Electrocardiography-
Part II
Kathy Glaze, RVT
Department of Small Animal Surgery College of Veterinary Medicine Texas A&M Universiry College Station, Texas
KEY POINTS
Ideally, the patient should be placed in right lateral recumbency during electrocardiography.
Heart rate is regulated primarily by a balance of sympathetic and parasympathetic (or vagal)
inputs.
Familiarity with normal rhythms (i.e., sinus rhythm, respiratory sinus arrhythmia, and
wandering pacemaker) is important in interpreting the electrocardiogram.
Although electrocardiography (ECG) may at first seem intimidating, technicians can learn to
produce electrocardiograms (ECGs) that are diagnostic and therefore useful. With practice, technicians
can also assist in the interpretation of ECGs. This article is the second pan of a three-pan presentation.
The first part discussed the essentials of basic ECG, such as the physiology of depolarization and
repolarization and the anatomy of the heart. The third part, which will appear in an upcoming issue of
Veterinary Technician®, will review use of the ECG to diagnose such cardiac disturbances as premature
contraction, sinus bradycardia and tachycardia, and atrial fibrillation.
Producing a Diagnostic Electrocardiogram Positioning and Lead
Placement
How is an ECG produced? First, ECG should be done in a quiet room in which the patient will
not be disturbed. The secret to success in recording a diagnostic ECG is proper positioning and
restraint of the patient and placement of the lead. The dog or cat should be in right lateral recumbency
and should be as comfortable as possible. For example, simply placing a blanket on a cold
stainless steel table is sometimes all that is necessary to greatly enhance patient comfort: it also must be
remembered that it is stressful for animals in respiratory distress to be placed on their sides In these
cast's. ECG can be done with the patient in sternal recumbency or even standing if necessary
Amplitude measurements f o r detection of chamber enlargement patterns done wi t h the patient in
either of those positions, however, w i l l compromise accuracy, although the ECG may be used for
rhythm and internal interpretation. It is important to remember that detection of chamber
enlargement requires standard lead placement and positioning, whereas detection of arrhythmia
requires only that the leads be attached to the patient. With the patient in right lateral recumbency
the person holding the patient places his or her right arm over the patient's neck and holds the
forelimbs with a finger placed between the legs so that they do not touch each other. With the left
arm, the hindlimbs are held in a similar manner. The front legs should be held perpendicular to the
spine of the patient. Flattened alligator clips (Figure 1) are then placed below the elbow on the
fore-limbs and below the stifle joint on the hindlimbs. In these positions (Figure 2), the clips are
usually far enough from the body to preclude chest motion from interfering with the tracing. With
longhaired animals, the technician must ensure that the clip is placed directly on the skin and not
on the hair. Contact solution must be used on the clips in order to ensure electrical conduction.
Options include alcohol or electrode pastes. Alcohol is a poor option because it evaporates too
quickly; pastes are used if there is a possibility that the session may be prolonged. During
application, runoff and puddles of the contact solution should be prevented; leads that lay in
puddles of contact solution produce artifacts on the ECG.
The first thing that must be checked on the ECG machine is the sensitivity setting. Most
machines have several options for this setting. The standard sensitivity setting of 1 makes a 1-cm
deflection of the pen on the graph paper when the millivolt button is pushed. That deflection can be
changed to 0.5 cm or 2.0 cm by changing the sensitivity selector. The standard sensitivity of 1 mV = 1
cm is appropriate for most ECGs. If the standard setting causes the pen to run off the page. sensitivity
should be cut in half so that 1 mV = 0.5 cm. If the P waves are not visible (this problem is common in
cats), sensitivity should be doubled so that 1 mV = 2.0 cm.
