Syllabus: Electrocardiography

Revised by V’19 cardio group from Dr. John Rush

Updated 2017


Note: In the following sections, the dog is the model for “normal vs arrhythmia” unless otherwise noted. Most descriptions are readily applicable to other species, if you take into consideration the normal heart rates and complex intervals for the different species.

General guide to the P-QRS-T complex: 

  • To understand ECG, it is necessary to understand how electrical impulses are carried through the heart. The impulse, or action potential, originates in the sino-atrial (SA) node because the cells in this node have the fastest rate of spontaneous depolarization. Therefore, the SA node dictates the heart rate.
  • This impulse is then carried through the atria via the internodal tracts, and causes the atria to activate, or depolarize. The P wave on the ECG is a result of the activation of the atria. Next, the impulse reaches the atrio-ventricular (AV) node. The cells in the AV node depolarize slowly so there is a conduction delay at this point.  After the AV node, the impulse reaches the bundle of His which divides into right and left bundle branches. The right and left bundle branches carry the impulse down to the right and left ventricles, respectively. This causes ventricular depolarization. Depolarization of the ventricles results in the QRS complex on the ECG. The Q wave is
    the initial downward deflection, the R wave is the upward deflection (it’s always positive), and the S wave is the last downward deflection. Finally, the ventricles begin to repolarize. This wave of repolarization is represented by the T wave on the ECG.


  • P wave: Depolarization of the atria.
  • QRS complex: Consists of Q, R, and S waves. Represents the depolarization of the ventricles.
  • T wave: Repolarization of the ventricles. The ventricles are relatively refractory during this time and can, therefore, respond to an impulse if it is large enough. (Note: Repolarization of the atria occurs at the same time as ventricular depolarization and is, therefore, hidden within the QRS complex and not seen on ECG).
  • P-R interval: Measured from the beginning of the P wave to the first downward deflection of the QRS complex (the Q wave). The conduction delay that occurs at the AV node contributes to part of the P-R interval.
  • S-T interval: Interval between the end of ventricular depolarization and the beginning of ventricular repolarization. During this segment, the ventricles are absolutely refractory and cannot respond to any impulse. From a mechanical viewpoint, this is when the ventricles are contracting and pumping blood to the aorta and pulmonary artery.
Sinus Rhythms
  • Regular Sinus Rhythm
    • Normal sinus rhythm occurs when the impulse properly originates in the sinoatrial node and results in regular P wave formation. If conduction is carried properly through the atria, AV node, and ventricles, then the QRS-T portion of the complex will also be regular. Normal sinus rhythm only occurs if the following
      criteria are met:

      • I. Normal heart rate for the species
      • II. Regular P-P and R-R intervals
      • III. P-QRST complex relationship is normal (there is a P wave for
        each QRS-T and they are related at a constant interval)
    • Below is an example of normal sinus rhythm:
      • To determine that this ECG reading is normal:
        • First measure the heart rate to ensure that it’s within normal range for the
          species (in this case it’s a dog). Make sure you pay attention to the speed that
          the strip is recorded at because your heart rate calculation will differ based on
          this speed.

          • At 50 mm/sec paper speed, 10 big boxes/second
          •  Therefore, 3 seconds is 30 big boxes
          • Count the number of QRS in 3 seconds (or 30 big boxes)
          • 7 QRS complexes in 3 seconds
          •  Multiply by 20 to get heart rate
          •  7 x 20= 140 bpm
          • 140 bpm is within the range of 70-160bpm for adult dogs
          •  Therefore, this is a normal heart rate
        • Next, we must look at the P-P and R-R intervals to ensure that they are regular.
          Imagine that you have calipers to measure the intervals: the blue line indicates
          the P-P interval, and the yellow line indicates the R-R interval that you measured

