An experimental study of concealed conduction

Experimental andlaboratoryreports

An experimental Gordon K. Moe, M.D., J. A. A bildskov, M.D. C. Mendez, M.D. Utica, N. Y.

T

study Ph.D.*

he phenomenon of cardiac excitation penetrating conducting tissue but failing to traverse it completely was called ‘Lconcealed conduction” by Langendorf in 1948.’ Earlier investigators had repeatedly obtained evidence of the phenomenon (see Langendorf’ and Langendorf and Pick2 for references), but the first clear demonstration was that of Lewis and Master,3 who showed that the conduction time of propagated beats during 2 :l A-V block was longer than that during 1 :l transmission at half the atria1 frequency. The most extensive studies of incompletely propagated beats have been achieved by painstaking and elegant analysis of clinical electrocardiograms.1,2 Penetration of impulses which do not emerge from the A-V node can be inferred from their influence on subsequent events, namely, delay of conduction of a succeeding propagated response, block of a succeeding atria1 impulse which occurs at a time when the transmission system should have been excitable, delay of the espected discharge of an A-V nodal pacemaker, or, in some cases, facilitation or acceleration of a succeeding impulse. Recent direct studies in animal preparations by Hoffman and associates4 and by Scher and associate? suggest that the weak point in the transmission line from atrium From

the State Supported Received *.iddress

338

@f conedad

to ventricle lies in the atria1 margin of the node itself. Single-unit action potentials recorded at various levels in the node in vitro appear to show decremental conduction and failure proximal to the bundle of His, although records from open dog hearts in situ also show occasional evidence of failure of transmission at more periphera1 sites in the specialized conduction system.4 The studies by Scher and associates5 emphasize the atrionodal junction as the point at which delay and failure of transmission occur with either forward or retrograde excitation. Alanis and associates6 concluded, on the other hand, that the weakest link occurs at the junction between the node and the bundle of His, and that only at higher frequencies of stimulation does atrionodal failure occur. In the studies reported here we have undertaken to describe in detail the temporal relations between atria1 and ventricular events as influenced by incomplete penetration of the A-V node from both directions, and to define some of the circumstances under which concealed conduction can be demonstrated. Methods Mongrel dogs which weighed from 10 to 20 kilograms were anesthetized with sodium pentobarbital. The heart was exposed

Masonic Medical Research Laboratory, Utica, N.Y.. and the Departments of Physiology and University of New York, Upstate Medical Center, Syracuse, N.Y. by grants from the Life Insurance Medical Research Fund and the American Heart Association. for publication April 15. 1963. correspondence to Gordon Ii. Mue, M.D., Masonic Medical Research Laboratory, Utica 2, N.Y.

Medicine.

Experimental

through a mid-sternal incision under artificial respiration. Stimulating and recording electrodes were attached to the right atrium and to the surface of the right and/or left ventricle. In some animals the sinoatrial node was clamped to permit observations at slow frequencies. The heart was driven at various frequencies, and complex trains of test stimuli were delivered from an impulse generator which permitted the application to atrium or ventricle of a series of 6 to 16 driving stimuli at a fixed frequency, followed by 2 to IO test stimuli with a variety of temporal patterns. The vagus nerves were sectioned, and in most animals the stellate and upper thoracic sympathetic ganglia were removed either at the outset or after prior control observations had been made. Electrical activity was monitored on a cathode-ray oscilloscope, and photographed from a parallel oscilloscope or recorded on an ink-writing polygraph. Results

A. Demonstration oj concealedconduction. As in the clinical studies, three criteria were available for the identification of beats concealed in the A-V transmission system: conduction delay of a subsequent propagated response, complete interruption of a subsequent response, and displacement of an A-V nodal pacemaker (Figs. 1 and 2). The three segments of Fig. l,A illustrate that the propagation of the Aa response to the ventricle was delayed by an interpolated AZ. In the upper segment, the earliest possible A2 caused a delay of 60 msec. in the propagation of the subsequent response, Aa. The response labeled I was the last of a series of driven beats at a basic cycle length of 280 msec., close to the maximum 1:l driving frequency. The response labeled 3 was propagated in the absence of, and 3’ in the presence of, the interpolated concealed beat, 2. In the middle segment, constructed from the same experiment, the AP response was placed 35 msec. later. The subsequent third-order beat was delayed by 85 msec.; the conduction intervals with and without the concealed response are indicated as 3 and 3’. The zone during which AZ could be concealed after the last driven beat ex-

study of comealed condwtion

339

tended from the end of the atria1 refractory period, estimated at 170 msec., until 250 msec., a range of 80 msec. In the lower segment of Fig. l,A, drawn similarly from a different experiment, the position of Aa was fixed at 285 msec. after the primary atria1 beat, and Al was applied at several positions, two of which are illustrated. Propagation of Aa in the absence of A2 is indicated by the earliest ventricular response (broken linel; the early position of At, 150 msec. after Ar, caused a delay of 45 msec., and the later position, 16.5msec. after Al, caused a delay of 85 msec. in the propagation of the thirdorder beat (3 and 3’, respectively). The zone during which AZ could be concealed in this experiment extended from the limit of the atria1 refractory period, 145 msec., to 212 msec. after the primary response, a range of 67 msec. Complete failure of transmission of a third-order response after a concealed AP is illustrated in Fig. 1,B. In the upper half of the figure the last basic response is followed by a premature AP, and in turn by Aa. Neither of these responses reached the ventricle. In the lower half, A3 was delivered in the same position, but AZ was deleted; A3 was propagated to the ventricle with a conduction time of 155 msec., the same as the primary response. The third evidence of extinction of an impulse within the A-V transmission system, displacement of an A-V nodal pacemaker, is illustrated in Fig. 2. In this experiment the S-A node was destroyed, and a very stable A-V nodal pacemaker assumed control. When the atrium was driven at a frequency slightly higher than the spontaneous rate, cessation of the regular driving stimuli was followed by a spontaneous discharge occurring at the approximate time expected, assuming that the pacemaker was discharged and reset by the last propagated response. The interval which followed the last driven response was scanned at intervals of 10 to 20 msec. with a premature test response (upper segment, Fig. 2). After each test stimulus, time was allowed for the appearance of a spontaneous beat from the A-V pacemaker. The responses labeled a’ through e’ represent the spontaneous discharges after the last driven beat, a, and

340

Moe, Abildskov,

and Mmdez

the corresponding premature responses, b through e. The premature responses b and c failed to reach the ventricle, but both of them reached, discharged, and reset

