Cardiac Physiology

Cardiac Physiology

Heart Structure and Function

• The heart acts as two serial pumps, sharing electrical and mechanical components.
• The right heart circulates blood to the lungs for oxygenation, and the left heart circulates it to the rest of the body.
• Atria are thin-walled structures acting as priming pumps for ventricles, which provide most of the energy to maintain circulation.
• Ventricles are thick-walled structures adapted to circulating blood through large vascular beds under pressure.

Radiological Features

• The heart occupies less than 50% of the transthoracic diameter in the frontal plane on a chest X-ray.
• The cardiac silhouette on the left is formed by the aortic arch, pulmonary trunk, left atrial appendage, and LV.
• The cardiac silhouette on the right is formed by the RA, superior and inferior venae cavae, and the lower right border is formed by the RV.

Coronary Circulation

• The conventional arterial supply to the heart arises from the right and left coronary arteries, which originate from the root of the aorta.
• The right coronary artery (RCA) gives off conus, acute marginal, AV nodal, and posterior descending arteries.
• The left coronary artery (LCA) gives off LAD, septal, diagonal, LCx, and obtuse marginal branches.
• Dominance of circulation is determined by whether the RCA or LCA supplies the posterior descending artery (PDA).
• Right-dominant circulation: PDA and at least one posterolateral branch arise from RCA (80%).
• Left-dominant circulation: PDA and at least one posterolateral branch arise from LCA (15%).
• Balanced circulation: Dual supply of posteroinferior LV from RCA and LCA (5%).

Conduction System

• The sinoatrial (SA) node is located at the junction of SVC and roof of RA, governing pace-making.
• The SA node is supplied by the SA nodal artery, which may arise from RCA (60%) or LCA (40%).
• The AV node is located within the triangle of Koch and is supplied by the AV nodal artery, which may arise from RCA (90%) or LCA (10%).
• The AV node connects to the bundle of His, which divides into LBB and RBB.
• LBB further splits into anterior and posterior fascicles.
• RBB and fascicles of LBB give off Purkinje fibers conducting impulses into ventricular myocardium.

Cardiovascular Innervation

• Sympathetic nerves innervate the SA node, AV node, ventricular myocardium, and vasculature, increasing heart rate and contractility.
• Parasympathetic nerves innervate the SA node, AV node, and atrial myocardium, slowing heart rate and reducing AV conduction.

Cardiac Output

• Cardiac output (CO) is the volume of blood pumped by each ventricle in one minute, normally 5 L/min.
• CO = HR x SV.
• CO depends on heart rate, preload, afterload, and contractility.

Stroke Volume and Ejection Fraction

• Stroke volume (SV) is the volume of blood pumped by each ventricle in one beat, normally 70 mL.
• Ejection fraction (EF) is the percentage of blood pumped out of the ventricle with each beat, normally 55-70%.
• LVEF is the measurement of how much blood is being pumped out of the left ventricle.
• RVEF is the measurement of how much blood is being pumped out of the right side of the heart to the lungs.
• An EF of less than 40% may confirm a diagnosis of heart failure.