'Expected infarct size without thrombolysis', a concept that predicts immediate and long-term benefit from thrombolysis for evolving myocardial infarction.

BACKGROUND Thrombolytic therapy should only be used when expected benefits outweigh the risks. In order to obtain a precise estimation of prognosis, with and without thrombolytic therapy, we postulated that mortality reduction by thrombolytic therapy is a function of the area of myocardium at risk for necrosis. We developed a model to estimate the myocardial area at risk for necrosis from clinical parameters readily available upon hospital admission. This model was validated in relation to long-term prognosis and benefits of thrombolytic therapy. METHODS Enzymatic infarct size with and without thrombolysis was predicted from the haemodynamic state and the electrocardiogram on hospital admission by multivariate regression analysis in 885 patients in the rt-PA placebo and rt-PA/PTCA trial of the European Cooperative Study Group. This multivariate function was used to validate the 'expected infarct size without thrombolytic treatment' in a test population of 533 patients from the Intracoronary Streptokinase trial of the Interuniversity Cardiology Institute of The Netherlands (ICIN) and 1741 patients from the Intravenous Streptokinase in Acute Myocardial Infarction (ISAM) study, both trials with a non-thrombolysed control group. RESULTS Expected infarct size correlated well with the actual enzymatic infarct size in the non-thrombolysed patients of the latter two series. Limitation of infarct size by thrombolytic therapy was greatest in patients with a large 'expected infarct size' and absent in patients with a small area at risk. Similarly, one year mortality reduction was greatest in patients with a large 'expected infarct size without thrombolysis'; four deaths were prevented per hundred (95% confidence interval 0 to 9) if the area at risk was large, vs one death (95% confidence interval -2 to 3) in patients with a small area at risk. Benefit was most pronounced in patients with a large area at risk who were treated early within 3 h of symptom onset. A score for the determination of 1 year mortality with and without thrombolytic therapy is presented to help the clinician determine who to treat with thrombolytic therapy. CONCLUSION 'Expected infarct size without thrombolysis' is a useful tool for clinicians to estimate the amount of myocardium at risk of necrosis in individual patients and to decide whether thrombolytic therapy is warranted. It is the only validated parameter of myocardium at risk for necrosis that is readily available for all patients with myocardial infarction and does not need high-tech equipment.