111e nodal pacemaker. In the lower segmerit of the figure, two successive concealed beats are diagrammed. The earl)- premature b was followed by C, which, in turn, failed

Fig. 1. Evidence for concealment in three experiments. Part A: Diagrams of transmission from atrium (aabooe) to ventricle (below) in experiment of Jan. 23, 1960 (upper two segments) and of May 26, 1959 (Zmuer segment). Abscissae, time in milliseconds. In upper two segments, 1 indicates last of a series of driven beats; 2 indicates nonpropagated premature atria1 response; 3 indicates the A-V conduction time when 2 was absent; and 3’ represents A-V conduction time when 2 was present, Lower segment, 2 and 2’ indicate two different temporal positions of premature nonpropagated atrial response; A-V propagation of 3 in the absence of 2 is indicated by the broken line, and A-V propagation of 3 in the presence of 2 or Z’ is indicated by the solid lines 3 and 3’. Part B: Electrograms of right atrium (above) and right ventricle. In upper segment, atria1 response 2 prevents propagation of 3. In lower segment, 3 propagates to ventricle in absence of 2.

0

200

400

600

800

1000

Fig. 2. Displacement of nodal pacemaker. Experiment of Feb. 7, 1956. Yart A: Atria1 responses above; ventricular responses below. “Level” of the nodal pacemaker is indicated by the intermediate horizontal line. Propagation of of the last of a series of driven responses is indicated by a; spontaneous discharge of the nodal pacemaker in the absence of premature atria1 excitation is indicated by a’. Interval a-a’ (measured at the horizontal line which indicates the level of the pacemaker) corresponds closely with free-running frequency of A-V pacemaker. Premature atria1 response at 2, fails to traverse node, but discharges pacemaker; next spontaneous firing at b’. When premature response was delayed to c, d, or e, spontaneous nodal discharge was correspondingly delayed to G’, d’, e’. Part B: a, fast driven response; a’ corresponding nodal discharge. Premature response b is followed by nodal firing at b’. When b and c were delivered in sequence, neither impulse traversed the node, but G also discharged the pacemaker, as indicated by shift to c’.

Experimental

to reach the ventricle. Yet each of these beats resulted in discharge of the nodal pacemaker, as indicated by the time of occurrence of the spontaneous beats b’ (when b was applied alone) and c’ (when b and c were both present). Moving the third-order response later while continuing b in the same position yielded the propagated response, d. It is clear that under the conditions of this experiment the early premature responses must have penetrated at least to the depth of the pacemaker and were, therefore, concealed, and, furthermore, that the refractory period of the pacemaker itself was less than the effective refractory period of some more distal element of the A-V transmission system. In most experiments the A-V nodal discharges which followed cessation of a period of rhythmic atria1 or ventricular stimulation were too erratic to permit systematic study. As a general rule, then, the diagnosis of concealed conduction was made on the basis of conduction delay or block of an immediately succeeding beat. B. Efects of sympathetic denervation. In animals prepared by anesthesia and vagal section, but not subjected to sympathectomy, the earliest possible premature atria1 response was usually propagated to the ventricles, and, on occasion, even a third-order impulse carried through. At low driving rates the earliest AZ was invariably propagated, and demonstration of concealment was possible only when the transmission system was taxed by the introduction of two or more successive premature beats, or by an increase in the driving rate to the maximum frequency permitting 1:l conduction. Even under these conditions the range of time during which a premature atria1 response failed to traverse the node was very narrow. In a11 experiments, however, it could be shown that the nonpropagated responses during 2:l A-V block were indeed concealed, i.e., they caused delay of the succeeding propagated responses. Because concealment could not be easily induced in the presence of the sympathetic nerve supply, the stellate and upper thoracic ganglia were routinely removed. After sympathectomy, the maximum 1:l frequency was reduced, the atria1 refrac-

study of concealed conduction

341

tory period was correspondingly increased at the slower frequency, but concealment was regularly demonstrable. C. Influence cf frequency. The influence of frequency upon the duration of the zone of concealment after sympathectomy is illustrated in Fig. 3. At the slowest frequency studied (cycle length = 380 msec.) the earliest premature AI? was propagated to the ventricle. At higher frequencies, concealment occurred and the period or zone during which failure of nodal transit was demonstrable widened progressively

260-

lM220

1111,,,,,,,,,, 243 260 28 300 320 340 AI AI CYCLE LENGTH

3wmsE!c,

Fig. 3. Zone of concealment related to basic cycle length when atrium was driven at various frequencies between maximum 1~1 frequency (basic cycle, AIAI = 260 msec.) and the highest frequency at which no zone of concealment was demonstrable (AIAI = 380). Experiment of May 5, 1959; thoracic sympathectomy. Earliest A 2 indicates atria1 functional RP (earliest possible premature atria1 response, AtAz on ordinate scale, at the corresponding basic cycle lengths). A 2 propagated indicates earliest premature response propagated to the ventricle. Min. VI Vz indicates the functional RP of the A-V transmission system (minimal obtainable interval between two ventricular responses both propagated from the atrium). AIVI indicates A-V conduction time for basic responses at the corresponding driving frequencies. Vertical lines labeled PI and Pe indicate the earliest possible atria1 response (lower points) and the earliest propagated atria1 responses following premature propagated atria1 responses initiated at an AIA~ interval of 222 msec. in a basic cycie of 340 (PI) and at 228 msec. in a basic cvcle of 260 (Pz).

Table I. Range of concealment ajicr u concealed brai; cealed As Experiment March

April

May,

May

May

May

23, 1959

29, 1959

5, 1959

8, 1959

11, 1959

25, 1959

B&z

ryde

d ,A 2

i

iv@wn~r

Enrly A ,A 3

q/’ t:pmpoyn/

piac~~l~lc~v~t

&tle =1,:I :

qf

f&z il@

270

155 162 172 182 185

292 289 309 331 326

305 307 317 339 315

13 18 8 8 19

250

153 160 177 187

266 282 302 321

317 306 313

51 21 11

260

140 1.51 168 188 205

230 250 272 302 322

325 326 323 334 351

95 76 5f 32 35

275

I56 170 187

277 303 327

306 310

29 7

162 172 188 223

278 292 336 385

332 333

183 190 206

323 338 362

335

250

280

All times are expressed as milliseconds. AtA2: Time of initiation concealed &. Late A&: Latest possible concealed AZ. Where was zero. Table I is continued on page 343.