          • The lines you drew are the same length, indicating that
            the intervals are consistent.
        • The last step is to determine that there is a P wave for every QRS-T and that
          they are at a constant interval. Looking at the strip above there is, in fact, a P
          wave for every QRS-T. Next you count the number of boxes in the P-Q interval
          and see that between each P and Q wave, there are 4 little boxes (or 0.08
          seconds). This indicates that the P-QRS-T intervals are consistent. (Green line
          drawn above shows the P-R interval).
  • Sinus Arrhythmia
    • Sinus arrhythmia is NORMAL in the dog and the horse, due to changing vagal
    • ECG signs:
      • I. Heart rate slow to normal
      • II. Cyclic irregularity of the rhythm (irregular P-P/R-P intervals)
      • III. In respiratory sinus arrhythmia, the heart rate increases with inspiration and decreases with expiration
      • IV. Normal P-QRS-T complex with normal P-QRS-T relationship
      • V. If “wandering pacemaker” exists, the P wave will regularly and gradually change it’s configuration as the rate speeds during inspiration and slows during expiration
    • Below is an example of sinus arrhythmia:
      • Signs that this ECG strip demonstrates sinus arrhythmia:
        •  There is cyclic irregularity of the rhythm. Grossly, you can tell that pattern is: complexes start infrequently (red arrow), become more frequent/concentrated (orange arrow), become less frequent again (yellow arrow), and become more frequent/concentrated again (green arrow). When you look more closely you can see that the PP intervals (blue lines) are irregular.
        • The P-QRS-T complexes are normal. There is a P wave for every QRS-T and the relationship between them is consistent (P-R interval labeled by purple line).
        • Note: There is no wandering pacemaker in this strip because the P
          waves don’t change configuration.
  • Sinus Bradycardia
    • Sinus bradycardia is seen with excess vagal tone, head trauma, brainstem lesions, increased CSF pressure, “toxemias,” hyperkalemia or other electrolyte disturbances, hypothermia, drugs (beta-blockers, calcium channel blockers, sedatives) are all associated with sinus bradycardia.
    • ECG signs:
      • I. Heart rate less than 60 bpm (dog), 160 (cat), 46 (bovine), 24 (equine).
      • II. Regular sinus rhythm or sinus arrhythmia present
      • III. Normal P-QRS-T complexes and relationship
    • Below is an example of sinus bradycardia:
      • In this strip the P-QRS-T complexes and relationship are normal- there is a P wave for every QRS-T and the P-R interval is regular (blue line).
        • Notice the paper speed is 25 mm/sec:
          • At 25 mm/sec, 5 big boxes/sec
          • Therefore, 3 seconds is 15 big boxes
          •  Count the number of QRS in 3 seconds (or 15 big boxes)
          •  2 QRS complexes in 3 seconds
          • Multiply by 20 to get heart rate
          •  2 x 20= 40 bpm
          •  40 bpm is below 60 bpm for this dog
          • Therefore, this is a slow heart rate (bradycardia)
  • Sinus Tachycardia
    • Sinus Tachycardia may be seen with excitement, pain, fear, heart failure, fever, hyperthyroidism, shock, and other causes of sympathetic nervous system discharge are associated with tachycardia.
    • ECG signs:
      • I. Heart rate greater than 160 bpm – toy dogs) (>60 in horses) 25 mm/sec
      • II. Regular rhythm (slight difference in R-R intervals occasionally)
      • III. Normal P-QRS-T complexes and relationship)
      • IV. P and T waves may superimpose due to rapid rate
    • Below is an example of sinus tachycardia:
    • In this strip the P-QRS-T complexes and relationship are normal- there is a P wave for every QRS-T and the P-R interval is regular (yellow line).
      •  Notice the paper speed is 25 mm/sec:
        • At 25 mm/sec, 5 big boxes/sec
        • Therefore, 3 seconds is 15 big boxes
        • Count the number of QRS in 3 seconds (or 15 big boxes)
        • 10 QRS complexes in 3 seconds
        •  Multiply by 20 to get heart rate
        • 10 x 20= 200 bpm
        •  200 bpm is above 160 bpm for this dog
        •  Therefore, this is a fast heart rate (tachycardia)
        • Notice that since the rate is so rapid, the P and T waves have superimposed (labeled by blue arrows). The T wave of the complex that has just occurred is superimposed with the P wave of the following complex since the complexes are occurring so closely together.
Premature Depolarizations and Tachyarrhythmias 
  • Supraventricular Premature Depolarizations (premature impulses originating in the atria or AV junction)
    • Causes may include chronic valvular or myocardial heart disease, toxemia, digitalis intoxication, atrial
      stretch, pulmonary disease, and pulmonary hypertension are associated with
      premature atrial depolarizations.
    • ECG signs:
      • I. Irregular R-R intervals due to premature impulses
      • II. Premature QRS is relatively normal and usually associated with an “ectopic P” wave (which may be “buried” in the previous T wave)
      • III. The P-R interval is usually different than normal (altered A-V
