Sequence of repolarization on the ventricular surface in the dog

Experimental

and laboratory

Sequence of repolarization surface in the dog

reports

on the ventricular

Gernot Autenrieth, M.D.* Borys Surawicz, M.D. Chien Suu Kuo, M.D.** Lexington, Ky.

The morphology of the T-wave depends on the sequence of ventricular repolarixation. This sequence is determined by the shapes and durations of the individual ventricular action potentials. Our knowledge of the normal sequence of ventricular repolarization stems almost exclusively from the measurements of local refractory periods in the dog heart.1-3 It has been assumed that the functional refractory periods reflect the durations of action potentials in the ventricular myocardium.3 This assumption is justified when all ventricular action potentials: (1) exhibit the same relation between the recovery of excitability and the membrane potential, (2) have the same slope of phase 3, and (3) differ from each other in duration only because of different durations of phase 2. If these conditions are fulfilled, and if the shape of the transmembrane action potential is known, the Twave morphology can be derived from the sequence of the functional refractory periods4 Successful recordings of transmembrane action potentials (TAP) with capillary microelectrodes from a heart in situ is technically very difficult. It is less difficult if the recording of monophasic action potentials is done (MAP) using suction electrodes. It has been shown that the From the Department of Kentucky College Received

for publication

of Medicine, of Medicine, May

Division Lexington.

of Cardiology,

University

20, 1974.

Reprint requests: Borys Surawicz, M.D., Center, College of Medicine, Department Kentucky, Lexington, Ky. 40506.

Albert B. Chandler Medical of Medicine, University of

*Former Research Fellow, Kentucky rieth’s present address is: University cine, GBttingen, Germany.

Heart Association. Hospital, Department

**Former United States Public Health Training Grant No. HE-05771.04,051, ation Research Grant.

Cardiovawular and Kentucky

April,

1975, Vol. 89, No. 4, pp. 463-469

Dr.

Trainee Heart

Autenof Medi(NIHL Associ-

shape and duration of the MAP is the same as those of the TAP.6 We have applied suction electrodes to map out the sequence of repolarixation on the surface of the dog ventricle. The purpose of this paper is to report this sequence in relation to the T-wave and to the ventricular gradient (VG). Methods

Studies were done on 21 mongrel dogs weighing 13.5 to 25 kilograms, and anesthetized with sodium pentobarbital(30 mg. per kilogram), administered intravenously. The chest was opened by a midsternal incision, and the animals were ventilated with a respirator (Harvard Apparatus). To gain access to the posterior wall of the heart, an additional lateral incision was made in several dogs. To slow the heart rate, we performed bilateral upper thoracic sympathectomy, removing the paravertebral sympathetic chain cephalad to the fifth thoracic vertebra, including the stellate ganglia. The heart was suspended in a pericardial cradle and a bipolar Grass E2B platinum electrode was attached to the right atria1 appendage. Another bipolar electrode was sutured to the right ventricle. The atria were paced (Medtronic Pacemaker, Model 58001 at the slowest rate required to overdrive the spontaneous rhythm.The exposed areas on the cardiac surface were kept as small as possible and were covered with sponges soaked in warm saline. These sponges were replaced before each recording. The exposed cardiac surface was heated by means of two model L510 explosion-proof surgical lamps (12 inches in diameter) (WiImont Castle Co., Rochester, N. Y.) illuminating the site of incision. The distance between the heat-reflecting lamp surfaces and the heart was adjusted to

American

Heart Journal

463

Autenrieth,

Surawicz,

and Kuo

PB

20

i if Ill!/ ,lj I I”l/ ’ 189 24 199

23

191

31

198 13

187

Fig. 1. Six monophasic action potentials (MAP) recorded from five different areas in the same dog. Abbreviations: AM = anterior middle; A = apex; PB = posterior base; PM = posterior middle; and AB = anterior base. The numbers under each tracing indicate the activation time in milliseconds on the left and the MAP duration in milliseconds on the right. In the right lower corner the MAP’s are retraced and placed in a row to illustrate the parallel slopes of repolarixation in MAP’s of different duration. The drawing in the right upper corner illustrates the method of determination of the onset (i) and the end (e) of the MAP. The horizontal line represents the baseline. The onset of MAP marked by the arrow is the point of intersection between the baseline and the extension of the initial portion of the MAP upstroke (vertical line). The end of MAP marked by the second arrow is the point of intersection between the baseline and the tangent to the steepest portion of repolarixation slope (oblique line).