as the basic driving rate was increased. In Fig. 3 the zone of concealment is indicated as the shaded area bounded below by the minimum AiAz interval (the atria1 refractory period), which defines the earliest possible input to the A-V node, and above by the earliest AZ which was capable of transmission to the ventricle. The zone widened from zero (no concealment) at the longest basic cycle length to 61 msec. at the shortest cycle at which 1:l A-V transmission could be maintained. A still wider zone was evident after a premature but propagated AZ (ALA2 interval 228 msec.) introduced into a basic cycle of 260 msec. As evidence that concealment is associated with some depression of nodal conductivity, the A-V propagation time of the basic responses is plotted on

of concealed “propagated”

cofl-

Propagated

Propagated 54 41 Propagated Propagated 11 Propagated Propagated AZ. Early AnA3: Earliest appears, no concealment

possible A3 response was possible, i.e., the

after range

the same time scale. At the slowest frequency, the AlVr interval was 149 msec. At the highest regular driving frequency the conduction interval was increased to 180 msec. Widening of the zone of concealment at higher driving frequencies was due in part to abbreviation of the atria1 refractory period, from 164 msec. at a cycle length of 380, to 132 msec. after the premature cycle of 228. This would imply that the refractory period (RP) at the atria1 boundary of the node remains as short as or shorter than the atria1 RP at all frequencies. The increase in the A,Az interval for propagated responses at shorter cycle lengths, delineating the ultimate temporal boundary of the zone of concealment, is clearly not the result of an increase in the

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Experimental

over-all RP of the system, for the A-V nodal functional RP was systematically diminished at the shorter cycle lengths (upper curve, min. V,V,, of Fig. 3). In 3 experiments an attempt was made to determine the degree of cumulative effect of frequency. The zone of concealment was measured at a basic frequency just under the maximum 1 :l, under conditions which permitted a series of 10 to 12 regular driving pulses before each test stimulus. The duration of the zone of concealment was recorded, after which the high-frequency driving pulses were replaced stepwise by pulses delivered at longer intervals. It was found that only two successive short cycles were necessary to maintain the zone of con-

343

study of concealed conduction

cealment. Some evidence of the lack of major cumulative effect can be seen at the left of Fig. 3. The two vertical lines, PI and Pz, indicate the duration of the zone of concealment which followed propagated premature atria1 responses introduced in basic cycles of 340 msec. (PI) and 260 msec. (P2). The ranges, 78 and 82 msec., respectively, are not significantly different. If a cumulative or “fatigue” effect were important, the range for Pz should have exceeded that for PI. D. Repetitive concealed beats. In many experiments the concealment of a premaby ture response, At, was demonstrated failure of propagation of a subsequent response, As. Under these circumstances it

Table I-Cont’d Experiment

Basic cycle

A,Az

Early ALA3

Late

A,Aa

Range

May

26, 1959

270

150 164 180 203

256 283 308 333

273 286 322 362

17 3 14 29

June

10, 1959

250

147 187

258 321

277 373

19 52

June

16, 1959

260

168 190

289 324

329

5

Propagated

June

26, 1959

250

132 142 177

217 245 280

266 277 320

49 32 40

July

7, 1959

240

147 157 163 175

257 280 278 ,295

274 281 289 296

17 1 11 1

Aug.

6, 19.59

270

155 170 175 19.5 208

293 300 300 333 346

297 308 320 347 366

4 8 20 14 20

Aug.

10, 1959

270

160 200

275 327

359

13.5 181

240 302

313

11

171 189 206 227 238

290 31.5 341 376 392

314 330 360 419 435

24 1.5 19 43 43

Aug.

Jan.

13, 1959

23, 1960

280

Propagated 32

Propagated

could be shown that the third-order response was also concealed. It seems logical to assume that a late concealed A2 response should penetrate the transmission system farther than an early concealed beat, and that the later beat should, therefore, be followed by a longer refractory wake. It follows that the late concealed response should provide a greater opportunity for serial concealment of the thirdorder response, i.e., it should be followed by a longer zone of concealment. This hypothesis, tested in the 15 experiments listed in Table I, was not uniformly supported by the results. The experiments recorded in Table I were performed at least 30 minutes after thoracic sympathectomy. The atrium was driven at close to the maximum 1:l A-V limiting frequency. The A-V recovery period after the last of a series of driving stimuli was studied by scanning at intervals of 5 to 10 msec. with a test stimulus. The earliest AZ response (i.e., the shortest possible ArAz interval) was recorded, and it was ascertained that the A2 response entered the node as a concealed impulse. The latest nonpropagated AZ was recorded in order to define the limit of the zone of concealment, and a series of early propagated responses was recorded in order to permit estimation of the shortest possible VrV2 interval. The results of such scans are illustrated in Figs. 4 and 5 as the curves labeled A. After the preliminary scan in each experiment, AZ was placed as early as possible after the last driving pulse. The recovery phase of the node which followed the nonpropagated early AP was similarly scanned at intervals with a test stimulus in order to determine the minimum VIV3 interval. This process was repeated with At as late as possible without propagation, and, when the AZ zone of concealment was sufficiently long, with AZ in one or more intermediate positions. The curves labeled B, C, D, E, and F in Fig. 4 illustrate the increments in the minimum V1V3 interval resulting from the several interpolations of a concealed ‘42. In this experiment the minimum VrVz inreterval (i.e., the effective functional fractory period of that element of the conduction pathway having the longest RP) was 250 msec., and the primary zone

of concealment, indicated by the horizontal bracket, was nearly 70 msec. The earliest possible Xp, initiated at the end of the atria1 RP, was concealed, and the minimal ViV, interval incorporating the concealed AZ was 360 msec. (curve B), an increase of 110 msec. over the ViVz interval. The concealed AZ was, in turn, followed by a secondary period of 95 msec. (horizontal bracket of curve B) during which propagation failed. Successively later application of AZ, represented by curves C and D, did not further prolong the minimum VrV, interval, which suggests that the obstructing element in the conduction pathway was not progressively displaced in time by the concealed AZ. The zone of concealment which followed ‘42 was narrowed by the later positions of A2, but only at the proximate limit defined by the end of the atria1 RP itself. As ‘42 was still further delayed, however, the minimum VrVg interval was increased stepwise to 374 msec. (curve E) and 387 msec. (curve F), and the ultimate boundary of the secondary zone of concealment was displaced to the right. The duration of the secondary zone of concealment was reduced to about 35 msec. for the scans of curves E and F. In the experiment of May 5, 1959, as in 5 others listed in Table I, an early concealed AZ provided a wider range for concealment of Aa than did later positions. In 4 of these experiments the earliest Aa after a late concealed A2 was invariably propagated, i.e., there was no secondary zone of concealment at all. In 7 additional experiments, however, the latest position of Aa provided a slightly greater opportunity for extinction of an Aa response, and in 2 (experiments of March 23 and June 26, 1959) the duration of the secondary zone of concealment was not greatly influenced by the position of Az. In 3 experiments, concealment of Aa was impossible when AS was early, but a narrow zone existed when AP was as late as possible. The experiment of Jan. 23, 1960, illustrated in Fig. 5, was one in which the latest concealed AZ was followed by a longer zone of concealment (curve C) than the earliest AZ (curve B). The primary zone of concealment (curve A) extended from the end of the primary atria1 refractory period, at 170 msec., to about 250 msec.,

