Br J: first published as 10.1136/hrt.32.2.172 on 1 March 1970. Downloaded from British Heart gournal, I970, 32, I72. Atrial parasystole

H. David Friedberg and Leo Schamroth From the Cardiac Section, Veterans Administration Center, Wood (Milwaukee), Wisconsin; Marquette School of Medicine, Milwaukee, Wisconsin, U.S.A.; and the University of the Witwatersrand,Johannesburg, South Africa

Two cases of atrial parasystole showing the various manifestations of the are presented. Analysis of the underlying mechanisms shows that atrial parasystolic with 'reversed' coupling is a form of escape-capture begeminy, sinus escapes being followed by an ectopic capture of the atria. Reasons are given for the rarity of atrial fusion beats. The similarities and differences between atrial and ventricular parasystole are explored. It is suggested that an atrial parasystolic pacemaker may lie within a major atrial preferential conducting pathway, and may consist of a congenitally ectopic fragment of sinus nodal tissue. The clinical significance of the arrhythmia is discussed; the associated diseases apparently represent a cross-section of medical ward experience.

Parasystole is a dual rhythm in which the parasystolic focus is in the atria, and thus in parasystolic pacemaker is protected from the the same bi-atrial chamber' as the sinus effects of the dominant, usually faster pace- pacemaker, the resulting disturbance of maker: this protection, which is the essence rhythm is modified, though the essential of the arrhythmia, is situated within the characteristics of the arrhythmia are not immediate vicinity of the parasystolic focus, changed. In particular, coupling intervals and is operative throughout its entire cycle vary less in atrial parasystole: a fixed bigeminy, (Schamroth, I964). When a parasystolic closely resembling an extrasystolic bigeminy ventricular and a sinus pacemaker coexist, with nearly constant coupling, may occur. http://heart.bmj.com/ a characteristic arrhythmia appears. The Atrial fusion beats are rare. The manifesta- ventricular beats bear no constant relation tions of atrial parasystole will be presented to the preceding QRS complexes (i.e. the in this paper and the underlying mechanisms coupling intervals vary) and indeed may well analysed. occur so late in diastole that they fuse with the ensuing sinus QRS: the ectopic beats do, Case reports however, bear a consistent relation to each Case x The electrocardiogram (Fig. i, a con- other, so that the interectopic intervals are tinuous recording of standard lead II) was ob- on September 25, 2021 by guest. Protected copyright. in simple multiples of the basic ectopic cycle tained from a 75-year-old man with severe length. In those rare instances in which the obstructive lung disease. The arrhythmia was 1 Electrophysiologically, both atria usually behave as a single chamber, and will be referred to as the bi-atrial Received 8 April I969. chamber.

FIG . I Case I. A continuous strip of standard lead II. Premature atrial beats, marked with a black dot, occur with varying coupling. The interectopic intervals are simple multiples of I20-124, and thus the rhythm is atrial parasystole. |1...... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.

< 248_ 2x124 > < 123 > < 369- 3 x 123 > < 120 > <

124 > < 120 >< 248 2x 124 > < 120 >< 246: 2 x 123 > Br Heart J: first published as 10.1136/hrt.32.2.172 on 1 March 1970. Downloaded from Atrial parasystole 173

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~....N.

...... ~~~..~ ~~~~

F ... j- > -0% .,Al,. U'rnM;rn.@A-----

FIG. 2 Case x. Strips of standard lead III and aVF. Atrial premature beats occur in bigeminy. The second last P wave in standard lead III arises from AVjunctional focus. The last P wave in this lead occurs with a different coupling interval, but maintains a constant interectopic interval, thus revealing the rhythm to be atrial parasystolic bigeminy with reversed coupling. (See text for discussion.)

