Are the arrhythmias due to myocardial ischaemia and infarction dependent upon the sympathetic system?

See article by Du et al. ([10], pages 919 –929) in this arteries. In other species (e.g. dog, guinea pig), the issue. occurrence of collateral arteries results in non-uniform ischaemia or even only partial ischaemia [2]. Such differMany studies have implicated the sympathetic nervous ing levels of ischaemia could have different sensitivities to system as being of importance in the genesis of the possible arrhythmogenic influences of the sympathetic arrhythmias induced by myocardial ischaemia and infarcsystem. tion [1]. This can involve increased activity in the sympaIn the following examination of the possible arrhythmothetic system or the local release of norepinephrine from genic importance of the sympathetic system, we have sympathetic nerve endings exposed to the extracellular chosen to consider only evidence obtained in the rat and conditions that occur in ischaemic tissue. Evidence to will differentiate between conditions of ischaemia and support the role of the sympathetic nervous system has those of infarction. Arrhythmias arising from reperfusion included the use of drugs to modify the system or are not considered. The necessity of differentiating beextirpation of nerves, but rarely mimics or increases tween ischaemia and infarction cannot be overstated since sympathetic activity. It has to be remembered that, regardthere may be great differences between the two. For less of the manner in which evidence is accumulated, purposes of differentiation, we consider ischaemia to occur activation of the sympathetic system is always arrhythmoin the early period after loss of blood flow, before heart genic and that this in itself exacerbates or even triggers cells are irreversibly injured. Infarction is when the arrhythmias due to other causes, and will reveal any majority of cells are irreversibly injured or dead. It has to underlying tendency for arrhythmias. be recognised that an infarction zone contains a small Despite a large number of experiments, the available percentage of injured but surviving cells. evidence both supports and denies the importance of the Within the above definitions, we can consider the sympathetic system. Some of the contradictions may arise evidence in rat hearts, both in vitro and in vivo, for a role from the methods or species used, or whether the for the sympathetic system in causing arrhythmias due to pathological condition being considered is ischaemia, ischaemia and infarction. infarction, or reperfusion following a period of ischaemia. We will first consider the evidence for involvement of The latter is a distinct condition in which the mechanisms the sympathetic system in the genesis of arrhythmias due inducing arrhythmias may be very different. Furthermore, to ischaemia in vivo. The evidence can be divided into that species may be an important factor. In some species (e.g. obtained with drugs and that with surgical denervation [3]. rats, rabbits, pigs), occlusion of a coronary artery results in Beta-blockers have been shown in many studies to reduce complete ischaemia in the area downstream of the site of arrhythmias following induction of ischaemia in anaesthetocclusion since, in such species, coronary arteries are true ised rats. However, such protection was not seen in end arteries, i.e. they do not have coronary collateral conscious rats [4]. This apparent paradox may be resolved by studies which show that the protective effect of betablockers in anaesthetised rats is due to an indirect action in *Corresponding author. Tel.: 11-604-822-9531; fax: 11-604-822which beta blockade in non-cardiac tissue allows for a rise, 9578. E-mail address: [email protected] (M.J.A. Walker) from depressed levels, of serum potassium and it is this