Electrophysiologic Study of the Conduction System in Normal Children

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Electrophysiologic

Study

of the Conduction

System

in Normal

Children

Nigel K. Roberts, M.D., and Paul C. Gillette, M.D.

From the Department of Pediatrics (Division of Cardiology), University of California Los Angeles School of

Medicine, and the Department of Pediatrics (Section of Cardiology), Baylor College of Medicine and Texas Children ‘s Hospital, Houston

ABSTRACT. Values for cardiac conduction intervals obtained from normal children are reported so that the data will be available for comparison with patients who are suspected of having abnormalities. Sinus node recovery time correlated linearly with the resting PP interval. The mean intra-atnal conduction was considerably shorter in children

(< 25 msec) than in adults (42 msec). The atrioventricular

node had similar electrophysiologic properties in the child and adult. With aging, the His bundle to Purkinje fiber time increased significantly (P < .01). Pediatrics 60:858-863, 1977, CARDIAC CONDUCTION SYSTEM, ELECTROPHYSIOLOGY,

His BUNDLE ELECTROCRAM, ATRIAL PACING.

Today cardiac arrhythmias are more commonly

recognized in children. Both increased awareness

and cardiac surgery contribute to this

observa-tion. ‘ Intracardiac electrographic recording has

greatly aided in the understanding and

manage-ment of arrhythmias in adults and children.58

Several published series of conduction intervals

derived from intracardiac recordings in children

have yielded conflicting results.9 ‘ ‘ These

pre-vious studies have focused mainly on children

with congenital heart disease and “normal”

conduction.

In this article we are presenting a compilation

of new and previously reported data on

conduc-tion intervals and conduction system function

derived mainly from children with normal hearts,

so that these data will be available for comparison

with patients suspected of having abnormalities.

METHODS

Electrophysiologic studies were performed

after informed consent at the time of diagnostic

cardiac catheterization between 1970 and 1976

by the authors in laboratories in Toronto

(Hos-pital for Sick Children), Los Angeles (University

of California Los Angeles Medical Center), and

Houston (Texas Children’s Hospital). During this

period approximately 7,500 children were

cathe-terized and about 10% (750) had intracardiac

electrophysiologic studies. From this large group

of patients, there were 75 children who had had

cardiac catheterizations because of intracardiac

murmurs and clinically suspected heart disease,

but who were subsequently found to be normal

after the cardiac catheterization. Also included,

for the studies on sinus node function and on

atrioventricular nodal response to atrial pacing,

were 15 children with minimal (less than 10 mm

Hg) pressure differences across either the aortic

or pulmonary valves. In addition, for sinus node

function studies only, there were five patients

each with tetralogy of Fallot and ventricular

septal defect and one each with idiopathic

hyper-trophy subaortic stenosis, total anomalous

pulmo-nary venous drainage, and truncus arteriosus. The

only cardiac surgery in this latter group of

chil-dren included pulmonary to systemic anastomosis

for two children with tetralogy of Fallot and a

banding of the pulmonary artery in one child who

had a ventricular septal defect. The ages of all the

children ranged from 3 days to 19 years.

Each child was premedicated routinely with

Received October 4, 1976; revision accepted for publication January 31, 1977.

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meperidine hydrochloride, 1 to 2 mg/kg;

chlor-promazine, 0.5 mg/kg; and promethazine

hy-drochloride, 0.5 mg/kg and at the time of study

was not receiving any other medications.

Since the children were studied to determine if

there was a structural cardiac abnormality, it was

usual to insert only one electrode catheter. In 15

children, however, two catheters were inserted

and it was possible to measure atrial and

atrioven-tricular node refractory periods.

The electrode catheter was generally used in

the bipolar mode although it was convenient to

use multipolar catheters. Initially between 1970

and 1973 one of us used predominantly a unipolar

electrode catheter system.

For atrial pacing studies either the His bundle

electrode catheter was moved to the lateral

superior vena caval-nght atrial wall, or a second

electrode catheter was inserted into the above

position. The pacemakers used in the atrial

pacing studies were a battery-operated modified

pulse generator (Medtronics 5837) or a stimulator

with a built-in isolator (Grass SD 9). The stimulus

applied was 2 msec in duration and twice the

diastolic threshold. In the refractory period

stud-ies, a modified pulse generator (Medtronics 5837)

was used. The intracardiac recordings were

passed through an isolation unit (Electronics for

Medicine) and were displayed along with three

external ECG leads on an oscilloscope

(Elec-tronics for Medicine). The His bundle

electro-gram was obtained and recorded as previously

reported.9 The signals were filtered below 12 to

40 Hz and above 500 to 2,000 Hz. Permanent

records were obtained either by a direct writing

system or upon photographic paper driven at 100

mm/sec. Sinus node automaticity was tested by

measuring the degree of suppression after rapid

atrial 23

Sinus node recovery time (SNRT) was

measured (in milliseconds) from the last pacing

artifact to the onset of the first spontaneously

occurring P wave of sinus origin as seen in any of

the ECG leads. This value was also expressed as a

percentage of the resting PP interval and as a

corrected sinus node recovery time (CSNRT). The

CSNRT was calculated by subtracting the resting

PP interval from the SNRT.