maintain the cardiac surface at a uniform temperature (Tele-thermometer, Yellow Springs Instrument Company, Inc.) ranging in individual experiments from 34” to 37” C. Two electrocardiographic leads and two monophasic action potentials (MAP) from the epicardial surface of the ventricular myocardium were recorded simultaneously with a multichannel direct writing recorder (Hewlett Packard) on a paper moving at 100 mm. per second. We recorded standard limb Leads II and Lead aV,in the initial experiments (12 dogs), and the orthogonal X- and Y-leads of the system designed for the dog by McFee and Parungao6 in the subsequent nine dogs. The ventricular complexes in Leads II and Y were nearly identical, while the complexes in Lead aV, and Lead X differed from each other. We measured Q-T interval in both leads, and used the value from the lead with the longer interval. The ventricular gradient (VG) was determined planimetrically in both leads after enlargement and retracing. MAP’s were recorded with bipolar suction elec-

464

trodes. One electrode pole was the tip of a stainless-steel wire (diameter 0.07 mm.) inserted into the lumen of a polyethelene tube (ID, 1.3 mm.; OD, 1.8 mm.). This wire tip touched the epicardium when suction was applied (negative pressure equals 75 mm. Hgl. The other pole was a cotton thread soaked in saline wound on the outside of the tube at its tip. Ventricular MAP’s were acceptable if the course of the repolarization was smooth, and the record was not distorted by the QRS and T-wave deflections. To test the influence of the ventricular electrocardiogram (ECG) complex on the MAP we compared the tracings recorded during atria1 pacing with those recorded during ventricular pacing. The MAP’s were acceptable if: (1) both atria1 and ventricular pacing produced identical tracings, (2) the MAP amplitude exceeded 25 mV., and (3) ten or more consecutive MAP’s were identical. In some experiments, only one of the two simultaneously recorded MAP’s was acceptable. The activation time was measured as the inter-

April, 1975, Vol. 89, No. 4

Repolarization

I. Activation

Table

Region

time (AT) of monophasic

No. of Dogs

No. of MAP’s

17 21 18 11 16

44 50 35 14 35

Posterior base (1) Posterior middle (2) Apex (3) Anterior middle (4) Anterior base (5)

action potentials

AT Average t S.D. in milliseconds 29.1 25.5 21.6 20.6 25.9

k + k f f

6.4 5.5 6.7 6.1 7.9

(MAP’s)

3t, 4* 3t 2t, 51: 5* 4$

surface

on the ventricular AT Average it S.D. (IS per cent of QRS duration

Significant differences between regions 2t, It, It, 1*, 3$,

on ventricular

in dogs

surface

i

! -Significant 1 differences 1 between regions

57.5 + 11.8 49.2 -t 10.1 41.5 + 12.6 41.6 +- 13.0 50.9 -+ 15.5

2*, I -, I*. 1*, 1 t,

3*. 3’. 2*, 2t 3’.

4*, 5t 4z 5* 5$ 4*

*p ( 0.001. tp ( 0.01. *p < 0.05.

val from the beginning of the QRS complex to the onset of the steep upstroke of the MAP (MAP,), and the MAP duration as the interval from the MAP, to the end of MAP (MAP,). The latter point represented the intersection of the baseline with the tangent to the steepest part of the MAP downstroke (Fig. 11.’ The interval between the end of MAP and the end of T-wave (MAP, - TJ was assigned a positive sign when MAP, occurred earlier than T,. The accuracy of the measured intervals was within 5 msec. This was determined by repeated measurements made by the same observer, and by independent measurements made by two different observers. We divided the surface of the heart into sixteen areas of approximately equal size, and attempted to explore each area in each dog. However, this was not feasible, and we obtained acceptable records only from five to 14 areas per dog. Since the numbers of records obtained from each of the small areas were not sufficient to analyze the results statistically, we grouped the data from several neighboring small areas. We combined the small areas into the following five larger areas: apex (A), anterior middle (AM), posterior middle (PM), anterior base CAB), and posterior base (PB). The landmarks separating anterior and posterior surfaces consisted of the obtuse cardiac margin on the left side, and the acute cardiac margin on the right side of the heart. The apical area was bound superiorly by a circular line perpendicular to the long axis, drawn at a distance equal to one fourth of the long axis. The line separating the middle area from the base bisected the remaining threefourths of the long axis. Standard statistical tests

