METHODS OF ANESTHESIA ADMINISTRATION
Anesthetics produce anesthesia because they are delivered to the brain at a high partial pressure that enables them to cross the blood–brain barrier. Relatively large amounts of anesthetic must be administered during induction and the early maintenance phases because the anesthetic is recirculated and deposited in body tissues. As these sites become saturated, smaller amounts of the anesthetic agent are required to maintain anesthesia because equilibrium or near equilibrium has been achieved between brain, blood, and other tissues.
Anything that diminishes peripheral blood flow, such as vaso-constriction or shock, may reduce the amount of anesthetic re-quired. Conversely, when peripheral blood flow is unusually high, as in the muscularly active or the apprehensive patient, induction is slower and greater quantities of anesthetic are re-quired because the brain receives a smaller quantity of anesthetic.
Liquid anesthetics may be administered by mixing the vapors with oxygen or nitrous oxide–oxygen and then having the patient inhale the mixture (Townsend, 2002). The vapor is administered to the patient through a tube or a mask. The inhalation anesthetic may also be administered through a laryngeal mask (Fig. 19-1), a flexible tube with an inflatable silicone ring and cuff that can be inserted into the larynx (Fortunato, 2000). The endotracheal technique for administering anesthetics consists of introducing a soft rubber or plastic endotracheal tube into the trachea, usually by means of a laryngoscope. The endotracheal tube may be in-serted through either the nose or mouth. When in place, the tube seals off the lungs from the esophagus so that if the patient vom-its, stomach contents do not enter the lungs.
General anesthesia can also be produced by the intravenous in-jection of various substances, such as barbiturates, benzodiaze-pines, nonbarbiturate hypnotics, dissociative agents, and opioid agents (Aranda & Hanson, 2000; Townsend, 2002). These med-ications may be administered for induction (initiation) or main-tenance of anesthesia. They are often used along with inhalation anesthetics but may be used alone. They can also be used to pro-duce moderate sedation. Intravenous anesthetics are presented in Table 19-2.
An advantage of intravenous anesthesia is that the onset of anesthesia is pleasant; there is none of the buzzing, roaring, or dizziness known to follow administration of an inhalation anesthetic. For this reason, induction of anesthesia usually begins with an intravenous agent and is often preferred by patients who have experienced various methods. The duration of action is brief, and the patient awakens with little nausea or vomiting. Thiopental is usually the agent of choice, and it is often administered with other anesthetic agents in prolonged procedures.
Intravenous anesthetic agents are nonexplosive, they require little equipment, and they are easy to administer. The low inci-dence of postoperative nausea and vomiting makes the method useful in eye surgery because vomiting would increase intraocular pressure and endanger vision in the operated eye. Intravenous anesthesia is useful for short procedures but is used less often for the longer procedures of abdominal surgery. It is not indicated for children, who have small veins and require intubation because of their susceptibility to respiratory obstruction.
A disadvantage of an intravenous anesthetic such as thiopen-tal is its powerful respiratory depressant effect. It must be ad-ministered by a skilled anesthesiologist or anesthetist and only when some method of oxygen administration is available imme-diately in case of difficulty. Sneezing, coughing, and laryngospasm are sometimes noted with its use.
Intravenous neuromuscular blockers (muscle relaxants) block the transmission of nerve impulses at the neuromuscular junction of skeletal muscles. Muscle relaxants are used to relax muscles in abdominal and thoracic surgery, relax eye muscles in certain types of eye surgery, facilitate endotracheal intubation, treat laryngo-spasm, and assist in mechanical ventilation.
Purified curare was the first widely used muscle relaxant; tubocu-rarine was isolated as the active ingredient. Succinylcholine was later introduced because it acts more rapidly than curare. Several other agents are also used (Table 19-3). The ideal muscle relax-ant has the following characteristics:
· It is nondepolarizing (noncompetitive agent), with an onset and duration of action similar to succinylcholine but with-out its problems of bradycardia and cardiac dysrhythmias (Townsend, 2002).
· It has a duration of action between those of succinylcholine and pancuronium.
· It lacks cumulative and cardiovascular effects.
· It can be metabolized and does not depend on the kidneys for its elimination.
Regional anesthesia is a form of local anesthesia in which an anes-thetic agent is injected around nerves so that the area supplied by these nerves is anesthetized. The effect depends on the type of nerve involved. Motor fibers are the largest fibers and have the thickest myelin sheath. Sympathetic fibers are the smallest and have a minimal covering. Sensory fibers are intermediate. Thus, a local anesthetic blocks motor nerves least readily and sympa-thetic nerves most readily. An anesthetic cannot be regarded as having worn off until all three systems (motor, sensory, and au-tonomic) are no longer affected.
The patient receiving spinal or local anesthesia is awake and aware of his or her surroundings unless medications are given to produce mild sedation or to relieve anxiety. The nurse must avoid careless conversation, unnecessary noise, and unpleasant odors; these may be noticed by the patient in the OR and may con-tribute to a negative view of the surgical experience. A quiet en-vironment is therapeutic. The diagnosis must not be stated aloud if the patient is not to know it at this time.
