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Review of: Brown JP, Mahmud E, Dunford JV, et al: "Effect of Prehospital 12-Lead Electrocardiogram on Activation of the Cardiac Catheterization Laboratory and Door-to-Balloon Time in ST-Segment Elevation Acute Myocardial Infarction." American Journal of Cardiology. 101(2):158-161, 2008.
The Science
This study from San Diego examined the effect of obtaining a prehospital 12-lead ECG and activating the cath lab for patients with suspected ST-elevation myocardial infarctions (STEMI). The time from arrival at the hospital to cath lab (door to balloon) was compared to that of patients who presented on their own to the emergency department (ED) (control group) who were diagnosed with a STEMI.
In this study, 25 prehospital patients were felt to be STEMI patients, and the cath lab was activated. Five were excluded as three 12-leads indicated an old bundle-branch block, and two had left-ventricular hypertrophy. The door-to-balloon time for the EMS patients was 73 +- 19 minutes and 130 +- 66 minutes for the control group. EMS scene times increase an average of 73 seconds for 12-lead acquisition.
The majority of patients with field STEMI achieved door-to-balloon times of <90 minutes (80% field STEMI, 25% controls).
The Street
This study is almost four years old, and I suspect it was a "proof of concept" study. It clearly demonstrated that prehospital 12-leads coupled with cath lab activation leads to faster entry of the patient into the system. The goal for reperfusion using interventional procedures (balloon, stents, etc.) is 90 minutes from arrival at the hospital. It has long been recognized that a large percentage of acute MIs do not call 9-1-1 opting instead to drive themselves to the hospital after waiting prolonged periods for their symptoms to improve or for family members to convince them to seek medical attention. Once at the hospital, they may or may not be properly triaged quickly and further delays may result in them not receiving a diagnostic ECG in a timely manner.
Although the number of EMS STEMI cases in this study is relatively small, it's interesting to note that they inappropriately activated the cath lab five times for a false positive rate of 20%. This is much higher than many other studies have reported. While over-triage of trauma tends to be 20-30%, the fiscal impact of calling in a cath lab can be a major impediment to a highly effective prehospital STEMI program. Some have advocated mandatory telemetry of 12-leads to lower the false positive rate.
The Allina Medical Transport program in Minneapolis/St. Paul will soon release an abstract of its experience, which shows similar times savings to this study with a false-positive rate of less than 5%. Although this study didn t explain how the medics determined what constituted a STEMI, I suspect they relied on the machine-generated interpretation, which tends to be fooled by bundle-branch blocks and ventricular hypertrophy. Various strategies to improve provider education along with the use of very stringent STEMI criteria have proven that EMS providers can accurately diagnose STEMI with low false-positive rates.
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