present during two weeks only, when a small an extrasystolic bigeminy with a regular dominant dose of digitalis was being given. The basic rhythm and fixed coupling intervals, the inter- is punctuated by atrial premature ectopic intervals will also be constant. This is not beats. The ectopic P' waves resemble the sinus parasystole, but due simply to the constancy of P waves, but begin more sharply, and rise more the post-extrasystolic pause and the constant steeply to a higher peak. The premature P' coupling intervals. In Fig. 2, the true situation is waves are not accurately coupled to their pre- revealed by the last pair of P waves in standard ceding sinus P waves, the coupling intervals lead III. The first P wave of this pair is inverted, varying from 421 to 70. The intervals between arising from an AV junctional focus, which anti- the P' waves - the interectopic intervals - are all cipates the sinus impulse. The second P wave in simple multiples of I20 to 124, thereby of this pair arises from the same atrial focus as indicating that these beats are related to each the earlier ones, but now the coupling is con- other. This combination of inconstant coupling spicuously different - 54 instead of 32 to 40. and simply related interectopic intervals is the The interectopic interval, however, is constant http://heart.bmj.com/ hallmark of parasystole. at i i6. It is clear that the atrial focus was not Fig. 2 was recorded two days later, and con- discharged by the AV junctional impulse, and sists of strips of standard lead III and lead aVF. is parasystolic. It is noteworthy that the para- A bigeminy is present, every second beat being systolic focus is discharged neither by the sinus an atrial premature beat. As the coupling inter- impulse nor by the lower junctional impulse, and vals are nearly constant, this closely resembles is thus protected from below as well as from the commoner extrasystolic atrial bigeminy. In above. 1 All time intervals are expressed in hundredths of a The electrocardiogram in Fig. 3 is part of the on September 25, 2021 by guest. Protected copyright. second, i.e., 22 = 0.22 second. same long tracing of standard lead II shown in

FIG. 3 Case I. Part of the same tracing as Fig. i. The sixth P wave in the upper strip, marked F, is an atrialfusion beat. (See text.) P' in the lower strip is interpolated (see Fig. 7). F

* * 124 o 124 0 124 0 es----24-- - -33723x 124 ) -- > - 120--

Pi P P2 P3

0 0* 124 0 Br Heart J: first published as 10.1136/hrt.32.2.172 on 1 March 1970. Downloaded from 174 Friedbergl-4and Schamrothw

II -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~t-I*~--

0 115 0 120 * 1 0 116 118 0

hi A:::~...... :1

7 l W ------4~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

118 a 117 117 0

F IG.- 4 Case 2. Strips of standard leads II and III. Ectopic P waves with varying coupling but constant interectopic intervals occur, showing atrial parasystole. Some of the parasystolic P waves are not conducted to the ventricles, some are conducted with aberrant ventricular conduction, and some are conducted normally.

i. The Fig. parasystolic cycle ranges from 120 to had suffered a fracture of the femur. All strips 12.4 (mean 23). The first, third, and eighth P waves were recorded at the same session; the bottom are parasystolic in origin. The beginning of two strips are continuous. P waves from many these P waves rises abruptly, forming a sharp different supraventricular foci are present. Some angle with the baseline. The beginning of the of these P waves (marked with a dot) closely sixth P wave, marked F, is smoothly rounded, as resemble the parasystolic P' waves shown in is that of the sinus P waves. However, this P Fig. 4. wave is a little taller than the sinus P waves, and In the three upper strips, the interectopic has a slightly steeper descending slope. It is thus intervals are a simple multiple of 98 (range 94 to intermediate in form between the sinus and the ioo). In the bottom strip, the interectopic http://heart.bmj.com/ ectopic P waves. Furthermore, this P wave begin intervals are I45, 148, and i44. These are not a o-o6 sec. before the ectopic P' wave is due (as simple multiple of 98. All the figures, however, indicated by the half open circle). Therefore, this are simple multiples Of 49 ± i. An atrial para- P wave results from a fusion between the atrial is therefore present, the basic cycle length parasystolic and the sinus impulses. The second of which is 49 ± i, and which is complicated by P' wave in the lower strip is interpolated. an exit block. If every second impulse reaches the atrial muscle to inscribe a P' wave, the apparent Case 2 The electrocardiogram (Fig. 4) was cycle length is 98 2, and 2:'i exit block is recorded a of mental during period confusion present. If every third impulse reaches the atria on September 25, 2021 by guest. Protected copyright. from a 74-year-old man with a history of ischae- and is manifest, the apparent cycle length is mic heart disease. The arrhythmia disappeared 147 ± 3, and 3: i exit block is present. with rest and sedation, but recurred temporarily after he broke his femur some months later. At Mechanisms no time did he have or take digitalis. For the most part, sinus P waves and ectopic P' (a) Parasystolic bigemiiny: 'reversed' waves alternate in an atrial bigeminy with con- coupling. In atrial parasystole, both the spicuous variation of the coupling intervals, protected atrial pacemaker and the unpro- which range from 36 to 6o. The interectopic tected sinus pacemaker are situated in the intervals range from 115 to i2o; the intersinus same bi-atrial chamber. As a the intervals range from 11I4 to 30. In standard lead result, sinus node is III, a supraventricular premature beat from a vulnerable to, and will fre- different focus (arrow) interrupts the sinus quently be prematurely discharged by., the rhythm, but does not affect the regularity of the ectopic impulse. The sinus cycle will be P' series. Some of the P' waves fall so early as consequently reset,, while the parasystolic to fail to the AV others are pass junction, con- cycle continues undisturbed. This is illus- ducted with varying degrees of ventricular aber- trated in Fig. 6., in which the 'build-up' of ration, and others are conducted normally. the sinus impulse toward a threshold is pic- Though the ectopic cycle is a little variable, the tured above a representation of events at the interectopic intervals vary much less than do atrial level. In I of the coupling intervals; and all the hallmarks of Diagram Fig. 6, the para- atrial parasystole are present. systolic impulse Ei reaches the sinus node The electrocardiogram in Fig. 5 was recorded and aborts the immature impulse there at from Case 2 four months later, shortly after he point Sdi., interrupting and resetting the Br Heart J: first published as 10.1136/hrt.32.2.172 on 1 March 1970. Downloaded from Atrial parasystolk 175

