Understanding 12 Lead EKGs A Practical Approach Brenda M. Beasley, RN, BS, EMT- Paramedic Michael C. West, RN, MS, EMT-

1. Understanding 12 Lead EKGsA Practical ApproachBrenda M. Beasley, RN, BS, EMT- ParamedicMichael C. West, RN, MS, EMT- Paramedic Chapter 3 Basic Electrophysiology Brady Understanding 12 Lead EKGs Ch.3

2. 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 Brady Understanding 12 Lead EKGs Ch.3

3. Basic Electrophysiology • Objectives [ continued ] • Describe the movement of ions • Describe cardiac depolarization • Describe cardiac repolarization • Define refractory period • Describe the absolute refractory period • Describe the relative refractory period Brady Understanding 12 Lead EKGs Ch.3

4. 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 Brady Understanding 12 Lead EKGs Ch.3

5. Basic Cell Groups • Specialized Pacemaker Cells • do 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 Brady Understanding 12 Lead EKGs Ch.3

6. 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 Brady Understanding 12 Lead EKGs Ch.3

7. Primary Cardiac Cell Characteristics • Possess four primary cell characteristics 1. Contractility - [ mechanical function ] is also referred to as rhythmicity, is the ability of cardiac cells to shorten and cause cardiac muscle contraction in response to an electrical stimulus 2. Automaticity - [ electrical function ] ability of cardiac pacemaker cells to spontaneously generate own electrical impulses without external stimulation Brady Understanding 12 Lead EKGs Ch.3

8. Primary Cardiac Cell Characteristics 3. Excitability - [ Electrical Function ] ability of cardiac cells to respond to electrical stimulus is also referred to as irritability. 4.Conductivity - [ Electrical Function ] ability of cardiac cells to receive an electrical stimulus and then to transmit the stimulus to other cardiac cells, they function collectively as a unit Brady Understanding 12 Lead EKGs Ch.3

9. Major Electrolytes That Affect Cardiac Function • Myocardial cells bathed in electrolyte solutions • Electrolyte- substance or compound whose molecules dissociate into charged components, orions • Produce + charged ions [ cation ] and - charged ions [ anion ] when placed in water Brady Understanding 12 Lead EKGs Ch.3

10. 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 Brady Understanding 12 Lead EKGs Ch.3

11. 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 Brady Understanding 12 Lead EKGs Ch.3

12. Cardiac Depolarization • When impulse develops and spreads throughout the myocardium, changes occur in heart muscle fibers • Cardiac Depolarization - sodium ions rush into cell, changing interior charge to + after cell stimulated • Cardiac Repolarization - sodium ions return to outside of cell, potassium returns to inside of cell. Brady Understanding 12 Lead EKGs Ch.3

13. Ion Shifts Fig. 3-1 • Ion shifts during depolarization and repolarization Brady Understanding 12 Lead EKGs Ch.3

14. Resting MembranePotential - 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 Brady Understanding 12 Lead EKGs Ch.3

15. 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 Terms to Remember Brady Understanding 12 Lead EKGs Ch.3

16. 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 Brady Understanding 12 Lead EKGs Ch.3

17. Two Stages of Repolarization • Absolute RefractoryPeriod- cardiac cell unable to respond to new electrical stimulus, cannot spontaneously depolarize Brady Understanding 12 Lead EKGs Ch.3

18. Two Stages of Repolarization • Relative RefractoryPeriod - repolarization is almost complete, cardiac cell can be stimulated to contract prematurely if stimulus is stronger then normal Brady Understanding 12 Lead EKGs Ch.3