ECG of the Month

Brenda L. Weissman Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610.

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Amara H. Estrada Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32610.

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A7-year-old castrated male German Shepherd Dog was evaluated at the Veterinary Medical Center of the University of Florida because of coughing, anorexia, and weight loss. Prior history included pacemaker implantation 1 year previously for third-degree atrioventricular (AV) block. At that time, echocardiography revealed volume overload and hyperkinesis of the left ventricle; no other abnormalities were identified echocardiographically. A transvenous, bipolar, dual-chamber VDDa,b (singlelead atrial synchronous and ventricular inhibited) pacing system was implanted in the right ventricular apex (RVA). The owner had not returned the dog for scheduled reevaluation following implantation.

On physical examination, a large amount of fluid was detected via abdominal palpation. A grade 3/6 left apical systolic murmur and a grade 2/6 right systolic murmur were ausculted. An irregular rhythm was detected, and an ECG was obtained (Figure 1). Echocardiographic evaluation revealed moderate left ventricular and left atrial volume overload. The pacing lead was observed across the tricuspid valve and implanted in the RVA. The mitral valve appeared normal; however, there was a central jet of mitral regurgi tation that was judged to be moderate in severity. Mild tricuspid valve regurgitation associated with the lead crossing the tricuspid valve was detected. Left ventricular motion was dyskinetic with normal apical contraction but minimal basilar movement. Left ventricular end-systolic dimension was mildly increased, and fractional shortening was 47% (range in clinically normal dogs, 25% to 45%).1 The E-point–to–septal separation was greatly increased at 1.94 cm (upper reference limit, 0.6 cm).1 Thoracic radiography revealed proper lead placement (without evidence of lead dislodgement) and severe pulmonary edema. Interrogation of the pacing system revealed functionally normal battery and lead impedance, excellent threshold values, and no loss of ventricular capture following pacing spikes.

Figure 1
Figure 1

Surface ECG from a dog with atrial fibrillation and complete heart block in which a VDD pacing system (single-lead atrial synchronous, ventricular inhibited) had been implanted transvenously in the right ventricular apex (RVA). Pacing spikes are visible (arrows). No P waves are detected, and there is undulation of the baseline trace because of fibrillation waves. Pacing occurs at irregular intervals in response to atrial-sensed events. Ventricular depolarizations are wide and bizarre in appearance because of the site of impulse initiation (the RVA). Paper speed = 50 mm/s; 5 mm = 1 mV.

Citation: Journal of the American Veterinary Medical Association 228, 7; 10.2460/javma.228.7.1025

Figure 2
Figure 2

Programmer strip printout obtained from the dog in Figure 1. The surface ECG (top trace) indicates an irregular-paced rhythm and lack of identifiable P waves. The marker channel (middle trace) indicates multiple atrial-sensed events both following (AS) and during (AR) the atrial refractory period. Ventricular pacing (VP) occurs irregularly in response to these multiple atrial-sensed events. The intracardiac electrogram (bottom trace) shows multiple fast and irregular atrial-sensed events consistent with atrial fibrillation. For the surface ECG, paper speed = 25 mm/s; 1 mm = 0.2 mV. For the intracardiac ECG, paper speed = 25 mm/s; 1 mm = 0.1 mV.

Citation: Journal of the American Veterinary Medical Association 228, 7; 10.2460/javma.228.7.1025

ECG Interpretation

Electrocardiographic evaluation revealed an irregular rhythm with a mean heart rate of 110 beats/min. P waves were not identifiable, and undulations were visible in the baseline, consistent with fibrillation waves. Small pacing spikes were identified immediately prior to the ventricular depolarizations. Ventricular depolarizations were wide and bizarre in appearance because of the site of impulse initiation (the RVA). Impulses generated from the RVA cannot use the specialized conduction tissue and thus must be propagated from cell to cell within the ventricular myocardium. Interrogation of the pacing system by use of a programmerc allowed a definitive diagnosis of atrial fibrillation that resulted in an irregular, ventricularpaced rhythm (Figure 2). The intracardiac electrogram displayed continuous, fast, and irregular atrial-sensed events. Ventricular pacing occurred at irregular intervals as a result of these irregular atrial-sensed events.

