Electrocardiography lesson                                       


  • know the indications for electrocardiography
  • to understand the sequence of the conduction system of the heart
  • to understand how the conduction system generates the electrocardiogram
  • be able to correlate heart sounds and heart function with features of the electrocardiogram
  • to understand what a "lead" is
  • be able to recognize the features of a normal lead II
  • be able to recognize the features of arrhythmias in lead II
  • to understand why arrhythmias are important
  • to understand the causes of arrhythmias

Reading assignment-

Sorry!!!-- it is in bits in pieces throughout the textbook. You will have to look for the appropriate paragraphs to read on these pages:

McCurnin D and Bassert J: Clinical Textbook for Veterinary Technicians 7th edition, pp. 908-913 ("Monitoring during Anesthesia) 606,and 1168 f.-- for some ECG samples see Advanced Life Support pp. 1167-1170 and Urinary Obstruction pp. 1186-1188

Optional text--Hanie, E: Large Animal Clinical Procedures for Veterinary Technicians, pp. 297-299

Writing assignments

There are two assignments associated with this lesson that you need to submit:

1. Generate an ECG at your practice site and use the form linked at the end of the lesson to write up your interpretation. If you are unable to generate an ECG at your practice, you will have an opportunity to do this at the lab session.

2. There is the usual writing assignment at the very end of the lesson  (after websites and references)with a list of review questions.


Electrocardiography is the study of the electrical stimulus of heart muscle activity. The electrocardiogram is a written record of the electrical activity that gives us some information about heart muscle function and physical condition. Although other parameters must be used also when making a clinical assessment of cardiac conditions in an animal.  

Special properties of cardiac muscle relating to electrocardiography

Cardiac muscle has special properties that make the study of electrocardiography useful. These properties are contractility, automaticity, excitability, refractoriness, and conductivity. We can see that  these properties are related to electrocardiography and are clinically significant.

  • Contractility- the heart contracts in response to an electrical current. If an electrical current is detected in the heart, we assume that there is a heart muscle contraction occurring. 
  • Automaticity- the heart automatically contracts without conscious control by the animal. A specialized node of tissue called the sinoatrial node (SA node)  is the pacemaker of the heart, which automatically generates an electrical stimulus, or impulse, that causes automatic heart contraction at a fairly constant rate. The pace of the electrical stimulus can change somewhat in response to stimuli. For example, stress can cause the pacemaker to increase the rate temporarily.
  • Excitability- the heart muscle is excited to contract by the electrical stimulus of the pacemaker and the conduction system. As the electrical stimulus is sent out by the pacemaker and conducted through the heart, depolarization occurs. Depolarization is actually the movement of electrolytes across the cell membranes which results in contraction of the muscle fibers. After contraction, there is repolarization. Repolarization is the movement of electrolytes back to the original position before contraction, which results in muscle fiber relaxation. The electrical activity that occurs during depolarization (contraction) and repolarization (relaxation) is measured by the electrocardiograph. If an animal has a problem with electrolyte imbalance, then depolarization and repolarization will be affected, and  the electrocardiogram will be abnormal. 
  • Refractoriness- cardiac muscle does not respond to other stimuli during the actual contraction, it has to repolarize first before another contraction can occur. This means the heart must repolarize  and get ready before it is capable of responding to another electrical impulse. Therefore, once the impulse is sent and conducted, the heart will contract in a predictable and consistent way.
  • Conductivity- contraction of one muscle cell incites the adjacent cell to contract and so on like a domino effect, or wave of contraction going through the heart. Since the electrical stimulus from the pacemaker occurs from the same anatomical site each time, the electrical activity and subsequent contraction wave occurs in the same sequence with each electrical impulse. If there is a deviation from the normal sequence, the electrocardiogram will record the change in conduction. We will recognize the change as abnormal electrical activity. When there is a change in conduction, there may also be a change in contraction that will impair the heart function. 

