ECG of the Month

Vittorio Saponaro 1Unité de Cardiologie d'Alfort, Université Paris-Est, Ecole Nationale Vétérinaire d'Alfort, Centre Hospitalier Universitaire Vétérinaire d'Alfort, Maisons-Alfort, F-94700, France.

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Peggy Passavin 1Unité de Cardiologie d'Alfort, Université Paris-Est, Ecole Nationale Vétérinaire d'Alfort, Centre Hospitalier Universitaire Vétérinaire d'Alfort, Maisons-Alfort, F-94700, France.

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Jean-Louis Pouchelon 1Unité de Cardiologie d'Alfort, Université Paris-Est, Ecole Nationale Vétérinaire d'Alfort, Centre Hospitalier Universitaire Vétérinaire d'Alfort, Maisons-Alfort, F-94700, France.

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Valérie Chetboul 1Unité de Cardiologie d'Alfort, Université Paris-Est, Ecole Nationale Vétérinaire d'Alfort, Centre Hospitalier Universitaire Vétérinaire d'Alfort, Maisons-Alfort, F-94700, France.
2U955 - IMRB Inserm, Ecole Nationale Vétérinaire d'Alfort, Université Paris-Est-Créteil, Maisons-Alfort, F-94700, France.

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A 4-month-old 18.6-kg (41-lb) spayed female Great Pyrenees was referred for a second opinion regarding an arrhythmia that had been fortuitously detected at the time of vaccination. The dog did not have any clinical signs and was not receiving any cardiac medication. On physical examination, the dog was alert with apparently normal mental status and had a body condition score of 5/9. Its rectal temperature was 38.3°C (100.9°F). Femoral pulses were irregular in rate and intensity. Cardiac auscultation revealed an irregular rhythm with a heart rate of 150 beats/min but no detectable heart murmur. The initial diagnostic assessment included echocardiography a CBC, serum biochemical analysis, and assessment of circulating electrolyte and serum cardiac troponin I concentrations. Echocardiography did not reveal any structural heart disease, and hematologic and serum biochemical variables were within the reference intervals. The serum cardiac troponin I concentration was 0.02 ng/mL (reference range, < 0.06 ng/mL). Six-lead ECG was performed to evaluate the dog's irregular heart rhythm.

ECG Interpretation

The 6-lead ECG recording (Figure 1) revealed a high heart rate (150 beats/min) with premature complexes (identified by R' waves; Figure 2). These premature complexes were characterized by narrow QRS complexes (duration, 40 milliseconds; reference range, < 70 milliseconds) that were not preceded by P waves; the physiologic electrical axis was +100° (reference range, +40° to +100°). These features were consistent with junctional premature complexes (JPCs) organized in a bigeminal or trigeminal pattern. The coupling interval with sinus complexes (R-R' interval) appeared constant (duration, 240 milliseconds) along the entire ECG tracing. The JPCs were not followed by a pause; thus, they were interpolated without affecting the normal sinus cycle duration1 (P-P interval). Occasionally a change in QRS complex morphology was observed, indicating the presence of fusion beats. Interestingly these JPCs were sometimes followed by a prolonged PR interval (150 milliseconds; reference range, 60 to 130 milliseconds) or by a nonconducted P wave, indicative of first- and second-degree atrioventricular (AV) block, respectively. Severity of the arrhythmia was assessed by 24-hour Holier monitoring, which confirmed junctional bigeminy and trigeminy during most of the recording. Only a short period of uninterrupted sinus rhythm was recorded between 5:04 am and 6:31 am. Twenty-two months after the evaluation, the dog was free of any clinical signs, and, in accordance with the owners' wishes, not receiving any treatment. An ECG follow-up examination was not performed.

Figure 1—
Figure 1—

Six-lead surface ECG recording obtained from a 4-month-old Great Pyrenees with no structural heart disease detected during echocardiographic examination. Paper speed = 25 mm/s; 1 cm = 1 mV.