American

Heart Journal

Table II. Differences between monophasic action potential durations in 73 simultaneously recorded pairs

fl!ff?L-& PBvs.PM PB vs. A PB vs. AB PM vs. A PBvs. AB A vs.AB ABvs. AM

5 3 4 6 3 6 7

11 6 7 14 4 8 23

were used to evaluate results.

+ 9 - 4 -28 +23 - 2 +13 +18

to to to to to to to __-

-12 --27 + 4 -35 -12 - 3 - 6

0.0 -13.3 -10.7 -- 8.7 - 8.5 + 6.3 + 0.6

-t -+ l?r k j, + +-

7.2 8.1 10.5 14.2 4.4 5.5 5.6

.-

the significance

of the

Results T-wave. In agreement with previous studies, we found that the bilateral sympathectomy had only a slight and inconsistent effect on ventricular repolarization.* The shape of the T-wave was similar to that recorded in other studies of anesthetized dogs in the supine position.4,9 The VG in the frontal plane was generally positive, i.e., greater than zero. In 86 out of 106 pairs of ventricular complexes recorded simultaneously with acceptable MAP’s, the VG was positive in both leads. In the remaining 20 pairs the VG was positive in one lead and negative in the other lead. In all dogs the R-wave was the major QRS deflection in Leads II and Y. In these leads, the Twave was upright with a single or bifid peak in 69 tracings, and inverted in 37 tracings. Monophasic

Activation

action

potentials.

time. The duration

of the QRS com-

465

Autenrieth,

Surawicz,

and Kuo

I lmu

I

2emv

Awl. ilaw M~Tln mssc 29 3s

msec 274 252

Fig. 2. Typical experiment illustrating the differences between two MAP’s from two different areas. T-wave is upright and bifid. The arrow marks the end of the T-wave. Heart rate is kept constant by atria1 pacing.

plex averaged 50.7 f 4.9 msec. Table I shows the activation times (AT) in five areas on the ventricular surface, expressed both as absolute durations and as per cent of the QRS duration. The earliest AT was in the anterior middle and apical areas, and the latest AT in the posterior basal area. Duration. Table II shows the differences between MAP durations in simultaneously recorded MAP pairs from two different regions. An example of such a pair is shown in Fig. 2. Although the range of differences in Table II is wide, most of the average differences between regions just attain significance (Table IV). In those experiments in which only one of the two simultaneously recorded MAP’s was acceptable, we expressed the duration of each MAP in per cent of the longest MAP duration in each dog (Table III). Table III shows that the longest MAP duration was at the apex and anterior base, and the shortest MAP duration at the posterior base. These results were similar to those in which pairs of MAP were recorded simultaneously. However, Table IV shows that most of the average MAP duration differences between regions (Table III) are more significant than the average differences between the pairs of MAP’s (Table II), probably

466

due to the larger sample size in Table III than in Table II. The slope of terminal Slope of repolarization. repolarization ranged from 0.3 to 0.54 volt per second. In the same dog the repolarization slopes of different MAP’s were nearly parallel (Fig. 11, and did not deviate by more than 10 per cent from the average slope. Interval between the end of MAP and the end of the T-wave. Of 178 MAP’s, two ended 4 msec. and 5 msec., respectively, after the end of the T-wave and one ended simultaneously with the end of the T-wave. The remaining 175 MAP’s ended before the end of the T-wave (Fig. 3). The average MAP, - Te intervals in different areas ranged from 27.4 f 11.7 msec. at the posterior base to 21.3 + 9.8 at the anterior base. The difference between the pairs of MAP’s (Table II), probably due to the larger sample size in Table III than in Table II. MAP duration and the onset of activation. To analyze the relation between the AT and the duration of MAP independently of the area, we arbitrarily subdivided all MAP’s into three groups: one with an early (8 to 20 msec.), one with an intermediate (21 to 30 msec.), and one with a late ( ) 31 msec.) AT. Table V shows that both the MAP’s in Group I and in Group II are significantly longer than the MAP’s in Group III. Discussion Sequence of depolarization and repolarization.

