chapter
Anaesthetics – general
Pages 4

Be aware that ↓ dose may be needed

NITROUS OXIDE ANTICANCER AND IMMUNOMODULATING DRUGS – METHOTREXATE

≠ antifolate effect of methotrexate

≠ toxicity of methotrexate Nitrous oxide is usually used for relatively brief durations when patients are anaesthetized, and hence this risk during anaesthesia is minimal. However, nitrous oxide may be used for analgesia for longer durations, and this should be avoided

ANAESTHETICS – GENERAL ANTIDEPRESSANTS

ANAESTHETICS – GENERAL MAOIs Some cases of both ≠ and ↓ BP on induction of anaesthesia. Mostly no significant changes

Uncertain Some recommend stopping MAOIs 2 weeks before surgery. Others suggest no need for this; monitor BP closely, especially during induction of anaesthesia

ANAESTHETICS – GENERAL TCAs A few cases of arrhythmias Uncertain Monitor ECG, PR and BP closely

INTRAVENOUS – KETAMINE ANTIDEMENTIA DRUGS – MEMANTINE

≠ CNS side-effects Additive effects on NMDA receptors

Avoid co-administration

INTRAVENOUS ANTIHYPERTENSIVES AND HEART FAILURE DRUGS

Risk of severe hypotensive episodes during induction of anaesthesia

Most general anaesthetics are myocardial depressants and vasodilators. Additive hypotensive effect

Monitor BP closely, especially during induction of anaesthesia

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ANAESTHETIC DRUGS ANAESTHETICS – GENERAL

Uncertain Monitor EGC and BP closely. Consider using intravenous agents for maintenance of anaesthesia

INTRAVENOUS – THIOPENTONE

ANTIPLATELET AGENTS – ASPIRIN

↓ requirements of thiopentone when aspirin (1 g) used during premedication

Uncertain at present Be aware of possible ↓ dose requirements for thiopentone

ANAESTHETICS – GENERAL ANTIPSYCHOTICS Risk of hypotension Additive effect Monitor BP closely, especially during induction of anaesthesia

INTRAVENOUS ANAESTHETICS (e.g. thiopentone sodium, propofol)

BETA-BLOCKERS Risk of severe hypotensive episodes during induction of anaesthesia (including patients taking timolol eye drops)

Most intravenous anaesthetic agents are myocardial depressants and vasodilators, and additive ↓ BP may occur

Monitor BP closely, especially during induction of anaesthesia

ANAESTHETICS – GENERAL BRONCHODILATORS

INHALATIONAL – HALOTHANE

TERBUTALINE, THEOPHYLLINE

Cases of arrhythmias when these bronchodilators are co-administered with halothane

Possibly due to sensitization of the myocardium to circulating catecholamines by the volatile anaesthetics to varying degrees

Risk of cardiac events is higher with halothane. Desflurane is irritant to the upper respiratory tract, and ≠ secretions can occur and are best avoided in patients with bronchial asthma. Sevoflurane is non-irritant and unlikely to cause serious adverse effects

INTRAVENOUS – KETAMINE THEOPHYLLINE Risk of fits Uncertain A careful risk-benefit assessment should be made before using ketamine. However, there are significant benefits for the use of ketamine to anaesthetize patients for emergency management of lifethreatening asthma

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Mechanism Precautions

hypotensive/bradycardic effects of diltiazem and verapamil

Additive hypotensive and negative inotropic effects. General anaesthetics tend to be myocardial depressants and vasodilators; they also ↓ sinus automaticity and AV conduction

Monitor BP and ECG closely

INHALATIONAL AND INTRAVENOUS ANAESTHETICS

DIURETICS ≠ hypotensive effect Additive effect as the anaesthetics cause varying degrees of myocardial depression and/or vasodilatation, while diuretics tend to ↓ circulatory volume

Monitor BP closely, especially during induction of anaesthesia

HALOTHANE ERGOT DERIVATIVES ↓ efficacy of ergometrine on uterus

Halothane ↓ muscle tone of the pregnant uterus; generally, its use in obstetric anaesthesia is not recommended as it ≠ risk of postpartum haemorrhage, for which ergot derivatives are commonly used

Use alternative form of anaesthesia for surgery requiring use of ergotamine

ANAESTHETICS – GENERAL MUSCLE RELAXANTS – BACLOFEN, TIZANIDINE

≠ hypotensive effect Additive hypotensive effect. Tizanidine also has a negative chronotropic effect and may cause additive bradycardia with beta-blockers and calcium channel blockers

Monitor BP closely, especially during induction of anaesthesia

ANAESTHETICS – GENERAL NITRATES ≠ hypotensive effect Additive effect (vasodilatation and/or depression of myocardial contractility)

Monitor BP closely, especially during induction of anaesthesia

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ANAESTHETIC DRUGS ANAESTHETICS – GENERAL

patients taking oxytocin

Uncertain; possibly additive effect. High-dose oxytocin may cause hypotension and arrhythmias

Monitor PR, BP and ECG closely; give oxytocin in the lowest possible dose. Otherwise consider using an alternative inhalational anaesthetic

ANAESTHETICS – GENERAL PERIPHERAL VASODILATORS – MOXISYLYTE

≠ hypotensive effect Additive effect Monitor BP closely, especially during induction of anaesthesia

ANAESTHETICS – GENERAL POTASSIUM CHANNEL ACTIVATORS

≠ hypotensive effect Additive effect Monitor BP closely

ANAESTHETICS – GENERAL SYMPATHOMIMETICS

ANAESTHETICS – GENERAL DIRECTLY ACTING SYMPATHOMIMETICS (e.g. epinephrine)

1. Risk of arrhythmias when inhalational anaesthetics are co-administered with epinephrine or norepinephrine 2. Case report of marked ≠ BP when phenylephrine eye drops were given during general anaesthesia

1. The arrhythmogenic threshold with injected epinephrine is lower with halothane than isoflurane or enflurane, which is attributed to sensitization to beta-adrenoceptor stimulation 2. Uncertain. Phenylephrine produces its effects by acting on alpha-adrenergic receptors; possible that these effects are enhanced

1. Use epinephrine in the smallest possible dose (when using 1:100 000 infiltration to ↓ intraoperative bleeding, no more than 10 mL/10 minutes and less than 30 mL/hour should be given) 2. Avoid use of phenylephrine eye drops during anaesthesia

ANAESTHETICS – GENERAL INDIRECTLY ACTING SYMPATHOMIMETICS (e.g. methylphenidate)

1. Risk of arrhythmias when inhalational anaesthetics are co-administered with methylphenidate 2. Case report of ↓ sedative effect of midazolam and ketamine by methylphenidate

1. Uncertain; attributed by some to sensitization of the myocardium to sympathomimetics by inhalational anaesthetics 2. Uncertain at present; possibly due to CNS stimulation caused by methylphenidate (hence its use in narcolepsy)

Avoid giving methylphenidate on the day of elective surgery