Discussion

Transvenous pacing of the RVA is a common procedure performed in humans and other animals for the treatment of third-degree AV block and other conduction disturbances or bradyarrhythmias. It is a minimally invasive surgical procedure that involves vascular cut down access to the external jugular vein for implantation (with fluoroscopic guidance) of a pacing lead in the endocardium of the RVA. The use of dual-chamber pacing systems, in which 1 lead is located in the atrium and another is placed in the ventricle, has recently become more popular in veterinary medicine. A VDD pacemaker paces only the ventricle, yet is able to sense activity in both the atrium and ventricle. The advantage of such a system over a single-chamber ventricular (VVI) pacemaker is the ability to initiate ventricular contraction in response to atrial events, thereby maintaining AV synchronization.2 In a studyd to compare VVI and VDD pacing in dogs with third-degree AV block, pulmonary capillary wedge pressure and pulmonary arterial systolic pressure were decreased in dogs with VDD pacing systems. Results of that studyd in dogs support findings of investigations in humans3,4 that suggest that maintenance of AV synchrony generally allows for improved cardiac performance.

In the dog of this report, the ability to respond to atrial-sensed events led to irregular ventricular pacing and contraction because the pacemaker sensed and responded to the multiple rapid atrial depolarizations during atrial fibrillation. Many of these atrial depolarizations occurred during the programmed refractory period of the atrium; therefore, a ventricular pacing impulse was not triggered. The atrial events that were sensed at times other than the programmed atrial refractory period were followed by an appropriately timed ventricular pacing impulse.

Recently, interest in the sequence of ventricular electrical activation has grown because of increasing evidence that chronic pacing at the conventional pacing site, the RVA, is associated with pathologic changes in the heart. During normal sinus rhythm or atrial pacing, impulse activation of the ventricles is characterized by minimal asynchrony, activation of the left ventricle earlier than the right ventricle, and earlier apical than basal activation. During RVA pacing, the action sequence deviates considerably from the physiologic one and is associated with a notable depression of left ventricular function.5 Results of studies6,7 in animals have indicated that the abnormal electrical activation induced by RVA-based pacing leads to a depression of systolic and diastolic left ventricular function, which is what we believe was occurring in the dog of this report.

During RVA pacing, mitral valve regurgitation can develop because of the early activation of the septum, which results in leftward motion of the septum while the papillary muscles are still passive because transseptal conduction is slow.8 Activation of the right ventricle earlier than the left ventricle causes a premature rise in right ventricular pressure, which results in paradoxical septal motion.9 This paradoxical movement may be associated with development of mitral regurgitation and poor left ventricular function. We believe that all of these factors contributed to the development of atrial fibrillation and biventricular congestive heart failure in the dog of this report.

Follow-up evaluations of the dog were not performed during the year following pacemaker implantation. Thus, we do not know the time course of development of the cardiac changes. However, we hypothesize that this dog represented a subset of dogs in which RVA-based pacing may lead to notable structural and functional cardiac dysfunction as a result of a nonphysiologic mode of pacing.

a.

Medtronic 5032 VDD pacemaker lead, Medtronic Inc, Minneapolis, Minn.

b.

Medtronic 8968i Thera VDD pulse generator, Medtronic Inc, Minneapolis, Minn.

c.

Medtronic 9790 programmer, Medtronic Inc, Minneapolis, Minn.

d.

Bulmer BJ, Oyama MA, Sisson DD. Acute hemodynamic consequences of physiologic VDD pacing in dogs with naturally occurring third degree atrioventricular block (abstr), in Proceedings. 20th Annu Forum Am Coll Vet Intern Med 2002;341.

References

  • 1.

    Kittleson MD, Kienle RD. Echocardiography. In: Small animal cardiovascular medicine. St Louis: Mosby Year Book Inc, 1998;104, 107.

  • 2.

    Bulmer BJ, Oyama MA, Lamont LA, et al. Implantation of a single-lead atrioventricular synchronous (VDD) pacemaker in a dog with naturally occurring 3rd-degree atrioventricular block. J Vet Intern Med 2002;16:197200.

    • Search Google Scholar
    • Export Citation
  • 3.

    Rediker D, Eagle K, Shunichi H, et al. Clinical and hemodynamic comparison of VVI versus DDD pacing in patients with DDD pacemakers. Am J Cardiol 1988;61:323329.

    • Search Google Scholar
    • Export Citation
  • 4.

    Nowak B, Voigtlander T, Himmrich E, et al. Cardiac output in single-lead VDD pacing versus rate-matched VVIR pacing. Am J Cardiol 1995;75:904907.

    • Search Google Scholar
    • Export Citation
  • 5.

    Prinzen FW, Peschar M. Relation between pacing induced sequence of activation and left ventricular pump function in animals. Pacing Clin Electrophysiol 2002;25:484498.

    • Search Google Scholar
    • Export Citation
  • 6.

