Original ArticlePrognosis in patients achieving ≥10 METS on exercise stress testing: Was SPECT imaging useful?
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
Section snippets
Methods
Prospectively collected data from the University of Virginia Nuclear Databank (UVAND) were analyzed in a cohort of consecutive patients who underwent exercise testing and SPECT MPI through the University of Virginia Health System.
Follow-Up
Follow-up data on all-cause mortality were obtained in 463 of the 509 patients (91%) with interpretable ECGs who reached ≥10 METS of exercise workload (Figure 1). The cohort without follow-up data available were younger (mean age 45.0 vs 53.7, P < .001) and were more likely to use tobacco (43.5% vs 26.4%, P = .014). Otherwise, the baseline demographics of patients lost to follow-up did not significantly differ from the overall cohort. Data on nonfatal events such as MI and late
Discussion
This study shows that a consecutive group of patients referred for stress testing with MPI who achieve a workload of ≥10 METS have an excellent prognosis with an annualized cardiac mortality of 0.1% and a combined cardiac death and nonfatal MI rate of 0.4%. Furthermore, the patients with either hard or soft cardiac events were all in the group which achieved their target heart rate. None of the event patients had either >5% or >10% ischemia or were in the subset of patients who manifested
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
Acknowledgment
The authors have indicated that they have no financial conflicts of interest.
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Dr Bourque is funded by an NIH NRSA Training Grant: T32 EB003841-04.