EP ROUNDS
An Uncommon Manifestation of Atrioventricular Block: What is the Mechanism? REGINALD T. HO, M.D. From the Department of Medicine, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
electrophysiology – clinical, atrioventricular block, supernormality
Case Presentation A 55-year-old man with pulmonary sarcoidosis presented with fatigue, dyspnea, and the 12-lead electrocardiogram (EKG) shown in Figure 1. Echocardiography revealed normal left ventricular function and pulmonary artery systolic pressures. Because of the concern for cardiac sarcoidosis, he underwent insertion of a dualchamber implantable cardioverter defibrillator with a temporary pacing catheter positioned at the His bundle region for rate support during implantation. During sinus rhythm and AAI pacing at 90 beats/min, he was in 2:1 atrioventricular (AV) block (Fig. 2, top panel). When the pacing rate was increased to 100 beats/min, he developed a repetitive pattern of alternating bigeminal QRS complexes that was originally seen on his presented 12-lead EKG and which abruptly disappeared when the pacing rate was increased further to 110 beats/min (Fig. 2, bottom panels). The bigeminal pattern re-emerged when the rate was lowered to 100 beats/min before disappearing again when pacing was discontinued and sinus rhythm resumed. What is the mechanism for the curious pattern of alternating bigeminy? Discussion The 12-lead EKG (Fig. 1) shows sinus arrhythmia (75–112 beats/min) with predominant 3:2 and a single episode of 2:1 AV conduction. The first conducted QRS complex of each bigeminal cycle shows right bundle branch block (RBBB [rsR ])/left anterior fascicular block (LAFB) preceded by a prolonged PR interval (249–282 ms). The second conducted QRS complex demonstrates a different RBBB (rSr ) morphology with left posterior
No financial support was required. Address for reprints: Reginald T. Ho, M.D., Department of Medicine, Thomas Jefferson University Hospital, 925 Chestnut Street, Mezzanine Level, Philadelphia, PA 19107. Fax: 215503-3976; e-mail: [email protected] Received December 23, 2013; revised January 9, 2014; accepted January 21, 2014. doi: 10.1111/pace.12378
fascicular block (LPFB). Advanced AV block in the setting of RBBB and alternating hemiblock indicates block below the His bundle confirmed by His bundle recordings (Fig. 2). However, the PR and His-ventricular (HV) intervals preceding the second conducted QRS complex are paradoxically shorter than the first. One explanation for this paradoxical finding is that the second QRS complexes (RBBB/LPFB) are actually ventricular premature depolarizations arising from the left anterior fascicle (LAF) resulting in foreshortened PR and HV intervals. However, their dependence on a critical atrial pacing rate especially during underlying AV block argues against this possibility. Alternatively, it is possible that the first QRS complexes (RBBB/LAFB) of each cycle are ventricular escape complexes originating from the left posterior fascicle (LPF) giving the appearance of long HV intervals. By retrogradely concealing into the LAF escape complexes facilitate subsequent LAF conduction by “peeling back refractoriness” or Wedensky facilitation resulting in shorter (although abnormal) HV intervals with RBBB/LPFB morphology QRS complexes.1 However, the relatively constant PR intervals beginning each cycle despite variability in pause lengths also argues against this explanation. Classically, several mechanisms have been proposed to explain the phenomenon of unexpected or even better-than-expected conduction on an EKG and include resolution of phase 4 block, gap phenomenon, and supernormality.2 In a patient with underlying RBBB (due to conduction failure) and LPFB (due to conduction delay), phase 4 block in the LAF could cause marked HV prolongation and LAFB with the first conducted QRS complex that follows each pause. Resolution of phase 4 block with the second conducted QRS complex allows LAF conduction and resumption of underlying RBBB/LPFB. Phase 4 block alone, however, fails to explain the occurrence of AV block following the third P wave of each cycle. While the combination of both Mobitz 2type behavior and resolution of phase 4 block might explain these findings, their coexistence implies not only two different mechanisms of
©2014 Wiley Periodicals, Inc. 900
July 2014
PACE, Vol. 37
ATRIO-VENTRICULAR BLOCK AND ALTERNATING HEMIBLOCK
Figure 1. Twelve-lead electrocardiogram. A long cycled sinus P wave (*) falling at the end of the T wave fails to conduct to the ventricle and interrupts the 3:2 bigeminal cycle causing 2:1 atrioventricular conduction.
