The myocardial supply:demand ratio--a critical review

Myocardial ischemia occurs when there is an imbalance between myocardial oxygen demand and supply, and it is usually entirely or predominantly subendocardial. Animal experiments have shown that relative subendocardial ischemia (a reduced inner:outer flow ratio) can be predicted quite accurately from the ratio of two pressure-time areas:DPTI, the area between diastolic aortic and left ventricular pressures, and SPTI, the area beneath the systolic left ventricular pressure curve. Although the importance of relating supply and demand is obvious, care is needed in applying the results of these animal experiments to man. Recent work has shown that the critical DPTI:SPTI ratio below which subendocardial ischemia occurs is about 0.4 to 0.5 rather than 0.7 to 0.8, as originally reported. On the other hand, the critical ratio may be raised to an unknown extent by myocardial edema or hypertrophy, or by thickened or narrowed coronary arteries. Furthermore, the critical ratio is not independent of absolute coronary diastolic pressure: It is much lower than 0.4 when coronary pressures are high, perhaps because intramyocardial diastolic pressures are much higher than once thought. Further work is required to allow an important physiologic concept to be used in making decisions about patients with heart disease.