Functional 2:1 A-V Block within the His-Purkinje System
Simulation of Type
II
Second-Degree A-V Block
BY ANTHONY N. DAMATO, M.D., P. JACOB VARGHESE, M.D., ANTHONY R. CARAcrA, M.D.,
MASOOD AKHTAR, M.D., AND SUN H. LAU, M.D.
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SUMMARY
In eight subjects in whom the effective refractory period of the His-Purkinje system was determined to be longer than that of the A-V node or atrium, abrupt acceleration of the atrial rate
resulted in either 2:1 block within the His-Purkinje system or 1:1 A-V conduction. The former
conduction pattern simulated a type II second-degree A-V block. The occurrence of either 2:1
block within the His-Purkinje system or 1:1 A-V conduction was determined primarily by (1) an
effective refractory period of the His-Purkinje system which exceeded that of the A-V node,
(2) the coupling interval of the first atrial capture beat and its effect or lack thereof on the
refractory period of His-Purkinje system as it relates to changes in ventricular cycle length, and
(3) the relative speed of A-V nodal conduction time. The distinction between this functional 2:1
block within the His-Purkinje system and a true type II second-degree block is discussed from
both the clinical and electrophysiologic points of view. Functional 2:1 block within the HisPurkinje can also result when sudden acceleration of atrial rate occurs spontaneously such as in
atrial flutter or A-V nodal reentrant atrial tachyeardias.
Additional Indexing Words:
Atrial pacing
Atrial tachyeardias
Premature atrial stimulation
A-V node
His bundle recording
His-Purkinje system
G ENERALLY, A-V block occurs in that portion
of the A-V conducting system which has the
longest effective refractory period. In most subjects,
the effective refractory period of the A-V node
exceeds that of the His-Purkinje system, and
therefore 2:1 block at the A-V nodal level results
whenever a sufficiently high rate of stimulation is
applied to the atria.' 2 In such patients, A-V nodal
block occurs irrespective of whether the atria are
stimulated at a high rate abruptly or by increments.
The specific rate at which 2:1 block occurs varies in
different individuals but, in general, in normal
subjects it is greater than 120 beats/min.
This report deals with the findings in eight
subjects in whom abrupt acceleration of the atrial
Refractory periods
rate resulted in either 1:1 A-V conduction or 2:1
A-V block within the His-Purkinje system. The latter
simulated a type II (Mobitz) second-degree A-V
block. Those factors which are responsible for this
"functional" form of block within the His-Purkinje
will be analyzed.
Methods
Right heart catheterization was performed with the
patient in the postabsorptive, nonsedated state. Informed consent was obtained. The patients were not
taking any cardiovascular drugs. Using local anesthesia,
a tripolar electrode catheter was percutaneously
inserted into a femoral vein and fluoroscopically
positioned in the region of the tricuspid valve in order
to record bundle of His electrograms as previously
described.3 A quadripolar electrode catheter was
similarly inserted into an antecubital vein and
fluoroscopically positioned against the lateral wall of
the right atrium at its junction with the superior vena
cava. The proximal pair of electrodes was used to
record a high right atrial electrogram, and the distal
pair used to stimulate the right atrium. Three standard
electrocardiographic leads were simultaneously recorded. Atrial stimulation was performed using a programmed digital stimulator which delivered rectangular
pulses of 2-msec duration at twice diastolic threshold
through an isolation unit. In each patient two modes of
atrial stimulation were performed. The extrastimulus
method was used to determine the refractory periods of
From the Cardiopulmonary Laboratory, U. S. Public
Health Service Hospital, Staten Island, New York.
Supported in part by the Federal Health Program Service,
U. S. Public Health Service Project Py 72-1 and National
Heart and Lung Institute, Projects HE 11829 and HE
12536.
Address for reprints: Anthony N. Damato, M. D.,
Cardiopulmonary Laboratory, U.S.P.H.S. Hospital, Staten
Island, New York 10304.
Received September 28, 1972; revision accepted for
publication November 12. 1972.
534
Circulation, Volume XLVII, March 1973
535
A-V BLOCK
recording paper at a speed of 150 mm/sec. Time lines
were inscribed at 10- and 100-sec intervals.
the A-V conduction system during sinus rhythm, and
this involved the introduction of a single atrial
premature depolarization at progressively shorter coupling intervals within the cardiac cycle.4' 5In the second
method of stimulation the atrial rate was abruptly
accelerated. Careful attention was paid to the
grounding of all equipment.