VF

* * 99 * 9 9 0 ______----_------_---- _-- 297

9;*0O * 94 O * I198-98-- 196 --

*98 100 - 100 *------294-

o 145 * 148 * 146

FIG. 5 Case 2. Strips of lead aVF and standard lead II. The bottom two strips are continuous. P' waves are indicated by a black dot. The interectopic intervals are at first in multiples of 98 ± 2, and change abruptly to 147 ± 3, indicating atrial parasystole with varying exit-block. (See text for discussion.) http://heart.bmj.com/ sinus cycle. The recharge of the sinus im- occurs if the ventricular impulse is com- pulse now begins anew, which on reaching pletely conducted retrogradely through the threshold, discharges spontaneously at point AV junction, atria, and SA junction to dis- S2, after the expiry of the sinus cycle (sc). charge the sinus node. Such complete retro-

Now, if the duration of the ectopic cycle (ec) grade conduction is the exception. The rela- on September 25, 2021 by guest. Protected copyright. is a little greater than the sinus cycle plus tively slowly conducting AV junction will the refractory period of the sinus pacemaker usually protect the sinus node and effectively (rp), the next ectopic impulse (E2) will again prevent the maintenance of a bigeminy. discharge the sinus pacemaker (at point Sd2) Atrial parasystolic bigeminy thus depends and the sinus cycle is again reset. In this upon a fortuitous arithmetical relation be- manner, the sinus discharge is causally tween the rates of the two centres of impulse related and linked to the protected ectopic formation, of which one is protected and the discharge, and the bigeminy is perpetuated other is vulnerable. Once begun, the bige- (see also Fig. 8B). This is a form of escape- miny tends to persist. The bigeminy may be capture bigeminy (Bradley and Marriott, interrupted and the true nature of the I958; Schamroth and Dubb, I965), but one arrhythmia revealed under the following in which a sinus escape is followed by an circumstances. (i) If there is sufficient sinus ectopic capture of the atria. arrhythmia to make the ectopic impulse so The time of appearance of the sinus P early that it falls during the absolute refrac- waves is thus dependent on, and coupled to, tory period of the atria induced by a sinus the parasystolic P' waves. This is the reverse impulse (see also Interpolation below); (2) if of the situation in an extrasystolic arrhythmia, a different ectopic impulse discharges the in which the ectopic complexes are dependent sinus node and further dislocates its rhythm on, and coupled to, the dominant beats. A (see Fig. 2); (3) if the parasystolic impulse similar 'reversed' coupling may occasionally suffers exit-block and fails to leave its focus appear during ventricular parasystole. This and invade the atria. Br Heart J: first published as 10.1136/hrt.32.2.172 on 1 March 1970. Downloaded from 176 Friedberg and Schamroth

'a- - -sc ...... I ----- sc- - ... . Sdl _~~~= L~~~~.1 Sd2 Sd3

I.

