ECG Interpretation for the Veterinary Technician

Transcription

Amanda Adams, LVT, VTS (ECC)
VCA Veterinary Specialty Center of Seattle
An electrocardiogram (ECG) is a graph of the heart`s electrical current, which allows evaluation of heart
rate, rhythm and conduction. Identification of conduction problems within the heart begins with a
general understanding of the heart`s anatomy, conduction system and how the electrical activity
corresponds to waveforms on an ECG. Veterinary technicians should be proficient at obtaining a
diagnostic ECG, identifying normal and abnormal complexes on an ECG tracing, and recognizing when
medical intervention might be needed.
Obtaining a diagnostic ECG:
The heart’s electrical activity creates currents that radiate through the patient`s surrounding tissue. The
current travels through electrodes placed on the patient`s body and is transmitted to an ECG monitor.
The information is then transformed into a waveform which allows for interpretation of the conduction
system.
The patient should be placed in right lateral
recumbency. Electrodes should be placed
directly on skin and wet with electrode gel or
alcohol. Ideally the electrodes are placed distal
to the elbows and hock, and under rather than
over the chest wall, which will remove any
baseline artifact from movement of the chest.
Attempt to keep the patient calm and
immobilized to prevent motion artifact. View
the tracing in lead II at 50mm/sec on the
monitor. Ensure that the top and bottom of the
complexes are visible in the tracing. If any
waveform appears too large or too small adjust
the size until all waves on the complex are
visible. Print the rhythm strip. If an arrhythmia is noted, multiple strips should be printed for
evaluation. In the event that waveforms need to be measured for diagnostic purposes, the strip needs
to be printed with 1cm = 1mV amplitude standard.
Generation of the Waveform:
In veterinary medicine, lead II is most commonly used to identify arrhythmias. A lead refers to the axis
drawn across the body connecting a negatively charged pole to a positively charged pole. There are a
total of 12 axes that can be measured across the body, which allow for alternate views of electrical
currents across the body. In lead II, the left hind electrode is the positive pole and the right axillary
electrode is the negative pole. As impulses from the heart travel towards the positive electrode, an
upward deflection on the ECG is recorded. Alternatively, impulses traveling toward the negative
electrode are traced as a negative deflection on the ECG. Isoelectric or flat line on a tracing is produced
either when there is no electrical activity or the electrical forces are equal.
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Fig. 1
The Conduction System:
Conduction begins with generation of an impulse at the sinoatrial node within the right atrium. The
sinoatrial node (SA), the primary pacemaker of the heart, generates impulses from 160-240 bpm in the
cat and 70 – 180 bpm in the dog (dependent on size and breed). The impulse spreads rapidly through
the atria along Bachman`s bundle (left atrium) and through the internodal pathways (right atrium) to
the atrioventricular (A-V) node. The rapid transmission of the impulse from the SA node to the A-V node
causes the atria to contract. The impulse is then slowed in the A-V node allowing for completion of
ventricular filling. As the impulse passes through the Bundle of His, rapid conduction resumes. The
impulse spreads through the left and right bundle branches and ends at the Purkinje fibers. The passage
of the impulse through the ventricles ultimately results in ventricular contraction. A precise conduction
pathway within the heart maximizes filling time during diastole and controls speed of conduction to
maximize strength of contraction.
Cardiac tissue is unique in that myocardial cells have the ability to spontaneously initiate electrical
impulses (automaticity). Special cells within the cardiac muscle that depolarize rhythmically are
appropriately named pacemaker cells. Specialized pacemaker cells are found along the conduction
route in the junctional tissue around the AV node, around the Purkinje fiber in the ventricles, as well as
other areas along the conduction pathway. As a general rule, the farther down the conduction tract
from the SA node, the slower the intrinsic pacemaker rates become. Under normal circumstances, the
faster rate of the SA node overrides the slower rate of the other sites. When an impulse fails to be
generated at the SA node, or conduction is blocked as it travels down the conduction pathway, one of
the lower pacemakers may become the primary pacemaker. When cardiac tissue becomes irritated
either as a primary cardiac condition, or secondary to systemic illness/trauma, an ectopic pacemaker
(group of excitable cells outside of normal conduction system) may generate impulses faster than the
intrinsic rate of the SA node. The site generating the faster impulses will drive the heart causing an
arrhythmia.
