Principles of Electrocardiography PDF

VETM 5291 ♥ Cardiovascular, Respiratory & Hemolymph Systems II Mandy Coleman, DVM, DACVIM (Cardiology) [email protected] By the end of this hour, you will be able to: § Label the x- and y-axes of the electrocardiogram (ECG) § Describe cell surface events when an upward deflection is recorded...

VETM 5291 ♥ Cardiovascular, Respiratory & Hemolymph Systems II Mandy Coleman, DVM, DACVIM (Cardiology) [email protected] By the end of this hour, you will be able to: § Label the x- and y-axes of the electrocardiogram (ECG) § Describe cell surface events when an upward deflection is recorded on the ECG § List the normal cardiac activation sequence from memory § For each component of the cardiac conduction system: § Describe relative velocity of conduction § Recall whether normal pacemaker activity is expected § Describe the method for recording a 6-lead ECG in a dog, and a base-apex ECG in a horse § When provided a normal ECG, label individual waves and explain the cellular events that each represents Voltage (mV) § Electrocardiogram (ECG or EKG) - records heart’s electrical activity from the body surface § Records extracellular signals produced by movement of depolarization/repolarization waves through time (sec) cardiac myocytes § Graph of voltage (mV, y-axis) over time (sec, x-axis) § Changes in voltage are recorded as waves/complexes, named by letters (P,QRS,T) Voltage (mV) § Evaluation of ECG gives clinician insight into: § Heart rate § Disturbances of heart rhythm and conduction time (sec) § Relative size of heart chambers (small animals only) § NOTE: ECG does NOT record mechanical activity and so can’t give insight into: § Whether the heart is contracting § Strength of cardiac contractions § Presence/absence of heart failure Remember: cardiac tissue is excitable! § At rest, myocytes are polarized – membrane is negatively charged (inside vs. outside) § When stimulated, resting myocyte depolarizes wave of depolarization (i.e., membrane polarity reverses) § Depolarized cell stimulates adjacent cell to depolarize § Depolarization “impulse” spreads as a wave § Extracellular currents associated with wave (depolarized cell) of depolarization are detected by the electrocardiogram § Cells must repolarize so this process can from: Mohrman DE, Heller JH. Cardiovascular Physiology, 8e happen again and again  wave of depolarization Negative portion Positive portion of electrical field of electrical field (depolarized cell) from: Mohrman DE, Heller JH. Cardiovascular Physiology, 8e  ECG screen/paper: 0 - + - + When ECG electrodes are placed on Electrocardiograph Negative electrode Positive electrode either side of a wave of depolarization, - + the electrical field can be measured wave of depolarization Negative portion Positive portion of electrical field of electrical field VERY IMPORTANT CONCEPT: By convention, if a wavefront of negative extracellular charges moves TOWARD the POSITIVE electrode, an upward deflection is recorded on the ECG! (depolarized cell) from: Mohrman DE, Heller JH. Cardiovascular Physiology, 8e At rest - outside of cell membrane is positively charged myocardial strip * Remember: a wavefront of negative charges moving toward the positive electrode = positive deflection  Effect of changing depolarization wavefront orientation relative to recording lead axis: = wavefront of depolarization A B C D E Recorded ECG deflection When a wavefront moves directly toward an electrode, in parallel with the lead axis, the largest possible deflection will be recorded (A and E)  Wavefront of depolarization (negative charges) - - Recording lead axis - + Lead A Negative Positive electrode electrode + + Lead C Lead B  Normal activation sequence RA LA LV RV Quiz yourself before moving on!  