El E2 - ec------. ..

FIG. 6 I. A diagram to show the mechanism of atrial parasystolic bigeminy. The upper saw-tooth line represents the 'build-up' of the sinus impulse towards threshold. A = atrial and are sinus are level; SI, S2, S3 impulses; Ei, E2, and E3 ectopic impulses; http://heart.bmj.com/ rp = the refractory period of the sinus node; sc = the duration of the sinus cycle; ec = the ectopic cycle. (See text.) II. Interpolation of an atrial premature beat. The conventions are the same as in I. Ectopic impulse E2 invades the atria before the end of the refractory period of the sinus node, so that the sinus cycle is not reset. S3 then occurs when anticipated, and an is sandwiched between two sinus beats. Note: The AVjunctional and ventricular levels have not been drawn. Conduction delay across the sino-atrial junction is shown in Fig. 7. on September 25, 2021 by guest. Protected copyright.

F IG. 7 A diagram to show the characteristic allorhythmia of an interpolated atrial premature beat. PI, P', P2, and P3 refer to the P waves so marked in the lower strip of Fig. 3 (see text). S = sinus node; S-A = sino-atrial junction; A = atrial level. AVjunctional and ventricular levels have not been drawn.

Si S2 S3 S 7 I I

S-A

A I

P1 P' P2 P3 Br Heart J: first published as 10.1136/hrt.32.2.172 on 1 March 1970. Downloaded from Atrial parasystole rL77

(b) Interpolation If the ectopic impulse conspicuously different; and because the falls early in the sinus cycle, at a time when relatively slowly conducting AV junction the atria have recovered excitability but when effectively isolates the two rhythms, coupling the sinus node is still refractory, the ectopic intervals vary greatly, and end-diastolic impulse will fail to reach and reset the sinus ectopic beats are common. pacemaker. The spontaneous discharge of Several factors militate against the occur- the sinus node then occurs when anticipated. rence and the recognition of atrial fusion Interpolation of the ectopic impulse between beats during atrial parasystole. two sinus impulses results. This is illustrated i) Lack of P wave detail The P wave is less in Diagram II of Fig. 6, where impulse detailed, less complex, and coarser than the E2 is interpolated between S2 and S3. This QRS complex. Subtle changes are therefore phenomenon is evident electrocardiographic- more difficult to detect. Furthermore, the P ally in the lower strip of Fig. 3. Incomplete waves from the sinus and the atrial pace- penetration of the S-A junction (the junc- maker are not usually conspicuously different, tional delay area between the S-A node and so that the recognition of fusion by changes the surrounding atrial myocardium) by the in shape alone is very difficult. Nor are atrial ectopic impulse may render it partially repolarization waves - Ta waves - sufficiently refractory to the next sinus impulse. This distinct to help. In Fig. 3, atrial fusion is produces a characteristic disturbance of PP postulated because the P wave concerned has intervals (Langendorf et al., I962): the inter- an initial vector typical of the sinus P waves val between a pair of sinus P waves enclosing in that lead, and is of intermediate height an interpolated atrial ectopic beat is longer and terminal slope, and because the timing than the intersinus interval. The subsequent is correct. PP interval is foreshortened. This is illus- trated in Fig. 7, a diagram of the conduction 2) Limited duration of opportunity for fusion sequence of the interpolated parasystolic In the sixth beat of Fig. 3, probable atrial impulse in the lower strip of Fig. 3. Im- fusion is recognized when the ectopic P' pulses SI, S2, and S3 are regular consecutive wave was due oo6 sec. after the onset of the sinus impulses resulting in P waves PI, P2, sinus P wave. Yet, in the ninth beat of the and P3, respectively. P' is the parasystolic upper strip of Fig. i, fusion did not occur impulse which penetrates the S-A junction though the ectopic P' wave was due o-o8 sec. http://heart.bmj.com/ but fails to reach and reset the sinus pace- after the start of the P wave. This suggests maker. This renders the junction partially that within o-o8 sec., the sinus impulse has refractory. Conduction of S2 through the SA spread to the tissues surrounding the atrial junction is therefore prolonged, and the parasystolic focus, i.e. this is the conduction inscription of P2 is delayed. But for the time from the sinus to the ectopic focus. effect of P', P2 would have been inscribed Therefore, in this case, if the sinus fires first, as indicated by the dotted line. The next the time available for possible fusion is not greater than sinus impulse S3 - P3 is not so delayed. P2 o0o7 sec. Should the ectopic on September 25, 2021 by guest. Protected copyright. is thus shifted towards P3, producing the focus discharge first, a similar limit for the characteristic allorhythmia of interpolation. duration of opportunity for fusion exists. A strictly analogous variation of RR inter- While this is not known, it should also be vals occurs with an interpolated ventricular about o0o7 sec., but may well be less. In parasystolic or extrasystolic beat (Schamroth, both these cases, and in many of the others (i967b). reported, the sinus and parasystolic P waves are similar, with some differences in their (c) Atrial fusion Ventricular fusion beats initial contour, but with little difference in have certain characteristics which lead to their terminal contour. Thus, once the easy recognition (Marriott, Schwartz, and ectopic impulse has inscribed the initial Bix, I962). Fusion is suspected if an ectopic part of the P wave, the contribution of the QRS is due near the time of a sinus QRS, sinus impulse to the shape of this wave may and an intermediate QRS is seen. Changes not be recognizable. Moreover, the time of in QRS configuration are easily detected appearance of the more variable sinus im- because of the finely inscribed, detailed, and pulse may not be known. Though a sinus complex QRS deflections. Thus, very small impulse may fuse in the atria with an earlier degrees of fusion may be deduced from subtle appearing ectopic impulse, this fusion may changes in the direction of the initial or well not be diagnosable. terminal QRS vectors, or in the . 3) Resetting of sinus node As shown above, Detection is further facilitated by the fact the presence of two centres of impulse forma- that the two complexes concerned are usually tion in the same bi-atrial chamber facilitates Br Heart J: first published as 10.1136/hrt.32.2.172 on 1 March 1970. Downloaded from 178 Friedberg and Schamroth