block but also two different sites of block in the left-sided conduction system and LAF. The leftsided blocked site during Mobitz 2 (truncal left bundle or combined LAF + LPF) must be proximal to the diseased segment of the LAF exhibiting phase 4 block in order to expose this segment to sufficiently long diastolic intervals allowing phase 4 LAFB at the start of each bigeminal cycle. In a patient with underlying RBBB and LAFB (due to phase 3 block), conduction delay proximal to the LAF could allow distal conduction over the LAF (gap) with development of RBBB/LPFB QRS complexes, provided that conduction is faster over the LAF than LPF. His bundle recordings, however, do not show visible delay proximal to the LAF prior to RBBB/LPFB (atrio-His intervals and His bundle durations are constant) and the gap phenomenon cannot account for the sizeable (70 ms) paradoxical shortening of PR and HV intervals. Left anterior fascicular conduction that occurs only after preceding QRS complexes and which is highly sensitive to the atrial rate suggests a temporal relationship between ventricular activation and subsequent P-wave conduction raising the possibility of supernormality. Such conduction only occurs when critically timed P waves fall within a narrow window of repolarization reaching the LAF during its supernormal period of recovery. During each bigeminal cycle, the first P wave conducts over a diseased LPF resulting
PACE, Vol. 37
in a prolonged HV interval and RBBB/LAFB. Retrograde conduction and penetration from LPF into the LAF allow for development of a supernormal phase during recovery of the LAF. The second P wave falls into this supernormal window conducting over the LAF with a shorter HV interval and RBBB/LPFB QRS complexes. Consecutive activation of the LAF causes a leftward shift in its repolarization period and therefore supernormal window, so that the third P wave falls beyond the period of supernormality and fails to conduct to the ventricle. The change in RBBB morphology when switching from LAFB to LPFB results from a difference in the initial site (LPF or LAF) of left ventricular activation. Two hundred and seventeen RP intervals (onset of QRS complex to atrial electrogram on His bundle catheter) during LAF conduction (n = 27) and block (n = 190) were examined during His bundle recordings. Every P wave with an RP interval between 292 ms and 348 ms conducted over the LAF (Fig. 3). Conversely, for all P waves falling before (AAI 110) or after (normal sinus rhythm and AAI 90), this 56-ms window failed to conduct over the LAF. While other mechanisms cannot be definitively excluded, the critical temporal relationship between preceding RBBB/LAFB QRS complexes and subsequent P wave conduction implicates supernormality as the most likely explanation for these observations.2–4 Three- totwo AV block with paradoxical shortening of
July 2014
901
HO
Figure 2. His bundle recordings during normal sinus rhythm and AAI pacing at 100 beats/min. Note that a single episode of far-field oversensing of ventricular activation on the atrial lead resets the atrial timer resulting in a long cycled fused (sinus rhythm/atrial pacing) P-wave complex (*) that fails to conduct to the ventricle and interrupts bigeminy.
the HV interval due to supernormality in the distal His bundle has been described but the unusual finding of alternating hemiblock was absent.5 RBBB with alternating LAFB/LPFB is an
902
uncommon form of bilateral bundle branch block with a high risk for AV block and has also been referred to as Rosenbaum’s syndrome.6,7 We report advanced AV block with underlying RBBB/LAFB
July 2014
PACE, Vol. 37
ATRIO-VENTRICULAR BLOCK AND ALTERNATING HEMIBLOCK
Figure 3. Graph of conducted and nonconducted RP intervals plotted against the preceding RR interval. Conduction only occurred during a critical 56 ms window of RP values (292–348 ms) and never occurred with RP values outside this range.
and intermittent supernormal conduction over the LAF causing HV interval shortening and
RBBB/LPFB as one mechanism of this rare syndrome.
References 1. Schamroth L, Friedberg HD. Wedensky facilitation and the Wedensky effect during high grade A-V block in the human heart. Am J Cardiol 1969; 23:893–899. 2. Fisch C, Knoebel SB. Electrocardiography of Clinical Arrhythmias. Armonk, NY, Futura Publishing Co. Inc., 2000, pp. 237–251. 3. Massumi RA, Amsterdam EZ, Mason DT. Phenomenon of supernormality in the human heart. Circulation 1972; XLVI:264–275. 4. Schamroth L. “Supernormal” phase in hemiblock conduction. J Electrocardiol 1989; 22:257–261.
PACE, Vol. 37
5. Ho RT, Rhim ES, Pavri BB, Greenspon, AJ. An unusual pattern of atrioventricular block. J Cardiovasc Electrophysiol 2007; 18:1000– 1002. 6. Rosenbaum MB, Elizari MV, Lazzari J, Nau GJ, Levi RJ, Halpern MS. Intraventricular trifascicular blocks. Review of the literature and classification. Am Heart J 1969; 78:306– 317. 7. Bayes de Luna A. In Memorium Dr. Mauricio Rosenbaum. Rev Esp Cardiol 2003; 56:1029–1030.
