Clinical Significance of Exit Block* IRANY M. DE AZEVEDO, M.D. YOSHIO WATANABE, M.D. LEONARD S. DREIFUS, M.D. From the Departments of Medicine, Physiology, and Biophysics, Hahnemann Medical College, Philadelphia, Pennsylvania The confinement of an ectopic discharge to its ily reserved for ectopic pacemakers, rather than the focus, and its consequent inability to invade the ad­ sinus node. jacent myocardium when falling outside of the re­ High Grade Atrioventricular Block. In vi rtu­ fractory period of the heart, is a well established ally all instances, exit block occurs in the presence phenomenon called "exit block." This cardiac ar­ of higher degrees of A-V block. Failure of the rhythmia was originally described by Kaufmann impulse to propagate from the subsidiary ectopic and Rothberger ( 8) to explain the failure of a para­ focus to either the ventricles, atria, or both is char­ systolic focus to activate the heart. All pacemakers acteristic of this form of exit block. Several exam­ are subject to exit block (9, 13, 15, 14, 11 , 16, 12, ples are illustrated. In figure 2, there is high grade 3, 10, 7, 1), however, by convention, conduction A-V block causing A-V dissociation. The atria are disturbances involving the sinus node (S-A block) under the control of the sinus node at a rate of 71 are usually excluded from this concept, and the term per minute, and the ventricles are controlled by a is reserved for ectopic pacemakers. Recent electro­ subsidiary ectopic pacemaker, probably originating physiological and clinical studies have shown that in the right bundle branch at a rate of approximately exit block may complicate reentrant arrhythmias and 40 per minute. In A VL the two first R-R intervals may be of either Wenckebach type I or II (2, 5). measure 1.46 seconds after the third QRS complex; a Several examples of exit block will be discussed in an long pause of 2.74 seconds occurs, probably due to attempt to show some different clinical aspects of this exit block from the Purkinje pacemaker, and only mechanism. atriai activation is seen. In another instance of Sinoatrial Block. In figure 1, precordial leads, high-grade A-V block (fig. 3), sinus tachycardia is Vl-3 demonstrate a sinus rhythm at a rate of ap­ present at a rate of 105 per minute. There is com­ proximately 65 per minute. The P-R interval mea­ plete A-V dissociation, and the ventricles are un­ sures 0.13 seconds, and the QRS complexes are of der the control of a sub-junctional pacemaker, prob­ normal contour and duration. The first P-P inter­ ably located in the right bundle branch system, at val in V3 measures 0.84 seconds, the second, 1.76 a rate approximately of 25 per minute. In aVF and seconds, the third P-P, 0.96 seconds, the fourth, Vl, the R-R intervals are almost the same with a 0.86 seconds, and the fifth P-P interval, 1.84 seconds. duration of 2.40 seconds. However, the first R-R in­ This is an instance of sinoatrial block. The terval in V2 is 2.64 seconds and suggests concealed second and fifth P-P intervals are longer than twice conduction to the level of the subsidiary pacemaker. the shorter P-P intervals. Such variation of P-P in­ The second R-R interval in V2 measures 5.72 sec­ tervals could be explained by Wenckebach conduc­ onds and is longer than twice the R-R intervals in tion from the sinus node to the atria. Sinoatrial block aVF and Vt, probably due to a combination of is a true form of exit block, but the term is ordinar- both exit block and concealed conduction as is shown In * Presented by Dr. Dreifus at the Symposium on by the diagram. figure 4, three records are taken Cardiac Arrhythmias, June 8, 1972, at Virginia Beach, from a patient with an inferior myocardial infarc­ Virginia. tion. A high degree of A-V block is present. The 26 M CV QUARTERLY 9(1): 26-32, 1973 DE AZEVEDO ET AL: CLINICAL SIGNIFICANCE OF EXIT BLOCK 27 -'--......--'-"-· • -'--------'---- · ~_'.__. _. ~_:·------"---'---"---'---'--:-,-c- , --,­ ·--'-'----~ - ,-- ~ I -'--·_: . .....J--"--~--......;__---:----'---"---o-_;,---,---, i '/' ~-.·ptr~'·'.".;b~4 i ~iii• i.. •...~'1 1%1~rrr~r . 