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. 1
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. 2
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. 3
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. 4
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. 5
Practice. Case 1: A 4 year-old bulldog presents for a routine surgery. An ECG is obtained as part of a preoperative database. 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. 2 beats X 20 = 40 bpm, bradycardia Rhythm? Regular (further evaluation of the strip) P waves: Uniform, precede each QRS, sinus rhythm QRS: Appears normal and uniform. QRS follows each P wave 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. 6
Case 3: A 7 year-old Labrador presents with pale mucus membranes and collapsed in the back yard. 12 beats X 20 = 240 bpm, tachycardia Rhythm? Regular P waves: Uniform, precede each QRS, sinus rhythm QRS: Appears normal and uniform. QRS follows each P wave 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. 8 beats X 20 = 160 bpm Rhythm? irregular P waves: Irregular P wave preceding the 4 th complex from the left. All other P waves are uniform QRS: Appears normal and uniform. QRS follows each P wave. P-R interval: The 4 th 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. 7
Case 5: A 4 year old Doberman presents with exercise intolerance and lethargy. 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. 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. 8
Case 7: A 13 year-old Fox Terrier presented for a 24 hour history of weakness and collapse. 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. 9
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. 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 P- R 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. Rhythm? P waves: QRS: P-R interval: Name: Treat? 11
Case 10: Post-op hemoabdomen 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