Background: The evaluation of acute chest pain in patients who are at low risk of acute coronary syndrome (ACS) is the most common indication for short-term hospitalization and it frequently involves cardiac stress testing. Clinical prediction rules have been proposed to identify patients who are at an even lower risk for cardiac events and who may be discharged safely without the need for provocative testing. We evaluated the utility of the modified North American Chest Pain Rule (NACPR) in guiding the decision to perform cardiac stress testing in patients with acute chest pain that is considered low-risk for ACS.

Methods: Using a prospective observational design, patients presenting with acute chest pain to the observation unit of an 805-bed tertiary teaching hospital were recruited in the study. As per hospital protocol, these patients were considered low-risk for ACS based on the absence of ongoing chest pain, dynamic ST-segment changes and elevated cardiac troponin I. Demographic and clinical data necessary to apply NACPR were collected at time of enrollment. The chest pain component of the original NACPR was modified by adopting the American Heart Association’s definition of typical angina. The attending physicians used their own clinical judgment when deciding to order a cardiac stress test. The occurrence of the primary composite outcome of myocardial infarction, coronary revascularization or death from any cause within 30 days of initial presentation was recorded through follow-up phone call and medical records review. Results were analyzed using chi-squared test to compare categorical variables.

Results: A total of 399 patients were enrolled and follow-up was complete for 284 (71.7%). Of the 284 patients included in the analysis, 218 (76.8%) underwent cardiac stress testing, of which 24 (11%) was positive and 5 (2.3%) was indeterminate for inducible ischemia. The primary composite outcome occurred in 5 (1.8%) patients. All NACPR predictors were absent in 64 (22.5%) patients, thus they were considered low-risk for significant cardiac events. The rate of performing stress tests was similar in low-risk and in non-low-risk patients, 68.8% and 79.1% respectively, p=.085. The stress test result was positive or indeterminate in 1 (2.3%) low-risk and in 28 (16.1%) non-low-risk patients, p<0.016. No primary outcome occurred among those who have a low-risk NACPR.

Conclusions: The rate of significant cardiac events is very low among patients undergoing evaluation for acute chest pain that is considered low-risk for ACS. The NACPR appears to be useful in identifying very low-risk patients who may be discharged without further provocative investigation.