Intra-atrial conduction was obtained by the

measurement from the earliest P wave deflection

to the peak of the initial rapid A deflection on the

His bundle electrogram. Effective and functional

atrial refractory periods were obtained by an

extra stimulus technique.8 The extra stimuli were

applied after eight sinus beats. The coupling

distance between the last normal beat and the

extra stimulus was sequentially decreased. The

longest coupling interval at which atrial capture

fails is the effective refractory period and the

shortest interval between atrial activities is the

functional refractory period.

Atnoventricular nodal conduction is reflected

by the AH interval on the His bundle

electro-gram. This interval starts at the A wave and ends

at the H potential. Effective and functional

atrioventricular nodal refractory periods were

obtained by an extra stimulus technique in a

similar fashion as described above. The longest

coupling interval at which atrioventricular nodal

conduction fails is the effective refractory period

and the shortest interval between His bundle

potentials is the functional refractory period.

His bundle to Purkinje fiber conduction was

measured from the H potential to the earliest

component of the ventricular complex.

RESULTS

Sinus Node Function

In 20 children with no evidence of conducting

system abnormality, the maximum absolute

SNRT after atrial pacing was 1,280 msec (Table

I). There was a positive linear correlation

between the absolute SNRT and the resting PP

interval. The upper limit of normal (mean + 2

SD) for the SNRT expressed as a percentage of the

resting PP interval is tabulated with the cycle

lengths. A cycle length of 500 msec represents a

heart rate of 120 beats per minute, 400 msec

represents 150 beats per minute, 333 msec

represents 180 beats per minute, and 300 msec

represents 200 beats per minute (Table I). The

CSNRT data are included in Table I.

Intra-Atrial

Conduction

It was possible to measure accurately

intra-atrial conduction as reflected by the PA interval

in 54 normal children. These children were

divided arbitrarily into three age-related groups:

less than 3 years (n = 11), 3 to 8 years (n = 23),

and 9 to 18 years (n = 20). The respective values

for the intervals are listed in Table I. For children

aged 3 to 8 years (n = 6) and 9 to 18 years

(

n = 9), the atrial effective and functional

refrac-tory periods are listed in Table I. (Data included

here also include figures reported by Dubrow et

al.’4 on refractory period measurements in normal

children who have no heart disease.)

Atrioventricular Nodal Function

The atrioventricular nodal conduction in

normal children less than 3 years of age (n = 13),

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TABLE I

NORMAL CONDUCTING SYSTEM MEASUREMENTS IN CHILDREN AND ADULTS

No.

-

Children

,

<3yr 3-8yr 9-18 yr

Sinus node

Absolute sinus node recovery time (maximum) (msec)

Sinus node recovery time (cycle length [CL] & % of resting CL)

20#{176} 20#{176}

1,280 500:163%-400:161% 333:128%-300:120%

1,1001

...

Corrected sinus node recovery time (msec) 54#{176} 250 5252

Atrial conduction PAinterval(msec+SD)

Refractory period measurements (msec) Effective

54 15

19±9 20±7 24±7

... 190 180

42±1127

23514

Functional .. . 200 240 27414

Atrioventricular node AH interval (msec + SD)

Refractory period measurements (msec) Effective

64 15

69 ± 18 83 ± 19 84 ± 17

.. . 240 240

73 ± 1227

28914

Functional .. . 310 376 40614

Atrial pacing increase of AVN conduction 26#{176} (See text)

His bundle

HV interval (msec + SD) 64 32 ± 8 33 ± 11 38 ± 7 40 ± 427

#{176}Includes some children with hemodynamically insignificant lesions, but with normal 12-lead EGGs.

(

n 27), and in normal children between 9 and

18 years of age (n = 24) is listed in Table I. Listed

in Table I as for the atrial effective and functional

refractory periods are those obtained from the

atrioventricular node for children aged 3 to 8

years (n = 6) and 9 to 18 years (n = 9). (Data

included here also include figures reported by

Dubrow et on refractory period

measure-ments in normal children who have no heart

disease.)