The sequence of activation on the surface was from apex to base. This result is in accord with the results of previous detailed studies of ventricular depolarization in the dog, monkey, and in man.‘O We found that the slopes of terminal repolarization in MAP’s of different duration were approximately constant. We have also shown that this slope was the same as the slope of TAP from the excised papillary muscles and trabeculae of the hearts of the same anima1s.l’ These findings suggest that the intervals between the ends of MAP’s represent the sequence of repolarization. Our study shows that this sequence on the ventricular surface is complex and geographically nonuniform. This is in keeping with the results of previous studies with local bipolar electrograms I2 which indicated that inhomogeneities of repolarization occur within very small distances over the entire surface of the ventricles.12J3 April, 1975, Vol. 89, No. 4

Repolarization

on ventricular

surface in dogs

Table Ill. Duration

of the monophasic action potentials (MAP) expressed in per cent of the longest MAP in each dog No. of No. of Dogs MAP’s

Region Posterior Posterior Apex Anterior Anterior

base middle middle base

11 21 18 11 16

44 50 35 14 35

Average Range 81.2 82.4 87.5 88.1 89.0

-

+ SD.

100.0 100.0 100.0 99.1 100.0

90.1 93.3 96.2 94.5 96.0

++ + f t

4 4 4 3 3 50.45

INTERVAL.

Table IV. Statistical

significance of the regional differences between the durations of monophasic action potentials (MAP) Regions PB PB PB PB PM PM A A AB PM

vs. vs. vs. vs. vs. vs. vs. vs. vs. vs.

Pairs of MAP’s* P value

PM A AM AB A AM AB AM AM AB

NS (0.01 (0.05 ( 0.05 (0.05 NS (0.05

SingkMAPbt P value < 0.001 (0.001 ( 0.001 ( 0.001 ( 0.01 ( 0.01 NS NS NS < 0.01

*Table II. tTable III

40-36

36-31

30-26

25-a

THE END OF MAP

20-16

15-11

IO-6

5-1

o--5

AND THE END OF T WAVE IN MSEC

Fig. 3. Distribution of the intervals monophasic action potential (MAP) wave in 178 tracings (see text).

between the end of the and the end of the T-

V. Relation between the duration of monophasic action potential (MAP) expressed in per cent of longest MAP in each dog and the activation time (AT) on ventricular surface

Table

I II III

8-20 21-30 >31

14 17 21

46 97 41

95.1 94.4 92.5

rfI 3.4 -t 4.5 +- 4.7

III* 111t I*, 11t

*p < 0.01. tp ( 0.05.

AP duration on the ventriwlar VG. The MAP’s were generally

surface and the

shorter on the posterior than on the anterior surface of the ventricles, and shorter at the base than at the apex. The latter finding is opposite to the order of repolarization suggested by Haas and co-workers12 and several other older studies.14 However, our results are in agreement with a recent study of Burgess and co-workerq3 where the refractory records were longer at the apex than at the base by 5 to 20 msec. in seven experiments. Our study suggests that progressive MAP shortening in progressively later activated areas is determined by the time of activation rather than by the site of activation. This distribution of the recovery properties confirms the results of Burgess and co-workers,3 and supports the validity of the concept of the ventricular gradient WGL The relation of MAP duration on the ventricular surface in this study agrees with the direction of VG in our ECG’s.

