Intravenous anesthetics are typically used when the patient re-quires general anesthesia for just a short period such as during outpatient surgery. They’re also used to promote rapid induction of anesthesia or to supplement inhalation anesthetics.
The drugs used as intravenous anesthetics are:
· barbiturates (methohexital, thiopental)
· benzodiazepines (midazolam)
· dissociatives (ketamine)
· hypnotics (etomidate, propofol)
· opiates (fentanyl, sufentanil).
Intravenous anesthetics are lipid-soluble and well-distributed throughout the body, crossing the placental barrier and entering breast milk. These drugs are metabolized in the liver and excreted in the urine.
Opiates work by occupying sites on specialized receptors scat-tered throughout the CNS and by modifying the release of neuro-transmitters from sensory nerves entering the CNS. Ketamine acts directly on the cortex and limbic system of the brain, producing a profound sense of dissociation from the environment.
Barbiturates, benzodiazepines, and etomidate seem to enhance re-sponses to the CNS neurotransmitter, gamma-aminobutyric acid. This inhibits the brain’s response to stimulation of the reticular ac-tivating system, the area of the brain stem that controls alertness. Barbiturates also depress the excitability of CNS neurons.
Because of the short duration of action of intravenous anesthet-ics, they’re used in brief surgical procedures such as outpatient surgery.
Barbiturates are used alone in surgery that isn’t expected to be painful and as adjuncts to other drugs in more extensive proce-dures. Benzodiazepines produce sedation and amnesia, but not pain relief.
Etomidate is used to induce anesthesia and to supplement low-potency inhalation anesthetics such as nitrous oxide. The opiates provide pain relief and supplement other anesthetics.
I.V. anesthetics, particularly ketamine, can produce a variety of drug interactions.
· Verapamil enhances the anesthetic effects of etomidate, causing respiratory depression and apnea.
· Administering ketamine together with halothane increases the risk of hypotension and reduces cardiac output (the amount of blood pumped by the heart each minute).
· Giving ketamine and nondepolarizing drugs together increases neuromuscular effects, resulting in prolonged respiratory depres-sion.
· Using barbiturates or opioids with ketamine may prolong recov-ery time after anesthesia.
· Ketamine plus theophylline may promote seizures.
· Ketamine and thyroid hormones may cause hypertension and tachycardia (rapid heart rate). (See Adverse reactions to I.V. anes-thetics.)