Most ECGs for dogs and cats are run at 50 mm/sec. The speed selection knob usually has two choices:
50 mm/sec or 25 mm/sec. When running an ECG, the technician places his or her left hand on the
position knob, keeping the tracing in the middle of the paper. It is essential for the pen to remain on
the paper. Lead selections can be made with the right hand. The machine indicates the lead
selections at the top of the paper. On some machines, lead I is marked with a short line (-), lead II
with two short lines (--), and lead ID with three short lines (---). Lead aVR is marked with a long line
(—), lead aVL with two long lines (--- ---), and lead aVF with three long lines (--- --- ---). If an
arrhythmia is noted at this time, a rhythm strip, or a long lead II reading, must be run. The rhythm strip
is run at a slower rate (25 mm/sec) for a sufficient amount of time to demonstrate the arrhythmia.
Artifacts
Artifacts during ECG are common. T he technician must be able to recognize and correct
these artifacts. Examples of common artifacts follow.
Sixty-cycle interference. Sixty-cycle interference (Figure 3) is an electrical interference pattern that
occurs when electrical equipment is not properly grounded. There are several methods to correct
the problem. Make sure the power cord is grounded, make sure that the clips are contacting the
skin ( i f there is any doubt, the clips should be reapplied); make sure the clips are clean and securely
attached to the cable: pull the plugs on other nearby equipment: turn off fluorescent lights: hold the
legs of the patient in the manner discussed to prevent them from touching; and make sure the cables
are not touching either the table or the person holding the patient. Any and all of these occurrences can
cause 60-cycle interference Sometimes this problem is very difficult to correct, but every effort
should be made to do so. Sixty-cycle interference can make the ECG very difficult to read.
Muscle tremors Muscle tremors (Figure 4) can result in rapid and irregular movements of the
baseline. To correct this problem, make sure the patient is calm and comfortable; readjust or reapply
the clips; [or place a hand on the trembling chest area and press firmly.] Purring in cats can also
cause this artifact.
Wandering baseline. A wandering baseline (Figure 5) is caused by changes in resistance between
the electrode and the patient. Respiratory movements are a common cause. To correct the problem,
place the animal in a standing position or in sternal recumbency if it is having respiratory difficulty; if
it is not, hold the mouth shut for 3 to 4 seconds.
Interpreting the Electrocardiogram
Once the tracing is completed, how is it interpreted? ECGs provide a wealth of information, such as
heart rate, cardiac rhythm, axis deviation, chamber enlargement, and conduction disturbances. With
some training, technicians can learn to recognize all types of cardiac disturbances in an ECG.
Heart Rate
Heart rate is a function of many variables. It is regulated by, among other things, a balance of
sympathetic and parasympathetic (vagal) inputs. Parasympathetic stimulation lowers the heart rate (as
in cases of bronchial disease), whereas sympathetic stimulation increases the heart rate (as in nervous
cats). Parasympathetic stimulation can also be influenced by the condition of the heart and circulatory
system itself. The sinoatrial node is the biological pacemaker of the heart; under normal circumstances, this node causes the heart to beat 80 to 120 times per minute. If the sinoatrial node fails, the
next pacemaker area is the atrioventricular junction; the atrio ventricular junction allows the
heart to beat 40 to 60 times per minute. If the sinoatrial node and atrioventricular junction both
fail, the ventricles take over and the heart drops to 20 to 40 beats/min. Drugs such as atropine or
digoxin can also affect heart rate.
There are three ways to calculate heart rate:



Count the R waves that have registered within 6 seconds. Multiply this
number by 10. This is a quick but inaccurate method
Count the number of large squares between two R waves. Divide this number into
300. This method loses accuracy when used to calculate very fast rates and can be
used only with regular rhythms.
Count the number of small squares between two R waves. Divide this number into
1500. This method is the most accurate but can only be used with regular
rhythms.
Interval and Amplitude Measurements
Interval and amplitude measurements are usually done with lead II. At 50 mm/sec, each
small box horizontally measures 0.02 seconds (Figure 6). Calipers, an instrument used for
measuring thickness and distance, can facilitate measurement. The P interval begins with the first
upward deflection from baseline ( the P-R interval) and ends with the return to baseline (a thorough
discussion of these intervals can be found in Part I). The P-R interval is measured from the first
upward deflection of the P wave to the first deflection of the QRS complex. The QRS complex is
measured from the first deflection of the Q wave to the end of the S wave. Low-amplitude QRS
complexes are caused by pleural and pericardial effusion as well as obesity.