Conduction system sequence

As the electrical impulse is conducted through the heart, the muscle contracts in an orderly fashion to pump blood in a mechanically efficient manner, then the muscle relaxes and the cycle begins again. The impulse normally originates at the sinoatrial node and then travels through the atria and ventricles in a specific way:

conductionseq1.jpg (26621 bytes) 1. Sinoatrial (SA) node--Initiation of the cardiac impulse is at the sinoatrial node, "the pacemaker", in the right atrium and travels through the atria to the AV node which is another area of specialized tissue at the junction of the atria and ventricles.

conductionseq2.jpg (27671 bytes) 2. AV node (atrioventricular node)-- The impulse travels to this node near the junction of the atria and ventricles and then on to fibers in the ventricle.

conductionseq3.jpg (26986 bytes) 3. The impulse continues from this bundle of fibers in the septum and spreads out through various branches into the ventricles.

Then the cycle automatically repeats itself in the same way each time.


The electrocardiogram is a  graphic recording of electrical activity of the heart which is also known as the "ECG" or "EKG". The ECG is commonly recorded on a strip of graph paper that is passed under a stylus that moves up and down in response to the voltages detected by the electrodes attached to the animal. You should notice that the graph paper is partitioned in small blocks of square millimeters and then sets of 5 blocks are more heavily outlined to help visualize and track calculations.

ecgpaper3.jpg (102991 bytes) Close up view of ECG graph paper. Each tiny square is a square millimeter.

The recording starts at a baseline. The baseline will appear across the paper as a rather flat straight line, or it may vary slightly as the animal moves or breathes. Each deflection from the baseline represents voltage variations occurring in specific areas of the heart as it depolarizes (contracts) and repolarizes (relaxes) in response to the electrical impulse traveling through the conduction system. The deflections will look like peaks or valleys and are called waves or waveforms. The height (or depth) of deflections is measured in millivolts. 

If the deflection occurs above the baseline it is said to be positive and if the deflection occurs below the baseline it is said to be negative. A group of deflections that occur during a single cardiac cycle (contraction and relaxation) are often referred to as a "complex". Since the impulse is conducted in the same way each time, the group of waveforms will tend to repeat in a predictable and uniform way from one contraction to the next. The characteristic waveforms are given letter names, starting with the letter "P" and are read on the paper strip going from left to right.  There is usually a visible baseline between each group of repeating waveforms.

ecg_strip.jpg (31305 bytes) An electrocardiogram of a dog (lead II). You should notice that there is a repeating pattern of complexes that are uniform in appearance from one group of deflections to the next.

Here is a dissection of the electrocardiogram through one cardiac cycle in lead II:

Pwavearrows.jpg (17606 bytes) The P wave is usually a small positive deflection, generated by the pacemaker (SA node) impulse and corresponds to atrial depolarization and contraction

QRSarrows.jpg (24966 bytes) The QRS complex: Q is the first negative deflection from the baseline

                                            R is the first large positive deflection after the Q

                                            S is the negative deflection that follows R

               The Q and the S are very close to the R and often seem to overlap it. The QRS complex corresponds to ventricular depolarization and contraction

Twave.jpg (25218 bytes) The T wave can be positive, negative, or biphasic (having two deflections, one negative, one positive); in this example it is positive. The T wave corresponds to ventricular repolarization or relaxation.

Therefore, you can appreciate that each PQRST complex corresponds to a single cardiac cycle of contraction and relaxation, which generates one heartbeat. Generally, the P wave gives us information about the atria and the QRS gives us information about the ventricles. 

Here is the link to the heart diagram to show you how all those things happen at the same time. 


Indications for electrocardiograms 

The purpose of electrocardiograms will vary with the clinical need:

  • Diagnosis of arrhythmias. Arrhythmias are caused by the abnormal rate, regularity, or site of origin of cardiac impulse (pacemaker), or a disturbance of the impulse and how it is conducted. These abnormalities will be detected by the ECG because they cause a deviation in appearance from the typical PQRST complex. You can often detect arrhythmias during auscultation. For many arrhythmias you can hear with the stethoscope an abnormally high or low heart rate, or an irregular heart beat, and also there may be pulse deficits present.


