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Michael R. O'Grady, Dana G. Allen, Andrew J. Mackin, Julian Van Gorder atrioventricular block (1). The first two PR intervals demonstrate a progressive prolongation of the PR interval preceding the blocked P wave (below 1 and 2 on the figure). This finding identifies the heart block as Mobitz type I second degree atrioventricular block or Wenckebach second degree atrioventricular block. After the longest pauses, a QRS complex occurs which is not preceded by a P wave (below 5 and 6 on the figure). These QRS complexes are known as escape beats (2). Escape beats are QRS complexes that result from specialized myocardial tissue that has the ability to discharge automatically if not already activated by another source. These areas of automaticity primarily involve cells of the conduction system. Escape beats represent a protective mechanism allowing for the initiation of a heart beat in the face of sinoatrial nodal disease thereby preventing cardiac standstill. There are two examples of P waves occurring in the ST segment (below 7 and 8 on the figure). In that the P wave morphology is identical to the other P waves, it appears that these two cases represent P waves that occur in the ST segment by serendipity. Note that the P waves that appear below 3 and 4 on the figure reveal a small positive deflection that immediately follows the negative deflection of the P wave. This small positive deflection represents atrial repolarization and is called a Ta wave (3). Close inspection of the complexes below 9 and 10 on the Department of Clinical Studies, University of Guelph, figure reveals a positive deflection after the T wave; NIG 2W1 (O'Grady, Allen, Mackin); Guelph, Ontario Limestone Valley Animal Hospital, 6634 Guelph Line, this positive deflection is most likely a Ta wave suggesting the presence of a P wave buried in each T wave. R.R. 2, Milton, Ontario L9T 2X6 (Van Gorder).
Athirteen-year-old spayed female West Highland White Terrier was referred to the Ontario Veterinary College Veterinary Teaching Hospital with a history of bradycardia noted by the referring veterinarian during a routine physical examination. This dog had a heart rate of 60-80/min while in an excited state. On physical examination the heart rate was 60/min, body temperature was 38.8°C, and the dog was panting. A grade III of VI pansystolic heart murmur was detected with a point of maximal intensity over the left cardiac apical region. The femoral arterial pulse was strong. No other cardiovascular abnormality was detected at this time. An electrocardiogram was recorded to determine the cause of the bradycardia. A lead II rhythm strip is presented recorded at a paper speed of 25 mm/sec and sensitivity of 0.5 cm/mv. The ventricular rate is variable ranging from 40-100/ min. The P waves are negative throughout. The negative P waves in lead II suggest an ectopic source (that is, a source distant from the sinoatrial node) responsible for the initiation of atrial depolarization. We expect the normal sinoatrial nodal pacemaker to result in a positive P wave in lead II. There are P waves present that are not followed by a QRS deflection (below 3 and 4 on Figure). This is called second degree
Can Vet J Volume 32, January 1991
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If portions of this tracing appear similar to third degree atrioventricular heart block, recall that in third degree heart block the atrial activity (P waves) occurs regularly and the ventricular activity (QRS complexes) also occurs regularly but at a slower rate (4). In third degree heart block the P waves are not related to the QRS complexes; both rhythms are completely independent. Thus third degree heart block is not present here. This electrocardiogram reveals bradycardia, ectopic P waves, second degree atrioventricular heart block, and escape beats. These findings indicate sick sinus syndrome (5). "Sick sinus syndrome refers to a constellation of signs, symptoms, and electrocardiographic criteria defining sinus node dysfunction in association with sinus bradycardia, sinus arrest, sinoatial block, alternating bradydysrhythmias and tachydysrhythmias, or carotid hypersensitivity. However, clinical signs and symptoms result from failure of escape pacemaker function, not from sinus node malfunction per se. Thus, the sick sinus syndrome may represent a generalized disorder of the conduction system of the heart, sinus node dysfunction being only one aspect." (5) Atropine sulphate, 0.04 mg/kg IV, was administered in order to determine if this bradydysrhythmia was a
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References 1. Tilley LP. Essentials of canine and feline electrocardiography. 2nd ed. Philadelphia: Lea & Febiger, 1985: 168-171. 2. ibid: 150-151. 3. ibid: 60. 4. ibid: 172-175. 5. Jordan JL, Mandel WJ. Disorders of sinus function. In: Cardiac Arrhythmias: Their Mechanisms, Diagnosis, and Management. Mandel WJ. Ed. 2nd ed. Philadelphia; JB Lippincott. 1987: 143-185. 6. Tilley LP. Essentials of canine and feline electrocardiography. 2nd ed. Philadelphia: Lea & Febiger, 1985: 180-183.
Y- :_Non-essential, continuous shedding is a h pet owners major prom the world. Grooming your pet with NoShed will reduce hair and dandruff in your living environment. Unnecessary hair loss is reduced without affecting the _ seasonal shedding process. veteriNoShed is marketed through narians and is ALREADY being offered to pet owners by thousands of veterinarians in the U.S., Canada and Europe.
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result of excessive vagal activity. This dog's rhythm disturbance did not change after atropine therapy. Most cases of sick sinus syndrome are not atropine responsive (6). It is intriguing that this dog was asymptomatic at the time of examination. It is to be predicted that syncope could be expected if bradycardia progresses. Permanent pacemaker therapy remains the only longterm treatment for such cases. The etiology of sick sinus syndrome is usually idiopathic (6). Disorders causing ischemia of the conduction system of the heart can be expected to result in sick sinus syndrome.
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Can Vet J Volume 32, January 1991