SA node Normal activation sequence Atrial muscle RA LA LV AV node RV His-Purkinje system (His bundle + bundle branches) Ventricular muscle  Quiz yourself before moving on! Conduction Pacemaker rate velocity (impulses/min) SA node Normal activation sequence Atrial muscle RA LA LV AV node RV His-Purkinje system (His bundle + bundle branches) Ventricular muscle  Conduction Pacemaker rate velocity (impulses/min) SA node 60-250 dominant pacemaker Normal activation sequence Atrial muscle fast None RA LA LV AV node slooow 40 – 60 RV Rescue pacemaker His-Purkinje system SUPER FAST 20 – 40 (His bundle + bundle Rescue pacemaker branches) Ventricular fast None muscle Atrial internodal tracts Rapidly-conducting tissue connecting SA and AV nodes Relatively resistant to effect of hyperkalemia (excess [K+]) Bundle of His/Bundle branches Divides into right and left bundle branches Rapidly conduct impulse to terminal Purkinje fibers Terminal Purkinje fibers Rapidly-conducting, subendocardial Penetrate inner ⅓ of myocardium in dogs and cats, so depolarization proceeds endocardium-to-epicardium Penetrate near-complete thickness in horses, cattle, birds § Patient in right lateral recumbency § Limbs parallel to one another, perpendicular to trunk § Calm environment, non-conducting surface to minimize artifacts § Electrode placement: § Forelimb electrodes (white/black) over elbows § Hindlimb electrodes (red/green) over stifles § Avoid contact with trunk and each other § “Black-and-white TV came before color; white on right and grass on the ground” § Some machines do not have a green (right hindlimb) electrode (grounding electrode) § Several standard leads used clinically § Lead = electrode pair (1 positive + 1 negative) § Lead axis = orientation of lead relative to heart § For this course, we will focus on lead II (used most frequently in the clinic) § Negative electrode on right forelimb (white) § Positive electrode on left hindlimb (red) § Lead axis oriented cranial-to-caudal, right-to-left Cranial Right Left Caudal LV RV  § No lead system is universally accepted in large animals - § Electrode placement for “base-to-apex” lead + + LA § Set machine to record in lead I (RA- to LA+) § Place white electrode (RA) over right jugular furrow or top of right scapular spine (base; “white on right”) - § Place black electrode (LA) over left apex beat + (“black on heart”) § Lead axis oriented cranial-to-caudal, right-to-left RA = Right Arm electrode LA = Left Arm electrode  Cranial Right Left Caudal blue = depolarized tissue pink arrow = direction of wave of depolarization  blue = depolarized tissue pink arrow = direction of wave of depolarization P wave = cell-by-cell atrial depolarization Positive in lead II + base-apex lead; frequently bifid (M-shaped) in horses  blue = depolarized tissue pink arrow = direction of wave of depolarization PR interval PR (PQ) interval: Includes depolarization of atria, AV node and His-Bundle PR interval approximates signal transmission through AV node Normal < 0.13 seconds (dog) or < 0.09 seconds (cat)  blue = depolarized tissue pink arrow = direction of wave of depolarization QRS complex = ventricular depolarization Should be tall, skinny and upright in lead II (in small animals) Normal < 0.06 seconds (dog), < 0.04 seconds (cat) In horses and ruminants, normal QRS complex is negatively deflected  blue = depolarized tissue pink arrow = direction of wave of depolarization ST segment ST segment Isoelectric (flat) line connecting the S and T waves All ventricular cells depolarized, no current flowing T wave = ventricular repolarization Ventricular repolarization is a complicated process T-wave may be negative, positive or biphasic – but must be there! Little emphasis placed on analysis in veterinary medicine  Ventricular depolarization Cranial Atrial Right Left depolarization Caudal Ventricular blue = depolarized tissue REpolarization pink arrow = direction of wave of depolarization  Bifid (notched) P waves are normal in horses − P QRS T P QRS T + negative electrode Negative QRS over right jugular furrow Normal sinus rhythm in a healthy horse complexes are normal in horses and cattle (base-apex lead) positive electrode over cardiac apex beat (left) T T P P QRS QRS Normal sinus rhythm in a healthy cow base-apex lead) From: Reddy BS, Sivajothi S. Electrocardiographic Parameters of Normal Dairy Cows during Different Ages. J Veter Sci Med. 2016;4(1): 5. By the end of this hour, you will be able to: § Label the x- and y-axes of the electrocardiogram (ECG) § Describe cell surface events when an upward deflection is recorded on the ECG § List the normal cardiac activation sequence from memory § For each component of the cardiac conduction system: § Describe relative velocity of conduction § Recall whether normal pacemaker activity is expected § Describe the method for recording a 6-lead ECG in a dog, and a base-apex ECG in a horse § When provided a normal ECG, label individual waves and explain the cellular events that each represents