There are many types of conduction blocks, depending on the nerve groups affected by the injection. Epidural anesthesia is achieved by injecting a local anesthetic into the spinal canal in the space surrounding the dura mater (Fig. 19-2). Epidural anesthe-sia also blocks sensory, motor, and autonomic functions, but it is differentiated from spinal anesthesia by the injection site and the amount of anesthetic used. Epidural doses are much higher be-cause the epidural anesthetic does not make direct contact with the cord or nerve roots.
An advantage of epidural anesthesia is the absence of headache that occasionally results from subarachnoid injection. A disadvantage is the greater technical challenge of introducing the anesthetic into the epidural rather than the subarachnoid space. If inadvertent subarachnoid injection occurs during epidural anesthesia and the anesthetic travels toward the head, high spinal anesthesia can result; this can produce severe hypotension and respiratory depression and arrest. Treatment of these compli-cations includes airway support, intravenous fluids, and use of vasopressors. Other types of nerve blocks include:
· Brachial plexus block, which produces anesthesia of the arm
· Paravertebral anesthesia, which produces anesthesia of the nerves supplying the chest, abdominal wall, and extremities
· Transsacral (caudal) block, which produces anesthesia of the perineum and, occasionally, the lower abdomen
Spinal anesthesia is a type of extensive conduction nerve block that is produced when a local anesthetic is introduced into the subarachnoid space at the lumbar level, usually between L4 and L5 (see Fig. 19-2). It produces anesthesia of the lower extremi-ties, perineum, and lower abdomen. For the lumbar puncture procedure, the patient usually lies on the side in a knee–chest po-sition. Sterile technique is used as a spinal puncture is made and the medication is injected through the needle. As soon as the in-jection has been made, the patient is positioned on his or her back. If a relatively high level of block is sought, the head and shoulders are lowered.
The spread of the anesthetic agent and the level of anesthesia depend on the amount of fluid injected, the speed with which it is injected, the positioning of the patient after the injection, and the specific gravity of the agent. If the specific gravity is greater than that of cerebrospinal fluid (CSF), the agent moves to the de-pendent position of the subarachnoid space. If the specific grav-ity is less than that of CSF, the anesthetic moves away from the dependent position. The anesthesiologist or anesthetist controls the spread of the agent. Generally, the agents used are procaine, tetracaine (Pontocaine), lidocaine (Xylocaine), and bupivacaine (Marcaine) (Table 19-4).
A few minutes after induction of a spinal anesthetic, anes-thesia and paralysis affect the toes and perineum and then grad-ually the legs and abdomen. If the anesthetic reaches the upper thoracic and cervical spinal cord in high concentrations, a tem-porary partial or complete respiratory paralysis results. Paralysis of the respiratory muscles is managed by mechanical ventilation until the effects of the anesthetic on the respiratory nerves have worn off.
Nausea, vomiting, and pain may occur during surgery when spinal anesthesia is used. As a rule, these reactions result from ma-nipulation of various structures, particularly those within the ab-dominal cavity. The simultaneous intravenous administration of a weak solution of thiopental and inhalation of nitrous oxide may prevent such reactions.
Headache may be an after-effect of spinal anesthesia. Several factors are involved in the incidence of headache: the size of the spinal needle used, the leakage of fluid from the subarachnoid space through the puncture site, and the patient’s hydration sta-tus. Measures that increase cerebrospinal pressure are helpful in relieving headache. These include keeping the patient lying flat, quiet, and well hydrated.
In continuous spinal anesthesia, the tip of a plastic catheter re-mains in the subarachnoid space during the surgical procedure so that more anesthetics may be injected as needed. This technique al-lows greater control of the dosage, but there is greater potential for postanesthetic headache because of the large-gauge needle used.
Infiltration anesthesia is the injection of a solution containing the local anesthetic into the tissues at the planned incision site. Often it is combined with a local regional block by injecting the nerves immediately supplying the area. The advantages of local anesthe-sia are as follows:
· It is simple, economical, and nonexplosive.
· Equipment needed is minimal.
· Postoperative recovery is brief.
· Undesirable effects of general anesthesia are avoided.
· It is ideal for short and superficial surgical procedures.
Local anesthesia is often administered in combination with epinephrine. Epinephrine constricts blood vessels, which pre-vents rapid absorption of the anesthetic agent and thus prolongs its local action. Rapid absorption of the anesthetic agent into the bloodstream, which could cause seizures, is also prevented. Types of local anesthetic agents are listed in Table 19-5.
Local anesthesia is the anesthesia of choice in any surgical pro-cedure in which it can be used. However, contraindications in-clude high preoperative levels of anxiety, because surgery with local anesthesia may increase anxiety. A patient who requests gen-eral anesthesia rarely does well under local anesthesia. For some surgical procedures, local anesthesia is impractical because of the number of injections and the amount of anesthetic that would be required (breast reconstruction, for example).
The skin is prepared as for any surgical procedure, and a small-gauge needle is used to inject a modest amount of the anesthetic into the skin layers. This produces blanching or a wheal. Addi-tional anesthetic is then injected in the skin until an area the length of the proposed incision is anesthetized. A larger, longer needle then is used to infiltrate deeper tissues with the anesthetic.The action of the agent is almost immediate, so surgery may begin as soon as the injection is complete. Anesthesia lasts 45 minutes to 3 hours, depending on the anesthetic and the use of epinephrine.
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