Atrioventricular nodal conduction during rapid

atrial pacing was assessed in 6 normal children

and in 20 children with the minimal intracardiac

lesions of bicuspid aortic valve, mild pulmonary

stenosis, and small ventricular septal defects.

Atrial pacing from slow resting heart rates (67 to

75 beats per minute or RR intervals of 900 to 801

msec, respectively) prolonged the nodal

conduc-tion by a mean of 22 msec/ 100 msec decrease in

RR interval; for heart rates between 76 and 86

beats per minute (RR intervals of 800 to 701 msec,

respectively), it was prolonged by a mean of 26

msec/ 100 msec decrease; and for heart rates of 87

to 100 beats per minute (RR intervals of 501 to

600, respectively) atrioventricular nodal

conduc-tion was prolonged by a mean of 32 msec per

decrease of 100 msec. There was no predictable

heart rate at which 1 : 1 atrioventricular

conduc-tion was lost and no correlation between age of

the child and paced rate at which atrioventricular conduction failed.

His Bundle Function

The His bundle to Purkinje conduction times

(HV interval) are listed in Table I for the normal

children less than 3 years of age (n = 13), for

those between 3 and 8 years (n = 27), and for

those between 9 and 18 years (n = 24). There was

a significant positive correlation with age

(P < .01) (Table I).

Rapid atrial pacing until loss of 1 : 1

atrioven-tricular capture resulted in a failure of conduction

at the atrioventricular node and not at the His

bundle in all the studied cases.

DISCUSSION

As investigative techniques for a cardiac

arrhythmia become more specific, It is important

to select the correct diagnostic maneuvers.

Inevi-tably, to interpret the data, it will be necessary to

compare them to normal values. To date, the

majority of invasive electrophysiologic studies

have been obtained in adult patients; thus, the

normal values may not be appropriate for

chil-dren. ‘#{149}“ Obvious differences among infants,

chil-dren, and adults are the degree of maturation of

the cardiac conduction system and the heart

rates. In addition in children, unlike the adult, it is

difficult to use more than one portal of entry for

the placing of an electrode or pacing catheter.

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TABLE II

PROTOCOL FOR Ir’vAsIvE INVESTIGATION OF AN ARRHYTHMIA

Problem No. of

Electrode Catheters

1 2

Site of Electrogr

#{163}

am#{176} P acing Proced

-ure Refractory

Period

HRA LRA CS H RV ,_

Atrial Ventricular

,

SNRTt

Congenital and/or acquired

First AV block + + + + + + +

Second AV block + + + + + + + +

Third AV block + + + + +

Bisfascicular block + + + + + + + +

Sinus bradycardia Supraventncular tachycardia Premature beats + + + + + + + + + + + + + + + + +

Pre-excitation syndrome + + + + + + + + + +

Special situations

Postoperative atrial septal defect Postoperative Mustard Postoperative Fallot + + + + + + + + + + + + + + + + + + + + +

#{176}HRA= high right atrial electrogram; LRA low right atrial electrogram; CS = coronary sinus electrogram; H His bundle electrogram; RV = right ventricular electrogram.

1

SNRT = sinus node recovery time.

in terms of numbers of catheters required for

certain investigative procedures.

Malfunction of the conduction system can arise

from both anatomic and electrophysiologic

abnormalities. For this reason, data that concern

both integrity of the structure and physiologic

function are reported

Sinus Node

The function of the sinus node is to provide the

impulse that initiates the sequence of cardiac

conduction. Sinus node activity itself cannot be

directly observed and can only be inferred from

observation of atrial activity by the vector,

morphology, and rhythm of the P waves. An ECG

rhythm strip that shows a normal sinus rhythm

and/or sinus arrhythmia excludes for the most

part abnormal sinus node function.

The pacemaker cells of the sinus node may

discharge normally but activity may not be

observed because of a functional block of the cells

around the node (exit block). The tissue

surrounding the pacemaker cells of the sinus node

has similar properties to that of the

atrioventric-ular node and these tissues normally conduct

more slowly than most cardiac conduction

1 7. 18 Usually, it is necessary to infer sinus

node exit block when the rhythmicity of the P

waves follows a typical Wenckebach cycle,

namely a progressive shortening of the PP

interval by decreasing increments followed by a

long PP interval which is mathematically related

to the underlying sinus rate. A sudden halving of

sinus rate suggests a 2: 1 sinus node exit block.

First-degree sinus node exit block and

third-degree sinus node exit block are more difficult to

confirm.