    • Below is an example of supraventricular premature depolarizations:
      • Blue arrows point to the supraventricular premature depolarizations. Notice that the R-R intervals are irregular due to the premature impulses. The blue lines show the R-R interval between normal sinus beats. Notice that the R-R intervals between the sinus and premature beats (green lines) are shorter.
        •  Notice that the P waves are abnormal- they are negative (yellow arrow).
        • Since the QRS-T portion of the complexes look normal, you know that the origin of this beat is supraventricular (impulses that originate from the ventricles do not have normal-looking QRS-T portions).
  • Supraventricular Tachycardia
    • Causes may include any cause of atrial dilatation or disease, atrial or A-V conduction disease or accessory conduction pathways, neoplasia, electrolyte disturbances are associated with supraventricular tachycardia.
    • ECG signs:
      • I. Rapid heart rate often paroxysmal (i.e. starts and ends abruptly)
      • II. Fairly regular impulse formation (R-R), although prolongation of atrial-ventricular conduction may result in variable R-R intervals
      • III. QRS is typically normal
      • IV. Ectopic P’ waves may be present and related to the QRS via anterograde or retrograde connections (any P wave configuration is possible)
    • Below is an example of supraventricular tachycardia:
      • The supraventricular tachycardia sequence is labeled by the blue arrow. Notice that it starts and ends abruptly, making it paroxysmal. You know that the tachycardia is supraventricular in origin because the QRS-T of the tachycardia looks normal.
        • Notice how short the R-R intervals (green lines) are during the run of tachycardia.
        • The yellow arrows point to ectopic P’ waves.
  • Atrial Flutter
    • Atrial flutter is due to rapid, regular, atrial activation that results in a rapid atrial rate (creating flutter waves rather than P waves). However, due to physiologic block of some of the impulses in the AV node, the ventricular rate is slower.
    • ECG signs:
      • I. Rapid or normal rate, depending on degree of physiologic AV block
      • II. Rapid atrial rate causing the P waves to be replaced by ‘sawtoothed’ flutter waves (‘F’ waves).
      • III. The ventricular response (R-R intervals) is irregular due to a variable degree of AV block. There is often a relationship between the number of flutter waves and QRSs, e.g. 3:2, 3:1, etc.
    • Below is an example of atrial flutter:
      • Blue lines indicate and areas where flutter waves are occurring. The green arrows point to some of the complexes indicating that the impulse was carried down to the ventricles. Notice that not all of the atrial impulses (which are producing the flutter waves) are conducted down to the ventricles.
  • Atrial Fibrillation
    • Cardiomyopathies of giant breeds (St. Bernard, Great Danes, etc.), chronic mitral valve disease, congenital heart diseases are associated with atrial fibrillation. Atrial fibrillation sometimes occurs spontaneously in horses and cattle.
    •  ECG signs:
      • I. Usually a rapid rate (sometimes normal)
      • II. No P waves
      • III. No consistent R-R interval
      • IV. Lack of organized P waves with consistent relationship to following QRS-T complex
      • V. QRS appears normal although slight differences between the complexes due to variable AV conduction are common
      • VI. Fibrillation (‘F’) waves (baseline undulations) are usually present
    • Below is an example of atrial fibrillation:
      • There are no P waves. Sometimes it can be confusing and seem like there is a P wave, but if you look for a relationship between what you think are P waves and the QRS-T complexes, you’ll notice that there is no consistent
      • While the QRS-T complex appears normal, there is no consistent R-R interval. If you take a measurement of the first R-R interval and then superimpose it over the rest of the R-R intervals, you’ll find that none of them match (the blue arrow indicates the first R-R interval and the following yellow arrows are the same length as that first blue arrow but superimposed over the following R-R intervals- notice that the first R-R interval differs from all of the following R-R intervals by varying amounts).
  • Ventricular Premature Depolarizations/Contractions (VPD/VPC)
    • Many associations including:
      • 1. Primary myocardial disease including tissue hypoxia due to heart failure, microscopic intramural myocardial infarction (MIMI), cardiomyopathy, toxic endomyocarditis, traumatic myocarditis, neoplasms, etc.
      • 2. Extracardiac diseases causing myocarditis or ischemia such as pyometra, sepsis, gastric dilatation, uremia, head trauma, pancreatitis, acidosis, hypoxia due to pulmonary disease, “autonomic imbalance”, etc.
      • 3. Electrolyte imbalances, particularly hypokalemia.
      • 4. Drug induced VPDs including digitalis, epinephrine, halothane, etc.
    • ECG signs:
      • I. Premature ectopic impulses originating from the ventricles
      • II. Typically wide and “Bizarre” appearance compared to normal QRS-T morphology (because the impulse originates in the ventricle)