The ST segment is measured from the end of the S wave to the first upward or downward
movement of the T wave. Although the duration of the ST segment is not generally of clinical
significance, changes in the amplitude or deflection can be very important. For example, changes in
the ST segment during anesthesia can signify suboptimal oxygenation of the patient.
The QT interval is measured from the beginning of the QRS complex to the final return of the T
wave to baseline. The QT interval is inversely related to the heart rate such that the faster the heart rate,
the shorter the QT interval.
All amplitude measurements are done from baseline (the P-R interval). At a standard deviation of
1 mV = 1 cm. each small box vertically measures 0.1 cm. Normal values for cats and dogs are shown in
Figure 6. Table 1 contains diagnostic differentials for abnormal values.
Determining What Is Normal
Familiarity with the three normal rhythms is crucial to reading an ECG. The first normal rhythm is
sinus rhythm(Figure 7). Sinus rhythm occurs when the heart rate falls between 60 and 140 beats/mm in
dogs and 120 and 200 heats/min in cats. In sinus rhythm, there is a P wave for every QRS complex,
the rhythm s regular and intervals have normal values.
The second normal rhythm is respiratory sinus arrhythmia (Figure 8). The criteria that apply to
sinus rhythm also apply to respiratory sinus rhythm, except that the heart rate is variable because it
corresponds with respiration. As the patient inhales, heart rate increases; as the patient exhales, the
heart rate decreases.
The last type of normal rhythm is called wandering pacemaker (Figure 9). This rhythm is
characterized by P waves with varied configurations and sizes in the same lead. The pacemaker
site may shift locations within the sinoatrial node, causing the vectors to shift slightly. This is
commonly seen concurrently with respiratory sinus rhythm.
Summary
Proper positioning is crucial for the ECG to be accurate: ensuring patient comfort is an
i mp or t a nt a s pe c t of positioning. Artifacts do occur during ECG; some of the more common
types are 60-cycle interference, muscle tremors, and a wandering baseline. Heart rate is a
function of many variables and is regulated by, among other things, a balance of sympathetic and
parasympathetic inputs. Evaluation of various intervals also plavs a critical rule in small animal
ECG.
Article #2 Review Questions
The article you have read is the equivalent of ln hour of study. To receive credit for your
study, choose only the one best answer to each of the following questions; then mark your
answers on the test form on the postage-paid envelope inserted in Veterinary Technician.
1. Success in recording a diagnostic ECG depends on:
a. positioning and restraint.
b. making the patient comfortable.
c. proper placing of leads.
d. all of the above
2. Which of the following measurements will be inaccurate if an ECG is recorded with
the patient in sternal recumbency?
a. heart rate
b.interval
c. amplitude
d. rhythm
3. Which of the following is the standard sensitivity for an ECG?
a. 1 mV = 10 cm
b. 1 mV = 0.5 cm
c. 1 mV = 1 cm
d. 2 cm = 1 mV
4. Lead II is indicated by what marking at the top of the ECG?
a. one short line
b. two short lines
c. one long line
d. two long lines
5. To correct 60-cycle interference:
a. turn off fluorescent lights.
b. press on the patient firmly
c. hold the patient's mouth closed
d. hold the alligator clips with the finger
6. Muscle tremor artifacts may be caused by:
a. shaking.
b. respiratory distress.
c. purring.
d. a and c
7. The primary biological pacemaker of the heart is the
a. atrioventricular node
b. right atrium.
c. sinoatrial node.
d. ventricles.
8. The baseline of an ECG is the
a. ST segment.
b. P-R interval.
c. P interval.
d. T wave.
9. Which of the following is considered a normal rhythm in dogs?
a. respiratory sinus arrhythmia
b. sinus arrest
;
c. wandering baseline
d. b and c
10. Low-amplitude QRS complexes are caused by
a. obesity.
b. pleural effusion.
c. pericardial effusion.
d. all of the above