McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 2 7 General and local anaesthetic agents

Therapeutic actions and indications Thiopentone is a barbiturate that is used for induction of anaesthesia, particularly in intravenous anaesthetics. Because it has no analgesic properties, the person may need additional analgesics after surgery. Pharmacokinetics Thiopentone has a very rapid onset of action, usually within 10 to 30 seconds, and an ultra-short recovery period of 5 to 8 minutes. Induction is generally smooth and rapid, but dose-related cardiovascular and respira- tory depression can occur. Awakening from a moderate dose of thiopentone is rapid because the drug redis- tributes into other tissues, particularly fat. However, metabolism is slow and sedative effects can persist for 24 hours. Repeated doses have a cumulative effect and recovery is much slower. Contraindications and cautions The drug should not be used until the anaesthetist and staff are ready and equipped for intubation and respira- tory support because of the rapid onset and because the drug can cause respiratory depression and apnoea. The drug should not be used during pregnancy or breast- feeding unless the benefit clearly outweighs the potential risk to the fetus or neonate because of the CNS depres- sive effects of this drug. This drug is lipophilic and can be rapidly absorbed through the placental barrier. Adverse effects The adverse effects associated with this drug are related to the suppression of the CNS with decreased pulse, hypotension, suppressed respirations and decreased GI activity. Nausea and vomiting after recovery are common. Prototype summary: Thiopentone Indications: Induction of anaesthesia, maintenance of anaesthesia; induction of a hypnotic state. Actions: Depresses the CNS to produce hypnosis and anaesthesia without analgesia. Pharmacokinetics: Route Onset Duration IV 1 min 20–30 mins T 1/2 : 3 to 8 hours; metabolised in the liver, excreted in the urine. Adverse effects: Emergence delirium, headache, restlessness, anxiety, cardiovascular depression, respiratory depression, apnoea, salivation, hiccups, skin rashes.

Induction Induction is the period from the beginning of anaes- thesia until stage 3, or surgical anaesthesia, is reached. The danger period for many people during induction is stage 2 because of the systemic stimulation that occurs. Often a rapid-acting anaesthetic is used to move quickly through this phase and into stage 3. NMJ blockers may be used during induction to facilitate intubation, which is necessary to support the person with mechanical ven- tilation during anaesthesia (see Chapter 28). Maintenance Maintenance is the period from stage 3 until the surgical procedure is complete. A slower, more predictable anaesthetic, such as a gas anaesthetic, may be used to maintain the anaesthesia once the person is in stage 3. Recovery Recovery is the period from discontinuation of the anaesthetic until the person has regained conscious- ness, movement and the ability to communicate. During recovery, the person requires continuous monitoring for any adverse effects of the drugs used and to ensure support of the person’s vital functions as necessary. ■■ General anaesthesia produces analgesia, amnesia and unconsciousness. ■■ General anaesthesia places the person at risk for problems because of its extensive CNS depression and widespread effects on other body systems. ■■ Balanced anaesthesia involves the administration of several drugs rather than a single drug to achieve analgesia, muscle relaxation, unconsciousness and amnesia. ■■ Induction of anaesthesia is the period ranging from administration of the anaesthetic to the point of surgical anaesthesia. GENERAL ANAESTHETIC AGENTS Several different types of drugs are used as general anaesthetics. These include barbiturate and non- barbiturate anaesthetics, gas anaesthetics and volatile liquids. See Table 27.1 for a list of general anaesthetic agents along with the anticipated adverse effects of these drugs. B arbiturate anaesthetics The barbiturate anaesthetics (Table 27.1) are intra­ venous drugs used to induce rapid anaesthesia, which is then maintained with an inhaled drug. The available drug in this group is thiopentone ( Pentothal ). KEY POINTS