  • Differentiation of disease signs- Dyspnea, shock, fainting/seizures, cyanosis, electrolyte disturbances (commonly potassium), or exercise intolerance may be due to conditions caused by heart problems that can be diagnosed by changes observed in the ECG.


  • Cardiac monitoring using the ECG may be a routine procedure in some instances or may be used to follow up on sick patients. Applications include anesthetic monitoring, evaluating response to antiarrhythmic or cardiac drugs, follow up on any previous diagnosis, critical care monitoring, and cardiopulmonary resuscitation
  • Cardiac evaluation- An ECG may be performed as part of any cardiac work up such as that which occurs when murmurs are detected, there are radiographic findings of heart disease or cardiomegaly, or for preoperative screening and geriatric work ups.

Heart sounds, heart rate, and the electrocardiogram

A single heart beat consists of two sounds- the normal heart beat for the dog and cat are heard as a "LUB-DUP". All four heart sounds sometimes may be heard in large animals, such as the horse. The "LUB-DUP" heart sounds are referred to as S1 and S2.

"S1"- The first heart sound "S1" corresponds to the closure of the atrioventricular valves (the mitral and tricuspid valves): "LUB" occurs just as the ventricles depolarize and contract during the QRS wave

"S2"- The second heart sound "S2" corresponds to the closure of the pulmonic and aortic valves: "DUP" occurs as the ventricles repolarize and relax to fill with blood for the next contraction during the T wave

ecgheartsounds.jpg (121314 bytes) Here is a typical ECG recording showing the corresponding timing of the heart sounds, as you would hear them while watching the complexes form on the screen or paper.

Since the heart contracts as a response to the electrical impulse, the heart rate can be calculated from the ECG. One heartbeat corresponds to one set of PQRST. The ECG paper passes by the stylus at a constant rate, so the horizontal axis of the paper actually records time in seconds, once you know the paper speed.

The ECG paper has vertical markings at the top white margin of the graph paper strip that occur every 75 millimeters. The paper travels through the machine during the recording session at a preset rate, depending on the setting made by the technician; most machines have setting choices of 25 mm/second or or 50 mm/second. This means that at 25 mm/second, the time lapse recorded between the marks will be 3 seconds. At 50 mm/second paper speed, the time lapse between the marks will be 1.5 seconds. Knowing the paper speed, one can calculate the heart rate by viewing the ECG and counting the number of PQRST complexes between the markings.

A three second interval on the ECG is traditionally used for calculations. Count the number of R waves between a set of marks that correspond to an interval of 3 seconds, then multiply by 20 to determine the number of beats per minute. It is very important to record the paper speed setting you are using, because it will markedly affect the calculations for heart rate. For example:

timemarks.jpg (38728 bytes)  Observe the vertical marks at the top margin. If this ECG was taken at 25 mm/sec paper speed, then the time between 2 marks is 3 seconds. There are 2 PQRST complexes between the marks, or 2 beats every 3 seconds. So the calculated heart rate would be 40 beats per minute. If this ECG was taken at 50 mm/second paper speed, the time between marks is 1.5 seconds instead.  There are then 2 beats every 1.5 seconds, or you could say 4 beats every 3 seconds. So the calculated heart rate would then be 80.

ECG equipment

Electrocardiograph machines are available in a variety of types. Some provide a paper print out of the electrocardiogram, and some provide a digital image on a screen. The electrical activity is detected using a certain number of electrodes, which are devices that are attached to the animal at specific places on the body. The electrical activity recorded between any two electrodes is referred to as a lead.  Some machines are capable of recording only one lead at a time and others can record several leads at a time. There are also computerized machines that can record and analyze the waveforms automatically. For long distance consultations telephone hook ups are available. There are also devices that can be worn during normal activities that monitor and record heart activity over several hours. Some of the machines will make a beeping sound to correspond to the heartbeat as the PQRST is recorded.

electrods.jpg (108754 bytes) Electrodes are color coded and, for animals, often are attached using alligator clips, as seen here. Other types using discs that are attached using adhesive conductive material  are also available.

