In low to moderate risk patients with chest pain, is a 6-hour protocol able to accurately rule out acute myocardial infarction ? - AMI - POEMs

Journal of Family Practice, Dec, 2001 by Emily A. Lagace

Herren KR, Mackway-Jones K, Richards CR, Seneviratne CJ, France MW, Cotter L. Is it possible to exclude a diagnosis of myocardial damage within 6 hours of admission to an emergency department? Diagnostic cohort study. BMJ 2001; 323:372-74

* BACKGROUND Many people come to physicians with acute chest pain, and it is difficult to sort out those who have potentially dangerous AMI and unstable angina from the many with more benign conditions that do not require hospital admission. Traditionally, this "role out" process has required a hospital stay with numerous measurements of cardiac enzymes often taking more than 24 hours and includes electrocardiogram (EKG) monitoring.

* POPULATION STUDIED The researchers of this study enrolled 383 consecutive patients presenting over a 12-month period to an inner-city emergency department in Manchester, England, with chest pain of low to moderate risk of AMI (the end prevalence of AMI in study participants was 18%). Patients were eligible if they were older than 25 years, had chest pain for less than 12 hours, had no history of trauma or other medical causes, no EKG evidence of AMI or ischemia, and no hypotension or arrhythmia.

* STUDY DESIGN AND VALIDITY This is a cohort study that entered consecutive patients in a 6-hour study protocol. This rule-out protocol consisted of continuous 12-lead ST segment monitoring and serial measurement of creatine kinase, myocardial bound (CK-MB) mass. CK-MB was measured 3 hours and 6 hours after the onset of pain, and after 6 hours of monitoring in patients with chest pain of less than 3 hours' duration. If chest pain had lasted 3 to 12 hours the CK-MB was measured on arrival and 3 hours later. Patients were ruled in if any of the CK-MB test results were positive or if an important change in the ST segment occurred, and ruled out if no changes occurred. Patients with a positive result were admitted, and discharged patients returned 2 days later for measurement of troponin T, which was used as the gold standard. A level of 0.1 [micro]g per mL indicated myocardial damage. Patients were also asked to return to the clinic at 1 month for a history, examination, and EKG.

The basic study design is a good way to look at this problem. Neither troponin T or CK-MB was adequate to identify those with non-AMi coronary syndromes, including those with unstable angina who have no enzyme or EKG changes but who do have increased morbidity and mortality. (1) This means that troponin T is a weaker gold standard for longer-term outcome in patients with chest pain.

* OUTCOMES MEASURED The primary outcome was to determine the number of patients with an elevated 2-day troponin T level given a diagnosis of AMI. Patients also followed up at 1 month with examination and EKG, but it is unclear how many did so.

* RESULTS Of the 383 patients who began the study protocol, 368 completed the initial 6-hour assessment. Fifty-three were protocol positive with either elevated CK-MB levels or ST segment changes; by the gold standard tests 18 patients had false-positive results. Only 292 had the follow-up 2-day troponin T value measured, with 11 people withdrawing from follow-up and the 65 others not returning for the 2-day blood test, making study results available for 76% of the original participants. Of the 239 people who had no initial CK-MB or ST segment changes, 238 had negative troponin T values, and one had a borderline increase in troponin T of 1.1 [micro]g per L. The sensitivity of the diagnostic test was 97.2% (95% confidence interval [CI], 95%-99%), and specificity was 93% (95% CI, 90%-96%). At a prevalence of 18%, which may be higher than some primary care populations, the protocol will correctly rule out patients without MI 99.6% of the time and correctly role in patients 66% of the time (positive likelihood ratio = 13.9; negative likelihood ratio = 0.03).

RECOMMENDATIONS FOR CLINICAL PRACTICE

In this study, an emergency department protocol using a 12-lead EKG and 2 to 3 CK-MB levels correctly identified 99.6% of the 18% of patients with chest pain who were later confirmed to have an AMI. This is promising information, but as family physicians we do not only need to know whether patients have an AMI but also whether the chest pain indicates that they have non-AMI coronary artery disease (eg, unstable angina) that will affect their lives. This kind of protocol can be used to role out MI, but how to manage thsee patients is still a problem. Some chest pain units that use a similar 6-hour protocol add troponin T levels and do some sort of noninvasive testing, such as exercise stress testing before discharge. In a low-risk population this has a risk of false-positive tests too. Replication of these results in other populations with longer-term follow-up would be useful before this protocol gets widespread use.

REFERENCE

(1.) O'Neil BJ, Ross MA. Cardiac marker protocols in a chest pain observation unit. Emerg Med Clin North Am 2001; 19:67-86

Emily A. Lagace, MD
Saint Paul, Minnesota
E-mail: elagace@faughnan.com