July 2014
903
An Uncommon Manifestation of Atrioventricular Block: What is the Mechanism? REGINALD T. HO, M.D. From the Department of Medicine, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania
electrophysiology – clinical, atrioventricular block, supernormality
Case Presentation A 55-year-old man with pulmonary sarcoidosis presented with fatigue, dyspnea, and the 12-lead electrocardiogram (EKG) shown in Figure 1. Echocardiography revealed normal left ventricular function and pulmonary artery systolic pressures. Because of the concern for cardiac sarcoidosis, he underwent insertion of a dualchamber implantable cardioverter defibrillator with a temporary pacing catheter positioned at the His bundle region for rate support during implantation. During sinus rhythm and AAI pacing at 90 beats/min, he was in 2:1 atrioventricular (AV) block (Fig. 2, top panel). When the pacing rate was increased to 100 beats/min, he developed a repetitive pattern of alternating bigeminal QRS complexes that was originally seen on his presented 12-lead EKG and which abruptly disappeared when the pacing rate was increased further to 110 beats/min (Fig. 2, bottom panels). The bigeminal pattern re-emerged when the rate was lowered to 100 beats/min before disappearing again when pacing was discontinued and sinus rhythm resumed. What is the mechanism for the curious pattern of alternating bigeminy? Discussion The 12-lead EKG (Fig. 1) shows sinus arrhythmia (75–112 beats/min) with predominant 3:2 and a single episode of 2:1 AV conduction. The first conducted QRS complex of each bigeminal cycle shows right bundle branch block (RBBB [rsR ])/left anterior fascicular block (LAFB) preceded by a prolonged PR interval (249–282 ms). The second conducted QRS complex demonstrates a different RBBB (rSr ) morphology with left posterior
No financial support was required. Address for reprints: Reginald T. Ho, M.D., Department of Medicine, Thomas Jefferson University Hospital, 925 Chestnut Street, Mezzanine Level, Philadelphia, PA 19107. Fax: 215503-3976; e-mail: [email protected] Received December 23, 2013; revised January 9, 2014; accepted January 21, 2014. doi: 10.1111/pace.12378
fascicular block (LPFB). Advanced AV block in the setting of RBBB and alternating hemiblock indicates block below the His bundle confirmed by His bundle recordings (Fig. 2). However, the PR and His-ventricular (HV) intervals preceding the second conducted QRS complex are paradoxically shorter than the first. One explanation for this paradoxical finding is that the second QRS complexes (RBBB/LPFB) are actually ventricular premature depolarizations arising from the left anterior fascicle (LAF) resulting in foreshortened PR and HV intervals. However, their dependence on a critical atrial pacing rate especially during underlying AV block argues against this possibility. Alternatively, it is possible that the first QRS complexes (RBBB/LAFB) of each cycle are ventricular escape complexes originating from the left posterior fascicle (LPF) giving the appearance of long HV intervals. By retrogradely concealing into the LAF escape complexes facilitate subsequent LAF conduction by “peeling back refractoriness” or Wedensky facilitation resulting in shorter (although abnormal) HV intervals with RBBB/LPFB morphology QRS complexes.1 However, the relatively constant PR intervals beginning each cycle despite variability in pause lengths also argues against this explanation. Classically, several mechanisms have been proposed to explain the phenomenon of unexpected or even better-than-expected conduction on an EKG and include resolution of phase 4 block, gap phenomenon, and supernormality.2 In a patient with underlying RBBB (due to conduction failure) and LPFB (due to conduction delay), phase 4 block in the LAF could cause marked HV prolongation and LAFB with the first conducted QRS complex that follows each pause. Resolution of phase 4 block with the second conducted QRS complex allows LAF conduction and resumption of underlying RBBB/LPFB. Phase 4 block alone, however, fails to explain the occurrence of AV block following the third P wave of each cycle. While the combination of both Mobitz 2type behavior and resolution of phase 4 block might explain these findings, their coexistence implies not only two different mechanisms of
©2014 Wiley Periodicals, Inc. 900
July 2014
PACE, Vol. 37
ATRIO-VENTRICULAR BLOCK AND ALTERNATING HEMIBLOCK
Figure 1. Twelve-lead electrocardiogram. A long cycled sinus P wave (*) falling at the end of the T wave fails to conduct to the ventricle and interrupts the 3:2 bigeminal cycle causing 2:1 atrioventricular conduction.