ECGmachine.jpg (55571 bytes) Here is an example of an ECG machine that makes a paper recording.

ECGmonitor.jpg (69209 bytes) Here is an example of an ECG machine in use that provides a continuous digital recording on a screen

Leads of the electrocardiogram

The electrocardiograph machine can measure electrical activity of the heart from different angles. One common ECG machine uses electrodes that are placed on the four legs and/or trunk of the animal. Measurements made by the ECG machine between any two electrodes are called a "lead".

The form the PQRST takes will vary from lead to lead because the deflections on the graph depend on the direction the cardiac impulse travels in relation to each set of electrodes.

Leads are designated by a standardized set of electrode placements and specific nomenclature:

lead1.jpg (71787 bytes) Lead I- reading from the right front leg to the left front leg

-lead2.jpg (53148 bytes) Lead II- reading from the right front leg to the left rear leg

lead3.jpg (85753 bytes) Lead III- reading from the left front leg to the left rear leg

leadavr.jpg (38376 bytes) aVR- reading from the right front leg to a point on the trunk

leadavl.jpg (43642 bytes) aVL- reading form the left front leg to a point on the trunk

leadavf.jpg (43618 bytes) aVF- reading from the left rear leg to a point on the trunk

Lead II is the standard lead used to screen for arrhythmias and is the one you are expected to be familiar with for this lesson.

ECG recording technique-

Electrodes are applied to the animalís legs and trunk to obtain readings from several leads. Areas where the electrodes are attached are often moistened with a conductant gel or alcohol. Most electrodes are labeled and color coded for humans, hence you will see that the electrodes are labeled for the arms and legs. For veterinary patients we use the arm electrodes for the front legs and the leg electrodes for the hind legs. For large animals, such as horses, the base apex lead placement technique is often used instead where the electrodes are attached to the neck and thorax. (see p. 298 in Hanie). Small animals are placed in right lateral recumbency, while large animals, such as horses, are often in a standing position.

widgetECG.jpg (65180 bytes) The small animal patient is placed in right lateral recumbency for the ECG. The black and white electrodes go on the front legs and the green and red electrodes attach to the rear legs. A way many technicians remember this is the saying "newspaper in the front" for the black print and white paper of the newspaper. The white electrode and green electrodes attach to the right side which is the recumbent side. Another saying to remember this positioning is "Snow and grass are on the ground".

When recording the ECG, the machine is set to record using a specific lead, paper speed, and sensitivity. These settings can be varied so it is important to record how the machine is set up for each ECG you perform. You must record the settings directly on the ECG strip if possible.

A standardization mark is made by pressing the sensitivity button before or after the recording. Standardization marks indicate how many squares in height equals 1 millivolt (mv). The mark will make a box that is the same height as what 1 mv would be. The machine can be set for different sensitivity levels. If the sensitivity is changed during the ECG, a new mark must be recorded to document the adjustment in amplitude. for the same animal, the complexes will change in appearance in height, or amplitude, but the actual millivoltage measurement will stay the same.

sensitivitymarks.jpg (35246 bytes) Here are a series of different standardization marks that show how the change in sensitivity setting changes how many squares in height equals 1 mv

The height of the deflections from the baseline records the amount of electrical activity in millivoltage (mv). Generally, the larger the heart, the higher the voltages. The sensitivity setting allows the operator to make the complexes appear larger or smaller to make visualization and measurements easier.

sensitivityset1.jpg (91415 bytes) The sensitivity is set for 1 cm = 1 mv. Each millimeter box is worth 0.1 mv. The height of the R wave ( the tall spike) is 19 millimeters. The R wave is 1.9 mv.

sensitivityset2.jpg (101790 bytes) Same dog- the sensitivity is set for 5 mm = 1 mv. Each millimeter box is worth 0.2 mv.  The height of the R wave is 91/2 millimeters. The R wave is 1.9 mv

sensitivityset3.jpg (66882 bytes)  Same dog- the sensitivity is set for 2.5 mm = 1mv. As the sensitivity is decreased, the complexes get smaller and it is more difficult to measure accurately.