A

B Sl...,...,...... - ... ~~ ~ ~ ~ ~ ...... I... C f D~~ff~

B ...... I......

......

FIG. 8 Relating the possibility of atrial fusion to the resetting of the sinus cycle. Impulse formation in the S-A node (S) and the ectopic focus (e) is shown by the black dots. http://heart.bmj.com/ The refractorytt...lperiod of the sinus node is represented by the stippled area. The sinus impulse does not penetrate into the parasystolic focus. The rates of the two pacemakers are assumed to be moderately steady. (A) The parasystolic cycle is a littk longer than the sinus cycle; occasional parasystolic beats appear. (B) The parasystolic cycle exceeds the swn of the sinus cycle and the sinus refractory period; parasystolic escape-capture bigeminy results. (C) The parasyitolic cycle equals two sinus cycles less the last coupling interval; atrialfusion results (markedf). (D) The parasystolic cycle equals the sinus cycle; repetitive fusion results. on September 25, 2021 by guest. Protected copyright.

the resetting of the unprotected sinus node. escape-capture bigeminy - of the atria The frequency of this resetting and the will ensue. resulting arrhythmic disturbance is depen- c) If the parasystolic cycle is shorter than dent on the relation of the two cycle lengths. the sinus cycle (not illustrated), the ectopic This is diagrammatically illustrated in Fig. 8. impulses anticipate every sinus discharge This theoretical analysis is based on the and thus usurp complete control of the assumption that the two rates remain moder- atria, resulting in an uncomplicated ectopic ately steady. atrial rhythm - an atrial parasystolic a) If the parasystolic cycle exceeds the . sinus cycle but is less than the sinus cycle The aforementioned relation with the re- plus the refractory period of the sinus setting of the node (Diagram A of there will be sinus cycle militates against the Fig. 8), occurrence of atrial fusion occasional 'captures' of the atria by the complexes. These will only occur under the following rare ectopic impulse. circumstances. b) If the parasystolic cycle exceeds the sum of the sinus cycle plus the ensuing a) If the parasystolic cycle equals two, or any refractory period of the sinus node (Dia- integral number of sinus cycles, less the last gram B of Fig. 8), parasystolic bigeminy - coupling interval, both impulses will simul- Br Heart J: first published as 10.1136/hrt.32.2.172 on 1 March 1970. Downloaded from Atrial parasystole 179 taneously invade the atria and fuse (Diagram Thus, in Fig. 4, an abrupt increase in the C of Fig. 8). common denominator from 98 tO I47 (i.e., b) If the parasystolic cycle equals the sinus il times 98) alows the diagnosis of a basic cycle, or any integral number of sinus cycles, ectopic cycle length of 49 with 2:I exit- fusion will recur regularly (Diagram D of block changing abruptly to 3: i exit-block. Fig. 8). The aforementioned principles may be Discussion modified under the following special circum- Incidence Well-documented cases of atrial stances: (a) in the presence of sinus arrhyth- parasystole are rare. In a search of the pub- mia; (b) in the presence of exit-block; (c) in lished material, Eliakim (I965) could find the presence of a variable ectopic cycle; and only I5 reported instances andaddedone more. (d) if the sinus cycle is reset by another - To these may be added that of Attinger different - ectopic impulse. (I940), two of Holzmann (I960), two of Fusion between atrial parasystolic and Moulopoulos and Sideris (I967), and Desh- sinus impulses then is rare because (i) it is pande (I968). The two reported here make difficult to recognize, (2) the coupling inter- a total of 24 cases. This is, then, a very much vals vary little, (3) there is only a