Cardiac Physiology L1.pdf

Cardiac Physiology Mohammed Ismael Dawood Assistant Professor of Medicine ❑ The heart acts as two serial pumps. ❑ These share several electrical and mechanical components. ❑ The right heart circulates blood to the lungs where it is oxygenated, and the left heart circulates...

Cardiac Physiology Mohammed Ismael Dawood Assistant Professor of Medicine ❑ The heart acts as two serial pumps. ❑ These share several electrical and mechanical components. ❑ The right heart circulates blood to the lungs where it is oxygenated, and the left heart circulates it to the rest of the body. ❑ The atria are thin-walled structures that act as priming pumps for the ventricles, which provide most of the energy required to maintain the circulation. ❑ The ventricles are thick-walled structures, adapted to circulating blood through large vascular beds under pressure. Radiologically: ❑ Normally, the heart occupies less than 50% of the transthoracic diameter in the frontal plane, as seen on a chest X-ray. ❑ On the patient’s left, the cardiac silhouette is formed by the aortic arch, the pulmonary trunk, the left atrial appendage and the LV. ❑ On the right, the silhouette is formed by the RA and the superior and inferior venae cavae, and the lower right border is formed by the RV. Coronary Circulation: ❑ Conventional arterial supply to the heart arises from the right and left coronary arteries, which originate from the root of the aorta RCA: ◆ conus artery ◆ acute marginal branches ◆ AV nodal artery ◆ PDA also known as PIV LCA: ◆ LAD – septal branches – diagonal branches ◆ LCx– obtuse marginal branches. ❑ Dominance of circulation determined by whether the RCA or the LCx supplies the 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 LCx (15%). ◆ balanced circulation: dual supply of posteroinferior LV from RCA and LCx (5%). ❑ The sinoatrial (SA) node is supplied by the SA nodal artery, which may arise from the RCA (60%) or LCA (40%). ❑ The AV node is supplied by the AV nodal artery, which may arise from the RCA (90%) or LCx (10%). Conduction system: ❑ SA node located at the junction of SVC and roof of RA governs pace-making; heartbeat originates here. ❑ Anterior-, middle-, and posterior-internal nodal tracts carry impulses in the RA with the atrial impulses converging at the AV node and along Bachmann’s bundle in the LA. ❑ AV node located within the triangle of Koch which is demarcated by: superior margin of the coronary sinus, tendon of Todaro, and hinge of the septal leaflet of the tricuspid valve. ❑ AV node is the conduit for electrical impulses from atria to ventricles, unless an accessory AV pathway (e.g. WPW syndrome) is present. ❑ Bundle of His: 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 fibres which conduct impulses into the ventricular myocardium. Cardiovascular innervation: ❑ Sympathetic nerves innervate the SA node, AV node, ventricular myocardium, and vasculature. increased activity of the SA node via the β1 receptor leads to increased HR via more frequent impulse from pacemaking cells (increased chronotropy - increased HR). cardiac muscle (β1) fibres increase contractility (inotropy - leads to increased SV). stimulation of β1- and β2-receptors in the skeletal and coronary circulation causes vasodilatation ❑ Parasympathetic nerves innervate the SA node, AV node, and atrial myocardium but few vascular beds. at rest, vagal tone dominates the tonic sympathetic stimulation of the SA node and AV node, resulting in slow AV conduction, and consequently a prolonged PR interval or second or third degree AV block (i.e. reduced dromotropy (if only affecting AV node conduction)). parasympathetics have very little impact on total peripheral vascular resistance. Cardiac output : (CO) It is the volume of blood pumped by each ventricle in one minute. CO = HR x SV. Normally : 5 liter / minute in the average ( 2.5 - 4.2 L / m2 / min. ). CO depends on: Heart rate. Preload (Volume load or diastolic load). Afterload (Pressure or Systolic load). Contractility (Inotropic State). Stroke volume : (SV) It is the volume of blood pumped by each ventricle in one beat. Normally : 70 ml in the average ( 40 - 70 ml / m2 ). Ejection fraction : (EF) ❑ Ejection fraction is a test that determines how well your heart pumps with each beat. ❑ Left ventricular ejection fraction (LVEF) is the measurement of how much blood is being pumped out of the left ventricle of the heart (the main pumping chamber) with each contraction. ❑ Right ventricular ejection fraction (RVEF) is the measurement of how much blood is being pumped out of the right side of the heart to the lungs for oxygen. ❑ In most cases, the term “ejection fraction” refers to left ventricular ejection fraction. Ejection fraction is usually expressed as a percentage. A normal heart pumps a little more than half the heart’s blood volume with each beat. A normal LVEF ranges from 55-70%. A LVEF of 65, for example, means that 65% of the total amount of blood in the left ventricle is pumped out with each heartbeat. The LVEF may be lower when the heart muscle has become damaged due to a heart attack, heart muscle disease (cardiomyopathy), or other causes. An EF of less than 40% may confirm a diagnosis of heart failure. Someone with diastolic failure can have a normal EF. An EF of less than 35% increases the risk of life- threatening irregular heartbeats that can cause sudden cardiac arrest (loss of heart function) and sudden cardiac death. An implantable cardioverter defibrillator (ICD) may be recommended for these patients.