Case Studies In Small Animal

Cardiovascular Medicine

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Case 14

Material Gathered After Pacemaker Implantation

ECG Taken After Pacemaker Implantation

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This is the dog's ECG following implantation of the permanent artificial pacemaker. Paper speed is 25 mm/second. There is a "spike" before each QRS complex. The spike is small in lead I and larger than the completely negative QRS complex in lead II. This electrical spike is generated by the pacemaker generator firing. Note that there is exactly 0.6 seconds (15 mm) in between each spike meaning the rate of the pacemaker is set at exactly 100 beats/minute (60 seconds/minute divided by 0.6 seconds/beat = 100 beats/minute).

Thoracic Radiographs Taken After Pacemaker Implantation

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The radiographs taken one day after pacemaker implantation show it to be well-positioned. On the lateral you can see the lead coursing from the heart up the cranial vena cava to the jugular vein and then subcutaneously back to the lateral thorax where the generator is positioned.

ECG Taken That Evening

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Later that same day, another ECG was recorded because an irregularity in the rhythm was noted on auscultation. The type of pacemaker placed in this dog is called a VVI pacemaker. This code means the pacemaker is pacing the heart in the ventricle (first V) and sensing spontaneous depolarizations from the ventricle (second V) and that these spontaneous and sensed depolarizations are supposed to inhibit (I) the pacemaker from depolarizing. This last feature allows the heart to take over its own rhythm if the sinus node decides to fire at a rate fast than the artificial pacemaker. The pacemaker is not performing this function properly at this time. The first two complexes are generated by the pacemaker. The third is a spontaneous sinus beat. Note that the pacemaker does not sense this at all but continues on at its rate of 100 beats/minute (i.e., there is no break in the rhythm). Compare this to the sixth complex where a spontaneous beat occurs that is sensed. The spontaneous depolarization is sensed by the pacemaker which is then reset. The spontaneous depolarization is then followed by a pause of 0.6 seconds before the pacemaker fires again (no hysteresis for you pacemaker buffs). The spontaneous depolarization sent a depolarization wave up the lead of the pacemaker to the generator. The generator sensed this beat and shut off for an appropriate interval. This is what is supposed to happen. The beats that are not sensed come in just a little bit earlier than the beats that are sensed. The pacemaker generator does have a period of time after it fires during which it is refractory to stimulation (i.e., it has a refractory period). If this was the problem, it would appear that the refractory period for this generator would be between 380 and 420 msec, a very long refractory period. It was also possible that the sensitivity setting was not set correctly. This setting was changed (via an external programming device) and the problem resolved suggesting that this was in fact the problem.

Houdi has gone on to do well with his pacemaker. Unfortunately, he also has a glomerulonephropathy that continues to cause problems. You should have noted in the history that Houdi had been on an ACE inhibitor but taken off because of renal problems. ACE inhibitors actually help protein loss in glomerulonephropathies by decreasing intraglomerular pressure. Consequently, it may have been a mistake to take him off his ACE inhibitor.

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