short period rarer arrhythmia than either ventricular or of opportunity for fusion, and (4) because junctional parasystole. Exit-block has been the mathematical relations between a pro- present in the cases of Katz et al. (1937), tected and an unprotected centre of impulse Holzmann (I96o), Deshpande (I968), and formation in the same chamber militate in one of the two in this paper, a total of four against fusion except in unusual circum- cases. Though figures for the frequency of stances. Scherf, Yildiz and De Armas (I959) exit-block in other forms of parasystole are commented that such fusion is rare. The not available, exit-block does not appear to only clear cases are those of Vedoya (I944, be less common in atrial parasystole. Case 5) and of Katz, Eschelbacher, and Strauss (I937). Clinical significance Most cases of atrial parasystole have been reported as curiosities, (d) Exit-block The parasystolic discharge and a review of well-substantiated cases does will activate the surrounding myocardium not reveal any particular clinical association. whenever the myocardium is not refractory It has been stressed that parasystole is a http://heart.bmj.com/ after activation by the other pacemaker. sign of a diseased heart and is associated Occasionally, however, calculation reveals with digitalis therapy (Faltitschek and Scherf, that, though the myocardium is responsive 1932; Scherf et al., 1959). On the other hand, at the time of the parasystolic discharge, Jervell (1932), Vedoya (I944), and Eliakim activation does not occur. This has been (i965) reported cases of atrial parasystole explained on the basis of exit-block - a without heart disease. Certainly, patients phenomenon in which an impulse is con- without heart disease are much less likely fined to its focus of generation by a 'block' to have long electrocardiographic strips taken on September 25, 2021 by guest. Protected copyright. at the ectopic-myocardial junction. Exit- and have atrial parasystole diagnosed. The block is also present if the calculated ectopic association of parasystole and heart disease cycle length is less than the cycle of the must be regarded as not definitely proven. dominant pacemaker, or if it is less than the Digitalis is often incriminated, but in no duration of any post-ectopic pause; for reported case is the appearance and regres- without exit-block an ectopic tachycardia sion of the arrhythmia so clearly related to would be present. the drug as in Case i above. Case 2 exempli- There is a further circumstance in which fies the apparent provocation of the arrhyth- exit-block may be diagnosed. Should a con- mia by an acute psychotic episode and by an stant 2: I exit-block change abruptly to one acute injury, in the absence of digitalis and with a 3: I ratio, the apparent common not necessarily related to ischaemic symp- denominator of the interectopic intervals toms. will change to one and a half times itself. Similarly, a change from a 3: I to a 4: I Site of the parasystolic pacemaker Of conduction ratio will result in an increase of the reported cases in which the parasystolic one-third in the apparent ectopic cycle focus is clearly atrial (as opposed to junc- length. That is to say, if the largest common tional, coronary sinus, or atrio-nodal), the denominator in a continuous recording P' waves of many bear a remarkable resem- changes abruptly to one with which it has a blance to one another. These include those simple fractional relation, exit-block with a reported by Jervell (I932), Attinger (I940), change in conduction ratio may be diagnosed. Scherf et al. (I959), Chung, Walsh, and Br Heart J: first published as 10.1136/hrt.32.2.172 on 1 March 1970. Downloaded from 18o Friedberg and Schamroth

