McKenna's Pharmacology for Nursing, 2e

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

sedation and produces a state of mental detachment. It also has antiemetic effects, reducing the incidence of nausea and vomiting in surgical and diagnostic proced­ ures. Ketamine has been associated with a bizarre state of unconsciousness in which the person appears to be awake but is unconscious and cannot feel pain. This drug, which causes sympathetic stimulation with increase in blood pressure and heart rate, may be helpful in situations when cardiac depression is dangerous. Propofol is often used for short procedures because it has a very rapid clearance and produces much less of a hangover effect and allows for quick recovery. It is also used to maintain people on mechanical ventilation. Pharmacokinetics Midazolam has a rapid onset but does not reach peak effectiveness for 30 to 60 minutes. Droperidol has an onset of action within 3 minutes and an ultra-short recovery period. Ketamine has an onset of action within 30 seconds and a very slow recovery period (45 minutes). Propofol is a very short-acting anaesthetic with a rapid onset of action of 30 to 60 seconds. Contraindications and cautions Midazolam is more likely to cause nausea and vomiting than are some of the other anaesthetics, and so it should be used with caution in any person who could be com- promised by vomiting. It has been associated with respiratory depression and respiratory arrest, and so life support equipment should be readily available whenever it is used. Droperidol should be used with caution in individuals with renal or hepatic failure and should be used with extreme care in people with prolonged QT intervals or who are at risk for prolonged QT intervals. Adverse effects People receiving any general anaesthetic are at risk for skin breakdown because they will not be able to move. Care must be taken to prevent decubitus ulcer forma- tion. People receiving midazolam should be monitored for respiratory depression and CNS suppression. During the recovery period, droperidol may cause hypoten- sion, chills, hallucinations and drowsiness. It may also cause QT prolongation, which puts the person at risk for serious cardiac arrhythmias. Ketamine crosses the blood–brain barrier and can cause hallucinations, dreams and psychotic episodes. Propofol often causes local burning on injection. It can cause bradycardia, hypotension and, in extreme cases, pulmonary oedema. Clinically important drug–drug interactions If ketamine and isoflurane are used in combination, severe cardiac depression with hypotension and brady- cardia may occur. If these agents must be used together,

the person should be monitored closely. Droperidol should not be used with other drugs that prolong the QT interval. If this combination is necessary, the person should be monitored continuously. Ketamine may also potentiate the muscular blocking of NMJ blockers, and the person may require prolonged periods of respira- tory support. Midazolam is associated with increased toxicity and length of recovery when used in combina- tion with inhaled anaesthetics, other CNS depressants, opioids, propofol or thiopentone. If any of these agents are used in combination, careful balancing of drug doses is necessary. Prototype summary: Midazolam Indications: Sedation, anxiolysis and amnesia before diagnostic, therapeutic or endoscopic procedures; induction of anaesthesia; continuous sedation of intubated people. Actions: Acts mainly at the limbic system and RAS; potentiates the effects of GABA; has little effect on cortical function; exact mechanism of action is not understood. Pharmacokinetics: Route Onset Peak Duration Oral 30–60 mins 12 hours 2–6 hours IM 15 mins 30 mins 2–6 hours IV 3–5 mins <30 mins 2–6 hours T 1/2 : 1.8 to 6.8 hours; metabolised in the liver, excreted in the urine. Adverse effects: Transient drowsiness, sedation, drowsiness, lethargy, apathy, fatigue, disorientation, restlessness, constipation, diarrhoea, A naesthetic gases Like all inhaled drugs, anaesthetic gases enter the bronchi and alveoli, rapidly pass into the capillary system (because gases flow from areas of higher concentration to areas of lower concentration) and are transported to the heart to be pumped throughout the body. These gases have a very high affinity for fatty tissue, and they are lipophilic, including the lipid membrane of the nerves in the CNS. The gases pass quickly into the brain and cause severe CNS depression. Once the person is in stage 3 of anaesthesia, the anaesthetist regulates the amount of gas that is delivered to ensure that it is suffi- cient to keep the person unconscious but not enough to cause severe CNS depression. This is done by decreas- ing the concentration of the gas that is flowing into the bronchi, creating a concentration gradient that results incontinence, urinary retention, bradycardia, tachycardia, phlebitis at IV injection site.