myocardial oxygen demand
Summary
Myocardial oxygen demand (MVO2) refers to the amount of oxygen required by heart muscle to sustain contractile function. It is determined by three primary factors: heart rate, contractility, and wall tension (afterload and preload). Understanding MVO2 is essential for managing ischemic heart disease and angina.
Detail
Myocardial oxygen demand is a key concept in cardiovascular physiology and pharmacology, especially relevant to ischemic heart disease. The heart has minimal anaerobic reserve and extracts nearly maximal oxygen from coronary blood even at rest (~70-80% extraction), so increased demand must be met by increased coronary blood flow rather than increased extraction.
The three major determinants of MVO2 are: 1. Heart rate (HR) - increased HR raises oxygen consumption due to more frequent contractions and reduced diastolic filling time (when coronary perfusion occurs) 2. Contractility - increased inotropic state raises energy expenditure 3. Wall tension - governed by Laplace's law (Tension = Pressure x Radius / Wall thickness), affected by: - Preload (ventricular volume/end-diastolic pressure) - Afterload (systemic vascular resistance, aortic pressure)
Clinical significance: In coronary artery disease, fixed atherosclerotic stenoses limit oxygen supply, while increased demand (from exertion, tachycardia, hypertension) can precipitate angina or myocardial infarction due to supply-demand mismatch. This is the basis for antianginal therapy:
- Beta-blockers: decrease HR and contractility, reducing demand - Calcium channel blockers: decrease contractility, HR (non-dihydropyridines), and afterload (vasodilation) - Nitrates: reduce preload (venodilation) and afterload (arterial dilation), decreasing wall tension - Ranolazine: reduces intracellular calcium overload, improving diastolic relaxation without significant hemodynamic effects
The rate-pressure product (HR x systolic BP) is often used clinically as an estimate of MVO2, particularly during stress testing, to gauge when ischemic threshold is reached (evidenced by ST depression or angina onset).
Understanding this concept is crucial for managing stable angina, acute coronary syndromes, and perioperative cardiac risk stratification, as interventions aim to either increase supply (revascularization, nitrates via coronary vasodilation) or decrease demand (beta-blockers, rate control).
Sources
- Guyton and Hall Textbook of Medical Physiology
- Katzung's Basic and Clinical Pharmacology
- First Aid for the USMLE Step 1
- Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine
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