EVALUATION OF ST SEGMENT ELEVATION CRITERIA FOR THE PREHOSPITAL ELECTROCARDIOGRAPHIC DIAGNOSIS OF ACUTE MYOCARDIAL-INFARCTION

Study objective: To determine retrospectively the diagnostic accuracy of various ECG ST segment elevation criteria for the prehospital ECG d iagnosis of acute myocardial infarction. Design and setting: During a six-month period, paramedics acquired prehospital 12-lead ECGs on adul t chest pain patients. Investigators interpreted ECGs independently, r etrospectively, and blinded to patient outcome. ECGs were classified a s meeting or not meeting the six ST segment elevation criteria regardl ess of ECG morphology if the criteria were present in two or more anat omically contiguous leads: 1 mm or more ST segment elevation; 2 mm or more ST segment elevation; 1 mm or more ST segment elevation in the li mb leads or 2 mm or more ST segment elevation in the precordial leads; and the first three criteria with the simultaneous presence of recipr ocal changes. ECGs that did not meet any ST segment elevation criteria were classified as normal, nonspecific ST/T wave changes, abnormal bu t not ischemic, and ischemic. Hospital charts were reviewed for final cardiac diagnosis. Type of participant: Four hundred twenty-eight stab le adult prehospital chest pain patients in whom paramedics acquired p rehospital 12-lead ECGs. Interventions: None. Measurements and main re sults: Of the 428 cases, 123 (29%) met 1 mm or more ST segment elevati on criteria. Sixty-three (51%) of these 123 patients did not have myoc ardial infarctions. ECG characteristics most frequently associated wit h these non-myocardial infarction ECGs were left bundle branch block ( 21%) and left ventricular hypertrophy (33%). The three criteria that r equired the presence of reciprocal changes had the highest positive pr edictive values (93% to 95%), with sensitivities ranging from 20% to 3 3%. The criteria of 1 mm or more ST segment elevation with the simulta neous presence of reciprocal changes had a positive predictive value o f 94% and included 18 of the 21 (86%) myocardial infarction patients w ho had ST segment elevation and received thrombolytic therapy within f ive hours after hospital arrival. Of the 428 cases, 305 (71%) did not meet any ST segment elevation criteria and had a sensitivity of 81% an d a negative predictive value of 49% for the absence of acute myocardi al infarction. Conclusion: Fifty-one percent of patients whose prehosp ital 12-lead ECG met 1 mm or more ST segment elevation criteria had no n-myocardial infarction diagnoses. ST segment elevation alone lacks th e positive predictive value necessary for reliable prehospital myocard ial infarction diagnosis. Inclusion of reciprocal changes in prehospit al ECG myocardial infarction criteria improved the positive predictive value to more than 90% and included a significant majority (62% to 86 %) of acute myocardial infarction patients with ST segment elevation w ho received thrombolytic therapy within five hours after hospital arri val. ST segment elevation criteria that include reciprocal changes ide ntify patients who stand to benefit most from early interventional str ategies.