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Fig. 2
Evaluating the ECG Tracing:
A normal ECG tracing will consist of multiple waveforms (complexes) in sequence. Each complex
represents one complete cardiac cycle and should consist of P, QRS, and T waves.
The P wave represents atrial depolarization and should precede the QRS complex. In lead II, if the
impulse originated in the SA node, the P wave should be upright, rounded and smooth. Variation of the
size and shape of the P wave on a single tracing may indicate that an impulse is originating from a site
within the atria other than the SA node.
The PR interval tracks the impulse from the atria through the A-V node. A prolonged PR interval may
indicate conduction delay through the atria or A-V junction.
The QRS complex follows the P wave and represents ventricular depolarization. The Q wave is the first
negative deflection following the P wave. The R wave is the first positive deflection following the P wave
and the S wave is the first negative deflection following the R wave. In some QRS complexes, not all
waves are visible. If no P is visible preceding the QRS complex, then the impulse for that complex may
have generated in the ventricles or in the junctional tissue surrounding the A-V node.
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The ST segment signifies the end of ventricular conduction/depolarization and the beginning of
ventricular repolarization. An ST segment should be consistent with the baseline. A significantly
elevated or depressed ST segment may indicate myocardial injury.
The T wave indicates ventricular repolarization and can have a positive or negative deflection on the
graph.
Fig. 3
Is the heart rate fast, slow or normal?
Calculate the heart rate. With a strip printed at
50mm/sec, count the number of complexes within
30 large boxes and multiply by 20. There are 7
complexes counted within 30 large boxes on this
strip. HR = 140bpm.
Is the rhythm regular?
Read the strip from left to right. Are the R-R
intervals regular? Regularly irregular? Use any
piece of paper to mark an R-R interval. Use the
paper to move from complex to complex. If the
rhythm is regular, the interval marked on the
paper should line up to each succeeding R wave as
it is moved down the strip.
Evaluate the P waves.
Are P waves present? Is there a P wave preceding
every QRS complex? Is there a QRS complex
following every P wave? When scanning the strip
from left to right, are the P waves uniform? Or do
they vary in shape or size? Again use a piece of
paper to mark two consecutive P waves. Slide the
paper down the strip to see if the P-P intervals are
consistent. This can help identify where a P wave
should be if the event of a superimposed P wave on
a QRS complex or a preceding T wave.
Evaluate the QRS complex.
Fig.4
If the QRS complex appears of normal height and duration, then we can assume that the initiating
impulse originated above the A-V node. When the QRS complex appears wide and bizarre, we can
assume that the impulse initiating that complex originated at an ectopic pacemaker within the
ventricles.
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Evaluate the relationship between the P waves and QRS.
On a normal ECG, there should be a P wave for every QRS, with a normal, consistent P-R interval.
Prolonged P-R intervals indicate a conduction delay through the A-V node. A P wave that is not followed
by a QRS complex indicates conduction through the atria, but a block occurred at the A-V node. There
are several forms of A-V block as depicted on the strips below. Short P-R intervals, where the P-wave is
positioned very close to the QRS complex indicate that the impulse was generated around the A-V node
potentially by the A-V junctional tissue or an ectopic focus close to the A-V node.
Evaluate the T wave.
With complicated arrhythmias, it may be difficult to discern a P wave from a T wave. It is important to
remember a T wave will always follow every QRS complex, and the same is not true for P waves which
may be buried in the complex or missing from the complex altogether.
ECG Classification
Arrhythmias are generally classified for disturbances of impulse formation, impulse conduction, or both.
Many times impulse formation disturbances can be characterized with words like: bradycardia,
tachycardia, premature (if the impulse is generated early) or fibrillation (rapid, unsynchronized
impulses). Impulse conduction disturbances are characterized by a block along the conduction pathway
and are usually named for place in which the block occurs and/or designated a degree to which the
block occurs. Alternatively some arrhythmias do not fit neatly into either classification.
Determine if the rhythm requires treatment.
