Bradycardias produced by surgical manipulation resulting in vagal stimulation. Severe bradycardia and asystole may result. It is more common in children because the sympathetic innervation of the heart is immature and vagal tone predominates. Bradycardias are most commonly seen in ophthalmic surgery due to the oculocardiac reflex. Generally the heart rate will improve when the surgical stimulus is removed.
Atrial fibrillation is common during thoracic surgery.
Halothane has a direct effect on the SA node and conducting system leading to a slowing in impulse generation and conduction and predisposes to re-entry phenomena. Catecholamines also have potent effects on impulse conduction, so the interaction of halothane and exogenous or endogenous catecholamines may cause ventricular arrhythmias. Ventricular ectopic beats are common. However rhythm disturbances such as ventricular tachycardia or rarely ventricular fibrillation may occur. Arrhythmias occurring during halothane anaesthesia can often be resolved by reducing the concentration of halothane, ensuring adequate ventilation thereby preventing hypercarbia, increasing the inspired oxygen concentration and providing an adequate depth of anaesthesia for the surgical procedure.