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BETWEEN

Heart Journal

Dispersion of repolarization. Van Dam and Durre? found that in the innermost layers the refractory periods were “sometimes. . . approximately 15 msec. longer than the middle and subepicardial layers.” In our study, the average maximum dispersion of MAP duration was 22 + 9 msec. This is similar to the MAP dispersion on the surface of the isolated cat heart which averaged 25 msec.16 These two studies suggest that the dispersion of AP duration on the entire surface exceeds the transmural differences of refractory periods. Some indication of the approximate maximum dispersion of repolarization in an individual dog can be obtained from the relation between the timing of the T-wave apex and the repolarization slope of MAP. The T-wave results from many simultaneous gradients between AP’s during repolarization. The peak of the T-wave is expected to coincide with the maximum of these local gradients. The maximum local gradient between two

467

Autenrieth,

Surawicz,

and Kuo

Fig. 4. Magnified ventricular complex from Fig. 2 (solid trace) with an additional four different T-waves derived from the “potential differences” between two superimposed MAP’s illustrated in Fig. 4, measured in arbitrary units at 10 msec. intervals during repolarisation. The solid circles mark the T-wave derived from the MAP’s superimposed in the same sequence as they-occurred in the experiment (MAP at the anterior base ends 17 msec. and MAP at the posterior base 33 msec. before the end of the T-wave). Other T-waves represent “potential differences” between MAP’s of the same configuration but after shortening, or lengthening, phase 2 of the MAP at the posterior base. The T-wave marked by crosses was constructed after the MAP at the posterior base was shortened by 14 msec.; this increased the MAP,-TB difference between these two MAP’s from 16 msec. to 30 msec. The T-wave marked by empty circles is constructed after the MAP at the posterior base was shortened by 34 msec.; this increased the MAP-TB difference between these two MAP’s to 50 msec. The T-wave marked by triangles was constructed after the MAP duration at the posterior base was increased by 26 msec.; this altered the MAP-T, sequence between these two MAP’s and produced a negative T-wave. Note that the aT-eT interval (second peak) of the recorded T-wave is 40 msec., and that the aT-eT intervals of the derived T-waves are: 44 msec, (dispersion 16 msec.), 55 msec. (dispersion 36 msec.), and 75 msec. (dispersion 50 msec.).

AP’s during repolarization may be expected during the inscription of the steepest repolarization slope of the AP which terminates earlier. If we apply this to the entire heart the peak of the Twave should not occur before the onset of rapid repolarization in some portion of the ventricle. This in turn suggests that the duration of the interval between the peak of the T-wave (a?‘) and the end of the T-wave (eT) bears a certain relation to the dispersion of repolarization in the entire heart. In our study, the interval between the peak (second peak if the T-wave was bifid) and eT in Leads II or Y, averaged 33 f 12 msec. The precise relation between the dispersiorrof repolarization and the aT-eT interval is not known, but knowing the repolarization slope of the individual MAP we can calculate the probable upper limits of dispersion in an individual case. An example of such a calculation is illustrated in Fig. 4. This figure shows one ventricular complex from Fig. 2. The MAP at the anterior base ends 17 msec. and the MAP at the posterior

468

base ends 33 msec. before the end of the T-wave. Thus, the dispersion of repolarization between these two MAP’s was 16 msec. The shape of the T-waves was derived from the plot of “potential differences” produced by superposition of these two MAP’s when the dispersion of repolarization was unchanged (16 msec.) and when the dispersion was shortened or prolonged by “shortening” or “lengthening” the MAP at the posterior base. Fig. 4 shows that, unlike the recorded T-wave, the derived T-waves have only one peak. The timing of this peak is within the observed range of aT-eT intervals when the dispersion of repolarization is 10, 16, or 30 msec. but the peak occurs earlier than observed in any experiment when the dispersion is 50 msec. This suggests that the dispersion of repolarization in dogs is less than 50 msec. Duration of MAP and QT interval. In the study of Yanowitz, Preston, and Abildskov’s,g unilateral stellate ganglion stimulation was followed by an apparent lengthening of the QT interval. This

April, 1975, Vol. 89, No. 4

Repolarization

suggested that certain AP’s terminate after the end of the T-wave, but do not generate deflections because they end simultaneously and undergo cancellation.g We found no evidence in support of such “silent repolarization” on the ventricular surface. This result is in agreement with our previous studies of MAP on the ventricular surface of isolated rabbit hearts (A. P. Zumino, L. S. Gettes, and B. Surawicz, unpublished observation) and the endocardial surface of human hearts.7

4.

