How serious is myocardial ischemia?

Maybe very serious. Ischemia means an area of the heart is not getting enough blood usually because of a blockage in an artery. The severity depends on whether it is a major artery or not.
Ischemia... Ischemia is serious. On myocardial perfusion stress tests, ischemia is usually characterized by extent, severity, and reversibility. These three terms help characterize ischemia. If you have ischemia it should be dealt with stat since ischemia can lead to heart attack. If you have a small amount of ischemia it may be medically managed. Large amount of ischemia may require surgical intervention.