Case Studies In Small Animal

Cardiovascular Medicine

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Case 14

Case Discussion

This dog has frequent episodes of sinus arrest on its ECG. Sinus arrest is caused either by a dying sinus node ("Sick Sinus Syndrome") or markedly increased vagal tone. Atropine was administered subcutaneously and another ECG was recorded at least 20 minutes later. Since there was no response, a vagally-mediated arrhythmia was ruled-out. Consequently, sick sinus syndrome was the diagnosis. Because the dog was having frequent and long periods of sinus arrest and because he was showing clinical signs compatible with weakness caused by cessation of blood flow (i.e., stumbling in the caudal limbs), pacemaker implantation is warranted at this time. This recommendation was made to the owner and the possible complications (e.g., infection, lead dislodgment requiring replacement and more cost, unknown generator life with need for replacement at some time, etc.) were outlined. He agreed that he wanted us to implant the pacemaker. Houdi also had moderate mitral regurgitation. His left atrium was moderately enlarged on both his echocardiogram and on his radiographs. As long as myocardial failure is not present and the regurgitation is not acute, the severity of the regurgitation is based on the size of the left atrium. Moderate mitral regurgitation was not a contraindication to pacemaker implantation in Houdi.

This picture shows the conduction system of the heart (in gold). The diseased portion of Houdi's heart, the sinoatrial (SA) node, sits at the very top of the conduction system in this diagram.

Houdi was atropinized and taken into a fluoroscopy room the following day. A catheter introducer was placed in the left lateral saphenous vein and a temporary pacing lead (i.e., plastic coated wire) was advanced from the left rear leg up through the caudal vena cava, and into the right ventricle using fluoroscopy to guide the catheter placement. A temporary generator was attached to the lead and the heart was paced at a rate of 100 beats/minute. To further document that the sinus node was not functioning properly, the heart was paced for several minutes at a rate greater than the dog's own inherent rate and then the temporary generator was turned off and the sinus node recovery time (time from last paced beat to first sinus node depolarization) was measured. In a normal dog, the recovery time is very short, usually in the neighborhood of the dog's inherent heart rate (e.g., if the heart rate is 60 beats/minute the sinus node would depolarize one second after turning off the pacemaker). In a dog with a diseased sinus node, the cells are bombarded by the depolarizations from the pacemaker and undergo a process called overdrive suppression. The net result is that they take a longer time to recover, often a very long time. Our record for sinus node recovery time is 24 seconds. This dog had two responses shown below.

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This first response was reasonably normal. The first four beats are from the pacemaker which is then turned off. The paper speed is 25 mm/sec so the sinus node takes a little more than 0.8 seconds (20 mm) to recover which corresponds to a heart rate of around 75 beats/minute (60 seconds/minute divided by 0.8 seconds per beat = 75 beats/minute). The sinus node rate after that depolarizes at approximately a rate of 100 beats/minute (0.6 seconds in between each beat). The dog had multiple sinus node recovery times measured. About half the time the recovery was like this one. The other half of the time it was like the one below.

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The pacemaker was turned off after the first three beats on this recording. The sinus node now does not recover before the AV node finally produces an escape beat (the fourth QRS complex appears to be normal [supraventricular in origin] but has no P wave in front of it making it most likely AV nodal in origin). The AV node obviously is also abnormal since a normal AV node should produce an escape beat within one to 1.5 seconds (it depolarizes at a rate of 40 to 65 beats/minute in a normal dog). The AV node, however, does appear to quickly recover to produce the fifth complex 1.4 seconds later (rate = 43 beats/minute) and then produces the sixth complex 0.94 seconds later (64 beats/minute). Finally, the sinus node recovers to produce the seventh complex (P wave in front). This clearly documents that the sinus node is diseased and shows why a measurement like this needs to be repeated to document what is really occurring. This sinus node apparently has some cells that can still respond normally at times but clearly are diseased enough that they cannot respond normally at all times.


Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.