V011wle Number

67 3

Experimental

345

study of concealed coladzlction

300 A, 260 1

,

1 I

1

I

140

I

I

180

Ir/‘

220

I

I

I

260

I

300

I

I

I

I

340

I

380

i

420

I

1

460

Fig. 4. Experiment of May 5, 1959. L’, Vt and VI V, intervals (ordinates) plotted against AIA2 and ALAs intervals. Curve A: Scan of basic cycle; zone of concealment is indicated by horizontal bracket. Curves B, C, D, E, and F: Scans after AZ responses delivered at progressively later times within the primary zone of concealment. Position of A2 for curves B-F is indicated by the vertical arrows. Diagonal broken line indicates equality of atria1 and ventricular intervals.

460-

1 a2-c

1w

Jw

44u,,I

4w-

A'

360 - '+ A: V,V, v.s psi, B: VA!3 *.&A3

//

32rJ-

c: v,vs = &A3 D: yv4 s &II4 A. 260260-

II

, I

180 Fig.

,+d

,

,

,

,

,

,

,

,

,

A,”

I

220

5. Experiment

260 of Jan.

300 23, lY60.

a range of 80 msec. The minimum VIVZ interval was 275 msec. When Az was placed early, the minimum VIV, interval was 344 msec., and the secondary zone of concealment was 24 msec. When AZ was concealed as late as possible (curve C). the minimum VrV3 interval was increased to 465 msec., and the zone of concealment was widened

340

380

Conventions

420 as in Fig.

464 4. Explanation

500 in text.

to 43 msec. Curve D represents the results of a scan with Ad when At and AS were both concealed as early as possible. The earliest A4 was propagated, and the minimum VIV4 interval, 441 msec., was briefer than VrV3 when A:! was late. In many experiments, including those illustrated in Figs. 4 and 5, serial extinc-

346

dloe, Abildskov,

und Men&z

tion of three or more beats was possible. The experiments of Figs. 4 and 5 were chosen for illustration of this phenomenon in Fig. 6, because, although they represent situations in which the early and late placements of A2 exerted opposite effects upon the duration of the secondary zone of concealment, the conditions necessary for the serial extinction of 4 successive atria1 responses were the same in both. In each portion of the figure the scans labeled a, b, C, d, and e represent the nodal recovery after 0, 1, 2, 3, and 4 serially concealed atria1 responses. In each experiment, each additional nonpropagated atria1 response delayed the nodal recovery by approsimately 150 msec. (range, 108 to 197). In each experiment the extinction of As (the fourth concealed beat) was possible when As, AS, and A4 were all placed as late as possible in their corresponding zones of concealment, but in neither experiment was it possible to conceal As. In 3 esperiments it was possible to conceal 6 successive beats, but since each successive response had to be placed precisely, it was not possible to attain s&icient stability of the heart to permit systematic scanning of the final cycle. E. Effect of vagal stimulation. Stimulation of the cervical vagi at intensities sufficient to prolong A-V conduction time and to reduce the maximum 1:l frequency caused significant prolongation of the zone of concealment in 4 experiments in which it was tried. One of these experiments is illustrated in Fig. 7. As in Fig. 3, the range of concealment at various cycle lengths is indicated as the time between the earliest premature atria1 response and the earliest possible propagated A?. A brief zone of concealment was demonstrable after stellectomy and vagotomy, but only at basic cycle lengths of less than 280 msec. During vagal stimulation, the minimum cycle length (at maximum 1 :I driving frequency) was increased to 270 msec., and the zone of concealment at that frequency was increased to 95 msec. At longer basic cycle lengths the duration of the zone of concealment diminished gradually to a value of 15 msec. at a cycle length of 470. During vagal stimulation, conduction intervals were not sufficiently stable to permit a precise study of repetitive concealed beats.

F. Ketrogmdt~ concealment. The possibility of retrograde concealmeltt, well documented in clinical studies, could not always be tested directIS,, because 1 :l retrograde transmission was not always possible at frequencies sufficiently rapid to prevent accession of a supraventricular pacemaker. 1. V-h CONDUCTION DI:KING 2:1 BLOCK. This was studied in 2 experiments. V-A conduction time was measured at various frequencies yielding 1 :l and 2:l transmission. The influence of the nonpropagated beat upon conduction time wzs reIatively slight. In one esperiment the conduction time during 2:l block was prolonged by only 20 msec. at the highest frequency, and was not significantly altered at lower driving rates. In the other experiment no significant effect was apparent before stellectomy, and there was only a slight effect after stellectomy. It is possible that these slight changes could be accounted for by a delay in conduction in relatively refractory tissue within the ventricle itself, without penetration of the node by the intervening blocked beats. 2. CONDUCTrON DELAY AFTER RLOCKED BEATS. In 3 experiments the right ventricle was driven at close to the maximum 1:l V-A frequency. The interval which followed a nonpropagated Vz was scanned with a V, response. In all cases the earliest possible V, was propagated, but with a significant delay in conduction. One of these experiments is illustrated in Fig. 8. The scan represented by curve A relates V1V2 intervals (abscissae) to the corresponding *AlAs intervals (ordinates) at a basic frequency of 2 per second. The basic cycle included a zone of concealment which extended from 213 msec. to about 320, as indicated by the horizontal bracket. When a premature concealed beat, V.L, was interposed at 215 msec. after VI, the scan B was obtained. The earliest Vs was propagated, i.e., there was no secondary zone of concealment. The minimum AlA, interval, 485 msec., exceeded the minimum AIAr interval by 93 msec. Later positions of Vz up to and including the series represented by curve C did not change the minimum A,As interval, which suggests that V2 was in each case arrested at the same level and at the same interval after

Volume Number

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study of concealed conduction

347

Fig. 6. Serial concealment of 4 beats in Experiments of May 5, 1959 (part A) and Jan. 23, 1960 (part B). Curwes a: Scans of basic cycle. Curves b, G,d. and e: Scans after serial concealment of 1, 2, 3, and 4 beats, respectively. -

225 -

BASIC CYCLE

LENGTH,

STELLECTOM! VAGAL STIM.

