The role of coronary collaterals in protecting the myocardium from ischaemia and necrosis has been extensively investigated. Difficulties in assessing the true extent of collaterals relate in part to their capacity to rapidly adapt in size and function according to the demand of the ischaemic territory. Therefore, in addition to the angiographic quantification of collaterals by the Rentrop score,1 studies have used additional techniques such as collateral flow index to quantify collateral flow, albeit in an artificial situation of induced ischaemia or coronary occlusion.2

The protective effects of collateral arteries in the setting of acute myocardial infarction (MI) include a reduction in infarct size, prevention of fatal arrhythmias through QT interval prolongation, improved ventricular function and prevention of aneurysm formation.3 ,4 Perhaps due to the difficulties in correctly assessing collateral circulation, studies investigating the prognostic significance of coronary collaterals have yielded conflicting results. In particular, the influence on mortality is a matter of ongoing debate. Recently, a meta-analysis of 12 studies including 6529 patients analysed the impact of collateral circulation on all-cause mortality in a mixed population of stable coronary artery disease (CAD) and acute and subacute MI.5 Results showed a reduced mortality in patients with ‘high collateralisation’ compared with patients with ‘low collateralisation’ (RR 0.64, 95% CI 0.45 to 0.91, p=0.012). Although the RR was statistically significant for patients with acute and subacute MI, the p value for interaction was not statistically significant. However, due to …