5.

6. I. 8.

Summary

Our study provides a reasonably detailed “repolarization map” of the ventricular surface in the dog heart. The approximately constant repolarization slope of all MAP’s on the surface supports the validity of the assumption that the sequence of ventricular repolarization can be derived from the sequence of refractoriness. Our results agree with the studies of functional refractory periods and show that the duration of recovery tends to shorten during the course of activation. This sequence is consistent with the positive ventricular gradient in the electrocardiogram. REFERENCES

10.

11.

12.

13.

1. Reynolds, E. W., Jr., and Vander Ark, C. R.: An experimental study of the origin of T-waves based on determinations of effective refractory period from epicardial and endocardial aspects of the ventricle, Circ. Rec. 7:943, 1969. 2. Van Dam, K. T., and Durrer, D.: Experimental study on the intramural distribution of the excitability cycle and on the fo&n of the epicardial T-wave in the dog heart in situ, AM. HEART J. 61:537,1961. 3. Burgess, M. J., Green, L. S., Millar, K., Wyatt, R., and Abildskov, J. A.: The sequence of normal ventricular recovery, AM. HEART J. 84:660,19’72.

American

9.

Heart Journal

14.

15.

on ventricular

surface in dogs

Burgess, M. J., Harumi, K., and Abildskov, J. A.: Application of a theoretic T-wave model to experimentally induced T-wave abnormalities, Circulation 34:660, 1966. Hoffman, B. F., Cranefield, P. F., Lepeschkin, E., Surawicz, B., and.Herrlich, H. C.: Comparison of cardiac monophasic action potentials recorded by intracellular and suction electrodes, Am. J. Physiol. 196:1297, 1959. McFee, R., and Parungao, A.: An orthogonal lead system for clinical electrocardiography. AM. HEART J. 62:93, 1961. Shabetai, R., Surawicz, B., and Hammill, W. A.: Monophasic action potentials in man, Circulation 38:341, 1968. Wallace, A. G., Schaal, S. F., Sugimoto, T., Rozear, M., and Alexander, J. A.: The electrophysiologic effects of beta-adrenergic blockade and cardiac denervation, Bull. N. Y. Acad. Med. 43:1119, 1967. Yanowitz, F., Preston, J. B., and Abildskov, J. A.: Functional distribution of right and left stellate innervation to the ventricles: production of neurogenic electrocardiographic changes by unilateral alteration of sympathetic tone, Circ. Res. 18:416, 1966. Scher, A. M.: Excitation of the heart: a progress report, in: Advances in Electrocardiography, Scblant, R. C., and Hurst, J. W., editors. New ‘York 1972, Grune & Stratton, pp. 61-71. Kuo, C. S., Autenrieth, G., Arita, M., and Surawicz, B.: Effect of isoproterenol on ventricular monophasic action potential, &a-membrane action potential, and Twave in dogs, Circulation 46:(Suppl II) 179, 1972. (Abstr.1 Haas, H. G., Bloemer, A., Ley, M., and Schaefer, H.: Experimentelle Untersuchungen am Hunderhenen zum Problem des Ventrikelgradienten, Cardiologia 3766, 1960. Schaefer, H., and Haas, H. G.: Electrocardiography, in: Handbook of Physiology: Circulation, Hamilton, W. F. and Dow, P., editors. Washington, D. C.. 1962, American Physiological Society, vol. 1, Sect. 2, pp. 323-415. Surawicz, B.: The pathogenesis and clinical significance of primary T-wave abnormalities, In: Advances in Electrocardiography, Schlant, R. C., and Hurst, J. W., editors. New York, 1972, Grune & Stratton, pp. 377.421. Sarachek, N. D., Roberta, J., and Leonard, J. L.: A new method to measure nonuniformity in the intact heart. J. Electrocardiol. 5:341, 1972.