Cardiac arrhythmias require treatment if they adversely affect hemodynamics resulting in decreased
cardiac output, hypotension or hypoperfusion. Additionally any benign arrhythmia should be monitored
for progression into more unstable rhythms. Other causes for cardiac arrhythmia (electrolyte disorders,
systemic disease, toxicity, pain, etc.) should be considered and ruled out before attributing the
arrhythmia to primary cardiac disease.
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Practice.
Case 1: A 4 year-old bulldog presents for a routine surgery. An ECG is obtained as part of a preoperative database.
HR?
6 (beats counted in 30 large boxes*) X 20 = 120bpm
Rhythm? Irregular, speeds up and slows down, complexes are identical
P waves: Uniform, precede each QRS, sinus rhythm
QRS: Appears normal and uniform. QRS follows each P wave
P-R interval: Consistent, appropriate duration**
*Many ECG tracings will print with hash lines at the top of a 50mm/sec strip
to mark the length of 15 large boxes.
** At 50mm/sec normal PR interval,
Dog: 3 to 6 and 1/2 small boxes
Cat: 2.5 to 4.5 small boxes
Name: Sinus arrhythmia, a.k.a. normal sinus arrhythmia. Especially common in brachycephalic breeds,
when respiratory rate speeds up during inhalation and slows during exhalation.
Treatment: No treatment required.
Case 2: A 14 year-old cocker spaniel presented with a 3 day history of vomiting and syncopal
episodes.
HR?
2 beats X 20 = 40 bpm, bradycardia
Rhythm? Regular (further evaluation of the strip)
P-R interval: Consistent, appropriate duration
Name: Sinus bradycardia. This rhythm is often is associated with increased vagal nerve tone, or other
systemic disease. This rhythm can be normal in resting well-conditioned large breed dogs.
Treatment: Yes, treat underlying cause. May respond to anticholinergics.
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Case 3: A 7 year-old Labrador presents with pale mucus membranes and collapsed in the back yard.
HR?
12 beats X 20 = 240 bpm, tachycardia
Rhythm? Regular
P-R interval: Consistent, appropriate duration
Name: Sinus tachycardia. Sinus tachycardia is the most common arrhythmia documented in dogs and
cats. Some common causes are pain, shock, anemia, hypoxia, exercise and anxiety.
Treatment: Yes, treat for underlying cause of tachycardia.
Case 4: A 9 year-old Cavalier King Charles Spaniel is hospitalized for CHF.
HR?
8 beats X 20 = 160 bpm
Rhythm? irregular
P waves: Irregular P wave preceding the 4th complex from the left. All other P waves are uniform
QRS: Appears normal and uniform. QRS follows each P wave.
P-R interval: The 4th complex has a slightly longer P-R interval than the rest
Name: Sinus rhythm with one atrial premature complex (APC). APCs are initiated by a supraventricular
impulse generated in the atria by a site other than the SA node (ectopic). This complex is called
premature because it is discharged early and out of sequence of the SA node impulses.
Treatment: In this case, the infrequent APCs noted on the strip are not affecting the patient`s
hemodynamic status. However, APCs can become frequent and may be precursors to atrial tachycardia
and atrial fibrillation. This patient should be monitored for progressive electrical instability.
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Case 5: A 4 year old Doberman presents with exercise intolerance and lethargy.
HR?
12 beats X 20 = 240 bpm
Rhythm? Irregularly irregular
P waves: No discernable P waves, bumpy baseline (fine fibrillatory waves)
QRS: Appears normal and uniform, however R-R interval is irregular and erratic
P-R interval: No discernable P-R interval
Name: Atrial fibrillation. Chaotic, asynchronous atrial impulses (F waves, which cause the bumpy
baseline) initiated by the atrial tissue bombard the AV node. The ventricles only respond to the
impulses that get through the AV node.
Treat? Yes. Rapid heart rate and erratic conduction decreases filling time of both the atria and
ventricles, severely affecting cardiac output. This patient requires antiarrythmic medication.
Case 6: A 10 year-old cat with hypertrophic cardiomyopathy and heart failure.
HR?
8 beats X 20 = 160 bpm
Rhythm? irregular
P waves: Two complexes not preceded by P waves, otherwise uniform
QRS: Two complexes appear wide and bizarre, the rest appear normal. There is a pause after each
abnormal complex
P-R interval: regular and appropriate duration (on sinus complexes)
Name: Sinus rhythm with two ventricular premature complexes (VPCs). Because the abnormal
complexes are not preceded by a P wave, we can call the complex premature and assume the impulse
was initiated within the ventricles.
