PHGY 1030 Electrical Activity of the Heart PDF

Electrical Activity of the Heart PHGY 1030 Asher Mendelson MD PhD FRCPC  OVERVIEW 1) The heart is autorhythmic. – specialized cells (pacemakers) trigger contractions – heart rate is determined by pacemaker activity 2) Cardiac contractions are coordinat...

Electrical Activity of the Heart PHGY 1030 Asher Mendelson MD PhD FRCPC  OVERVIEW 1) The heart is autorhythmic. – specialized cells (pacemakers) trigger contractions – heart rate is determined by pacemaker activity 2) Cardiac contractions are coordinated. - contractile order is required for efficient pumping - coordination is achieved through structure 3) ECGs provide information about cardiac performance - heart rates, rhythms, and function  The sinoatrial node is the normal pacemaker of the heart The majority of cardiac cells are contractile. They do not fire spontaneously. Autorhythmic cells are responsible for initiating and conducting cardiac action potentials. These include cells from the sinoatrial (SA) node, the atrioventricular (AV) node, the Bundle of His, and Purkinje fibres. Autorhythmicity is a consequence of their unique action potential characteristics. Pacemaker activity of cardiac autorhythmic cells  Different autorhythmic cells fire action potentials at different rates. The fastest rate (70-80 bpm) is observed for the SA node. The AV node fires action potentials at 40-60 bpm. The Bundle of His and Purkinje fibres fire action potentials at 20-40 bpm. The cells with the highest discharge rate are called pacemaker cells. Those with lower rates are called latent pacemakers. Occasionally, this normal hierarchy of pacemaking activity can be disrupted.  Action potential firing rates in cardiac autorhythmic tissue TISSUE FIRING RATE (BPM) SA node 70-80 (normal pacemaker) AV node 40-60 Bundle of His and 20-40 Purkinje fibres  Different Autorhythmic Rates Because cell A has a faster rate of depolarization, it reaches threshold more quickly than cell B and therefore generates action potentials more rapidly. Spread of Cardiac Excitation An action potential initiated at the SA node first spreads throughout both atria. Its spread is facilitated by the interatrial and internodal pathways. The AV node is the only point where an action potential can spread from the atria to the ventricles. From the AV node, the action potential spreads rapidly throughout the ventricles, hastened by a specialized ventricular conduction system consisting of the bundle of His and Purkinje fibers.  Cardiac contractions must be coordinated for efficient pumping. The heart functions as two separate pumps in series. Atrial excitation and contraction must precede ventricular contraction. Contraction of each chamber must occur as a unit. Contraction of the pair of atria and the pair of ventricles must occur simultaneously.  The coordination of cardiac excitation is achieved structurally. The SA node initiates contraction in the atria. Excitation occurs throughout the atria via cell to cell contact (gap junctions) and the interatrial pathway. The AV node is excited through the internodal pathway and by cell to cell contact. The atria and ventricles are electrically separated. Transmission of excitation occurs through the AV node. Slow conduction through the AV node ensures a delay between contraction of the atria and the ventricles.  The ventricular conduction system is highly organized. Ventricular excitation is rapidly conducted from the AV node through the Bundle of His to the Purkinje fibres. Excitation of the large ventricular mass then proceeds by cell to cell contact (gap junctions). The ventricular action potential has different characteristics than that of the SA and AV nodes. Action potential in contractile cardiac muscle cells Cardiac vs Pacemaker cells https://doi.org/10.1016/j.anclin.2011.05.001  In cardiac muscle, an electrical signal is converted into a contractile signal. This process is referred to as excitation- contraction coupling. The signal for contraction is an elevation of cytosolic Ca2+. Ca2+ is derived from two sources: the ECF and the sarcoplasmic reticulum. The elevated Ca2+ leads to cross-bridge cycling between actin and myosin. The relationship between the electrical event and the mechanical event prevents tetanus. Downloaded from: Cecil Textbook of Medicine (on 10 November 2005 05:10 PM) © 2005 Elsevier Downloaded from: StudentConsult (on 27 February 2006 07:17 PM) © 2005 Elsevier http://www.pathophys.org/physiology-of-cardiac-conduction-and-contractility/ Relationship between Electrical and Mechanical Event The cardiac action potential and contraction is prolonged. The action potential precedes contraction. This relationship protects the heart from tetanization. http://www.pathophys.org/physiology-of-cardiac-conduction-and-contractility/ Analogy of pacemaker activity  Autonomic innervation of the heart Heart is innervated by both parasympathetic and sympathetic afferent and efferent neurons. Sympathetic: postganglionic sympathetic fibers from paravertebral sympathetic ganglia associated with T1-T5 innervate the atria, ventricles, and conduction system. Parasympathetic: parasympathetic innervation is limited to vagal efferent fibers which innervate the SA node and the AV node; parasympathetic innervation to the ventricles is minimal. Both sympathetic and parasympathetic tone is exerted on the heart at rest, but parasympathetic tone predominates. Sympathetic neurons release norepinephrine, a catecholamine, which activates β1 receptors on cardiac myocytes, leading to the following effects (note: epinephrine, also a catecholamine, can be made by the adrenal glands and released into the circulation, and has the same effect on β1 receptors): Chronotropic: increased heart rate Dromotropic: faster conduction through AV node Inotropic: increased contractility Lusitropic: faster relaxation after contraction Parasympathetic neurons release acetylcholine, a cholinergic hormone, which activates muscarinic M2-receptors on cardiac myocytes, leading to just one main effect: Negative chronotropic: decreased heart rate http://www.pathophys.org/physiology-of-cardiac-conduction-and-contractility/  The ECG (electrocardiogram or EKG) records the overall spread of electrical activity in the heart. The ECG measures electrical signals from the heart conducted by body fluids. It is not a direct measure of cardiac electrical activity. The ECG represents the sum of all electrical activity in the heart, not the activity of single cells.  The ECG measures the difference in electrical potential between two different points on the body. For standard comparisons, twelve leads or electrodes are used to record the ECG.  Various components of the ECG correspond to specific cardiac events. The P wave represents atrial depolarization. The QRS complex represents ventricular depolarization. The T wave represents ventricular repolarization.  The PR segment shows AV nodal delay. The ST segment occurs when the ventricles are completely depolarized. The TP interval occurs when the heart is at rest.  The ECG can be useful for diagnosing abnormal heart rates, arrhythmias, and damage to heart muscle. Heart rate abnormalities include tachyarrhythmias and bradycardias. Arrhythmias include atrial flutter and fibrillation, heart block (complete or different ratios, e.g., 2:1 and 3:1), extrasystoles, ventricular fibrillation, cardiac myopathies (e.g. ischemia). Types of AV Block Figure 15-41 Atrial and ventricular fibrillation. Downloaded from: StudentConsult (on 10 November 2005 05:13 PM) © 2005 Elsevier Figure 15-41 Atrial and ventricular fibrillation. Downloaded from: StudentConsult (on 10 November 2005 05:13 PM) © 2005 Elsevier Figure 15-25 The cardiac conduction system. Downloaded from: StudentConsult (on 10 November 2005 05:13 PM) © 2005 Elsevier Electrophysiology  OVERVIEW 1) The heart is autorhythmic. – specialized cells (pacemakers) trigger contractions – heart rate is determined by pacemaker activity 2) Cardiac contractions are coordinated. - contractile order is required for efficient pumping - coordination is achieved through structure 3) ECGs provide information about cardiac performance - heart rates, rhythms, and function