1 I I :· I · ttt :I. : I ' -----;, SINUS NODE A v Fig. 1- Sinoatrial block. Three sinus beats are seen followed by a pause. The fourth P wave is not visible as 4: 3 block occurs between the sinus node and atria as seen in diagram. at ria are under the control of the sinus node at a the subsidiary pacemaker can be easily identified rate of 91 per minute, and the ventricles are under if the block occurs with a precise conduction ratio a control of a junctional pacemaker at a rate of 66 as seen in figure 5. In this example, there is atrial pe r minute. In B, longer pauses are observed, but fibrillation, and the ventricles are under the con­ the atrial rate is decreased to 83 per minute, and trol of a junctional pacemaker at a rate of 64 per the ventricular rate is 63 per minute. In C, the atrial minute. In V2, the R-R intervals become more pro­ rate is 97 per minute, and the ventricular rate is 32 · longed and measure 1.83 seconds, almost twice per minute. The R-R intervals measure 1.87 that seen in I and II, and are probably due to exit seconds, .twice the R-R interval seen in B, probably block from the subsidiary junctional pacemaker. An­ due to exit block of the junctional pacemaker. other example of this problem is illustrated in figure In ?1any instances, high-grade A-V block may 6. Junctional tachycardia is present at a rate of be associated with atrial fib rillation. Exit block from approximately 125 per minute. The atrial activity is 28 DE AZEVEDO ET AL: CLINICAL SIGNIFICANCE OF EXIT BLOCK LL3 ):\li __J __ TI l · J_ __ :__ LL~.J-! -l L L--~ ·, f __[ J -!-l-t- L-~--,\- i-f -J_J-++-t-+++- : I . I ' : : ,. I : I I i ' I _l,~ -;.-:. _: t~i- r- -i--H-+-r---;~ 1 ~-:·.·.-.._. [~ +~, -:,~_11-_-_:_:l- ··i,-~.·. ;:~-:-- -..-. :... -.·.. :,-·_··. :·····- :,/,21.·_-__: -~_;_ -~.._i_ _ . J -'~~I . i : i I , •• : • : I - \ <- '·-,: ,r +T ~- !-~-¥-~--+- - 1 i , , : 1 t I i i 1 !. 1 1 i _i + i .0 .. L~.t- - _·t_ aVL.v"t--;--i -. -i··- - - :···--J·· , . : . ; - : I I : I 1 ·r- i l T 1·r1 L I J I : :--::rtJ-t :--:J : :~ 1~1 : :~ :;; ;~; :=l~1~~;~fli~ ITT[- -lt-rr+--,- t ·1-1- , - ; -:·-- i ·+. i- r --,- r t '. - -- i--n-I l-rt f -- _!_ A \ \ \ \ \ \ \ \\ A-V p v Fig. 2- High-grade A-V block is present. In strip AVL a long pause is seen, indicating exit block from a subsidiary Purkinje pace­ maker, probably located in the right bundle branch system. hardly seen, and atrial fibrillation is present. After the ent types of QRS complexes are seen in the upper third, ninth, and twelfth QRS complexes, long record, the first one upright and the other one pauses of 0.86 seconds, sometimes twice the R-R predominantly downward. There are two indepen­ interval of the basic rhythm, are present. These dent rhythms, sinus rhythm and ventricular para­ pauses are engendered by exit block from the sub­ systole, as shown in the diagram. The parasystolic sidiary junctional pacemaker. In sharp contrast to pacemaker has a cycle length of 580 per msec, and these last two examples, an irregular ventricular the variation of R-R intervals during the parasystole response in the presence of atrial fibrillation can is due to Wenckebach phenomenon in the trans­ cause some diagnostic confusion unless Wencke­ mission of the impulse from the ectopic parasystolic bach conduction from the subsidiary pacemaker is pacemaker to the ventricles. When exit block of the considered. In figure 7, atrial fibrillation is present. parasystolic pacemaker occurs with a ratio of 4 : 3, The ventricles are under the control of a junctional sinus impulses are transmitted to the ventricles. pacemaker at a rate of approximately 136 per min­ The coupling intervals show slight variation beats in ute. The R-R intervals as seen by the diagram vary 2, 6, 10, 14, 18. slightly. After the seventh QRS complex, right carotid Among the clinical examples, figure 1 was pre­ sinus pressure was applied, and the R-R intervals sented to demonstrate a similar mechanism be­ became prolonged from 0.44-0.46 to 0.88 seconds, tween sinoatrial block and exit block. Hence, only twice the R-R intervals seen previously. This is due the failure of an ectopic pacemaker to activate ad­ to exit block from a subsidiary junctional pace­ jacent myocardium is considered exit block. How­ maker. At the end of record B, 2 : 1, 3: 2, and 4: 3 ever, other authors admit that exit block can occur ratios are seen. This variation of R-R intervals in in all excitable tissues of the heart with pacemaker B is caused by Wenckebach conduction from the or latent pacemaker properties ( 2, 6) . Thus, if the subsidiary junctional pacemaker. latter assumption is accepted, it would be very hard Ventricular Parasystole. In figure 8, two differ- to differentiate sinoatrial block from exit block of DE AZEVEDO ET AL: CLINICAL SIGNIFICANCE OF EXIT BLOCK 29 3 V1 ~-1--'----'---'----'--'-~--i-.,--'--'--'- : i -+--'-+-~tt+---'---+--c'--'--+--'-----'-:...C.,.--"-'~ "'--'lf-±'--i-~~~!-~t,~t.,Lt.-.-~ t+. ~-f j TJ_~I~-------_._.,..._~~ ~..,-+-- V2 I I I \ A I).