Of 2886 patients monitored during acute myocardial infarction, 500 were observed within one hour of the onset of symptoms. Half of the early admission group were admitted in response to emergency 999 calls and 435 of them travelled in resuscitation ambulances, where surveillance for arrhythmias was instituted. Pulmonary oedema occurred in 130 patients (26%), cardiogenic shock supervened in 60 (12%), and 115 (23%) died in hospital. Ventricular fibrillation was observed in 98 patients (20%). Forty two of them survived to be discharged, including 20 of the 24 with primary fibrillation which had occurred first in hospital. In only one case did primary ventricular fibrillation occur after the first 10 hours of onset of illness. Sinus bradycardia, atrial fibrillation, ventricular tachycardia, and ventricular fibrillation were all observed more frequently in patients admitted within one hour after the onset of symptoms than in those admitted later.An element of selection is inevitable when early admission is encouraged by the existence of a resuscitation ambulance system; this will depend in part on the early recognition of risk and the geographical location of the attack. These factors may bias the group towards relatively high risk. Nevertheless, prompt admission after myocardial infarction should improve survival by permitting successful management both of ventricular fibrillation and of other arrhythmias which may influence short term and long term prognosis. IntroductionLittle information is available in world reports on the complications of myocardial infarction when patients come under observation very early after the onset of symptoms. A series representing an unselected population cannot be obtained because data are collected only from those who seek treatment. In 1971 Adgey and colleagues reported' on the incidence of arrhythmias among 284 patients observed within one hour in a mobile coronary care unit manned by medical staff and, at that time, intended to operate in response to calls from general practitioners. Selection may not necessarily be similar in a community encouraged to make use of the emergency (999) telephone system for patients with severe chest pain or collapse; the Belfast results may also have been influenced by the availability of medical skills.Information on the results of early intervention is of particular value because of a resurgence of interest in prehospital care and early hospital admission. In Britain the Department of Health no longer actively discourages the creation of new coronary care and resuscitation ambulance programmes.2 3 District health authorities considering setting up such schemes within financial constraints will wish to know the potential benefits of early intervention.Taking advantage of data collected since the creation of an ambulance system4 -6 based on emergency services without direct medical intervention, we have reviewed retrospectively the records of 500 patients with confirmed myocardial infarction observed within 60 minutes of the onset of chest ...