The development of myocardial necrosis after occlusion of the coronary artery is time dependent. Animal data suggest that irreversible damage occurs within three to four hours,1 but the pattern in clinical myocardial infarction is more complex as coronary occlusion may be a stuttering event and myocardial viability may be preserved by collateral flow.2

The relation between delay to thrombolytic treatment and survival benefit was first investigated adequately by the Gruppo Italiano per lo Studio della Streptochinasi nell' Infarto Miocardico and in the second international study of infarct survival.3,4 The Italian investigators found that the risk of death fell by a quarter in patients treated within three hours of the onset of symptoms and by a fifth in those treated three to six hours after the onset of symptoms. An analysis of patients treated within one hour suggested that the risk was halved. Similar trends were evident in the second international study of infarct survival. Neither trial, however, was designed specifically to evaluate the relation between the time of treatment and survival, and the trends were not significant.

The delivery …