Interaction between arrival time and thrombolytic treatment in determining early outcome of acute myocardial infarction.

Background: Shortening prehospital delay has been identified as an important means of improving responses to reperfusion treatment. If this increases the risk profile of the population delivered to hospital, it may paradoxically cause a deterioration in hospital mortality.

Objective: To examine the interaction between arrival time (time from onset of chest pain to arrival at hospital) and thrombolytic treatment in determining the early outcome of acute myocardial infarction.

Methods: Prospective cohort study of 1723 patients with acute myocardial infarction who were potentially eligible for thrombolytic treatment (ST elevation on ECG; arrival time < or = 12 hours).

Results: All patients were eligible for thrombolysis but only 1098 (80%) received it. Patients who did not receive thrombolytic treatment were older (66 (58-73) v 61 (53-70) years, p < 0.001), more commonly female (32.1% v 24.8%, p < 0.01), and had higher frequencies of previous infarction (28.6% v 15.6%, p < 0.001) and left ventricular failure (37.5% v 26.9%, p < 0.01) than patients who received thrombolytic treatment. For the group as a whole, 30 day mortality was 11.7% and was unaffected by arrival time, but in patients who did not receive thrombolysis an arrival time of < or = 6 hours was associated with significantly higher 30 day mortality than an arrival time of 6-12 hours (24.3% v 2.6%, p = 0.002). Conversely, in patients who did receive thrombolysis an arrival time of < or = 6 hours was associated with a lower 30 day mortality than an arrival time of 6-12 hours (8.5% v 14.5%, p < 0.02).

Conclusions: Shortening prehospital delay in acute myocardial infarction will tend to increase the risk profile of patients presenting to emergency departments. The data presented here indicate that this may increase hospital mortality if underutilisation of thrombolytic treatment among high risk groups is not diminished.