Journal of the Louisiana State Medical Society
DIAGNOSIS: Sinus rhythm; high-grade second-degree atrioventricular block with a junctional escape rhythm and three capture complexes, each with right bundle branch block aberration; possible septal myocardial infarct of indeterminate age; ST-T and U wave changes suggesting hypokalemia. In most cases of second degree atrioventricular block, the longer the R-P interval, the more likely is the P wave to be conducted to the ventricles. Thus, in this ECG, the 2nd, 6th, 10th, and 12th Pwaves, with R-P intervals of 0.67 to 0.81 seconds, would be the most likely ones to be conducted; and the 4th and 8th P waves, with R-P intervals of 0.42 to 0.50 seconds would also have a good chance to be conducted. None of them is. Instead, the 3rd, 7th, and 13th P waves with much shorter R-P intervals, 0.19 to 0.23 seconds, are conducted. The 1st, 5th, 9th, and 11th P waves either occur simultaneouslywith a junction-initiatedQRS or 0.08 seconds after one and are not conducted. Thus, there appears to be only a short window of time soon after junction-initiated QRSs when atrioventricular conduction can occur. This seeming paradox needs explanation. In times past, supernormality often has been invoked to explain unexpected excitability or conductivity in cardiac tissue. Pick et al. defined the supernormal phase as, “…a short, early, and limited period of the cardiac cycle during which a stimulus elicits either a totally unexpected response, or one that is less abnormal than expected considering the state of recovery from the preceding impulse.” 1 They then stated that they had collected 18 cases demonstrating a supernormal phase of atrioventricular conduction in the preceding five years, in addition to 10 cases that they had described previously. 2-5 All were cases of advanced atrioven- tricular block, and the authors concluded, “…supernormal AV conduction is an abnormal phenomenon encountered only in the presence of AV block – in other words, onlywhen the absolute or relative refractory period of the conduction system is unduly prolongedmay a phase of supernormality become manifest.” 1 In reviewing the previous literature, including the 10 cases they reported earlier, 2-5 but none of those in their cur- rent publication, 1 they decided, “…that a supernormal phase was definitely in action in 32 instances… In 14 additional ones, alternative interpretation appears possible…” 1 Alter- native explanations included vagal effects, second-degree AV block with escape complexes, dual AV nodal pathways, and “…a prolonged rest period in the upper AV junction following retrograde conduction…” 1 Their final conclusion stated, “The application of the concept of supernormal con- duction in conjunction with that of concealed conduction and of unidirectional block permits a satisfactory interpre- tation of some otherwise inexplicable features of AV block encountered in clinical electrocardiography.” 1 Six years after the paper of Pick et al, 1 Moe and col- leagues wrote a seminal paper demonstrating that in most, if not all, published cases of supernormality of AV conduc- tion alternative mechanisms could explain the observed phenomena. 6 They divided the alternative mechanisms into
three main classes: “…(1) occult 2:1 A-V block in which an idioventricular beat ‘retracts’ an otherwise refractory barrier within the A-V node; (2) alternation between dissociated in- tranodal transmission pathways; and (3) ‘ventriculophasic’ (vagal) depression of nodal conductivity.” 6 Although there is now general agreement that true supernormal conduction does not occur in the AV node, su- pernormal conduction has been demonstrated in the bundle branch-Purkinje system of the dog. 7-8 Furthermore, Spear and Moore have demonstrated supernormal conduction in the distal bundle of His and proximal bundle branches, but they could not demonstrate it in the proximal portion of the bundle of His. 9 They ascribed this difference to the rela- tively low transmembrane resting potential of the proximal bundle of His, similar to that in the AV node, whereas the electrophysiological properties of the distal bundle of His are similar to those in bundle branches. 9 Supernormal AV conduction allowing transmission of P waves with R-P intervals of 0.19 to 0.23 s, but not those with shorter or longer R-P intervals, would explain the unexpected AV conduction in our patient. If, as described above, supernormality does not occur in the AV node, could supernormality in the distal His bundle following His-initiated complexes of the escape rhythm explain the unexpected conduction? In fact, Moe et al. theorize such a possibility in section F of their paper. 6 Whatever the fine points of the patient’s atrioventricu- lar block, it was successfully treated with an electronic AV sequential pacemaker that captured 100% of the time, usu- ally in the P-synchronous mode. None of the patient’s other ECGs before the pacemaker implantation suggested a septal myocardial infarct as the one in the Figure did. Furthermore, his echocardiogramshowed normal left ventricular function, but also showed moderate aortic stenosis. REFERENCES 1. Pick A, Langendorf R, Katz LN. The supernormal phase of atrioventricular conduction: I. Fundamental mechanisms. Circulation 1962;26:388-404. 2. Mack I, Langendorf R, Katz LN. The supernormal phase of recovery of conduction in the human heart. AmHeart J 1947;34:374- 389. 3. Pick A, Fishman AP. Observations in heart block. Supernormality of A-V and intraventricular conduction and ventricular parasystole under the influence of epinephrine. Acta Cardiol 1950;5:270-287. 4. Katz LN, Pick A. Clinical Electrocardiography I. The Arrhythmias. Philadelphia: Lea & Febiger;1956. 5. Langendorf R. Alternation of A-V conduction time. Am Heart J 1958;55:181-191. 6. Moe GK, Childers RW, Merideth J. An appraisal of ‘supernormal’ A-V conduction. Circulation 1968;38:5-28. 7. Arbel E, Sasyniuk BI, Moe GK. Supernormal ventricular conduction in dog heart (Abstract). Federation Proc 1971;30:553. 8. Spear JF, Moore EN. Supernormal excitability and conduction in the His-Purkinje system of the dog. Circ Res 1974;35:782-792. 9. Spear JF, Moore EN. Supernormal conduction in the canine bundle of His and proximal bundle branches. Am J Physiol 1980;238:H300-H306.
76 J La State Med Soc VOL 166 March/April 2014
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