Citation: Journal of the American Veterinary Medical Association 256, 4; 10.2460/javma.256.4.427

Figure 2—
Figure 2—

Lead 11 ECG tracing obtained from the 6-lead surface ECG tracing in Figure 1 (A) with a corresponding ladder diagram (B) and a magnified area (outlined by red brackets) of the lead II ECG tracing (C). Notice the numerous junctional premature complexes (R') that have a fixed coupling interval of 240 milliseconds with the preceding sinus complexes (R). A fusion beat (F) is also present in the ECG tracing. In the highlighted detail of the ECG tracing, notice that when the R'P interval is 170 milliseconds, the following PR interval (blue bar) is 150 milliseconds, thereby generating pseudo-first-degree atrioventricular (AV) block (white arrow). When the R'P interval is 160 milliseconds, the P wave is not conducted, which results in pseudo–second-degree AV block (black arrow). In the ladder diagram, the upper zone represents atrial activation (A), the middle zone represents AV junction activation (AV), and the lower zone represents ventricular activation (V). The black dots represent the sites of impulse origin that can occur in the sinus node (within the atria [A]) or ectopically (in the AV junction). The black lines represent the direction of propagation of the impulses through the heart. Interruption of a line with a dash indicates that the impulse does not propagate further than that point. Two opposing dashes represent the point of collision of 2 opposite wave fronts as occurs in the case of a fusion beat. Paper speed = 25 mm/s; 1 cm = 1 mV.

Citation: Journal of the American Veterinary Medical Association 256, 4; 10.2460/javma.256.4.427

Discussion

The surface ECG recording obtained from the dog of the present report had evidence of subclinical supraventricular allorhythmia (ie, frequent bigeminal and trigeminal JPCs that contributed to a high heart rate) alternating with first- and second-degree AV blocks (the latter, conversely potentially contributing to slow the heart rate). The AV block alternated from first to second degree depending on the time when the sinus complex following the JPC occurred. The shorter the R'P interval, the more the AV conduction was affected. This variation in the R'P interval was likely attributable to sinus arrhythmia because the sinus cycle duration was unaffected by the interpolation of JPCs (absence of compensatory pauses following JPCs).1

Junctional complexes are usually retrogradely conducted to the atria, providing negative P waves on the surface ECG recording that are positioned before, inside, or after the QRS complexes, depending on the conduction delay and modality in the AV node.2 For the dog of the present report, no negative P waves preceding or following JPCs were observed. Nevertheless, the sinus complexes were sometimes blocked when they were generated too soon after JPCs (nonconducted positive P waves in lead II). Thus, it was likely that the retrograde wave front provided by JPCs was unable to emerge from the AV node and depolarize the atria but did prolong the refractory period of the AV node, thereby avoiding the conduction of sinus complexes. This phenomenon is called concealed conduction and refers to a rare electrophysiological condition in which a so-called concealed impulse is not sufficiently strong to pass through the whole AV node to be conducted but has the ability to make the AV node refractory and prevent conduction of the following impulses.3 In other words, the concealment refers to a nonrecordable event (the concealed impulse) on the surface ECG tracing, which is uniquely unmasked by its effect (AV block) on the following impulse (recordable event).4 In humans, concealed conduction frequently occurs when a retrogradely conducted interpolated ectopic impulse enters the AV node.3–10 Often, these premature ectopic impulses originate in the His bundle.5–9 The sinus beat following the ectopic impulse is then not conducted to the ventricle or is conducted with a prolonged PR interval because of the increased refractoriness of the AV conduction system.3 In the case described in the present report, refractoriness of the AV node typically depended on the time when the postextrasystolic sinus complex occurred (R'P interval duration). The postextrasystolic sinus complex was delayed if it occurred during the late refractory period of the AV node, thereby leading to pseudo-first-degree AV block (R'P interval of 170 milliseconds). However, if the postextrasystolic sinus complex occurred earlier (R'P interval of 160 milliseconds), it reached the completely refractory AV node and resulted in pseudo-second-degree AV block. There are several reports4–11 about concealed conduction related to interpolated JPCs (as detected in the dog of the present report) or ventricular premature beats in humans and experimental animals. However, to the authors' knowledge, no clinical data are currently available on this topic in the veterinary medical literature.