block but also two different sites of block in the left-sided conduction system and LAF. The leftsided blocked site during Mobitz 2 (truncal left bundle or combined LAF + LPF) must be proximal to the diseased segment of the LAF exhibiting phase 4 block in order to expose this segment to sufficiently long diastolic intervals allowing phase 4 LAFB at the start of each bigeminal cycle. In a patient with underlying RBBB and LAFB (due to phase 3 block), conduction delay proximal to the LAF could allow distal conduction over the LAF (gap) with development of RBBB/LPFB QRS complexes, provided that conduction is faster over the LAF than LPF. His bundle recordings, however, do not show visible delay proximal to the LAF prior to RBBB/LPFB (atrio-His intervals and His bundle durations are constant) and the gap phenomenon cannot account for the sizeable (70 ms) paradoxical shortening of PR and HV intervals. Left anterior fascicular conduction that occurs only after preceding QRS complexes and which is highly sensitive to the atrial rate suggests a temporal relationship between ventricular activation and subsequent P-wave conduction raising the possibility of supernormality. Such conduction only occurs when critically timed P waves fall within a narrow window of repolarization reaching the LAF during its supernormal period of recovery. During each bigeminal cycle, the first P wave conducts over a diseased LPF resulting
PACE, Vol. 37
in a prolonged HV interval and RBBB/LAFB. Retrograde conduction and penetration from LPF into the LAF allow for development of a supernormal phase during recovery of the LAF. The second P wave falls into this supernormal window conducting over the LAF with a shorter HV interval and RBBB/LPFB QRS complexes. Consecutive activation of the LAF causes a leftward shift in its repolarization period and therefore supernormal window, so that the third P wave falls beyond the period of supernormality and fails to conduct to the ventricle. The change in RBBB morphology when switching from LAFB to LPFB results from a difference in the initial site (LPF or LAF) of left ventricular activation. Two hundred and seventeen RP intervals (onset of QRS complex to atrial electrogram on His bundle catheter) during LAF conduction (n = 27) and block (n = 190) were examined during His bundle recordings. Every P wave with an RP interval between 292 ms and 348 ms conducted over the LAF (Fig. 3). Conversely, for all P waves falling before (AAI 110) or after (normal sinus rhythm and AAI 90), this 56-ms window failed to conduct over the LAF. While other mechanisms cannot be definitively excluded, the critical temporal relationship between preceding RBBB/LAFB QRS complexes and subsequent P wave conduction implicates supernormality as the most likely explanation for these observations.2–4 Three- totwo AV block with paradoxical shortening of
July 2014
901
HO
Figure 2. His bundle recordings during normal sinus rhythm and AAI pacing at 100 beats/min. Note that a single episode of far-field oversensing of ventricular activation on the atrial lead resets the atrial timer resulting in a long cycled fused (sinus rhythm/atrial pacing) P-wave complex (*) that fails to conduct to the ventricle and interrupts bigeminy.
the HV interval due to supernormality in the distal His bundle has been described but the unusual finding of alternating hemiblock was absent.5 RBBB with alternating LAFB/LPFB is an
902
uncommon form of bilateral bundle branch block with a high risk for AV block and has also been referred to as Rosenbaum’s syndrome.6,7 We report advanced AV block with underlying RBBB/LAFB
July 2014
PACE, Vol. 37
ATRIO-VENTRICULAR BLOCK AND ALTERNATING HEMIBLOCK
Figure 3. Graph of conducted and nonconducted RP intervals plotted against the preceding RR interval. Conduction only occurred during a critical 56 ms window of RP values (292–348 ms) and never occurred with RP values outside this range.
and intermittent supernormal conduction over the LAF causing HV interval shortening and
RBBB/LPFB as one mechanism of this rare syndrome.
References 1. Schamroth L, Friedberg HD. Wedensky facilitation and the Wedensky effect during high grade A-V block in the human heart. Am J Cardiol 1969; 23:893–899. 2. Fisch C, Knoebel SB. Electrocardiography of Clinical Arrhythmias. Armonk, NY, Futura Publishing Co. Inc., 2000, pp. 237–251. 3. Massumi RA, Amsterdam EZ, Mason DT. Phenomenon of supernormality in the human heart. Circulation 1972; XLVI:264–275. 4. Schamroth L. “Supernormal” phase in hemiblock conduction. J Electrocardiol 1989; 22:257–261.
PACE, Vol. 37
5. Ho RT, Rhim ES, Pavri BB, Greenspon, AJ. An unusual pattern of atrioventricular block. J Cardiovasc Electrophysiol 2007; 18:1000– 1002. 6. Rosenbaum MB, Elizari MV, Lazzari J, Nau GJ, Levi RJ, Halpern MS. Intraventricular trifascicular blocks. Review of the literature and classification. Am Heart J 1969; 78:306– 317. 7. Bayes de Luna A. In Memorium Dr. Mauricio Rosenbaum. Rev Esp Cardiol 2003; 56:1029–1030.
July 2014
903
