Patients with Normal Exercise Stress Myocardial Perfusion Studies Who Achieve Less Than 75 Percent of Maximum Predicted Heart Rate Are at Increased Risk for Cardiac Events

Objectives To investigate patients with normal stress MPI who achieve < 75% Maximal Predicted Heart Rate (MPHR) with the Bruce protocol are at higher risk for cardiac events compared to those who achieve higher levels of MPHR.

Methods 1000 patients (614 male, 386 female; mean age 60 ±12y) with normal MPI in whom Bruce protocol stress MPI was performed. The patients were divided into two groups based on MPHR; 116 patients with MPHR< 75% were placed in the Group 1 and 884 patients with MPHR ³75% were placed in Group 2. Follow-up data over ~2±0.3 year were obtained from hospital records. Associated cardiac risk factors, medications and prior cardiovascular medical history were compared in these two groups, and prognostic value of exercise treadmill stress variables analyzed.

Results There were a total of 30 adverse cardiac events. In Group 1, with less than 75%MPHR, there were 16 events (16 / 116=14.0%). In Group 2, with ≥75% MPHR, there were 14 events (14/ 884=1.5% ). The difference in number of events in the two groups was highly significant (p< 0.001). Group1 patients were more likely have a history of MI, PTCA, diagnosed CAD, symptoms of typical angina, smoking history, hypertension, or treatment with β-blockers. Resting HR, peak HR, peak SBP, METS, frequency of ST depression ≥1 mm and EF were significantly higher in Group 2 patients; the Duke score was significantly lower in the Group 2 patients. Stepwise regression analysis demonstrated that the variables most likely to be related to cardiac events were MPHR, followed by ischemic ECG changes, and METS. Duke score and systolic blood pressure did not have a strong influence on adverse cardiac events.

Conclusions Patients with normal exercise MPI with MPHR below 75% should be evaluated with pharmacological stress testing or other appropriate procedures for protection from future adverse cardiac events.