A method to determine the pacemaker

func-tion of cells of the sinus node is to overdrive

electrically the pacemaker cells and on ceasing

the rapid stimulation measure the “recovery”

time.’9’2#{176}The absolute recovery time can then be

measured. The resultant time which is

rate-dependent can then be expressed as a percentage

of resting RR interval’3 or the CSNRT (SNRT

minus KR interval).2’ Sinus node entrance block

or sinus node exit block will, however, mask the

actual response of the sinus node to rapid

stimu-lation, the former by protecting the node from

stimulus and the latter by delaying the observable

effect (P wave appearance). It is, however,

theo-retically possible to measure the degree of exit (or

entrance) block from the sinus node. The

conduc-tion time for a premature impulse to travel into

and back out of the sinus node is theoretically

twice the one-way conduction time. An actual

premature stimulus is introduced and if the

succeeding cycle is reset, the AA interval is

measured and from it is subtracted the

prestim-ulus AA interval. This resulting conduction

interval represents not only conduction into but

out of the sinus node (sinoatrial conduction time

[SACT]). Upper limits of normal for these

inter-vals in adults are reported as 120 msec and 140

msec.22 If the assumption that antegrade and

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antegrade conduction time is, therefore, less than

70 sec. So far no such measurements have been

reported in children, although our preliminary

results indicate that SACT measurements are

similar to those found in normal adults. A similar

inverse relationship as that which exists between

the atrial pacing rate and atrioventricular nodal

conduction has been reported between sinus

cycle length and SACT.2i

Atrial Conduction

Normal atrial conduction velocities may be

inferred from the measurement of the PA interval

on the electrogram and the knowledge of the

atrial anatomy. An approximate mean value of 25

msec for the PA interval and a distance of 4 cm

between sinus node and atrioventricular node

give a value for intranodal conduction of 1 6

m/sec, a figure that compares favorably with

experimental results.24 However, note that during

the time of maximum cardiac growth there is no

significant difference between the PA intervals

measured in children aged less than 3 years and

those aged 9 to 18 years. These findings may be

attributed to the cable properties of internodal

conduction. (From the cable equation the

conduction velocity is proportional to the square

root of the radius. Thus, there is an exponential

relationship between increase in radius and

velocity of conduction.25) As the heart gets larger,

so may preferential conduction pathways; thus

conduction velocity between the nodes increases.

The mean normal adult figure for this interval of

42 msec is considerably longer than the

corre-sponding figure from the child (Table I). The

slowing conduction velocity between the nodes

may possibly be on the basis of aging.

Effective and functional refractory period

measurements of the atrium are shorter than in

the normal adult (Table I).

Atrioventricular Node

Conduction through the atrioventricular node

is reflected by the AH interval of the His bundle

electrogram. The values are similar in both

chil-dren and adults. Refractory period measurements

likewise are similar (Table I). Rapid atrial pacing

prolongs atrioventricular nodal conduction time

in a predictable way. Eventually gradual increase

of the pacing results in a loss of conduction, which

occurs at the atrioventricular node.

His Bundle

Conduction through the His bundle is reflected

by the HV interval on the His bundle

electro-gram. This interval also includes the conduction

time through the bundle branches, The HV

interval normally increases with age, which

probably reflects the ircrease in ventricular

size.

In conclusion, we have reported data from

either normal children or in certain instances

children with cardiovascular abnormalities on

cardiac conduction intervals and normal function

of the sinus node, atrium, atrioventricular node,

and His bundle. These data have been contrasted

with comparable data reported for normal adults,

and we have included a table (Table II) that we

use for the investigation of a child with an

arrhythmia.

The principal thought in the design of these

protocols has been to limit the number of portals

of entry for the catheters, a situation that is not

necessary with an adult patient. One electrode

catheter is indicated when either an electrode

recording is required from the atrium of His

bundle or a pacing procedure such as overdrive

atrial pacing is needed to obtain an SNRT. Two

catheters are required when it is necessary to

observe atrioventricular nodal function and His

bundle function under controlled or stressed heart

rates, such as situations of the investigation of

bifascicular block or a pre-excitation situation.

REFERENCES

1. Godman MJ, Roberts NK, Izukawa T: Late post-operative conduction disturbances after repair of ventricular septal defect and tetralogy of Fallot:

Analysis of His bundle recordings. Circulation

49:214, 1974.

2. Gillette PC: Electrophysiologic studies of surgical

complete atrioventricular block. Cardiovasc Dis

1:95, 1974.

3. Gillette PC, El-Said C, Sivarajan N, et a!: Electrophys-iological abnormalities after Mustard operation for transposition of the great arteries. Br Heart J

36:186, 1974.