      • III. Large T wave in the opposite direction of the QRS
      • IV. ST segment slurring – no real ST segment shelf
      • V. Compensatory pause – SA node is not reset
    • Below is an example of a VPC:
      • The blue arrow points to the VPC. Notice that the complex looks wide and “bizarre”. Since it doesn’t look like the normal P-QRS-T complex, this is your clue that the impulse did not properly originate in the sinoatrial node (it
        originated in the ventricle instead).
      •  The QRS (yellow arrow) is in the opposite direction as the large T wave (green arrow). You cannot really determine where the ST segment is because there is ST segment slurring.
      • You know there is a compensatory pause (meaning the sinoatrial node is not reset) because the pause during the premature beat is equal to two times the length of the normal R-R interval. The orange line indicates a normal R-R
        interval. If you superimpose two of the R-R intervals together you see that the sinus beat after the VPC is occurring exactly two R-R intervals after the sinus beat that occurred before the VPC.
    • To illustrate the difference between a supraventricular premature depolarization and a ventricular premature depolarization (VPD) see below:
      • The blue arrow points to the supraventricular premature depolarization. Notice how the QRS complex looks normal, indicating that the impulse originated from the atria.
      • The green arrow points to the ventricular premature depolarization. Notice that the QRS complex looks wide and “bizarre”, indicating that the impulse originated from the ventricle. The QRS is in the opposite direction as the
        large T positive wave.
  • Ventricular Tachycardia
    • The same etiologies associated with VPCs are associated with ventricular tachycardia.
    • ECG signs: Ventricular tachycardia occurs when there are at least three VPCs firing in series.
    • Below is an example of ventricular tachycardia:
      • The blue arrow indicates three VPCs in a row and the orange arrow indicates 5 VPCs in a row. Since at least 3 VPCs have occurred in series, this qualifies as ventricular tachycardia.
    • Notice the difference between supraventricular tachycardia and ventricular tachycardia:
      • The top picture shows supraventricular tachycardia and the bottom picture shows ventricular tachycardia. The easiest way to tell them apart is to look at the appearance of the QRS complex. In supraventricular tachycardia,
        the QRS should look the same as in a normal sinus beat, the beats are just squished closely together. In ventricular tachycardia, the QRS complex looks abnormal and is largely negative. Look for the large downsweeping
        negative QRS waves (some of them are highlighted by blue arrows) to determine that it is ventricular tachycardia.
      • If you are having trouble, try finding a normal sinus beat to compare to the portion of tachycardia (normal sinus beats are highlighted by orange lines in both pictures). Notice how the tachycardia looks similar to the sinus
        beats in supraventricular tachycardia, while the tachycardia looks very different form the sinus beats in ventricular tachycardia.
  • Ventricular Fibrillation
    • Ventricular fibrillation occurs when there is disorganized generation of impulses from multiple places in the ventricles. It is an indication of widespread ventricular disease and is NOT compatible with life because cardiac output drops to zero
      (when there is no organized contraction in the heart, blood cannot be pushed forward to the body or to the lungs).
    •  ECG signs:
      • I. Lack of recognizable P-QRS-T complexes
      • II. Irregular undulations in the baseline
    • Below is an example of ventricular fibrillation:
      • There are no recognizable complexes anywhere. It just looks like a wavy line.
Rhythms Due to Failure of Impulse Formation
  • Sinoatrial Arrest
    • Sinoatrial arrest is when the sinus node does not discharge impulses for greater than two times the normal R-R interval. The period of arrest can be followed by a return to normal sinus rhythm, a junctional escape complex (originates at the atrioventricular junction), or a ventricular escape complex (originates at the purkinje fibers in the ventricle).
      • NOTE: Junctional escape complexes look exactly like supraventricular premature contractions, and ventricular escape complexes look exactly like ventricular premature contractions (VPC’s), which were covered in earlier sections. The difference is that these beats are not occurring prematurely, and are instead occurring after a period longer than the predominant R-R interval. Escapes are protective mechanisms to ensure that electrical-mechanical functions will continue. Even thought they don’t look like normal P-QRS-T complexes, escape beats keep the animal alive.
    •  ECG signs:
      • I. Slow or normal rate
      • II. Pauses greater than two R-R intervals without demonstrable atrial activity (no P waves)
      • III. P-QRS-T complexes predominate but junctional and ventricular escapes are common
    • Below is an example of atrial standstill:
      • First determine the normal R-R interval. The green arrow highlights the length of two R-R intervals. Then superimpose that line after the last R wave and you’ll notice that there is no P wave activity for greater than two
        R-R intervals.
Atrioventricular Conduction Disorders
  • First Degree AV Block
    • A minor delay in atrioventricular conduction causes lengthening of the P-R interval.
    • ECG signs:
      • I. The heart rate is variable
      • II. There is a P wave for each QRS-T and vice versa
      • III. The P-R interval is prolonged
        • Normal P-R length is 0.09 sec in cats and 0.13 sec in dogs
      • IV. Occasionally the P-R interval varies
    •  Below is an example of first degree AV block: 
      • First, make sure there is P wave for every QRS-T complex and vice versa (every QRS-T complex is preceded by a P wave). Blue arrows point to the P wave. Notice that P waves are occurring closely in time to the T wave
        from the preceding complex- don’t be confused by the shape resulting from this (green arrows point to T waves).
      • Now, measure the length of the P-R interval to see if it is normal:
        • The paper speed is 50 mm/sec, so 10 big boxes/sec, or 50 little boxes/sec, or each little box= 0.02 sec.
        • The P-R interval is indicated by the red line. Don’t forget to include the entire P wave (start your line at the very beginning of the P wave).
        • Count the number of little boxes in the P-R interval. There are 9 little boxes.
        • Multiply (9 little boxes) (1 little box/0.02 sec)= 0.18 sec.
        • 0.18 sec > 0.13 sec, therefore the P-R interval is lengthened and this is First Degree AV block.
  • Second Degree AV Block
    • Second degree AV block occurs when some of the impulses originating in the sinoatrial node do not get conducted down through the atrioventricular node to the ventricles. This manifests on ECG as some P waves without corresponding
      QRS-T complexes, or “dropped” P waves.
    • Second degree AV block can be subdivided into two types:
      • 1. Mobitz Type I (Wenchebach)- progressive P-R interval prolongation occurs before the “dropped” P wave
      • 2. Mobitz Type II- fixed P-R interval throughout (often a more advanced form of AV block)
    • ECG signs:
      • I. The rate is usually slow
      • II. Some P waves not followed by QRS-T complexes (the impulse is blocked at the AV node)
      • III. In Mobitz type I block, the PR interval prolongs until a P wave is not followed by a QRS-T
      • IV. In Mobitz type II block, the PR interval is constant and some P waves are not followed by a QRS-T
      • V. Escapes, either junctional or ventricular, may be noted after pauses
    • Below is an example of Second Degree AV Block Mobitz Type I:
      • First, you must determine that this is Second Degree AV block. Notice that some of the P waves do not get conducted down into the ventricles, resulting in P waves without a following QRS-T complex. Blue arrows point to “dropped” P waves.
      • Now that we know it’s second degree AV block, we need to decide if it’s Mobitz Type I or Type II. We know that it’s Mobitz Type I (Wenchebcah) because the P-R interval gets longer and longer as it approaches the “dropped” P wave (orange lines highlight the P-R intervals). This is a physiological event and is associated with a healthy heart.
        •  Notice the difference in length between the P-R interval closest to the “dropped” P wave (bottom) and an earlier P-R interval (top).
    • Below is an example of Second Degree AV block Mobitz Type II:
      • You know this is Second Degree AV block because some of the P waves are not conducted down to the ventricles (blue arrows).
      • Notice that the P-R interval (orange line) remains constant before “dropped” P waves, making this Mobitz Type II. This is a pathologic arrhythmia and is associated with disease of the AV node or bundle of His.
  • Third Degree AV Block
    • Third Degree AV block occurs when there is a persistent interruption in impulse conduction through the AV node, preventing every single P wave from being conducted down to the ventricles. As a result, a site located below the AV
      junction block determines the predominant rhythm. The administration of drugs (parasympatholytics or sympathomimetics) will not cause P waves to be conducted through the A-V node if there is anatomic disruption. Treatment for this is an implantable pacemaker.
    • ECG signs:
      • I. The heart rate is slow (usually less than 50/minute)
      • II. There are more P waves than QRS-Ts.
      • III. There is no consistent relationship between the P waves and the QRS-T. No P waves are conducted to the ventricle
      • IV. The QRSs are the result of pacemaker activity below the level of the AV block
    • Below is an example of Third Degree AV block:
      • You know this is Third Degree AV block because none of the P waves are being conducted down to the ventricles, or all of the P waves are “dropped” (indicated by blue arrows). The predominant rhythm is determined by ventricular escape beats (orange arrows), which originate from below the level of the AV block (they originate in the ventricles).