Frequent JPCs should be treated particularly if they trigger hemodynamic consequences (which were not observed in the dog of the present report),10,12 and sotalol could be a suitable treatment choice because of its ability to prevent automaticity or reentry. Moreover, an adverse effect on the AV block would have been unlikely because pseudoblock resulted from the concealed conduction secondary to the JPCs. However, because of financial constraints, the owners declined both treatment and follow-up evaluation of the dog.

To the author's knowledge, this is the first report of JPC-related concealed conduction in a client-owned dog; this condition had only been experimentally induced in this species previously.11 The case described in the present report has highlighted the fact that, in such a clinical setting, surface ECG findings can be equivocal or misleading regarding the presence of AV block.

References

  • 1. Santilli R, Moïse NS, Pariaut R, et al. Ventricular ectopic beats and rhythms. In: Santilli R, Moïse NS, Pariaut R, eds. Electrocardiography of the dog and cat: diagnosis of arrhythmias. 2nd ed. Milano, Italy: Edra, 2018;189202.

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  • 2. Sung RJ, Waxman HL, Saksena S, et al. Sequence of retrograde atrial activation in patients with dual atrioventricular nodal pathways. Circulation 1981;64:10591067.

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  • 3. Ari H, Doğanay K. Concealed conduction. In: Advances in electrocardiograms: methods and analysis. London: InTech Open Ltd, 2012. Available at: www.intechopen.com/books/advances-in-electrocardiograms-methods-and-analysis/concealedconduction. Accessed Mar 3, 2018.

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  • 4. Arι H, Arι S, Koca V, et al. Misleading ECG appearance of AV block due to concealed AV nodal conduction caused by interpolated ventricular ectopic beats. Türk Kardiyol Dern Arιs 2009;37:197200.

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  • 5. Rosen KM, Rahimtoola SH, Gunnar RM. Pseudo A-V block secondary to nonpropagated His bundle depolarization. Documentation by His bundle electrocardiography. Circulation 1970;42:367373.

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  • 6. Schamroth L, Surawicz B. Concealed interpolated A-V junctional extrasystoles and A-V junctional parasystole. Am J Cardiol 1971;27:703707.

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  • 7. Ameen A, Dharawat A, Khan A. His bundle extrasystoles revisited: the great electrocardiographic masquerader. Pace 2011;34:e56e59.

  • 8. Cannom DS, Gallagher JJ, Goldreyer BN. Concealed bundle of His extrasystoles simulating nonconducted atrial premature beats. Am Heart J 1972;83:777779.

    • Crossref
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    • Export Citation
  • 9. Fletcher E. Extrasystoles arising from the bundle of His. Br Heart J 1955;17:566568.

  • 10. Oh YZ, Tan VH, Wong KCK. Concealed conduction of premature ventricular complexes resulting in AV nodal block. J Arrhythm 2017;33:528529.

  • 11. Damato AN, Lau SH, Bobb G. Cardiac arrhythmias simulated by concealed bundle of His extrasystoles in the dog. Circ Res 1971;28:316322.

  • 12. Olgun H, Yokokawa M, Baman T, et al. The role of interpolation in PVC-induced cardiomyopathy. Heart Rhythm 2011;8:10461049.

  • Figure 1—

    Six-lead surface ECG recording obtained from a 4-month-old Great Pyrenees with no structural heart disease detected during echocardiographic examination. Paper speed = 25 mm/s; 1 cm = 1 mV.