The width of the deflections from the baseline records the time span of the electrical activity in seconds. Time intervals between deflections are also measured. The paper speed setting allows the operator to make the complexes appear farther apart or closer together to make visualization and measurements easier.

paperspeed50.jpg (32685 bytes) The paper speed is set for 50 mm/sec

paperspeed25.jpg (44296 bytes) The paper speed is set for the same dog at 25 mm/sec

The normal height and width of each deflection from the baseline has been determined for each species and serves as the standard for diagnosis. When taking measurements for the analysis of the ECG, the sensitivity and paper speed must be taken into account.

Recording technique:

  • Determine machine settings, center the stylus, and make the standardization mark for amplitude (for height in mv).
  • Position the animal on the table in right lateral recumbency. Make sure the table is padded with a towel or blanket. Turn all fluorescent lights off.
  • Attach electrodes and apply alcohol or gel.
  • Start recording the requested leads. As you change the lead setting, label the leads using the marker button which will make a dash at the top of the paper whenever it is pressed. You press it once for lead I, twice for lead II, three times for lead III, etc.
  • Once all the leads are recorded, make a 30-60 second recording of lead II.
  • Turn off machine and remove electrodes.

Troubleshooting the ECG

  • If the R waves are too tall- decrease the sensitivity
  • If the R waves are too small- increase the sensitivity
  • If the complexes are too close together, increase the paper speed
  • If the complexes look weird and are not recognizable, check to see if the electrodes are attached properly to the correct limbs. Keep the animal relaxed and quiet.
  • If the animal is very restless, readjust the electrodes or allow him to be in a different position, and make note of the alternative positioning as this will change interpretation.
  • If the stylus vibrates at the baseline- check connections and interference from electrical equipment, such as fluorescent lights)

The ECG record

Once the ECG tracing is generated, you mount and label representative strips of each lead onto a piece of paper or ECG form.

Find a representative PQRST and record the measurements for the various waveforms in lead II. The height is recorded in mv, and the width is recorded in seconds. Through years of research the normal range of parameters for each waveform have been established for various species of animals. When measuring the waveforms, we look for abnormalities in mv or time lapse, which may indicate certain disease states. For example: If a wave takes too long to form, then there may be a delay in conduction, such as a "heartblock". If a wave is too small (low mv), there may be fluid present. If a wave is too tall (high mv), the heart may be enlarged.

Keep in mind the settings for the machine at the time the ECG was performed when making your measurements so you know what each millimeter block is worth. Here are the basic steps for ECG measurements:

1. Identify the PQRST complex: First find an area on the strip that is representative and hold the strip so that it is right side up. The white margin containing the time marks are often at the top, depending on the brand of ECG paper. Identify the tall positive R wave spikes. Then you know the waveform to the left is the P wave and the waveform to the right will be the T wave.

   A diagram of the PQRST.

A lead II print out showing the PQRST complex. The S is difficult to appreciate in this tracing. And notice that the T is positive in this animal. Remember the T wave may be positive, negative, or biphasic.

2. Determine rate- there are several methods but try this one 

10 or 20 method- usually there are regular vertical marks at the top or bottom of the strip

            Count the number of complexes that occur in 3 seconds

At 50 mm/sec paper speed, the marks are 1.5 sec apart

            At 25 mm/sec paper speed, the marks are 3 sec apart


            Multiply the number of complexes in 3 seconds by 20= heart rate

Or        Multiply the number of complexes in 6 seconds by 10= heart rate

3. Record the height (vertical) and width (horizontal) measurements of the PQRST. The height will be recorded in millivolts (mv) and the width will be recorded in time (seconds). You will need to know the machine settings in order to know what each millimeter block is worth vertically and horizontally. You will find it easier to work with a strip that was recorded at 50 mm/sec. At 50mm/sec, each tiny square millimeter block will equal 0.02 seconds horizontally. At 1 mv = 1 cm, each square millimeter will equal 0.1 mv vertically.