Treat? Treat the symptomatic patient. Frequent VPCs can affect cardiac output leading to hypotension,
poor perfusion, weakness, etc. The patient`s ECG should be monitored for progressive electrical
instability leading to ventricular tachycardia or fibrillation.
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Case 7: A 13 year-old Fox Terrier presented for a 24 hour history of weakness and collapse.
HR?
4 beats X 20 = 80 bpm (actual atrial rate, P waves: 11 X 20 = 220)
Rhythm? irregular
P waves: Uniform. Not all P waves are conducting to a QRS.
QRS: All QRS complexes are preceded by a P wave
P-R interval: Consistent
Name: AV Block (Second-Degree, Mobitz Type II). Sinus impulses are being generated, however only
every third impulse is conduction through the AV node.
Treat? Yes, patient is symptomatic. Check for underlying systemic disease. Monitor ECG for progression
of arrhythmia. (See below for explanation of all AV block types.)
First-degree AV block:
First-degree AV block looks like a normal sinus rhythm, however, the P-R intervals are consistently long
and do not vary. The prolonged P-R interval indicates a delay in conduction through the AV node.
Generally patients are asymptomatic.
Second-degree AV Block, Mobitz Type I:
This rhythm is characterized by P-R intervals that become progressively longer with each complex until
finally the impulse fails to conduct through the AV node (dropped QRS complex). This is a predictable
rhythm with the next impulse conducting through the AV node and the cycle repeats.
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Second-degree AV Block, Mobitz Type II:
This rhythm is characterized by a regular atrial rate (uniform, consistent P waves); however some P
waves are not followed by a QRS complex. Because the ventricular rate may be significantly lower than
the atrial rate, cardiac output can be severely affected.
Third-degree AV Block (Complete
Block):
Impulses from the atria are completely blocked at the AV node. Since no impulses make it past the AV
node to the ventricles, the intrinsic ventricular rate kicks in and can be as low as 20 bpm. The wide and
bizarre complexes are ventricular escape beats.
Case 8: A 6 year-old DSH presents with two day history of straining in the litterbox.
HR?
approx. 120 bpm
Rhythm? regular
P waves: None
QRS: Deep S wave merging with tall spiked T wave
10
P-R interval: None
Name: Sine wave
(changes characteristic
of a hyperkalemic
patient)
Treat? YES!!! This is a
fatal arrhythmia.
As serum potassium
levels rise, T waves
peak, followed by
prolongation of the PR interval.
The QRS complex
begins to widen, while P waves flatten until ultimately they disappear.
Fig. 5
Once atrial activity has ceased, the next progression is unstable ventricular activity and ultimately death.
Practice:
Case 9: 3 year-old Golden retriever, hit-by-car 36 hours ago.
HR?
Rhythm?
P waves:
QRS:
P-R interval:
Name:
Treat?
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Case 10: Post-op
hemoabdomen
HR?
Rhythm?
P waves:
QRS:
P-R interval:
Name:
Treat?
Resources:
Battaglia, Andrea M. Samll Animal Emergency and Critical Care. Philadelphia: Saunders, 2001. Print.
Tilley, Larry P., Francis W.K. Smith, and Michael S. Miller. Cardiology. Denver: American Animal Hospital
Association, 1990. Print.
Tilley, Larry P., Michael S. Miller, and Francis W.K. Smith. Canine and Feline Cardiac Arrhythmias. Philadelphia:
Lea & Febiger, 1993. Print.
Tilley, Larry P., and Naomi L. Burtnick. ECG for the Small Animal Practioner. Jackson, WY: Teton NewMedia,
1999. Print.
Fig. 1 research.vet.upenn.edu
Fig. 2 Tilley, LP. Feline cardiac arrhythmias. Vet Clin North Am 1977;7:274
Fig. 3 ecgtest.org
Fig. 4 medicine.mcgill.ca
Fig. 5 Mayo Clinic Proceedings December 2007 vol. 82 no. 12 1553-1561
12

ECG Interpretation for the Veterinary Technician

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