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Advanced Cardiac Care in the Streets Understanding EKGs Ray Taylor Valencia Community College Electrophysiology Notice All rights reserved. Slide show used with permission only for the purposes of educating emergency medical providers (EMTs and Paramedics) No portion of this presentation may be reproduced, stored in a retrieval system in any form or by any means (including but not limited to electronic, mechanical, photocopying etc.) without prior written permission from the author Basic Electrophysiology Objectives State the two basic myocardial cell groups Describe the function of each myocardial cell group Discuss the four primary properties of cardiac cells List the three major electrolytes that affect cardiac function Basic Electrophysiology Objectives Describe [ continued ] the movement of ions Describe cardiac depolarization Describe cardiac repolarization Define refractory period Describe the absolute refractory period Describe the relative refractory period Cardiac Physiology Electrophysiology Cardiac Muscle Atrial Ventricular Excitatory and Conductive Fibers Intercalated discs Syncytium Atrioventricular Bundle Depolarization Basic Cell Groups Specialized do Pacemaker Cells not have the ability to contract responsible for controlling rate and rhythm by coordinating regular depolarization found in electrical conduction system of the heart primary function is generation and conduction of electrical impulses Basic Cell Groups The Myocardial Working Cells responsible for generating the physical contraction of heart cells muscular layer of atrial walls and thicker muscular layer of ventricular walls primary functions are both contraction and relaxation physical contraction of myocardial tissue actually generates blood flow Basic Cell Groups Threshold - point at which a stimulus will produce a cell response “All or None” phenomenon - stimulus is strong enough for cardiac cells to reach threshold, ALL cells will respond to this stimulus or none will respond. Syncytium Cardiac Physiology Cardiac Cell Characteristics Properties Automaticity Excitability Conductivity Contractility Primary Cardiac Cell Characteristics 1. Automaticity - [ electrical function ] ability of cardiac pacemaker cells to spontaneously generate own electrical impulses without external stimulation 2. Excitability - [ electrical function ] ability of cardiac cells to respond to electrical stimulus is also referred to as irritability Primary Cardiac Cell Characteristics 3. Conductivity - [ electrical function ] ability of cardiac cells to receive an electrical stimulus and then to transmit the stimulus to other cardiac cells, so they function collectively as a unit 4. Contractility - [ mechanical function ] also referred to as rhythmicity, is the ability of cardiac cells to shorten and cause cardiac muscle contraction in response to an electrical stimulus Major Electrolytes That Affect Cardiac Function Myocardial cells bathed in electrolyte solutions Electrolyte - substance or compound whose molecules dissociate into charged components, or ions. Produce + charged ions [ cation ] and - charged ions [ anion ] when placed in water Three Major Cations That Affect Cardiac Function Potassium [K+] - performs major function in cardiac depolarization and repolarization Sodium [Na] - performs vital part in depolarization of myocardium Calcium [Ca] - important function in depolarization and myocardial contraction Movement of Ions Ionic difference on two sides of cell Potassium ion -concentration greater inside cell Sodium ion -concentration greater outside cell Sodium-potassium exchange pump active transport, potassium / sodium moved in and out of cell through cell membrane During polarized or resting state, inside of cell electrically negative relative to outside of cell Cardiac Depolarization When impulse develops and spreads throughout the myocardium, changes occur in heart muscle fibers Cardiac Depolarization - sodium ions rush into cell (fast channel), changing interior charge to + after cell stimulated Calcium enters slow channel maintaining contraction Cardiac Repolarization - sodium ions return to outside of cell, potassium returns to inside of cell Cardiac Physiology/Ion Shifts Cardiac Depolarization Resting Potential Action Potential Repolarization Terms to Remember Resting Membrane Potential - state of cardiac cell- inside negative-outside positive Action Potential - change in polarity; produces change in cell electrical charge caused by stimulation of myocardial cells Terms to Remember Syncytium cardiac muscle cell groups that are connected together and function as a unit Polarized State resting state of cardiac cell, inside of cell is electrically negative relative to outside Refractory Periods Attempts to ensure muscle is totally relaxed before another action potential or depolarization can be initiated Atrial muscle = 0.15 sec. Ventricular muscle = 0.25 to 0.3 sec. Period of rest is referred to as Cardiac Repolarization Two Stages of Repolarization Absolute Refractory Period - cardiac cell unable to respond to new electrical stimulus, cannot spontaneously depolarize Two Stages of Repolarization Relative Refractory Period - repolarization is almost complete, cardiac cell can be stimulated to contract prematurely if stimulus is stronger than normal Thank you!