Measurement of the P wave. For the height, use the distance from the baseline to the tallest point. For the width, measure from the beginning to the end of the P wave deflection.

Measurement of the R wave, using the QRS complex. For the height, use the distance from the baseline to the tallest point. For the width, measure from the Q to the S. Often the Q and S are very close to the R wave.

Measurement of the P-R interval. Measure the distance from the beginning of the P wave to the beginning of the Q wave. Often you can't see the Q wave, or the Q and R wave are very close together, then use the beginning of the R wave.

Measurement of the T wave. Measure the first major deflection from the baseline that occurs after the QRS. Normally it is less than 25% the height of the R wave.

  Observe the ST segment. This is the area from the end of the S (QRS) to the onset of the T wave. It can be a little bit above or below the baseline. Note if it significantly dips below the baseline (depressed), goes above the baseline (elevated), or is isoelectric (travels along the baseline). 

Measure the QT interval. Measure the distance from the onset of  Q to the end of the T wave. This is used sometimes to evaluate subtle changes that can occur when there are problems with certain medications.

Then you will observe the entire strip for any artifacts or unusual waveforms that catch your eye because they do not fit the typical PQRST pattern. The next section will cover a few unusual waveforms that you should recognize.


A major reason ECGís are performed are to detect or characterize arrhythmias for diagnosis and treatment. When arrhythmias occur, the heart does not contract in a normal manner, cardiac output is not as efficient, and clinical signs such as fainting or weakness occurs. However, if the pacemaker is not working, then the arrhythmia that occurs with an abnormal electrical impulse conduction is better than nothing!

Sources of Arrhythmias

An arrhythmia is the abnormal rate, regularity, or site of cardiac impulse formation, or is a change in the normal sequence of the cardiac impulse. Arrhythmias happen when:

  • The SA  pacemaker (sinoatrial node) activity is depressed, fails, or if the SA pacemaker impulse is produced but does not conduct. Then sites other than the SA node serve as back up pacemakers.
  • The normal pacemaker is interrupted by abnormal activity of any of these alternate pacemaker sites.

Detecting arrhythmias by determining the site of impulse formation from the ECG-

The electrical impulse from an alternate pacemaker site will be conducted in a different direction than that from the usual SA node site. Each pacemaker site produces its own characteristic ECG because the impulse is always conducted in a certain way from that site!!! Some arrhythmias can be spotted on sight and do not require tedious measurements for detection.

If the impulse comes from the SA node, the ECG will appear normal, because it is normal.

SAimpulseorigin.jpg (19377 bytes) Sinoatrial node- normal dominant pacemaker, originates from specific tissue in the atrium, positive P waves with normal QRS

If the impulse comes from some other site in the atrium, the ECG will appear very close to normal; it will be slightly different in measurements from the SA node impulse. You will probably not notice this arrhythmia very easily because it appears so close to normal.

atrial.jpg (24858 bytes) Atrial- alternate atrial sites other than the SA node. Positive P waves with a normal QRS, P waves are slightly different height and width in appearance from the SA node impulse, but look rather normal, often not detected with a casual glance!

If the impulse comes from the AV node, the P wave will look different than those from the SA node, but the QRS will look rather normal.

