc- day, Wells inhaled the gas himself and, with the aid of a tures. The list of such compounds includes not only the colleague, had one of his own teeth extracted without classic anesthetic agents, such as the general and local any sensation of pain. Wells then began routinely using anesthetics, but also many central nervous system (CNS) nitrous oxide for dental procedures in his own practice. depressants, such as analgesics, sedative/hypnotics (bar- In 1845, he attempted to demonstrate the anesthetic biturates and benzodiazepines), anticonvulsants, and effects of nitrous oxide at the Massachusetts General skeletal muscle relaxants. Although various mechanisms Hospital in Boston. This demonstration was considered of action are attributed to these agents, ultimately they all to be a failure, however, because the patient cried out produce their anesthetic actions by interfering with con- in the middle of the procedure. Following this unfortu- duction in sensory neurons and sometimes also motor nate incident, the use of nitrous oxide was minimal until neurons. Many of these agents are routinely used today it resurfaced in dental practice during the mid-1860s, in clinical practice to facilitate surgical and medical pro- when it was combined with oxygen and made available cedures. This chapter will focus on those agents typically in steel cylinders. This gas is still commonly used today, classifi ed as “general” and “local” anesthetics. especially in combination with other anesthetic and anal- gesic agents. GENERAL ANESTHETICS The general anesthetic that gained greatest popularity shortly after the failed demonstration of Wells was diethyl Prior to the mid-1800s, pain-producing surgical and ether. William Morton, a Boston dentist, was familiar at dental procedures typically were undertaken without the time with the use of nitrous oxide by Wells. He also the aid of effective anesthetic agents. Chemical methods had heard of the interesting effects of diethyl ether and available at the time included intoxication with ethanol, began to experiment on animals and himself with this hashish (cannabis), or opium, whereas physical meth- volatile liquid. In 1846, he was allowed an opportunity ods included packing a limb in ice, creating ischemic to demonstrate the anesthetic actions of diethyl ether at, conditions with tourniquets, inducing unconsciousness again, the Massachusetts General Hospital. In the famed by a blow to the head, or the most common technique, “Ether Dome,” which still stands today, Morton admin- employing strong-armed assistants to hold down the help- istered diethyl ether with a specially designed delivery less patient during the entire painful surgical procedure. device to the nervous patient, and the surgical proce- Additionally, at this time, many practicing physicians had dure was performed without apparent pain. Following been erroneously taught that pain was a requirement for this demonstration, word of its success spread quickly, effective healing; therefore, the observation of a patient and soon, dental and medical practices throughout the in terrible pain was viewed as part of the normal healing United States and Europe were employing diethyl ether process. These factors, along with the lack of knowledge as a general anesthetic agent. Today, diethyl ether is no regarding aseptic techniques or the availability of suit- longer used in procedures because of its toxicity and able infection-fi ghting agents, made surgical procedures dangerous physical properties (e.g., it is fl ammable and a last resort approach to treating disease. explosive!). There have been many accounts of the fi rst dem- Other general anesthetic agents that enjoyed early pop- onstration by the Hartford dentist Horace Wells of the ularity were chloroform and cyclopropane. Chloroform use of nitrous oxide as a general anesthetic for surgery vapor depresses the CNS of a patient, allowing a doctor in 1844. Wells fi rst observed the anesthetic actions of to perform various otherwise painful surgical procedures. LLemke_Chap16.inddemke_Chap16.indd 550909 112/9/20112/9/2011 44:14:09:14:09 AAMM 510 PART III / PHARMACODYNAMIC AGENTS CLINICAL SIGNIFICANCE Anesthetics are a structurally of new agents is aimed at reducing adverse reactions, main- diverse class of medications that taining optimal physiologic conditions during procedures, and enable clinicians to perform sur- minimizing postoperative complications related to the proce- gery and other noxious procedures. dure itself. The study of medicinal chemistry gives us hope Understanding the essential components of for future treatment options, and knowledge of structure- the anesthetic state (i.e., immobilization, analgesia, and amnesia) activity relationships fosters the development of new medi- as well as the medicinal chemistry of the various anesthetic agents cations and administration techniques. New generations of allows the clinician to optimize therapy to meet patient specific drugs are being created by modifying the structures of exist- needs. The patient undergoing a minimally invasive ambulatory ing compounds to improve the side effect and pharmacoki- surgical procedure may only require a local anesthetic with adjunc- netic profiles. Clinicians will have to stay up to date with new tive pain control. Alternatively, patients undergoing a major surgi- developments in anesthesia practices, the molecular actions of cal procedure may require general anesthesia with several different anesthetics, and the pharmacokinetic properties of the drugs classes of anesthetics as well as several adjunctive medications to to provide the best therapeutic outcomes for patients. counteract deleterious emergence reactions related to the anesthet- ics. In both cases, a thorough understanding of the basic chemical Paul Arpino, RPh properties of the drugs and their respective mechanisms of action Harvard Medical School will prove invaluable to making appropriate clinical decisions. Department of Pharmacy The practice of anesthesia is typically not considered to be Massachusetts General Hospital therapeutic; therefore, the practice as well as the development Boston, MA In 1847, the Scottish obstetrician James Young Simpson and Gillespie subsequently further subdivided these fi rst used chloroform for general anesthesia during child- stages (Fig. 16.1), as described in the following sections. birth. The use of chloroform during surgery expanded rapidly thereafter in Europe. In the United States, chlo- Stage 1: Analgesia roform replaced diethyl ether as an anesthetic at the Characterized by a mild depression of higher cortical beginning of the 20th century; however, it was quickly aban- neurons, this stage is suitable for minor surgical pro- doned due to its cardio and CNS toxicity. Cyclopropane cedures that do not require signifi cant neuromuscular is a hydrocarbon with anesthetic properties like diethyl relaxation. Depression of thalamic centers probably ether, except it is also explosive and is no longer used. As accounts for the observed analgesia, because many of the described later in this chapter,