MSEC.

Fig. 7. Zone of concealment as a function of frequency before and during vagal stimulation. Min. AI.42 indicates trial refractory period. At profiugated indicates earliest propagated At.

VI. At the still later position of Vz represented by curve D the minimum AlAa interval was increased to 522 msec., which indicates either deeper penetration of Vz or later activation of the limiting segment of the conduction pathway. In 2 additional experiments, no direct evidence of retrograde concealment could be elicited prior to sympathectomy, although early premature beats failed to propagate to the atria. After stellectomy it was not possible in either case to drive

the heart from the ventricles at a frequency sufficiently rapid to maintain control. 3. DISPLACEMENT OF A-V NODAL PACEMAKER. In one experiment, displacement of a presumed nodal pacemaker was observed after a premature nonpropagated ventricular beat. The evidence is displayed in Fig. 9. In part A of the figure the last of a series of regular driving pulses, delivered to the ventricle at a frequency of about 2.5 per second, is diagrammed as

348

Mar,

Abildskov,

and Mendez

1

I

,

I

,

/

I /

550 VZI)

Fig.

8. Direct

evidence for retrograde concealment. Experiment of April 24, 1959. Curve A,Az to VjV2 for basic cycle; horizontal bracket indicates “primary” zone of concealment. Curves 3, C, and D: Scans relating AIA, to Viva when VZ is interpolated at positions indicated by arrows.

A: Scan relating

; VZ

Vl

Fig.

9. Failure

I

I

I

I

loo

288

3m

um

to demonstrate

Vd

VS

V2

vl 2 0

Vn

I

500

I

600

700 -___ msec.

retrograde

concealment. Experiment of June beats initiated in ventricle. Vz = nonpropagated ventricular premature response. V,A, = “A-V nodal” when beat in absence of Va; V,‘A.’ = A-V nodal beat from higher pacemaker Vz is present. Part B: Same experiment; %‘,A3 indicates retrograde conduction time with or without interposition ofVa.

30, 1958. Part A: VIAI = last of series of driven

VIA,. This was regularly followed by a spontaneous beat in which the ventricular complex, V,, preceded the corresponding atria1 activity, A,,, by 60 msec. When a premature nonpropagated response, Vy, was initiated in the ventricle 200 msec. after the last driving pulse, the spontaneous beat which followed was delayed, but the atria1 complex preceded the ven-

tricular response by 30 msec. (A,’ and V,‘). The pacemaker shift induced by VZ was observed repeatedly. By projecting lines back from atria1 and ventricular responses in these two situations, at the slopes characteristic for A-V and V-A conduction time in this animal, an approximation of the “level” of the two pacemakers was obtained. It is evident that

Experimental

Vz discharged the lower pacemaker, permitting the firing of a higher focus with a lower intrinsic frequency. This cannot, however, be interpreted as evidence for penetration of the A-V node by the premature ventricular beat, for the estimated time between spontaneous firing of the lower pacemaker and the arrival of activity at the ventricular surface was only 30 msec. It must be presumed that the lower pacemaker was not in the A-V node, but probably in the bundle of His. The nonpropagated Vz response in this experiment also failed to alter the conduction time of the earliest possible Va (Fig. 9,B). By the criteria available, then, penetration of the node by Vz was not demonstrable. A similar experiment, in which evidence of concealment of Vz could not be obtained by displacement of a nodal pacemaker nor by conduction delay of a subsequent V-A response, is illustrated in Fig. 10. When a train of driving stimuli applied to the ventricle was interrupted (Fig. l&A), the pacemaker regularly fired at approximately the expected moment (response a’). Premature ventricular beats, initiated between 20.5 (the earliest possible Vz response) and 270 msec. after the basic Vi response, failed to reach the atrium, and also failed to discharge the pacemaker. As in the experiment of Fig. 9, the latest nonpropa-

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gated premature beat also failed to alter the conduction time of a third-order response, Vz (Fig. 10,B). Concealment of VZ was, therefore, not demonstrated by these measurements. Further manipulation, however, provided indirect evidence that V2 must indeed have entered the A-V node (Fig. 10,C). Coincident with the last basic driving stimulus applied to the ventricle (VI), the atrium was stimulated (Ao). The arrival of Vi in the atrium in the absence of Ao, and the corresponding ventricular emergence of Ao in the absence of V1, are indicated by the broken lines. Collision of the two wave fronts must have occurred midway in the A-V nodal pathway at about 60 msec. after their simultaneous origin; A0 could not have reached the common bundle, nor could VI have reached the atrium. Yet when Ao was present, Vz was propagated to the atrium, and when Ao was absent (i.e., when Vi was allowed to propagate to the atrium), Vz failed to traverse the node. It seems obvious that the earlier excitation of the atrium and the proximal portion of the node by A0 removed a barrier to the transmission of Vr, and it is also apparent that that barrier must have been above the point of collision of Ao and Vi; in other words, V2 must have penetrated at ieast to that level in the absence of Ao, and must, therefore, have been concealed. This observation thus

Fig. 10. Indirect evidence of retrograde concealment. Experiment of Aug. 10, 1959. Before stellectomy. Part A: ViAi indicates propagation of last driven beat. Premature response at VZ or at Va’ fails to discharge nodal pacemaker. Part B: Premature nonpropagated V2 fails to alter conduction time of subsequent propagated response (V&a interval is the same as VIA,). Purl C: Early premature Vr propagated in presence of Ao.

