Drugs Used to Induce Anesthesia

Methohexital

Methohexital is generally administered intravenously at a concentration of 1% and a dose of approximately 1 to 2 mg/kg (also see Chapter 10 ).^[129] Problems associated with intravenous administration include burning, hiccups, apnea, and extrapyramidal-like movement. Methohexital has a shorter elimination half-life than thiopental does. It is also useful for providing sedation during radiologic procedures by rectal administration (10% solution).^[130] Usually, 25 to 30 mg/kg produces sleep within 8 to 10 minutes. Obstructive or central apnea may occur. Therefore, a means of ventilating the patient must be available, and appropriate monitoring with pulse oximetry is important.^[131] Because it can cause seizures, methohexital is contraindicated in patients with temporal lobe epilepsy.^[132] Children receiving seizure medications generally require larger doses.

Thiopental (also see Chapter 10 )

Intravenous bolus administration of 2.5% thiopental, 5 to 6 mg/kg, is sufficient to induce anesthesia in most healthy, unpremedicated pediatric patients. ^{[133] [134]} Termination of effect occurs through redistribution of the drug into muscle and fat; thiopental should be used in reduced doses (2 to 4 mg/kg) in children who have low fat stores, such as neonates or malnourished infants. Limiting the total dose to 10 mg/kg or less in older patients minimizes the possibility of prolongation of anesthesia caused by residual barbiturate sedation. Thiopental (30 mg/kg in a 10% solution) may also be administered rectally if methohexital is contraindicated.

Propofol

Propofol is highly lipophilic and promptly distributes into and out of vessel-rich organs; its rapid redistribution, hepatic glucuronidation, and high renal clearance account for the short duration of its effect. As with barbiturates, the induction dose is higher in younger patients (2.9 mg/kg for infants younger than 2 years) than older patients (2.2 mg/kg for patients 6 to 12 years old).^{[135] [136] [137]} This difference may be related in part to a larger central volume and greater clearance in younger patients.^[138] The major drawback of propofol is pain on intravenous administration, particularly through small veins. As little as 0.2 mg/kg lidocaine (mixed with the propofol) has been effective in reducing, but not eliminating, this discomfort.^[139] I will often place a tourniquet on the patient and administer 1.0 mg/kg of lidocaine 15 to 20 seconds before administering the propofol. I then administer the propofol with the intravenous line running, and once most of it has entered the vein, I let the tourniquet down. This "mini-Bier block" technique seems to be quite effective in reducing pain. Another method for minimizing pain is to use a small-gauge catheter (22 to 24 gauge) and administer the drug through a large antecubital vein.^[136] Propofol is particularly useful for the brief and repeated sedation needed for radiotherapy in children with central venous lines. A constant infusion is useful for sedating children undergoing radiologic procedures and as a means of maintaining anesthesia during transport from one location to another, for example, from computed axial tomographic scanning to the operating room.^[140] A modest reduction in systolic blood pressure often accompanies bolus administration.^[141] Propofol has been associated with a reduced rate of postoperative vomiting.^{[142] [143]} Because propofol contains egg and soy products, it may not be indicated for children with egg or soy allergies.

Ketamine (also see Chapter 10 )

Ketamine, a phencyclidine derivative, causes central dissociation of the cerebral cortex while providing analgesia and amnesia. In addition to the intravenous and intramuscular routes, ketamine may be administered rectally (10 mg/kg), orally (6 to 10 mg/kg), or intranasally (3 to 6 mg/kg).^{[144] [145] [146] [147]} The combination of oral ketamine (4 to 6 mg/kg), oral midazolam (0.5 mg/kg), and oral atropine (0.02 mg/kg) provides a well-sedated patient. Intravenous administration of doses as low as 0.25 to 0.5 mg/kg may be used to provide sedation/analgesia for painful procedures, whereas doses of 1 to 2 mg/kg produce sedation sufficient for a smooth transition to general anesthesia. Higher doses (up to 10 mg/kg intramuscularly) provide sufficient analgesia for insertion of invasive monitoring devices before induction of anesthesia (cardiac surgery) or in patients with limited venous access. Ketamine is useful for induction of anesthesia in hypovolemic patients. Patient-to-patient variability in response to this drug, however, is relatively large. A major side effect, increased production of secretions, usually requires administration of an antisialagogue. Other undesirable side effects include vomiting and postoperative "dreaming" or hallucinations; the incidence of dreaming may be reduced by concomitant administration of a benzodiazepine. Although spontaneous respirations and a patent airway are usually maintained, apnea and laryngospasm have been reported.

Contraindications to the use of ketamine in children include the presence of an active upper respiratory tract infection, increased intracranial pressure, open-globe injury, and the presence of a psychiatric or seizure disorder. Ketamine does not preserve the gag reflex and thus should not be used as the sole anesthetic for patients with a full stomach or hiatal hernia.^[148] Ketamine has also been used as an adjunct for epidural analgesia.^{[149] [150] [151]} However, because the preservative in ketamine is neurotoxic, one must administer only preservative-free ketamine in the epidural space.^[152]