The A-H interval was used as an approximation of
A-V nodal conduction time. Normal values in our
laboratory are 60-140 msec. The H-V interval measures
His-Purkinje conduction time, and our normal values
are 30-55 msec. The His-Purkinje system includes the
bundle of His, right and left bundle branches, and
peripheral Purkinje network.
Refractory Period Determinations
A single atrial premature depolarization (A2)
Results
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Table 1 lists the essential clinical data of the eight
patients of this report. QRS duration and axis were
normal in all patients except one who had a RBBB
pattern. For each patient the electrocardiogram at
the time of study was similar to that recorded 1-5
years earlier. Similarly, the electrocardiograms have
not changed over a 1-2-year period following
study.
Table 2 lists the electrophysiologic data obtained
during sinus rhythm. In all patients A-V nodal
conduction time, as measured by the A-H interval,
was normal during sinus rhythm. His-Purkinje
conduction time (H-V interval) was also normal.
The A-H interval demonstrated the normal progressive increase as the atrial rate was gradually
increased (by increments of 10 beats/min) to the
maximum rate which allowed 1:1 A-V conduction.
The H-V interval remained constant over this same
range of paced heart rates. At sinus rhythm, the
effective refractory period of the His-Purkinje
system was greater than that of the A-V node in all
patients. In five of the eight patients the effective
refractory period of the right atrium was reached
before that of the A-V node.
In every patient, sudden acceleration of the atrial
rate at cycle lengths which ranged between 380 and
415 msec resulted in either 2:1 A-V block within the
was
introduced after every eighth sinus beat (A1) at
progressively shorter coupling intervals (A1-A2 interval) up to the point of atrial refractoriness.
The effective refractory period (ERP) of the atrium
was defined as longest interval between the last atrial
sinus beat and the delivered stimulus (S) at which S
failed to capture the atrium. Effective refractory period
(ERP) of the His-Purkinje system-the longest H1-H2
interval at which H2 fails to propagate to the ventricles.
Effective refractory period (ERP) of the A-V nodethe longest A1-A2 interval at which A1 fails to conduct
to the bundle of His.
Relative refractory period (RRP)- of the His-Purkinje
system is determined by the H1-H2 interval at which H2
conducts with an H2-V2 interval which is longer than
the sinus beat (i.e., H2-V2 > H1-H2) or H2 results in
aberrant ventricular activation.
All tracings were taken on a seven-channel tape
recorder using a multichannel oscilloscopic photographic recorder and later transferred to photographic
Table 1
Clinical Data
Pt
Age (yr), sex
1
15 F
NHD; frequent PVBs
2
56 M
3
Diagnosis
P-R (see)
QRS complex
None
< 0.20
ASHD
AP
< 0.20
75 M
Occasional PVBs
None
< 0.20
4
63 M
Occasional PVBs; old inferior Ml
None
< 0.20
5
65 M
Diabetes; old anteroseptal MI
None
< 0.20
Normal duration
Normal axis
RBBB
Normal axis
Normal duration
Normal axis
Normal duration
Normal axis
Normal duration
Normal axis
6
35 F
PAT due to A-V nodal reentry
Palpitations
< 0.20
7
65 M
PABs; PVBs
None
< 0.20
8
60 M
Old inferior MI
None
< 0.20
Abbreviations: NHD
=
Symptoms
Normal duration
Normal axis
Normal duration
Normal axis
Normal duration
Normal axis
Sinus
CL (msec)
820- 960
810- 920
1130-1400
780- 945
800- 920
800- 920
850-1000
1000-1350
nio heart disease; ASHD = arteriosclerotic heart disease; YI = myocardial infarction; PVB = premaproxysmal atrial tachycardia; AP = angina pectoris; CL = cycle
ture ventricular beat; PAB = premature atrial beat; PAT =
length.
Circulation, Volume XLVII, March 1973
536
DAMATO ET AL.
Table 2
Electrophysiologic Data during Sinus Rhythm
Pt
1
2
3
4
5
6
7
8
A-H interval
(msec)
H-V interval
(msec)
80
70
140
85
110
100
90
110
40
40
ERP
A-V node
ERP
His-Purkinje
system
280
270*
480
440
35
290
440
50
300
450
45
47
40
50
<
< 290*
< 280*
< 300*
365
440
470
< 320*
570
Paced atrial cycle
length at which
2:1 A-V block or
1:1 conduction
occurred (msec)
430
380
380
430
450
400
415
500
*The atrium was most refract6rv.
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His-Purkinje system or 1:1 A-V conduction with
normal ventricular activation. In three patients
abrupt acceleration of the atrial rate resulted in 1:1
A-V conduction with aberrant ventricular activation.