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Am. Hart I. March, 1964

and Mcndez

Fig. II. Indirect evidence for retrograde concealment. Experiment of Feb. 17, 1959. Experimental design indicated by inset schema. Curwe A: Plot of “VZ”V~ against AzA3 when V,,Z is delivered at position A. Curse B: Similar plot when VU is delivered at position B. Curse C: VzVa against A?.~3 when V,Z is omitted. Ordinates and abscissae, time in milliseconds.

conforms with the statement of Hoffman and Cranefield that retrograde as well as normograde transmission may fail in the upper levels of the A-V node. It is also apparent that the lower portion of the node traversed by the concealed Vz remained refractory for a shorter period of time than the ventricle itself, for the transmission of Va (Fig. 10,B) was not significantly delayed when Vz was present. 4. INDIRECT ASSESSMENT OF RETROGRADE CONCEALMENT. The experiment of Fig. 10 suggested that retrograde penetration of the A-V node may be the rule rather than the exception. Therefore, a series of experiments was set up in which retrograde entry could be demonstrated even when 1:l control of the heart could not be achieved. In effect, these experiments resembled the clinical “experiments” in which penetration of the A-V node by ventricular premature beats was demonstrated by an effect upon subsequent A-V conduction.’ The heart was driven from the atrium at a frequency compatible with 1:l A-V transmission. A ventricular premature beat was introduced at a fixed interval after the last atria1 beat, at a time sufficiently early to permit collision of the two impulses within the node. The subsequent interval was scanned from the atrium to determine what influence, if any, the interposed

ventricular beat had upon A-V propagation. Repeated scans were made with various temporal positions of the ventricular beat. In order to provide room for relatively early placement of the ventricular response, it was usually necessary to stimulate the ventricle in anticipation of the last previous A-V response, i.e., to remove the limitation of the ventricular RP which followed the last driven beat. The schema used, and the results of typical scans, are shown in Fig. 11. In the schematic diagram, A”, Ai, and As represent the last three regular driving pulses in the atrium; Vo represents the propagated response to A,. V,l represents the premature response of the ventricle to a stimulus applied in advance of the anticipated response to Al, and V,Z represents the test response. The arrival time of Ae in the absence of V,r and V,z is indicated by the dotted line terminating at VB. The subsequent interval was scanned by Aa for various positions of VS2, and again when AZ was allowed to propagate. Curve C of Fig. 11 represents the reiationship between AZAS and VZV~ when AZ was allowed to reach the ventricle. The minimum V2V3 interval, representing the effective nodal RP was 346 msec. Curve B was recorded when V szwas delivered about 130 msec. prior to the expected ventricular

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On two occasions, however, we recorded instances of apparent failure of penetration by atria1 premature beats. The evidence from one of these experiments is displayed in Fig. 12. AIVr represents the last of a series of driven responses. N 1represents the subsequent spontaneous discharge of a nodal pacemaker. Az represents a premature atria1 response which, although not propagated to the ventricle, discharged and reset the pacemaker (Nz). Concealment of At was also demonstrated by the failure of propagation of Aa, delivered at a time when it would have successfully traversed the node in the absence of At. AS, however, failed to discharge the pacemaker (N3). It must be concluded that Aa (1) failed to enter the node, i.e., was blocked at the atria1 margin, or (2) entered the node but died before reaching the pacemaker, or (3) penetrated the node to the same depth but bypassed the pacemaker. The first possibility is unlikely, although proof of concealment by a scan of the succeeding interval was not attempted. The third possibility seems unlikely unless the pacemaker were located, so to speak, beside but not in the conduction pathway. We interpret the experiment to indicate that Aat following upon the heels of the concealed A*, was extinguished short of the level of the pacemaker, i.e., it was concealed in the refractory wake of the prior response. An additional observation which may be interpreted as failure of nodal entry is recorded in Fig. 13. Concealment of a premature AZ was demonstrated, as shown in

arrival of response Az. The “VZV$” interval for this scan, as well as for that of curve A, represents the time between the expected arrival time of AZ (“Vz”) and the actual occurrence of V3. Had Vs2 failed to penetrate the A-V node, there should have been little difference between the V2V3 intervals for the scans of curves B and C. In fact, the V2V3 interval was abbreviated by 46 msec. by the interpolation of V,z. Still earlier application of V,Z. in curve A, shortened the VzV3 interval by 92 msec. Direct appraisal of concealment in this experiment was impossible, for retrograde transmission to the atria could not be demonstrated, Nevertheless, it is clear from the display that the ventricular “premature” beat must have penetrated deeply into the A-V nodal condurtion pathway. G. Block without nod& entry. On every occasion when the recovery of the A-V transmission system after a nonpropagated beat was assessed by scanning the ensuing period, it was found that concealment of the nonpropagated response had occurred, even when it was the last of a series of such concealed beats. Repeated re-excitation of the atrium in such experiments may bring the atriai RP down to 90 msec. or less, and it follows that either the RP of the entire atria1 margin of the node must be abbreviated to the same degree under these circumstances, or fractionation and dephasing occur, permitting two or more portions of the upper levels of the node to respond alternately.

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Fig. 12. Failure of AJ to discharge a nodal pacemaker. NI, Spontaneous discharge after last driven response, AIVI. Nz, Spontaneous discharge after nonpropagated Aa. Nat Spontaneous discharge of pacemaker when A2 and AS are delivered in sequence.

Fig. 13,A; the earliest propagated Aa was delayed 85 msec. by the interpolation of AZ. Similarly, concealment of =in was demonstrated by a delay of 45 msec. in the propagation of Ad (Fig. 13,B). However, the earliest possible As was delayed only 12 msec. by the nonpropagated A,. Delay of this magnitude could have occurred as a result of propagation of L45 in relatively refractory atria1 tissue, i.e., it is possible that A, was blocked and did not enter the node. This conclusion, however, is not obligatory, for if At, Aa, and Aq were concealed at successively more prosimal levels within the node, it follows that As would encounter a shallower zone of refractory tissue and, therefore, would be delayed less in propagation than A, and Ah of A and B of Fig. 13. Despite many attempts to demonstrate block without concealment, the abovementioned equivocal results were the only evidences which could be so construed. Discussion

When an impulse eilters the A-V transmission system and dies within it, it is obvious that at some point the action potential becomes subthreshold for the succeeding element. Failure of transmission could be due to decrement of the action potential or elevation of the threshold, or both. Reduction of the action potential and elevation of the threshold are both features of propagation in relatively refractory tissue, and it can be said, therefore, in effect, that concealment occurs when an impulse dies in refractory tissue within the transmission pathway. Whatever the mechanism of conduction failure, the data presented above confirm in most respects the conclusions drawn from examination of records of complex arrhythmias in the human subject. A nonpropagated beat can be shown to delay or block the propagation of a subsequent impulse (Fig. 1). So long as it can be demonstrated that the delay of the subsequent propagated beat exceeds any possible delay in conduction in relatively refractory tissue within the atrium or ventricle, this provides the most certain evidence of nodal penetration. Impulses which fail to traverse the transmission s~~stemmay also discharge a

