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The threshold for myocardial ischemia varies in patients with coronary artery disease depending on the exercise
C E Garber1, R A Carleton, D N Camaione
1Department of Medicine (Division of Cardiology), Memorial Hospital of Rhode Island, Pawtucket 02860-4499.
Insights
The fixed threshold for myocardial ischemia during exercise may not be fixed. Lower exercise intensity significantly reduced the rate-pressure product at which ischemia occurred, even in patients who previously experienced angina.
Area of Science:
- Cardiology
- Exercise Physiology
Background:
- The established concept suggests myocardial ischemia and angina occur at a fixed heart rate-systolic blood pressure product.
- Recent evidence challenges this fixed threshold theory for ischemic events.
Purpose of the Study:
- To investigate how varying exercise intensity impacts the ischemic threshold in patients with coronary artery disease.
- To determine if a lower exercise intensity alters the rate-pressure product at which myocardial ischemia is detected.
Main Methods:
- Thirty-three patients with coronary artery disease and inducible myocardial ischemia underwent two treadmill tests.
- Tests included a symptom-limited incremental exercise test followed by a 20-minute submaximal test at 70% peak heart rate.
- Myocardial ischemia was documented using thallium-201 myocardial perfusion imaging and ST segment depression.
Main Results:
- All patients developed significant ST depression during the incremental test at a mean rate-pressure product of 19,130.
- During the submaximal test, 85% of patients showed significant ST depression at a lower mean rate-pressure product of 15,250.
- Importantly, 86% of patients with ST depression during the submaximal test were asymptomatic, including some who had experienced angina previously.
Conclusions:
- The ischemic threshold, defined by the rate-pressure product, is significantly influenced by exercise intensity.
- Lowering exercise intensity can reveal myocardial ischemia at a lower physiological stress level.
- This suggests the concept of a fixed ischemic threshold may be inaccurate and dependent on exercise conditions.
Abstract:
It is generally accepted that angina pectoris and, presumably, myocardial ischemia occur at a fixed heart rate-systolic blood pressure product in a given patient. This concept of a fixed threshold has recently been challenged. To evaluate the effects of varying exercise intensity on the ischemic threshold, 33 patients with coronary artery disease and provokable myocardial ischemia, documented by thallium-201 myocardial perfusion imaging, underwent two exercise tests 2 to 7 days apart. A symptom-limited incremental treadmill exercise test was followed by a 20 min submaximal treadmill test at an intensity approximating 70% of the peak heart rate attained during the incremental test. During the incremental exercise test, angina pectoris developed in 16 patients and 17 patients were asymptomatic. At least 0.1 mV of ST segment depression developed in all subjects during the incremental exercise test at a mean exercise duration of 5.3 +/- 2.6 min, a rate-pressure product of 19,130 +/- 5,735 and oxygen uptake of 19.6 +/- 7.0 ml/kg per min. During the submaximal exercise test, 28 (85%) of the 33 patients had significant ST segment depression. Of these patients, 24 (86%) were asymptomatic, including 10 patients who had previously reported anginal symptoms during the incremental test. The average time to onset of 0.1 mV ST segment depression during the submaximal test was 8.1 +/- 4.5 min. These changes occurred at a rate-pressure product of 15,250 +/- 3,705 and an oxygen uptake of 14.3 +/- 5.9 ml/kg per min, and were significantly (p less than 0.001) lower than values observed during the graded exercise.(ABSTRACT TRUNCATED AT 250 WORDS)