The ischemic cascade
The ischemic cascade describes the sequence of events during myocardial ischemia, beginning with perfusion abnormalities and ultimately leading to chest pain (angina). Myocardial perfusion imaging techniques, such as SPECT and PET, can detect perfusion abnormalities early in this sequence, before mechanical dysfunction (i.e. left ventricular systolic or diastolic dysfunction) or electrical changes (ST-T changes on ECG) become apparent. In contrast, modalities like echocardiography and ECG typically identify ischemia at later stages, when functional or electrical alterations are evident. Subjective symptoms (i.e. chest pain) appear as the last event in the cascade. Thus, ischemic episodes may pass asymptomatically.
Importance of heart rate during myocardial perfusion imaging
It is generally accepted that achieving 85% of the age-adjusted maximum heart rate (MHR) is sufficient to provoke overt ischemic signs during stress testing (Bourque et al.). Thus, patients who undergo exercise stress testing must achieve at least 85% of the age-adjusted MHR. Heller et al. demonstrated the following, among patients with known coronary artery disease:
- Achieving 85% of age-adjusted MHR resulted in myocardial perfusion defects in all patients,, anginal symptoms in 83%, and ECG changes in all patients.
- Achieving 70% of age-adjusted MHR resulted in perfusion defects in 89% of patients, anginal symptoms in 26% and ECG changes in 47%.
