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
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.
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:
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.
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.
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.
Here is a dissection of the electrocardiogram through one cardiac cycle in lead II:
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
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:
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
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:
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.
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.
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:
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.
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.
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.
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.
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.
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 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 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.
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:
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.
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- 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.
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.
Summary for the recognition of alternate pacemaker sites :
Criteria for arrhythmias on the ECG
When evaluating an ECG for an arrhythmia, there are set of questions to work through:
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.
Sinus ArrhythmiaThis 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.
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.
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.
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..
Ventricular tachycardia There is an entire run of wide and bizarre complexes and the rate is very rapid.
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.
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.
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!
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.
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.
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: firstname.lastname@example.org