A guide to electrocardiography Reading and Applying
Major Confusion!!! Reading an ECG and deciphering what it is telling you can be confusing ECG’s dictate the electrical impulses of the heart Electrical impulses are what make the heart contract This contraction is also known as depolarization
A Little Vocabulary•Artifact: A wave that arises from sources otherthan the heart or brain.•Cardiac Arrhythmia: Any electrical activity of theheart that differs from that of a healthy awakepatient. Can be innocuous or life threatening.•Deflection: movement from one side to the other.•Depolarization: Contractions as a result of cellsstimulated by electricity.•Ectopic: occurring in an abnormal position.•Lead: A pair of electrodes, connected by an axisthat provides a particular view of the of theelectrical activity of the heart.•Repolarization: Relaxation of cells afterdepolarization.
Cables Placement Black and white on front limbs (B=Left/W=Right) -Placed at the elbow region Green and red on back limbs (G=Right/R=Left) -Placed at the stifle region.*Alcohol, ECG paste or ECG pads can be used as conduction agents for more accurate readings. If your patient has long hair, it should be shaved at the site of cable placement in order to achieve adequate conduction in addition to the use of a conduction agent.
•Having the ECG machine Solid Ground well grounded is an important factor for obtaining an accurate reading.•Placing the patient on a rubber matand ensuring that the machine isplugged into a grounded outlet aswell as checking the ECG machinefor loose wires or crackedinsulation on wires are steps thatcan be taken for a good ground.
•The cables read the electrical activityLeads of the heart between two points •Selecting specific leads on the ECG machine will cause specific cables to•Lead I: causes right forelimb to become negative or positive poles. (mildly complicated)become a negative pole and the leftforelimb to become a positive pole.•Lead II: causes right forelimb tobecome negative and left rear limb tobecome positive•Lead III: the left hind limb becomes apositive pole and the left forelimbbecomes a negative pole.•What happened to the Green cable youask? It is only a reference point. Don’tforget it’s there, but concentrate on the Oh, no! Humans, who let them in here?main three!
Mind your PQRST’s P wave: represents atrial depolarization Therefore any changes in the P wave could indicate atrial dysfunction, or a degree of heart block QRS wave: represents ventricular depolarization So, any changes in the QRS wave could indicate ventricular fibrillation, lack of a QRS wave could indicate a form of heart block, and a wide QRS wave might indicate Premature Ventricular Contractions. T wave: Represents ventricular repolarization Myocardial hypoxia, and electrolyte disturbances can be evident with changes in the T wave.
Fill in the Blanks…•P wave: Created with the depolarization ofthe sinoatrial node, which spreads electricalactivity to the atria. This electrical impulseis spread downwards to the left, in thedirection of the positive pole of lead II.Because the electrical activity is moving ina downward deflection away from thenegative pole, the ECG tracing inscribes theP wave in an upward direction.
Between the Lines… The PR interval is inscribed due to a delay of depolarization at the atrioventricular node (allowing time for the ventricles to fill) and no wave is produced.
On your mark….•Q Wave•The wave of depolarization moves through theBundle of His and it’s branches, and then theinterventricular septum is depolarized. This isthe first part of the ventricular myocardium tobecome depolarized. This depolarization occursfrom left to right. The electrical current movesthrough the thin layered interventricular septumin a downward motion, resulting in the Q wavebeing inscribed on the ECG. The Q wave isaimed downward because the current movestoward the negative pole.
Get ready… R wave From the downward inscription of the Q wave on the ECG, depolarization spreads along the ventricular conduction system and depolarization occurs in both ventricles simultaneously. Because the Left Ventricle has greater mass than the right ventricle, most of the electrical impulse travels in a left and downward direction. This electrical activity moving in an upward deflection towards the positive pole of Lead II results in the inscription of the large R wave.
Set… S wave Electrical impulses move from the apex to the base of the heart, continuing ventricular depolarization. This results in the ECG inscription of the S wave.
Almost there…. ST interval After the ventricle is completely depolarized, few impulses take place during this time before repolarization (also called ventricular relaxation). This time is inscribed on the ECG as the ST interval. At this time the heart is susceptible to any electrical impulse and somewhat unstable. *This is the point where defibrillators are placed during human CPR.
Go! T wave Ahhhh! Repolarization. This occurs from the endocardium to the epicardium. Because the left ventricle has more mass than the right ventricle or atria, electrical impulses travel downward to the left. This produces an upward T wave in Lead II. A period of electrical inactivity follows repolarization, and is inscribed as a flat line until the SA node begins all over again!
Panic at the T Wave Sometimes the T wave can deflect downwards depending on the patient, so do not be alarmed if it looks like this on your reading.
So What?•Now that we know how the ECGof a normal, healthy heart isproduced and what it should looklike, we can detect abnormalities.•Excited?•Some cardiac disturbances whichcan be detected are PVC’s,myocardial hypoxia, electrolytedisturbances, fibrillation, sinusbradycardia, atrial dysfunction,and other arrhythmias.
Interference Remember that rubber mat and how you checked the machine for any causes of a bad ground to the machine? Interference can be caused by other machinery such as a pulse- ox, or BP monitor that is hooked to the animal, even fluorescent lighting. This is what interference looks like on the reading. Make a mental picture of how fuzzy it looks.
Muscle Tremor•If your patient is not calm andcomfortable, or just reallynervous and shaky… thereading may look like this.Also caused by happy, purringfeline friends.• Reapplying or readjusting theclips may help.•A towel or blanket can beplaced on top of patient to helpcalm them.•You can also place a hand onthe chest of your patient. Becareful not to apply too muchpressure or it will interfere with •Looks much different than interference right?the reading.