Figure 1 depicts the refractory period determination during sinus rhythm for patient 2. Figure IC
demonstrates that the effective refractory period of
the His-Purkinje system begins at an H1-H2 interval
of 440 msec. Figure 2A and B demonstrates that in
this patient both a 1:1 and 2:1 A-V conduction
response could be produced when the atrial rate
was suddenly accelerated to a cycle length of 380
msec. The sinus cycle lengths preceding the onset of
abrupt atrial stimulation vary by only 20 msec. The
A-H interval of the sinus beats measures 70 msec.
In figure 2A, the first atrial capture beat (A2)
occurs at an A1-A2 interval of 470 msec with an A2H2 interval of 110 msec. The corresponding H1-H2
interval is 510 msec and, since this value is greater
than the determined ERP of the His-Purkinje
system, conduction to the ventricles occurs. The
decrease in ventricular cycle length from 870 to 510
msec results in a corresponding decrease in ERP of
the His-Purkinje system to a value of less than 380
msec. This is reflected in the fact that all
subsequent atrial impulses are conducted at H-H
interval of 380 msec. In figure 2B, the first atrial
capture beat occurs at an A1-A2 interval of 720
msec. The corresponding H-H interval measures
750 msec. A decrease in the ventricular cycle length
from 890 to 750 msec does not significantly alter
ERP of the His-Purkinje system from its determined
value of 440 msec (fig. 1). This is reflected in the
fact that the next atrial capture beat is blocked
within the His-Purkinje system at an H-H interval
of 400 msec. Thereafter, 2:1 A-V block is maintained at H-H intervals of 380 msec.
Patient 3 demonstrated three types of A-V
conduction patterns at the same paced atrial cycle
length of 380 msec. Using the extrastimulus method
(as shown in fig. 1), the ERP of the His-Purkinje
system at sinus cycle lengths, which ranged from
800 to 950 msec, occurred at an H1-H2 interval of
440 msec. Figure 3 (continuous trace) demonstrates that the first atrial capture beat results in an
H1-H2 interval which is the same as the ERP of the
His-Purkinje system (440 msec) and consequently
block within this system occurs. Therafter, 2:1 A-V
block within the His-Purkinje system occurs at H-H
intervals of 380 msec.
Figure 4A illustrates that, at the same paced cycle
length of 380 msec, 1:1 A-V conduction resulted by
the same mechanism as in figure 2A. The first atrial
capture beat results in a shortening of the
ventricular cycle length from 800 to 590 msec. A
significant decrease in ERP of the His-Purkinje
system follows so that 1:1 A-V conduction was
maintained at H-H intervals of 435 msec to 380
msec.
Figure 4B demonstrates 1:1 A-V conduction with
aberration at a paced cycle length of 380 msec. The
first atrial capture beat shortens the ventricular
cycle length from 880 to 760 msec which in turn
shortens the refractory period of the His-Purkinje
system. However, the decrease in the refractory
period is less than that which occurs in figure 4A.
Consequently, the second atrial capture beat
which results in an H-H interval of 440 msec falls
within the relative refractory period of the HisPurkinje system and 1:1 conduction with LBBB
aberration results.
The functional nature of this type of His-Purkinje
block is further exemplified by the findings shown
in figures 5 and 6. Patient 4 (fig. 5) demonstrated
either 1:1 or 2:1 A-V conduction when the atria
Circulation, Volume XLVII, March 1973
A-V BLOCK
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Figure 1
Refractory period determination in patient 2 during sinus rhythm. The sinus cycle lengths (CL) vary
between 810 and 870 msec. In each panel, the tracings from top to bottom are electrocardiographic leads
I, II, III, a high right atrial bipolar electrogram (HRA), and His bundle electrogram (HBE). T denotes
time lines at 10 and 100 msec. A premature atrial beat (A2) is introduced after every eighth sinus beat
(A1). (A-C) The A,-A,, coupling interval is progressively shortened from 600 to 420 msec. (C) The A,
depolarizes the bundle of His (Hj) but fails to activate the ventricles. The resultant H1-H2 of 440 msec
defines the effective refractory period of the His-Parkinje system. Similar abbreviations will be used for
other figures.
were suddenly accelerated at a cycle length of 430
msec. The initial portion of figure SA shows a
period of 2:1 A-V block within the His-Purkinje
system. The third ventricular complex is a spontaC:rculaion, Volume XLVII,
March 1973
neous premature ventricular beat followed by 1:1
A-V conduction. The premature ventricular beat
retrogradely depolarizes the bundle of His as
evidenced by the fact that the fifth atrial beat is not
538
A
DAMATO ET AL.
