Intravenous Anesthesia Case File

Intravenous Anesthesia Case File

Lydia Conlay, MD, PhD, MBA, Julia Pollock, MD, Mary Ann Vann, MD, Sheela Pai, MD, Eugene C. Toy, MD

Case 1

A 6-year-old child is scheduled for an MRI to rule out a possible brain tumor. The child is terrified by the scanner’s noise and the closed space, and refuses to hold still. The patient is scheduled for general anesthesia. However, an anesthesia machine cannot be brought into the same room with the magnet.

➤ How can this patient safely receive general anesthesia?

ANSWER TO CASE 1:

Intravenous Anesthesia

Summary: A 6-year-old child needs an MRI scan and cannot hold still. The general anesthesia machine cannot be brought into the room due to the magnet.

➤ Method of Anesthesia: Intravenous anesthetics are used for a pleasant, rapid induction of general anesthesia. Intravenous agents are also useful in lower doses for sedation. They may also be used for maintenance of anesthesia in conjunction with inhalational anesthetics, or instead of inhalational anesthetics when the later are contraindicated (such as in malignant hyperthermia, case on strabismus) or where it is impractical for an anesthesia machine to be present.

ANALYSIS

Objective

Introduce the student to the more common intravenous anesthetics including their properties, uses, and potential side effects.

Considerations

This patient can have an anesthetic cream placed on the arm, and then an i.v. placed with minimal pain. During the procedure, he will be anesthetized with a continuous infusion of propofol, an intravenous agent, and intubated with an endotracheal tube prior to entering the MRI machine. A continuous infusion such as this does not require metal equipment such as an anesthesia machine, and is thus an option in this circumstance.

“Intravenous anesthetics” do not necessarily include intravenous opioids. Opioids are analgesics, and do not cause general anesthesia per se, unless used in very large doses. However, an opioid (eg, fentanyl) is often combined with an intravenous hypnotic drug for prevention of a response to a noxious stimulus (such as movement), and can thus aid in providing a rapid awakening and spontaneous ventilation at the end of a surgical procedure.

APPROACH TO

Intravenous Anesthesia

DEFINITIONS

Pharmacodynamics: The effects of a drug on the body, or relationship between the plasma concentration of a drug and the pharmacologic response to it.

Pharmacokinetics: The effects of the body on a drug, and are determined by the volume of distribution for the drug (Vd) and clearance of that drug from the body. Intravenous anesthetics exhibit multi-compartmental pharmacokinetics: that is, the drugs are distributed into peripheral tissues, and at the same time cleared from the body. The administration of an intravenous anesthetic obviously increases the plasma concentration. The concentration of the agent next peaks in the “vessel rich” group of tissues, such as liver and spleen, followed by the “muscle group,” and then, finally, into fat. (Please see Figure 1–1.) Plasma concentrations of intravenous agents are also affected by tissue uptake, renal excretion, and hepatic metabolism.

Volume of distribution (Vd ): The volume that relates the plasma concentration of a drug to the total amount of drug in the body. It can be thought of as the “size of the tank.” By rearranging the terms defining concentration, Vd becomes the dose of drug given intravenously divided by its plasma concentration.

Clearance: The amount of a drug removed by the kidneys and/or metabolized in the liver during a specified period of time (eg, mL/min).

Context-sensitive half-time: The time for the plasma concentration of a drug to decrease by 50% from an infusion that maintains a constant concentration. The context is the duration of the infusion.

Figure 1–1. Concentrations of anesthetics peak first in plasma, then in the “vessel rich group” (VRG), next the muscle group (MG), then in the fat group (FG). (Reprinted by permission from Macmillan Publishers Ltd.: Price HL, et al.The uptake of thiopental by body tissues and its relation to the duration of narcosis. Clin Pharmacol Ther. 1960;1:16.)

CLINICAL APPROACH

Propofol

Propofol is the most commonly used intravenous drug for induction of anesthesia. It has gained its popularity because it is associated with a pleasant, rapid loss of consciousness, rapid awakening, and seemingly fewer residual effects on patient’s brain. Its chemical structure is an alkyl phenol, and some of its behavioral effects seem to resemble those observed with alcohol.

Propofol is used as a bolus for the induction of anesthesia, in doses ranging from 1.5 to 2.5 mg/kg. The administration of propofol can cause significant pain upon injection, which can be attenuated by using an intravenous placed in a large vein, and/or administering lidocaine 0.5 to 1.0 mg/kg i.v. just prior to injecting the propofol. It is also advisable to warn the patient that some burning might occur during injection.

