McKenna's Pharmacology for Nursing, 2e

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P A R T 4  Drugs acting on the central and peripheral nervous systems

in the movement of gas in the opposite direction—out of the tissues and back to expired air. Anaesthetic gases include ethylene (red cylinder) and nitrous oxide (blue cylinder). (See Table 27.1.) Therapeutic actions and indications Nitrous oxide is a very potent analgesic. However, it is the weakest of the gas anaesthetics and the least toxic. It moves so quickly in and out of the body that it can actually accumulate and cause pressure in closed body compartments such as the sinuses. Because nitrous oxide is such a potent analgesic, it is used frequently for dental surgery and during labour. It does not cause muscle relaxation. Nitrous oxide is usually combined with other agents for anaesthetic use. Ethylene is less toxic than most of the other gas anaesthetics. It is not associated with bronchospasm, and there is usually less postanaesthetic vomiting with this drug. Pharmacokinetics All of these agents have a rapid onset of action, usually within 1 to 2 minutes, and a rapid recovery period. Timing of recovery depends on the other drugs being used. Contraindications and cautions Nitrous oxide and ethylene can block the reuptake of oxygen after surgery and cause hypoxia. Because of this reaction, it is always given in combination with oxygen. Susceptible individuals should be monitored for signs of hypoxia, chest pain and stroke. Adverse effects As with other general anaesthetics, people need to be monitored for skin integrity when they are not able to move for periods of time. Ethylene can leave the person with a headache and a very unpleasant taste in the mouth. Nitrous oxide can cause acute sinus and middle ear pain, bowel obstruction and pneumothorax because it so rapidly moves into and accumulates in closed spaces. Because nitrous oxide inactivates vitamin B 12 , people should also be monitored for low vitamin B 12 levels, including neurological, immune and haemato­ logical complications. Clinically important drug–drug interactions Caution should be used if these drugs are combined with any other drug that causes CNS depression. If halothane and ketamine are used in combination, severe cardiac depression with hypotension and bradycardia may occur. If these agents must be used together, the person should be monitored closely.

V olatile liquids Inhaled anaesthetics also can be volatile liquids —liquids that are unstable at room temperature and release gases. These gases are then inhaled by the person. Therefore, volatile liquids act like gas anaesthetics. Most of the volatile liquids in use are halogenated hydrocarbons such as desflurane ( Suprane ), isoflurane ( Aerrane, Forane ), methoxyflurane ( Penthrox ) and sevo- flurane ( Sevorane ). (See Table 27.1.) Therapeutic actions and indications Desflurane is widely used in outpatient surgery because of its rapid onset and quick recovery time. Isoflurane is widely used to maintain anaesthesia after inductions. It can cause muscle relaxation. Sevoflurane is used in outpatient surgery as an induction agent and is rapidly cleared for quick recovery. Pharmacokinetics Desflurane and isoflurane have a rapid onset—also within 1 to 2 minutes—and rapid recovery—usually within 15 to 20 minutes. Sevoflurane, the newest of the volatile liquids, has a very rapid onset of action—within 30 seconds—and a very rapid clearance—lasting only about 10 minutes. These drugs are all cleared through the lungs. Contraindications and cautions Desflurane use should be avoided in people with respira- tory problems and in those with increased sensitivity because of its irritation to the airways and tendency to cause respiratory depression. In addition, it is not recom- mended for induction in children because of its irritation of the airways. Isoflurane and sevoflurane should be used with caution in individuals with respiratory depression to avoid severe respiratory depression. All of these drugs have the potential to trigger malignant hyperthermia and Prototype summary: Nitrous oxide Indications: Induction and maintenance of anaesthesia. Actions: Depresses the CNS to produce anaesthesia and analgesia. Pharmacokinetics: Route Onset Duration IV 1–2 mins 20 mins T 1/2 : minutes; not metabolised, excreted in the lungs. Adverse effects: Cardiovascular depression, respiratory depression, apnoea, earache, sinus pain, vomiting, malignant hyperthermia.