Massie (i964, Cases I, 3, and 4), Moulo- Deshpande, S. Y. (I968). Atrial parasystole with exit poulos and Sideris and Deshpande block and post-parasystolic P and T wave changes. (i967), Diseases of the Chest, 54, I62. (I968), and the two reported here. In all Eliakim, M. (I965). Atrial parasystole. Effect of caro- these cases the P' wave arises more abruptly tid sinus stimulation, Valsalva maneuver and to a higher peak than the sinus P wave, while exercise. American journal of Cardiology, I6, 457. the later portions of the two waves are simi- Faltitschek, F., and Scherf, D. (I932). Klinischer Beitrag zur Parasystoliefrage. Wiener Archiv fur lar. Further, the P-R and the P'-R intervals innere Medizin, 23, 269. are equal or nearly so. Holzmann, M. (I960). Beitrag zur Kenntnis der Now, there is considerable evidence that Vorhof-Parasystolie. Cardiologia (Basel), 36, 223. intra-atrial conduction and internodal conduc- James, T. N. (I966). Anatomy of the conduction sys- tem of the heart. In The Heart, p. 64I. Ed. by tion occur via definite pathways, and are not J. W. Hurst and R. B. Logue. McGraw-Hill, radial, as was formerly thought (James, New York. I966; Merideth and Titus, I968). It seems Jervell, A. (1932). Ein Fall von Vorhofparasystolie. possible that the atrial parasystolic focus Acta Medica Scandinavica, 79, 239. Katz, L. N., Eschelbacher, J. L., and Strauss, S. lies within a major atrial conducting path- (I937). An unusual case of auricular parasystole way, and is close to the sinus node. The showing 'exit' block. American Heart journal, 14, 'family resemblance' and the rarity of atrial 571I. parasystole suggest that in these patients Langendorf, R., Lesser, M. E., Plotkin, P., and Levin, B. D. (I962). Atrial parasystole with interpolation; the focus is a portion of sinus tissue a little observations on prolonged sinoatrial conduction. distance from the main node itself. This American Heart Journal, 63, 649. 'ectopic sinus tissue' may be an embryonal Marriott, H. J. L., Schwartz, N. L., and Bix, H. H. rest. Interestingly, even the sinus node itself (I962). Ventricular fusion beats. Circulation, 26, 88o. may be parasystolically protected (Scham- Merideth, J., and Titus, J. L. (I968). The anatomic roth, I967a). If an atrial parasystole does atrial connections between sinus and A-V node. reflect a congenital anatomical variant, then Circulation, 37, 566. the clinical features of this condition would Moulopoulos, S. D., and Sideris, D. A. (I967). Time relation between two pacemakers in atrial para- reflect a cross-section of hospital practice, systole. British Heart journal, 29, 758. which does seem to be the case. Schamroth, L. (I964). The definition of parasystole. Cardiologia (Basel), 44, 37. References - (I967a). Sinus parasystole. American Journal of Cardiology, 20, 434. Attinger, E. (I940). Zur Pathologie des Vorhof- - (I967b). Interpolated extrasystoles. South Afri- http://heart.bmj.com/ rhythmus und der P. Zacke. Schweizerische medi- can MedicalJournal, 41, 919. zinische Wochenschrift, 70, 782. -, and Dubb, A. (I965). Escape-capture bigeminy. Bradley, S. M., and Marriott, H. J. L. (I958). Escape- Mechanisms in S-A block, A-V block, and re- capture bigeminy. Report of a case of A-V dis- versed reciprocal rhythm. British Heart Journal, sociation initiated by 2:i S-A block with result- 27, 667. ing bigeminal rhythm. American Journal of Scherf, D., Yildiz, M., and De Armas, D. (1959). Cardiology, I, 640. Atrial parasystole. American Heart Journal, 57, Chung, Koo-Young, Walsh, T. J., and Massie, E. 507. (I964). Atrial parasystole. American Journal of Vedoya, R. (I944). Parasystolia. A. L6pez, Buenos

Cardiology, 14, 255. Aires. on September 25, 2021 by guest. Protected copyright.