    Frohlig G, Schwaab B, Kindermann M. Selective site pacing: the right ventricular approach. Pacing Clin Electrophysiol 2004;27:855861.

  • 7.

    Zile MR, Blaustein AS, Shimizu G, et al. Right ventricular pacing reduces the rate of left ventricular relaxation and filling. J Am Coll Cardiol 1987;10:702709.

    • Search Google Scholar
    • Export Citation
  • 8.

    Simantirakis EN, Vardakis KE, Kochiadakis GE, et al. Left ventricular mechanics during right ventricular apical or left ventricular-based pacing in patients with chronic atrial fibrillation after atrioventricular junction ablation. J Am Coll Cardiol 2004;43:10131018.

    • Search Google Scholar
    • Export Citation
  • 9.

    Little WC, Reeves RC, Arciniegas J, et al. Mechanism of abnormal interventricular septal motion during delayed left ventricular activation. Circ Res 1982;65:14861490.

    • Search Google Scholar
    • Export Citation
  • Figure 1

    Surface ECG from a dog with atrial fibrillation and complete heart block in which a VDD pacing system (single-lead atrial synchronous, ventricular inhibited) had been implanted transvenously in the right ventricular apex (RVA). Pacing spikes are visible (arrows). No P waves are detected, and there is undulation of the baseline trace because of fibrillation waves. Pacing occurs at irregular intervals in response to atrial-sensed events. Ventricular depolarizations are wide and bizarre in appearance because of the site of impulse initiation (the RVA). Paper speed = 50 mm/s; 5 mm = 1 mV.

  • Figure 2

    Programmer strip printout obtained from the dog in Figure 1. The surface ECG (top trace) indicates an irregular-paced rhythm and lack of identifiable P waves. The marker channel (middle trace) indicates multiple atrial-sensed events both following (AS) and during (AR) the atrial refractory period. Ventricular pacing (VP) occurs irregularly in response to these multiple atrial-sensed events. The intracardiac electrogram (bottom trace) shows multiple fast and irregular atrial-sensed events consistent with atrial fibrillation. For the surface ECG, paper speed = 25 mm/s; 1 mm = 0.2 mV. For the intracardiac ECG, paper speed = 25 mm/s; 1 mm = 0.1 mV.

  • 1.

    Kittleson MD, Kienle RD. Echocardiography. In: Small animal cardiovascular medicine. St Louis: Mosby Year Book Inc, 1998;104, 107.

  • 2.

    Bulmer BJ, Oyama MA, Lamont LA, et al. Implantation of a single-lead atrioventricular synchronous (VDD) pacemaker in a dog with naturally occurring 3rd-degree atrioventricular block. J Vet Intern Med 2002;16:197200.

    • Search Google Scholar
    • Export Citation
  • 3.

    Rediker D, Eagle K, Shunichi H, et al. Clinical and hemodynamic comparison of VVI versus DDD pacing in patients with DDD pacemakers. Am J Cardiol 1988;61:323329.

    • Search Google Scholar
    • Export Citation
  • 4.

    Nowak B, Voigtlander T, Himmrich E, et al. Cardiac output in single-lead VDD pacing versus rate-matched VVIR pacing. Am J Cardiol 1995;75:904907.

    • Search Google Scholar
    • Export Citation
  • 5.

    Prinzen FW, Peschar M. Relation between pacing induced sequence of activation and left ventricular pump function in animals. Pacing Clin Electrophysiol 2002;25:484498.

    • Search Google Scholar
    • Export Citation
  • 6.

    Frohlig G, Schwaab B, Kindermann M. Selective site pacing: the right ventricular approach. Pacing Clin Electrophysiol 2004;27:855861.

  • 7.

    Zile MR, Blaustein AS, Shimizu G, et al. Right ventricular pacing reduces the rate of left ventricular relaxation and filling. J Am Coll Cardiol 1987;10:702709.

    • Search Google Scholar
    • Export Citation
  • 8.

    Simantirakis EN, Vardakis KE, Kochiadakis GE, et al. Left ventricular mechanics during right ventricular apical or left ventricular-based pacing in patients with chronic atrial fibrillation after atrioventricular junction ablation. J Am Coll Cardiol 2004;43:10131018.

    • Search Google Scholar
    • Export Citation
  • 9.

    Little WC, Reeves RC, Arciniegas J, et al. Mechanism of abnormal interventricular septal motion during delayed left ventricular activation. Circ Res 1982;65:14861490.

    • Search Google Scholar
    • Export Citation

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