Ischemic Heart Disease: O2 Supply-Demand Mismatch

Ischemic heart disease (IHD) results from an imbalance between myocardial oxygen supply and demand. The NP must understand the determinants of each side of this equation to guide pharmacotherapy. OXYGEN DEMAND is determined by four factors: heart rate (most modifiable and most important determinant), myocardial wall stress (determined by preload, afterload, and ventricular radius via the Law of LaPlace: wall stress = pressure x radius / 2 x wall thickness), contractility (inotropic state), and to a lesser extent, basal metabolic requirements. Tachycardia is particularly detrimental because it both increases demand (more contractions per minute) AND decreases supply (shortened diastole reduces coronary filling time). OXYGEN SUPPLY depends on coronary blood flow (determined by coronary perfusion pressure, which equals aortic diastolic pressure minus left ventricular end-diastolic pressure, and coronary vascular resistance) and oxygen-carrying capacity (hemoglobin concentration, oxygen saturation). Unlike other organs, the myocardium extracts approximately 70-80% of delivered oxygen at rest (the highest extraction ratio of any organ), meaning it cannot compensate for increased demand by extracting more oxygen -- the ONLY way to increase myocardial oxygen delivery is to increase coronary...