Halothane Adrenaline

Enflurane produces a dose-related decrease in systemic arterial blood pressure secondary to reductions in cardiac output and systemic vascular resistance. There is evidence that cardiac output is partially maintained by a compensatory increase in heart rate. This effect seems dependent on a degree of hypercardia and does not occur during controlled ventilation. Enflurane and halothane depress myocardial contractility to a similar extent and less than isoflurane. Enflurane does not sensitise the heart to the effects of catecholamines to any significant extent and adrenaline (epinephrine) may be given subcutaneously for control of bleeding. 

The duration of action of a local anaesthetic is proportional to the time that the drug remains bound to the sodium channels. Measures that prolong contact time will prolong the duration of the local anaesthetic effect. Cocaine has a vasoconstricting effect on blood vessels and prevents its own absorption. Many local anaesthetics are prepared with adrenaline (epinephrine) in order to achieve this effect. Concentrations are usually of the order of 1 200000 or more dilute than this. Care should be exercised when using adrenaline-containing solutions in the presence of halothane as it is known to sensitise the myocardium to the effects of catecholamines. 

When adrenaline is administered during halothane anaesthesia there is an increased risk of arrhythmia. 

Halothane and Adrenaline (S) Anaesthetic sensitises other halogenated (epinephrine) and myocardium to adrenaline,... 

Halothane and some other anesthetics sensitize patients to the risk of adrenaline-induced ventricular dysrhythmias and acute pulmonary edema, especially if hypoxia is present (18,19). 

Adrenaline used during anesthesia can cause ventricular dysrhjdhmias. The threshold dose of adrenaline for dysrhythmias is reduced by halothane, but not by desflurane the dose of adrenaline required to produce dysrhythmias in 50% of patients was three times that needed when anesthesia was with halothane (17). 

Cardiac dysrhythmias are generally considered to be less frequent, or at least less severe, with enflurane than with halothane (5,6). However, caution in the use of adrenaline is advisable, especially in patients with cardiac disease or hjrperthyroidism. Isorhythmic atrioventricular dissociation was seen in 16 of 105 patients after the use of 1.0-1.5% enflurane (7). 

Lidocaine is used in cardiovascular medicine for its antiarrhythmic properties (see Ch. 12). More recently, studies have looked at the i.v. administration of lidocaine to horses with colic (see Ch. 6). It appears that i.v. lidocaine (without epinephrine (adrenaline)) may have some desirable effects on jejunal distension and peritoneal fluid accumulation and is well-tolerated periop-eratively in horses with colic (Brianceau et al 2002). In addition, i.v. lidocaine reduced the halothane MAC significantly (Doherty Frazier 1998) in ponies. 

K., Yoshiya, I., 1996. Anti arrhythmic of rilmenidine on adrenaline-induced arrhythmia via central imidazoline receptors in halothane anaesthetized dogs. Br. J. Pharmacol. 117, 1744-1748. 

Isoflurane ha.s similar actions to halothane but is less cardiodepre.s-sant and doe.s not sensitize the heart to epinephrine (adrenaline). It causes dose-related hypotension by decreasing systemic vascular resistance. Only 0.2%i of the absorbed dose is metabolized and so isoflurane Ls very unlikely to cause hepatotoxiciiy,... 

Halothane is known to cause arrhythmias and it has been suggested that it may increase susceptibility to the adverse cardiac effect of beta-agonist bronchodilators, which can cause arrhythmias. Note that beta agonists such as terbutaline are sympathomimetics (see Table 24.1 , (p.879)), like adrenaline (epinephrine), which has also been shown to cause arrhythmias in the presence of halothane (see Anaesthetics, general + Inotropes and Vasopressors , p.99). 

Patients anaesthetised with inhalational anaesthetics (particularly cyclopropane and halothane, and to a lesser extent desflurane, enflurane, ether, isoflurane, methoxyflurane, and sevoflurane) can develop cardiac arrhythmias if they are given adrenaline (epinephrine) or noradrenaline (norepinephrine), unless the dosages are very low. Children appear to be less susceptible to this interaction. file addition of adrenaline to intrathecal tetracaine enhances the sedative effects of propofol. 

Children appear to be much less susceptible to these effects than adults. A retrospective study of 28 children found no evidence of arrhythmia during halothane anaesthesia with adrenaline doses of up to 8.8 micrograms/kg, and a subsequent study in 83 children (aged 3 months to 17 years) found that 10 micrograms/kg doses of adrenaline were safe. ... 

Phenylephrine is a sympathomimetic, and as such may carry some risk of potentiating arrhythmias if it is used with inhalational anaesthetics such as halothane -see Anaesthetics, general + Inotropes and Vasopressors , p.99. However, it is considered that it is much less likely than adrenaline (epinephrine) to have this effect, since it has primarily alpha-agonist activity. ... 

Cocaine has sympathomimetic actions (tachycardia, peripheral vasoconstriction, and hypertension). Combined use with sympathomimetics such as adrenaline increases these effeets, and the risk of life-threatening arrhythmias. This risk may be further inereased if halothane anaesthesia is used (two of the above patients reeeived halothane ). See also Anaesthetics, general + Anaesthetics, local , p.92 and Anaesthetics, general + Ino-tropes and Vasopressors , p.99. 

The local risks of vasoconstrictors in local anaesthetic solutions, particularly when the latter are used in the fingers or other extremities, have long been recognized. In addition, it is well known that the use of adrenaline or noradrenaline for this purpose can lead to marked rises in blood pressure, especially in patients who are taking MAO inhibitors the cardiovascular effects can be dangerous in patients with existing cardiovascular disease or where there is simultaneous treatment with either a tricyclic antidepressant (SED VIII) or with those general anaesthetics which sensitize the myocardium to the effects of catecholamines (e.g. chloroform, cyclopropane, halothane). 

The use of adrenaline drops to promote haemostasis during general anaesthesia induced with halo thane diould be avoided halothane, and also cyclopropane, cause the myocardium to become sensitized to beta adrenergic activity so that with the concomitant use of adrenaline there is a risk of cardiac arrhythmia. It is also to be noted that there is a greater likelihood of absorption into the systemic circulation when adrenaline drops are used in the presence of vascular tissue. 

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