EFFECT OF DURATION FROM SYMPTOM ONSET ON THE NEGATIVE PREDICTIVE VALUE OF A NORMAL ECG FOR EXCLUSION OF ACUTE MYOCARDIAL-INFARCTION

Study objective: We hypothesized that the negative predictive value of the ECG would improve with time and assessed the effect of time elaps ed from symptom onset to ED presentation on the negative predictive va lue, sensitivity, specificity, and positive predictive value of the in itial ECG in patients presenting with symptoms suggestive of acute myo cardial infarction (AMI). Methods: We conducted a cross-sectional stud y in a university teaching hospital. Our structured data instrument, c ompleted at the lime of presentation included demographics, time of on set of symptoms, history, laboratory, and ECG findings. AMI was diag n osed with the use of international diagnostic criteria. Patients were stratified according to duration of time from symptom onset at 3-hour intervals. Results: We enrolled 526 patients in the study group. The m ean age was 59 years; 40% were female. The mean time elapsed from symp tom onset to presentation was 185 minutes. A diagnosis of AMI was made in 104 patients (20%). The negative predictive values of a normal ECG for exclusion of AMI, stratified by duration of time from symptom ons et, were: 0 to 3 hours, 93.2% (95% confidence interval [CI], 87.4% to 96.1%); 3 to 6 hours, 93.0% (95% CI, 83.0% to 98.1%); 6 to 9 hours, 92 .6% (95% CI, 75.8% to 99.1%); and 9 to 12 hours, 94.1% (95% CI, 71.3% to 99.9%) (P=1.0). The sensitivity, specificity, and positive predicti ve value of the ECG were similar in all groups. Conclusion: The negati ve predictive value of a normal ECG for exclusion of AMI does not impr ove as the duration of time from symptom onset to presentation increas es. Normal ECG findings cannot be used to rule out an AMI, even those obtained up to 12 hours after symptom onset.