nodal pacemaker and delay its nest sporltaneous discharge (Fig. 2); however, this cannot be taken as absolute evidence of nodal penetration unless the pacemaker can be proved to lie within the nodal portion of the transmission pathway. In the experiment of Fig. 9, for example, it is probable that the pacemaker discharged by a premature ventricular beat was located in the bundle of His. It is also possible that pacemakers discharged by premature atria1 responses(Figs. 2 and 10) were, in fact, proximal to true A-V nodal tissue.8 The same considerations apply to the analysis of clinical electrocardiograms. Evidence of repetitive concealment was readily obtained : in a few experiments, as many as six successive responses were demonstrated to have penetrated the A-V transmission system without emerging in the ventricle. However, in the experiments of Rosenblueth and Rubio,g curves relating A-V conduction time to A-A intervals during 2:l block approached those for 1:l transmission at high frequencies. although precise convergence of the curves was not shown, it is implied that under these circumstances the intervening atria1 responses were blocked, not concealed. Convincing evidence of block without penetration of the node was never obtained under the conditions of our experiments. Before symthe earliest possible atria1 pathectomy, premature beat not only entered the node, but usually traversed it completely. The experiments of Figs. 12 and 13, recorded after sympathectomy, are only suggestive of atrionodal block. It is possible, of course, that block could occur at the junction between atrium and node, particularly under the influence of drugs which may prolong the refractory period of nodal tissue without affecting that of atria1 tissue. Under the conditions of the isolated preparations examined by Hoffman and associates,4,7 the duration of the action potentials recorded near the atria1 margin of the node did not exceed that of atria1 muscle. If the refractory periods of the two tissues are closely related to the durations of the action potentials, it should be expected that nodal entry should be the rule rather than the exception. Retrograde entry of the node could not be uniformly demonstrated by a direct

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approach, either because retrograde transmission at appropriate frequencies could not be achieved, or because a nonpropagated ventricular premature beat exerted no significant influence on subsequent events. In those experiments in which an indirect test was applied, however, entry of the node from the ventricles was reguIarly demonstrated (Figs. 10 and 11). We may assume, therefore, that under the conditions of our study the RP of both the atria1 and the ventricular margins of the node was, in effect, as short as that of the atrium and the bundle of His, respectively. Since it has been shown repeatedly that the over-all functional or effective RP of the A-V transmission system is usually considerably longer than that of either the atria or the ventricles,10-12 it follows that some element within the node must possess an RP longer than that of either extremity. Exceptions exist, however. For example, the limiting tissue in A-V transmission in very young animals has been shown to be the ventricles rather than any supraventricular element.13

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Evidence for facilitation of the transmission of subsequent beats as a result of supernormality after a concealed beat was not observed in any experiment. Relative supernormality has been invoked to explain certain features of A-V propagation in clinical studies,14 and it has also been cited in experimental studies.g Experiments in which A-V transmission was facilitated by a nonpropagated ventricular beat, or V-A transmission by a concealed atria1 response, may not be used as proof of supernormal conduction. In the experiment of Fig. lO,C, propagation of a ventricular premature beat succeeded only in the presence of a prior atria1 response, Ao. It does not follow, however, that A@ was followed by a supernormal phase which permitted the passage of Vt; it is more likely that A0 accomplished the prior excitation and, therefore, the earlier recovery of a refractory barrier which otherwise blocked the passage of Vz. Similarly, in Fig. 11, the ventricular beats Ve2, A and B, might be said to facilitate the passage of the subsequent Aa response; but it is

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Fig. 13. Repetitive concealment. In each segment, atrium above and ventricle below. Part A: A-V propagation time in presence (3’) and in absence (3) of nonpropagated atria1 response, 2. Pert B: Two serially concealed atria1 responses. Part C: Three successive nonpropagated atria1 responses. Experiment of June 16, 1959. Abscissae, time in msec.

354

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probable that V,s “peeled back” a refractory barrier. It is, of course, possible that supernormality may explain certain patterns of A-V transmission under other circumstances. lModeEs of A-V transmission. The circumstances which permit the extinction of impulses within the A-V transmission system are those in which nodal conduction may be said to be “depressed”: viz., frequency increased to a degree which results in prolongation of A-V conduction time; sympathetic denervation; and vagal stimulation. Of these three agencies, the latter two result in prolongation of the over-all functional refractory period of the A-V transmission system, whereas increased frequency has the converse effect. All three, however, increase the P-R interval. It seems appropriate, therefore, to consider the phenomena of concealed conduction in terms of depressed conductivity in relatively refractory tissue. A model of nodal behavior has been constructed by Grant,15 who proposed an explanation of Wenckebach periodicity in terms of coupling of two oscillators beating at disparate frequencies. The “slow diastolic depolarization” which is characteristic of pacemaker activity is an oscillatory phenomenon, and oscillations without spike discharge have been observed in intracellular records of isolated preparations exposed to low Na concentrations.16 No electrical evidence of oscillatory activity is apparent in extracellular recordings.6 Rosenblueth” has proposed that the Wenckebach-Luciani cycles represent a junctional phenomenon between a proximal element which has a short RP and a more distal element with a longer RP. Models which incorporate a dual A-V transmission system have been proposed by Rosenbluethi8 and by Moe and associ’ates.rg The simplest conceptual model of concealed conduction is a modification of Rosenblueth’s “Wenckebach” model, in which a distal element with a longer RP provides a barrier to propagation.” TO explain retrograde Wenckebach periodicity, and retrograde concealment, this element would have to be proximal to tissue with a shorter RP at the ventricular margin. In such a model, concealment of one beat from either end could be explained, but

serial concealment of two or more impulses would require additional stepwise barriers. In a study of A-V conduction as influenced by frequency, Rosenblueth and Rubies called attention to breaks in the curve relating A-V conduction time to input cycle length over the range of frequencies within which 2:l block was demonstrable. In some experiments, as many as four such breaks occurred, which suggests that at least four junctions might exist at which failure of transmission would be likely. Inasmuch as we have been able to conceal as many as six successive beats, it seems more likely that failure of transmission results from a gradation of properties rather than numerous steps in the conduction pathway. Another possible model can be conceived in which an impulse arrives with an action potential so attenuated that it fails to excite a succeeding element, even though that element has fully recovered its excitability. The direct observations of Hoffman and associates4s7 suggest attenuation of the action potential as the mechanism of intranodal delay, and there can be little doubt that decremental conduction may account for failure of transmission within the node, at least under the influence of acetylcholine. If this model were applicable, it follows that serial extinction of two or more impulses would be likely only if each succeeding impulse were initiated as early as possible, i.e., when the tissue at the critical junction is most likely to develop a subthreshold action potential. It would explain those experiments (Fig. 4) in which an early concealed beat was followed by a zone of concealment longer than that of a later beat, but it does not explain the fact that serial concealment of several impulses was possible only when each successive impulse was initiated as late as possible in the corresponding zone of concealment (Fig. 6). Therefore, this model, at least in its simplest form, cannot account for all of the phenomena of concealment. The hypothesis that the properties of the node are graded, either in the sense that the refractory period increases or the margin of safety diminishes from the superior and inferior margins toward the interior of the node is suggested by numerous

