The variety of possible presentations of myocardial infarction is well known, but chest pain is perhaps the most constant of symptoms. Evidence has linked duration of pain with size of infarct,' and Mitchell suggested that analgesic requirements should be used as an early guide to whether infarction has taken place.' I carried out a study to evaluate the diagnostic potential of analgesic requirements.
Patients, methods, and resultsAltogether, 217 consecutive admissions to the coronary care unit were considered for study. Reasons for exclusion (97 patients) included rapid death, muscular trauma, and chest pain apparently not due to myocardial ischaemia. Analgesia was given as intravenous diamorphine, usually after a trial of glyceryl trinitrate 0-5-1-0 mg sublingually.Infarction was assumed (80 patients) if the criteria of Rowley and Hampton for definite or probable myocardial infarction were fulfilled.3 The remaining patients were assumed not to have had myocardial infarction, and 40 were included in the study. Infarct size was estimated by plasma creatine kinase activity (highest of four estimations in three days), hydroxybutyrate dehydrogenase activity (highest of three estimations), and the number of electrocardiographic territories (inferior, anterior, lateral, posterior, or The time after admission of the final injection was the most useful variable: only one (250°,,) of the group without myocardial infarction required analgesia after two hours compared with half of the group with infarction. Values in patients with equivocal electrocardiographic changes and in the one or two territory groups were 5-7, 7-6, and 13-3 hours respectively; the correlation with creatine kinase activity was r= 0 40 (p < 0 001).
CommentThe results of this study were consistent with a relation between severity of myocardial infarction and amount of analgesia required in the first hospital day. One fifth of the patients with myocardial infarction, however, did not need any analgesia at all, and one patient without myocardial infarction required 20 mg diamnorphine. Moreover, there seems little clinical advantage in attempting to guess the size of myocardial infarction after only 24 hours. In the early decision of whether infarction has taken place analgesic requirements may be of greater value. This study suggests that if a second injection of analgesia is required in the first 24 hours the chances of myocardial infarction being excluded lengthen to one in eight and myocardial infarction is four times more likely. If three or more injections are needed myocardial infarction is 10 times more likely. If a second injection is required more than three hours after admission exclusion of myocardial infarction becomes a 40 to one chance and infarction is 20 times more likely. Thus if this three hour rule was applied it would indicate infarction with a specificity of 97 5%, and a similar predictive value. A negative result is less reliable, the sensitivity being 50%/ These results confirm that analgesic requirement can be related to the s...