Junctional.jpg (30575 bytes) Junctional or AV node region- negative or isoelectric (equally positive and negative readings results in a flat line) P waves, normal looking QRS

If the impulse comes from a site in the ventricle, there will be no P wave and the QRS will be large and abnormal looking. This arrhythmia is most easily recognized.

ventricular.jpg (25114 bytes) Ventricular tissue sites - no P waves, QRS is wide and bizarre

Summary for the recognition of alternate pacemaker sites :

  • Atrial-- looks very similar to normal
  • AV node-- funny looking P wave, QRS looks normal
  • Ventricular-- no P wave, QRS looks weird: wide, big, and bizarre

Criteria for arrhythmias on the ECG

When evaluating an ECG for an arrhythmia, there are set of questions to work through:

  • Heart rate- normal, slow, or fast? Too slow or fast-- possible arrhythmia
  • Rhythm- the R to R intervals are regular or irregular? Irregular R to R--possible arrhythmia
  • P waves- height and width are normal and consistent or are they variable or absent? Variable or absent--- arrhythmia
  • QRS- height and width are normal and consistent, or are wide and bizarre complexes present? Wide and bizarre complexes present- arrhythmia (ventricular)!
  • P and QRS relationship- is there a P for every QRS and is there a consistent and normal P to R time interval? If there are more Pís than QRSí--- "heart block" arrhythmia. If there is a longer than normal P to R interval--- "heart block" arrhythmia

Nomenclature of heart rhythms

Heart rhythms are named by the the source of the impulse and a descriptive term for the heart activity. The impulse will be designated either sinus (for SA node), atrial, junctional (for AV node), or ventricular. Then the term for the heart rate may be included, such as bradycardia or tachycardia.

Sinus rhythm- "Sinus" refers to the sinoatrial (SA) node as the source of the rhythm, which is normal.

normal_sinus.jpg (78562 bytes)

Sinus Arrhythmia This is a normal variant in some situations, especially dogs. The heart rate is regularly irregular because the change in rate coincides with respiration. The rate increases and decreases with inspiration and expiration.

sinusarr1.jpg (25021 bytes) In sinus arrhythmia, there are irregular R to R intervals, but these follow a pattern that coincide with respiration.

Here are a few selected arrhythmias:

Sinus Bradycardia This is a common arrhythmia when under anesthesia. The complexes appear normal and originate from the SA node, but the rate is abnormally slow.

Sinus tachycardia This is a common arrhythmia when an animal is fearful, as for a visit to the vet! The complexes are normal and originate from the SA node, but the rate is abnormally fast.

Junctional rhythm This is termed junctional because the impulse is from the AV node or junctional area between the atria and ventricles. You will recognize this rhythm by the unusual P waves in lead II, but the QRS looks normal.

junctionalrhythm.jpg (45632 bytes) Junctional rhythm. The P waves are upside down.

Atrial fibrillation This arrhythmia is easily ausculted. The rate is rapid but the lub- dup sound is of varying volume so it will be hard to count. At first you will think there is something wrong with your stethoscope or your ears. The sound is often described as that made by tennis shoes in the dryer machine. There will be pulse deficits or pulses of varying intensity. There is no single discernible P wave but just several repeated baseline deflections, or fibrillations, where the P wave should be.

atrialfib.jpg (79633 bytes)  atrialflutter.jpg (52746 bytes)   Atrial fibrillation

Ventricular premature complexes (VPC) There is one or a few wide and bizarre complexes present amongst the relatively normal complexes. They occur very soon after a normal complex, which is why they are termed premature. You may hear a lub-dup-dup which corresponds to the PQRST complex (lub-dup) and the VPC (dup). Then there is often a pause after the VPC before normal rhythm resumes, so you have the definite impression of an irregular heart beat on auscultation..

vpc.jpg (94365 bytes) VPC

Ventricular tachycardia There is an entire run of wide and bizarre complexes and the rate is very rapid.

Vtach.jpg (46043 bytes) Ventricular tachycardia

Ventricular asystole There is no activity, also known as "flatline".

Ventricular fibrillation The electrical activity is very disorganized and has no particular waveform. The ventricles are fibrillating.

First degree AV block The P-R interval is longer than normal. The SA impulse is delayed but does eventually get conducted.

heartblock1st.jpg (21867 bytes)  First degree heartblock. The P-R interval should be 0.06-0.13 sec (for the dog). At paper speed of 50 mm/sec, the P-R interval should be no longer than 3 to 61/2 tiny boxes wide. In this ECG the P-R interval is 10 boxes.

