Heart’s Blood Supply and Circulation

Heart's Blood Supply

• The heart has its own circulatory network, the coronary circulation, which arises from the heart's top portion.
• The right and left coronary arteries supply blood to the heart muscle.
• The left coronary artery has the greatest blood volume flow, feeding the left atrium and left ventricle, and the right ventricle.
• Blood leaves the tissues of the left ventricle through the coronary sinus and the right ventricle through the anterior cardiac veins, both dumping into the right atrium.
• Normal blood flow to the myocardium at rest is 200-250 ml/min (5% of total output).

Myocardial Oxygen Supply and Use

• At rest, the myocardium extracts 70-80% of oxygen from the blood in coronary vessels.
• Proportionate increase in coronary blood flow during exercise provides the sole mechanism to increase myocardial oxygen supply.
• Two factors increase myocardial blood flow:
  • Increase in metabolism dilates coronary vessels.
  • Increase in aortic pressure during exercise forces a proportionately greater volume of blood into the coronary circulation.

Impaired Myocardial Blood Supply

• The myocardium depends on adequate oxygen supply because it has limited anaerobic capacity and can't make an extensive switch.
• Extensive vascular perfusion supplies at least one capillary to each of the heart's muscle fibers.
• Tissue hypoxia provides a potent stimulus to myocardial blood flow, which can produce chest pains (angina pectoris).
• A blood clot in a coronary vessel can impair normal heart function, leading to a myocardial infarction, where heart tissue will die due to lack of oxygen.

Rate-Pressure Product (RPP)

• RPP is an estimate of myocardial workload and VO2.
• RPP is computed from the product of peak SBP measured at the brachial artery and heart rate.
• RPP is an index of relative cardiac work and relates closely to directly measured myocardial VO2 and coronary blood flow in healthy subjects over a wide range of exercise intensities.
• RPP ranges from 6000 at rest to greater/equal to 40,000 during exercise, depending on intensity and mode.

Myocardial Metabolism

• At rest, the heart muscle uses fat/lipids as fuel.
• During moderate exercise, glucose and lactate are used more.
• During intense exercise, lactate is oxidized.

Intrinsic Regulation of Heart Rate

• Unlike other tissues, cardiac muscle maintains its own rhythm.
• Left to its inherent rhythmicity, the heart would beat around 100 bpm.
• The sinoatrial (SA) node provides the innate stimulus for heart action and is the "heart's pacemaker".

Normal Route of Myocardial Impulse Transmission

• The sinoatrial node (SA) in the right atrium is the origin of the impulse.
• The impulse spreads across the atria, causing them to contract.
• The impulse passes to the atrioventricular node (AV), travels along the atrioventricular bundle into its two branches (right and left crus), and spreads into the ventricles, causing them to contract.
• Dissipation of the impulse causes the atria and ventricles to relax or dilate.

Electrocardiogram (ECG)

• The myocardium's electrical activity creates an electrical field throughout the body.
• ECG represents a composite record of the heart's electrical events during a cardiac cycle.
• These electrical events can monitor HR during physical activity and exercise stress testing.
• A valid ECG tracing requires proper electrode placement.
• ECG leads transmit the electrical signal to a recorder, which creates a composite electrical "3D picture" of myocardial activity.

ECG Waves

• P wave: depolarization of atria before atria contract.
• QRS complex: signals electrical changes from ventricular depolarization before ventricles contract.
• Atrial repolarization after P wave, but it's obscured by the QRS complex.
• T-wave: represents ventricular repolarization that occurs during ventricular diastole.

Electrocardiogram (ECG) and Heart Rate

• Typically, R to R measurement is used to measure HR, which is typically irregular, indicating decent recovery.
• Greater HRV (heart rate variability) is a better indication of allowing the system to relax and recover.

Electrocardiogram (ECG) and Pathology Detection

• Large low spaces on the ECG can indicate that the heart is not told to contract, which can be an issue (e.g., heart block).

Explore the coronary circulation network providing blood supply to the heart, including the right and left coronary arteries. Learn about the distribution of blood to different parts of the heart and the role of coronary sinus and cardiac veins in returning blood to the right atrium.