4. Yabek SM, Jarmakani JM, Roberts N; Postoperative trifascicular block complicating tetralogy of Fallot repair. Pediatrics 58:236, 1976.

5. Gillette PC, Nthill MR, Singer D: The electrophysiolog-ical mechanism of the short PR interval in Pompe’s

disease. Am J Dis Child 128:622, 1974.

6. Nasrallah AT, Gillette PC, Mullins CE, et a!: His bundle extrasystoles in children. Am J Cardiol 35:288, 1975.

7. Gillette PC, Gallagher JJ, Sealy W: Concealed cardiac pathways: An operable cause of supraventricular tachycardia in children. J Pediatr 90:427, 1977. 8. Gillette PC: The mechanisms of supraventricular

tachy-cardia in children. Circulation 54: 133, 1976. 9. Roberts NK, Olley PM: His bundle recordings in

chil-dren with normal hearts and congenital heart disease. Circulation 45:295, 1972.

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11. Bekheit S. Morton F, Murtagh JG, Fletcher E: Compar-ison of sinoventncular conduction in children and adults using bundle of His electrograms. Br Heart

J

35:507, 1973.

12. Gutgesell HP, Gillette PC, McNamara DG: The

response of the sinoatrial node to rapid atrial

stimulation. Pediatr Res 8:350, 1974.

13. Yabek SM, Jarmakani JM, Roberts NK: Sinus node function in children, factors influencing its evalua-tion. Circulation 53:28, 1976.

14. DuBrow 1W, Fisher EA, Amat-Y-Leon F, et al: Compar-ison of cardiac refractory periods in children and adults. Circulation 51:485, 1975.

15. Damato AM, Berkowitz WD, Patton RD. et al: A study of atrioventricular conduction in man using prema-hire atnal stimulation and His bundle recordings. Circulation 40:61, 1969.

16. Narula OS, Cohen LS, Samet P, et al: Localization of AV conduction defects in man by recording of the His bundle electrogram. Am J Cardiol 25:228, 1970. 17. Scher AM, Rodriguez MI, Liikane J, Young AC: The

mechanism of atrioventricular conduction. Circ Res 7:54, 1959.

18. Brooks CMcC, Lu HH: The Sinoatrial Pacemaker of the Heart. Springfield, Ill, Charles C Thomas Publisher, 1972, p 71.

19. Mandel WJ, Hayakawa H, Danzig R, Marcus HS: Evaluation of sino-atrial node function in man by

overdrive suppression. Circulation 44:59, 1971.

20. Mandel WJ, Hayakawa H, Allen HN: Assessment of sinus node function in patients with the sick sinus syndrome. Circulation 46:761, 1972.

21. Narula OS, Samet P, Javier lIP: Significance of the sinus node recovery time. Circulation 45:140, 1972. 22. Masini C, Dianda R, Graziina A: Analysis of sino-atrial

conduction in man using premature atrial

stimula-tion. Cardiovasc Res 9:498, 1975.

23. Reiffel JA, Bigger JT, Konstam MA: The relationship between sinoatrial conduction time and sinus cycle

length during spontaneous sinus arrhythmia in

adults. Circulation 50:924, 1974.

24. Yamada K, Honba M, Sakaida Y, et al: Organization and transmission of impulse in the right auricle. Jap Heart J 6:71, 1965.

25. Hodgkin AL: A note on conduction velocity. J Physiol 125:221, 1954.

26. Narula OS, Samet P, Javier BY: Significance of the sinus node recovery time. Circulation 45:140, 1972. 27. Roberts NK: Normal values of the component parts to

the P-R interval, in The Cardiac Conducting System and His Bundle Electrogram. New York,

Appleton-Century-Crofts, 1975, pp 49-56.

CRITICISM’S EASY-TO BE RIGHT IS DIFFICULT

Perhaps never before within the last century have we as Americans been so

aware of the arrogance, shallowness, and potential abuses of the vertical vision

by venal individuals who justify their special treatment and betray society’s

trust by invoking professional privilege, confidence, and secrecy. The question

for Americans is, How does society make professional behavior accountable to

the public without curtailing the independence upon which creative skills and

the imaginative use of knowledge depend? The culture of professionalism has

allowed Americans to achieve educated expressions of freedom and

self-realization, yet it has also allowed them to perfect educated techniques of

fraudulence and deceit. In medicine, law, education, business, government, the

ministry-all the proliferating services middle-class Americans thrive on-who

shall draw the fine line between competent services and corruption?

BURTON BLEDSTEIN

Submitted by Student

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1977;60;858

Pediatrics

Nigel K. Roberts and Paul C. Gillette