Second degree AV block The P-R interval is delayed and some delays are so long that another P wave is conducted. Some Pís do not have a QRS.

heartblock.jpg (100877 bytes) Second degree heartblock. There is an extra P without a QRS at the left of the strip.

Third degree AV block The SA node keeps trying to send an impulse but the P wave is completely blocked and conduction is not completed at all. A back up pacemaker in the ventricle takes over, and although it is at a slow rate, it is better than nothing!

heartblock3rdb.jpg (53195 bytes) Third degree heartblock. P waves are present, but there is no normal QRS paired with the P waves because the impulse never gets through.


Artifacts cause abnormal waveforms that may be confused for arrhythmias. These include purring, limb movement, heavy respiratory effort, panting, and electrical interference from other electrical equipment in the room such as fluorescent lights. Be observant and rule out these causes of artifacts before you finalize your ECG results.

respartifact.jpg (20884 bytes) Respiratory movement makes the baseline drift on the graph paper.

trembling.jpg (114984 bytes) Muscle tremors or trembling mimics atrial fibrillation

-limbmovement.jpg (28512 bytes) Limb movement will mimic wide and bizarre complexes.

60cycleinterference.jpg (120051 bytes) 60 cycle electrical interference from other equipment in use will mimic atrial flutter or fibrillation. However, the deflections from electrical interference will be smaller and very uniform. A similar artifact appears if the electrodes are not on securely. Check the connections and add more conductant.


Tilley LB and Burtnick NL: ECG for the Small Animal Practitioner Teton New Media


McCurnin DM and Poffenbarger EM: Small Animal Diagnosis and Clinical Procedures WB Saunders Company, Philadelphia, 1991, pp.53-63

Edwards NJ: ECG Manual for the Veterinary Technician WB Saunders Company Philadelphia 1993

Tilley LP: Basic Canine Electrocardiography The Burdick Corporation Milton, Wisconsin


Websites of Interest--(if there is a password window, keep canceling and you will get in)


Large animal ECG:


ECG of the month:


ECG tutorial for ECG under anesthesia:


Canine cardiology site:



Writing Assignment- includes 1) Task list assignment and 2) writing assignment from the lesson

1)Task list assignment ECG

ECG evaluation: Generate an ECG, preferably from a dog, at your practice site and use the form linked below to submit for grading. Please note that the reference values in the form are for dogs, so if you use a different species, you must use different reference values for your normal range when evaluating the ECG. You may scan it as a document and send via email attachment or mail it in.

Click here for the report form to use for the ECG assignment for this lesson.

2) When you have completed your study of the lesson, please answer the following questions and submit your answers via e-mail.  You may answer the questions directly in an e-mail or you may download the Word file, fill in your answers and return the Word file as an attachment.

Send to Dr. Bidwell: abidwell@nvcc.edu

ECG Lesson Writing Assignment MS Word file

  1. Name 3 indications for obtaining an ECG. 
  2. Define arrhythmia. 
  3. Name and describe the alternative sites for cardiac impulse formation other than the sinoatrial node. 
  4. How could there be circulation problems if the ECG looks normal? 
  5. As long as the heart is beating, why would we worry about circulation problems if there is an arrhythmia? 
  6. What are the letter designations of the waveforms on the electrocardiogram? 
  7. During what part of the ECG do S1 and S2 occur? 
  8. What does isoelectric mean?
  9.  What is a lead?

  10.  What is the favored lead that we studied most in class and lab? 

  11. Why does the ECG look different for each lead for the same cardiac impulse? 
  12. The horizontal axis of the ECG graph paper measures _________________. 
  13. The vertical axis of the ECG graph paper measures ___________________. 
  14. Name 5 criteria for determining if an arrhythmia exists. 
  15. What is sinus arrhythmia? 
  16. Are all arrhythmias bad? 
  17. Name 3 causes of artifacts.