Prognosis in patients achieving ≥10 METS on exercise stress testing: Was SPECT imaging useful?

Abstract

Background

The benefit of myocardial perfusion imaging (MPI) over exercise ECG stress testing alone is unclear in individuals attaining a workload of ≥10 METS. The purpose of this prospective study is to determine mortality and nonfatal cardiac events in patients at either intermediate pretest risk for CAD or patients with known CAD, achieving ≥10 METS regardless of peak exercise heart rate. The authors previously reported a low prevalence of significant ischemia in this patient cohort.

Methods

Baseline characteristics, ECG stress test findings, and perfusion and function results from quantitative gated ^99mTc-SPECT MPI were compared by achievement of a maximum age-predicted heart rate ≥85% in 509 consecutive patients who reached ≥10 METS. Events including all-cause and cardiac mortality, non-fatal myocardial infarction (MI), and late revascularization (>4 weeks after MPI) were prospectively collected.

Results

Of the 509 patients achieving ≥10 METS, follow-up for mortality was obtained in 463 (91%). Those lost to follow-up were older and had higher rates of tobacco use. The prevalences of CAD risk factors, prior known CAD, and MPI abnormalities were higher for the 68 patients failing to reach 85% of their target heart rate. The rate of ≥10% left-ventricular (LV) ischemia by MPI remained very low irrespective of attained heart rate (0.6% (3/463)). Six (1.2%) had an LVEF < 40%. Death occurred in 12 (2.6%) patients, one of which was classified as cardiac (0.1%/year). The other 11 deaths were related to cancer. Additionally, there were three nonfatal MIs (0.7 %) and one late revascularization (0.2%). Only one of these patients had any ischemia on MPI. No cardiac event patient had exercise ST depression or ≥5% LV ischemia.

Conclusions

Thus, patients at intermediate risk for CAD or known CAD achieving ≥10 METS have a very low prevalence of ≥10% LV ischemia and very low rates of cardiac mortality, nonfatal MI, and late revascularization, irrespective of heart rate achieved. Cardiac events did not correlate with abnormalities on the index MPI study. These results suggest that patients who attain ≥10 METS during exercise stress have an excellent prognosis over an intermediate term of follow-up, regardless of peak exercise heart rate achieved. The added value of MPI to standard exercise ECG testing in this population is questionable.

Introduction

Ischemic heart disease leads to a high morbidity and mortality. Exercise electrocardiographic (ECG) stress testing, although having limited sensitivity for coronary artery disease (CAD) detection, can identify those at high risk for cardiac events.¹ Despite receiving an appropriate indication for the evaluation of ischemia in most patients at intermediate risk,² the precise indications for performing myocardial perfusion imaging (MPI) in conjunction with symptom-limited exercise testing over exercise ECG testing alone is unclear. Because exercise MPI is more sensitive than exercise-induced ST depression for detection of ischemia,³ it is often combined at the outset for testing patients at an intermediate pretest likelihood of disease. On the other hand, it has been shown that patients who achieve high exercise heart rates and workloads without ischemic ST depression have an excellent prognosis.4, 5, 6, 7, 8, 9

The authors¹⁰ recently reported that in more than 470 consecutive patients who achieved 85% or greater of their maximum age-predicted heart rate (MAPHR) and a workload of ≥10 metabolic equivalents (METS) on symptom-limited exercise MPI, the prevalence of significant ischemia comprising 10% or more of the left ventricle (LV) was very low (0.4%). The authors also found that no patient in this cohort who achieved ≥10 METS without ≥1.0 mm of horizontal or downsloping ST depression had significant ischemia on single-photon emission computed tomography (SPECT) imaging. The prevalence of 5% to 9% ischemia of the left ventricle (LV) was also low in such patients (0.7%). Twenty percent of these patients had known CAD, and seventy percent had chest pain as the indication for testing. These data suggested that MPI could be eliminated in patients achieving ≥10 METS on exercise testing without loss of prognostic capability.

The purpose of this report is to assess all-cause and cardiovascular mortality and the incidence of cardiac events during follow-up of these individuals achieving a high exercise workload. The authors hypothesized that these patients achieving a high exercise capacity (≥10 METS) should have very low rates of cardiac mortality and nonfatal myocardial infarction (MI). Patients who achieved ≥10 METS but less than 85% of MAPHR were also followed-up for cardiac events. If the event rate was found to be very low in these patients, as the authors observed for the prevalence of significant ischemia,¹⁰ it would lend support to the notion that an imaging agent might not need to be injected for patients achieving ≥10 METS on exercise testing. Elimination of imaging would yield high cost savings since such patients represent about one-third of all patients referred for exercise SPECT MPI over a period of 1 year.¹⁰