Propofol can also be used for the maintenance of anesthesia. While it is possible to administer propofol for maintenance in multiple boluses, it is best used as continuous infusion since it has a short context-sensitive half-time. Perhaps surprisingly, administering propofol by multiple boluses actually consumes more of the drug than a continuous infusion (please see Figure 1–2). For the maintenance of anesthesia, a loading dose of 1 to 2 mg/kg can be followed by an infusion of 100 μg/kg/min to be titrated to effect.

Propofol is also useful in lower doses for sedation during regional and monitored anesthesia care, and for patients in the intensive care unit. Propofol has never been associated with a case of malignant hyperthermia; so it is the agent of choice for general anesthesia in this setting.

Etomidate

Etomidate is distinguished from the other intravenous agents by its paucity of effects on the cardiovascular system. It causes little or no change in systemic

Figure 1–2. Administering an intravenous anesthetic using multiple boluses actually consumes more of the drug than a continuous infusion. (Reprinted from Reeves JG. Profiles in anesthetic practice: Rational administration of intravenous anesthesia. In: Morgan GE Jr, Mikhail MS, Murray MJ, eds. Clinical Anesthesiology, 4th ed. New York, NY: McGraw-Hill, 2006:190-191.)

or pulmonary artery pressure, little or no change in heart rate, and only a mild effect on cardiac output. Thus, etomidate is the agent of choice whenever cardiovascular stability is potentially an issue. The possible side effects of etomidate are adrenocortical suppression, myoclonus, and the activation of seizure foci.

Thiopental

Originally known as “sodium pentothal,” thiopental is mainly used for induction of anesthesia in patients undergoing neurosurgery. Thiopental reduces the brain’s oxygen consumption, and may reduce ischemia-induced brain damage. It is also used intraoperatively for burst suppression, and for the treatment of increased intracranial pressure. There is absolutely no merit in the concept that thiopental was a “truth serum.”

Ketamine

Ketamine is an intravenous hypnotic drug that is chemically related to LSD. It produces a dissociative state accompanied by analgesia, unawareness, and nystagmus. When used for induction, it can be associated with “bad dreams” or emergence delirium, although this side effect can be prevented by pretreatment with a benzodiazepine.

Ketamine increases systemic blood pressure, heart rate, and cardiac output. It has no effect on ventilation, although pretreatment with an antisialagogue (eg, glycopyrrolate) may be warranted as ketamine causes an increase in respiratory secretions. Ketamine is contraindicated in patients with intracranial pathology, as it increases intracranial pressure and cerebral blood flow.

Comprehension Questions

1.1. A 60-year-old man undergoing an inguinal hernia repair is induced with propofol. Which of the following is most likely to be observed?

    A. Hyperthermia

    B. Hypetension

    C. Apnea

    D. Prolonged sedation

1.2. Match the anesthetic (A-D) to its effect (1-4).

    A. Propofol                  1. “Bad dreams”

    B. Sodium pentothal    2. Pleasant emergence and slight euphoria

    C. Ketamine                 3. Long-term sedation

    D. Etomidate                4. Few hemodynamic effects

ANSWERS

1.1.   C. Hypotension and apnea are the most common side effects of propofol. Answers A and D are incorrect. Propofol does not cause hyperthermia, indeed, it is the anesthetic of choice in patients at risk for malignant hyperthermia. Propofol is associated with a fast emergence, which is also an advantage of the drug.

1.2.  A, 2. Propofol is known for its pleasant anesthetic properties and occasional euphoria.

        B, 3. Sodium pentothal is a barbiturate known for its long-term sedation.

        C, 1. Ketamine is chemically related to LSD and is sometimes associated with “bad dreams.”

        D, 4. Etomidate is associated with the fewest cardiovascular side effects.

Clinical Pearl

➤ Intravenous anesthetics have life-threatening complications, so they should always be used in an environment with appropriately trained personnel and monitoring.

References

Glass PSA, Shafer SL, Reeves JG. Intravenous drug delivery systems. In: Miller, R. ed. Anesthesia. 5th ed. Philadelphia, PA: Churchill Livingstone; 2000: 390-398. 

Shafer SL, Varel JR. Anesthesiology. 1991;74:53-63.

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