  • Figure 2—

    Lead 11 ECG tracing obtained from the 6-lead surface ECG tracing in Figure 1 (A) with a corresponding ladder diagram (B) and a magnified area (outlined by red brackets) of the lead II ECG tracing (C). Notice the numerous junctional premature complexes (R') that have a fixed coupling interval of 240 milliseconds with the preceding sinus complexes (R). A fusion beat (F) is also present in the ECG tracing. In the highlighted detail of the ECG tracing, notice that when the R'P interval is 170 milliseconds, the following PR interval (blue bar) is 150 milliseconds, thereby generating pseudo-first-degree atrioventricular (AV) block (white arrow). When the R'P interval is 160 milliseconds, the P wave is not conducted, which results in pseudo–second-degree AV block (black arrow). In the ladder diagram, the upper zone represents atrial activation (A), the middle zone represents AV junction activation (AV), and the lower zone represents ventricular activation (V). The black dots represent the sites of impulse origin that can occur in the sinus node (within the atria [A]) or ectopically (in the AV junction). The black lines represent the direction of propagation of the impulses through the heart. Interruption of a line with a dash indicates that the impulse does not propagate further than that point. Two opposing dashes represent the point of collision of 2 opposite wave fronts as occurs in the case of a fusion beat. Paper speed = 25 mm/s; 1 cm = 1 mV.

  • 1. Santilli R, Moïse NS, Pariaut R, et al. Ventricular ectopic beats and rhythms. In: Santilli R, Moïse NS, Pariaut R, eds. Electrocardiography of the dog and cat: diagnosis of arrhythmias. 2nd ed. Milano, Italy: Edra, 2018;189202.

    • Search Google Scholar
    • Export Citation
  • 2. Sung RJ, Waxman HL, Saksena S, et al. Sequence of retrograde atrial activation in patients with dual atrioventricular nodal pathways. Circulation 1981;64:10591067.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3. Ari H, Doğanay K. Concealed conduction. In: Advances in electrocardiograms: methods and analysis. London: InTech Open Ltd, 2012. Available at: www.intechopen.com/books/advances-in-electrocardiograms-methods-and-analysis/concealedconduction. Accessed Mar 3, 2018.

    • Search Google Scholar
    • Export Citation
  • 4. Arι H, Arι S, Koca V, et al. Misleading ECG appearance of AV block due to concealed AV nodal conduction caused by interpolated ventricular ectopic beats. Türk Kardiyol Dern Arιs 2009;37:197200.

    • Search Google Scholar
    • Export Citation
  • 5. Rosen KM, Rahimtoola SH, Gunnar RM. Pseudo A-V block secondary to nonpropagated His bundle depolarization. Documentation by His bundle electrocardiography. Circulation 1970;42:367373.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6. Schamroth L, Surawicz B. Concealed interpolated A-V junctional extrasystoles and A-V junctional parasystole. Am J Cardiol 1971;27:703707.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 7. Ameen A, Dharawat A, Khan A. His bundle extrasystoles revisited: the great electrocardiographic masquerader. Pace 2011;34:e56e59.

  • 8. Cannom DS, Gallagher JJ, Goldreyer BN. Concealed bundle of His extrasystoles simulating nonconducted atrial premature beats. Am Heart J 1972;83:777779.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9. Fletcher E. Extrasystoles arising from the bundle of His. Br Heart J 1955;17:566568.

  • 10. Oh YZ, Tan VH, Wong KCK. Concealed conduction of premature ventricular complexes resulting in AV nodal block. J Arrhythm 2017;33:528529.

  • 11. Damato AN, Lau SH, Bobb G. Cardiac arrhythmias simulated by concealed bundle of His extrasystoles in the dog. Circ Res 1971;28:316322.

  • 12. Olgun H, Yokokawa M, Baman T, et al. The role of interpolation in PVC-induced cardiomyopathy. Heart Rhythm 2011;8:10461049.

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