Experimental

previous observations as well as by portions of the evidence presented here. In general, a late concealed AZ exerts a greater influence on the conduction time of a subsequent beat than does an early Az (Fig. 1). The direct studies which show decremental conduction within the node suggest that late premature beats might be expected to generate more adequate action-potential amplitudes and, therefore, penetrate more deeply than early ones. Such a graded system readily explains why each successive concealed beat must be placed as late as possible, for under these circumstances each successive beat would be extinguished at the most distal possible level, and “room” would be allowed for more proximal extinction of the subsequent response. This was, of course, the hypothesis tested in the experiments of Table I. The failure to achieve a yes or no answer in those experiments should not be sufficient grounds for rejecting the hypothesis in toto, for it is entirely possible that the properties of the system are graded, but that, as suggested by Alanis and assopoints exist ciates,6 one or more “weak” at which failure of transmission is prone to occur. In the experiment of Fig. 4, for example, the scan curves B, C, D, and possibly E, would be expected if the corresponding concealed AZ responses were all extinguished at the same level. Because of slow conduction or delay in relatively refractory tissue, the respective AZ responses could all have arrived at that level at the same interval after the primary response. Yet the curve F suggests that a still later AZ may have penetrated more deeply, and the same conclusion can be drawn from the B and C curves of Fig. 5. No one of the models presented will adequately fit all possible vagaries of A-V transmission. It is certain that the conduction pathway is nonhomogeneous; both anatomic and physiologic evidence support that conclusion.lgJO Premature entry of the node may expose either vertical or horizontal dissociation, or a combination of both. The temporal patterns which can be obtained in any individual animal may depend, therefore, upon the individual anatomic variant, upon nerve influences, and upon the chemical and physical environment of the transmission pathway.

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The specific temporal pattern will also depend upon the refractory periods of the atrium and ventricle, for these will determine the degree of prematurity with which impulses can be delivered to the transmission system. Summary Entry of the A-V node by impulses which fail to emerge was demonstrated by delay or block of a subsequent impulse, or by displacement of an A-V nodal pacemaker. Concealment of nonpropagated responses from either the atrium or the ventricle appeared to be the rule rather than the exception; clear evidence of block without nodal penetration was not obtained. Concealment was regularly demonstrable after thoracic sympathectomy, during vagal stimulation, or when the heart was driven at close to the maximum 1:l frequency. Some of the experiments reported here were performed with the collaboration of Dr. James Preston,

Dr.

Willard

Cohen,

and

Dr.

Robert

Vick.

REFERENCES 1. Langendorf, R.: Concealed A-V conduction: the effect of blocked impulses on the formation and conduction of succeeding impulses, Aaa. HEART T. 35542. 1948. 2. Langeniorf, R., ind Pick, A.: Concealed conduction. Further evaluation of a fundamental aspect of propagation of the cardiac impulse, Circulation 13:381, 1956. . 3. Lewis, T., and Master, A. M.: Observations upon conduction in the mammalian heart. A-V conduction, Heart 12:209, 1925. 4. Hoffman, B. F., Cranefield, P. F., and Stuckey, J. H.: Concealed conduction, Circulation Res. 9:194, 1961. 5. Scher, A. M., Rodriguez, M. I., Liikane, j., and Young, A. C.: The mechanism of atrioventricular conduction, Circulation Res. 754, 1959. 6. Alanis, J., L6pez, E., Mandoki, J,, and Pilar, G.: Propagation of impulses through the atrioventricular node, Am. I. Phvsiol. 197:1171, 1959. 7. Hoffman, B. F., and Cranefield, P. F.: Electroohvsioloev of the heart. New York. 1960. *McGrawyHill Book Company, pp. 163-i64. 8. Paes de Carvalho, A.: Cellular electrophysiology of the atria1 specialized tissues, in Proceedings, Symposium on the specialized tissues of the heart, Amsterdam, 1961, Elsevier, pp. 115-133. 9. Rosenblueth, A., and Rubio, R.: La influencia de la frecuencia de estimulacion sobre 10s tiempos de propagation auriculo-ventricular y ventriculo-auricular, Arch. Inst. Cardiol. Mexico 2553.5, 19.55. -

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and Mendrz

Krayer, O., Mandoki. J. J., and blendez, C.: Studies on V’eratrum alkaloids. XVI. The action of epinephrine and of veratramine on the functional refractory period of the auriculoventricular transmission in the heart-lung preparation of the dog, J. Pharmacol. & Exper Therap. 103:412, 1951. Mendez, C., Gruhzit, C. C., and Moe, G. K.: Influence of cycle length upon refractory period of auricles, ventricles, and A-V node in the dog, Am. J. Physiol. 184:287, 1956. Rosenblueth, A.: Functional refractory period of cardiac tissues, Am. J. Physiol. 194:171, 1958. Preston, J. B., McFadden, S., and Moe, G. K.: Atrioventricular transmission in young mammals, Am. J. Physiol. 197:236, 1959. Katz, L. N., and Pick, A.: Clinical electrocardiography. The Arrhythmias, Philadelphia, 1956, Lea and Febiger. Grant, R. P.: The mechanism of A-V arrhyth-

miss: with ari electrouic analogue of the humau A-V node, Am. J. Med. 20:334. 1956. 16. Wrest, 1‘. C.: Effects of chronotropic inHuences on subthreshold oscillations in the sino-atrial in Proceedings, Symposium on The node, specialized tissues of the heart, .Amsterdam, 1961, Elsevier, pp. 81-94. 17. Kosenblueth, A.: Mechanism of the Wenckebath-Luciani cycles, Am. J. Physiol. 194:491, 1958. 18. Rosenblueth, A.: Two processes for auriculoventricular and ventriculo-auricular propagation of impulses in the heart, Am. J. Physiol.

194:495, 1958. 19.

20.

Moe, G. K., Preston, J. B., and Burlington, Physiologic evidence for a dual A-V mission system, Circulation Res. 4:357, James, T. N.: Morphology of the human ventricular node, with remarks pertinent electrophysiology, AM. HEART J. 62:756,

H.: trans1956. atrioto its 1961.