The reason lies in its higher baseline oxygen need and limited maximal vasodilator response. Under baseline physiologic conditions, subendocardium has the highest metabolic rate due to the increased wall tension when compared to epicardium. This exaggerated wall tension is due to the combined effect of higher intramyocardial pressures in the subendocardium plus the contribution from the intraventricular pressures. Because of the increased demand at baseline, the ratio of endocardial to epicardial blood flow is approximately 1.25:1.0 in conscious dogs.
The maximal blood flow that endocardial blood vessels can provide is attenuated as compared to epicardial vessels primarily due to additive effects of limitation to flow to diastolic phase and the extravascular compressive forces. As previously discussed, the subendocardial blood flow is limited to the diastolic phase, and the extravascular compressive forces reach their maximum in the subendocardium. buy levaquin online
Increased baseline demand and decreased maximal supply to the endocardium leaves this region of the myocardium with least reserve and consequently most susceptible to ischemia. It is therefore imperative that at the time of myocardial ischemia, the qualitative changes in the factors influencing supply and demand be directed towards improvement of blood flow to the subendocardium.