Wandering Baseline Resistance between the patient and the electrode can cause a wandering baseline. An example would be respiratory movement. Placing the patient in sternal recumbancy or a standing position will help to reduce this resistance. Holding the patient’s mouth closed for a few seconds could also help reduce resistance.
Atrial Premature Contraction•The P wave of a Atrial Premature Contraction is different from thatof a normal sinus rhythm.•The origin of a Atrial Premature Contraction is the sinoatrial node ordisplaced locations in the atria, and may or may not conduct to theventricles; this depends on when the impulse reaches the AV node.•If depolarization occurs in the ventricles, the QRS wave will benormal.•If depolarization does NOT occur in the ventricles, and reaches theAV node before repolarization, a premature P waves w/o QRS waveswill read on the ECG.
Ventricular PrematureContraction Seen on ECG as wide, bizarre QRS complexes w/o associated P waves. (So this is going to be a weird and bizarre explanation.) This is because the origin of depolarization is a cell-to-cell depolarization and repolarization from the ventricular myocardium, not the SA node. In addition, if the impulse originated in the left ventricle, the wave of depolarization will be travel up, and the Ventricular Premature Contraction would be a downward deflection on the ECG. However, if the impulse originated from the right ventricle, the Ventricular Premature Contraction would travel downward and left. The ECG tracing will read an upward deflection.
Escape Beats Escape Beat is a safety mechanism in the heart which functions when the SA node fails to fire an impulse. They are the result of depolarization in other locations of the heart which cause the escape beat. When the AV node depolarizes, the current travels upward into the atria and then downward to the ventricles along the normal conduction system.
Escape Beats cont’d Depolarization through the ventricles produces a normal QRS complex. Depolarization through the atria causes a downward tracing of the P wave in lead II on the ECG. Depending on the speed of the upward wave of polarization, it A B may appear before, after or during the QRS complex. If the AV node fails to depolarize, the ventricles will take over and resemble a VPC as a wide, bizarre QRS complex w/ o an associated P wave. The difference between the two is that an escape beat is life-saving, and a VPC disturbs an otherwise healthy rhythm.
Tachycardia Can result from the use of drugs: ketamine, atropine, or epinepherine. Also due to surgical stimulation, but may not mean that anesthetic depth is too shallow unless accompanied by increased respiration, movement, or reflex reaction. Anesthetic complication? Hypoxia, hypotension, or hypercapnia Preexisting condition? Hyperthyroidism, anemia, circulatory shock, septicemia, cardiac dz,, or excitement in an awake patient Direct digital pressure can be applied to the eyeballs, or propranolol can be administered to slow the heart
Sinus Tachycardia Normal sinus rhythm. Characterized by a rhythm faster then 160 beats/min in dogs and faster than 240 beats/min in cats.
Ventricular Tachycardia Three or more ectopic ventricular complexes at a rate of 140 beats/min. May appear and then disappear during normal rhythm (paraxysmal ventricular tachycardia) or be present throughout. If the arrhythmia is present at all times, it indicates an irritable ventricular myocardium, and may precede ventricular fibrillation.
Atrial Flutter Seen as uniform, saw-tooth shaped wavelengths between QRS complexes. Can be difficult to identify. An atrial flutter is a rapid atrial depolarization which occurs at the rate of 250 -350 beats/min. The AV node becomes flooded with depolarizations from the atria. The ventricular rate varies from the atrial rate because it takes time for the AV node to catch up with all the impulses being fired at it by the atria. The ventricular rates for this arrhythmia are generally 140-150 beats/min and bizarre P waves my appear between QRS complexes.
Sinus Bradycardia A normal sinus rhythm. Originated from the SA node. Characterized by a rhythm less then 60 beats/min in dogs and 70-80 beats/min in cats. Based on an individual patient basis, and may be normal for some such as athletic dogs.
Atrial Fibrillation Lacking P wave Constant irregular rhythm Fast rhythm for the atria (350-600 beats/min) and ventricles (220-240 beats/min) Decreased cardiac output due to insufficient atrial contractions which lead to inadequate filling of the ventricles.
Ventricular Fibrillation Chaotic depolarization of the ventricles results in disorganized contractions Inscribed on ECG as small or large undulations of the baseline, but lacking true QRS complexes Ventricular Fibrillation WILL lead to cardiac arrest!!!
Heart Block: Electrical impulse isnot transmitted throughout the heart First degree heart block The interval between the P wave and the QRS complex is prolonged, but the sequence is normal Second Degree heart block P waves sometimes not followed by QRS complexes Third Degree heart block P wave, QRS complex pattern is irregular 2nd and 3rd degree heart block lower the ability for the heart to contract fully. Can be caused by Alpha-2 Agonists, increased vagal tone, hyperkalemia, and cardiac dz.
Created by Emily L. Dzek Fall 2007With MUCH help from:McKelvey, Diane and Hollingshead, Wayne K. Veterinary Anesthesia andAnalgesia. 3rd ed. St. Louis: Mosby, 2003. Glaze, Kathy. “BasicElectrocardiography Part I”. Veterinary Technician 17(1996):661-667. Glaze,Kathy. “Basic Electrocardiography Part II”. Veterinary Technician17(1996):719-725. Glaze, Kathy. “basic Electrocardiography Part III”.Veterinary Technician 18(1997).Many pictures from online sourcesRhythm strip examples from Veterinary Technician magazine listed above.
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