CL 380
*
"
_
l\
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H~~H
T
iH-H 89
38O1 l380
400
750
380
A
.iiil)tii.i
¢ii
,ti1,hit,ttit1szt
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Figure 2
Results of abrupt atrial pacing at a cycle length (CL) of 380 msec in patient 2. (A) Sudden acceleration of
the atrial rate results in 1:1 A-V conduction, while in B 2:1 block within the His-Purkinje system results.
followed by a His deflection. This results in a
shortening of the cycle length of the His-Purkinje
system and consequently a decrease in its effective
refractory period so that 1:1 A-V conduction
follows at the same H-H intervals that previously
produced 2:1 A-V block. In figure 5B a spontaneous
ventricular beat occurring at a slightly more
premature coupling interval is retrogradely concealed within the His-Purkinje system. Note that
the fifth atrial impulse is followed by a His
deflection. Since the cycle length of the HisPurkinje system was not altered by the premature
ventricular beat refractoriness of this system
remains the same and 2:1 A-V conduction persist.
Figure 6 demonstrates that during 1:1 A-V
conduction at a paced atrial cycle length of 400
msec failure to capture the atrium for one beat
(third stimulus) causes a sudden increase in the
ventricular cycle length which in turn produces an
increase in effective refractory period of the HisPurkinje system. Following this latter change 2:1
block within the His-Purkinje system occurs.
Discussion
In the present study, as contrasted with most
previous studies using His bundle recordings, atrial
acceleration was initiated abruptly rather than
incrementally." 2,6-8 In addition, the resultant
conduction patterns (2:1 A-V block within the HisPurkinje system, 1:1 A-V conduction with normal
ventricular activation, and 1:1 A-V conduction with
aberrant ventricular activation) were correlated
with refractory period studies using the extrastimulus method.
The occurrence of any one of the three A-V
conduction patterns was determined primarily by
(1) an effective refractory period in the HisPurkinje system which exceeded that of the A-V
node, (2) the coupling interval of the first atrial
capture beat and its effect or lack thereof on the
refractory period of the His-Purkinje system as it
relates to changes in ventricular cycle length, and
(3) the relative speed of A-V nodal conduction
time.
The fact that both the effective and relative
refractory periods of the His-Purkinje system can be
altered by changes in ventricular cycle length
explains why at the same paced rate, and with
identical H-H intervals, 2:1 A-V block or 1:1 A-V
conduction (with or without aberration) can result.
The effective and relative refractory periods of the
Circulation, Volume XLVII, March 1973
539
A-V BLOCK
CL 380
vi
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880
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Figure 3
Continuous record of patient 3 in whom 2:1 block within the His-Purkinje system resulted when the atrial
rate was suddenly accelerated to a CL of 380 msec. The first atrial capture results in an Hi-H interval
of 440 msec which is within the effective refractory period of the His-Purkinje system.
His-Purkinje'system are greater at longer ventricular cycle lengths and decrease when the ventricular
cycle length is shortened.5' 9,
This same relationship between ventricular cycle length and refractoriness also explains the perpetuation of either the
2:1 or 1:1 A-V conduction patterns throughout the
pacing procedure.
During 2:1 A-V block, the nonconducted atrial
impulses encounter an area of maximum refractoriness within the His-Purkinje system. The exact area
of maximum refractoriness could not be determined
in this study but may be within the common bundle
itself, the bundle branches, or peripheral Purkinje
network. Since the area of maximum refractoriness
and the conducting tissues distal to it are not
depolarized, they effectively maintain a long cycle
length which is equivalent to that of the conducted
atrial impulses. The longer ventricular cycle length
is associated with a longer effective refractory
period and, consequently, 2:1 A-V block is maintained. However, if during 2:1 A-V block, the distal
area of maximum refractoriness is prematurely
10
Circulation, Volume XLVII, Marcb 1973
depolarized,
retrogradely by a
(fig. 5), its refractory
period is shortened. Thereafter, the shorter refractory period permits 1:1 A-V conduction at the same
H-H interval which previously caused 2:1 A-V
block. This same relationship between cycle length
and refractoriness can be invoked to explain 1:1
A-V conduction when the cycle length is sufficiently
shortened by the first atrial capture beat (figs. 2A
and 4A).
The relative speed of A-V nodal conduction time
during atrial pacing also contributed to the
development of 2:1 A-V block within the HisPurkinje system. The A-H intervals during abrupt
acceleration of the atrial rate were not markedly
prolonged relative to the A-H interval during sinus
rhythm. This lack of significant A-V nodal conduction delay permitted the paced atrial impulses to be
delivered to the His-Purkinje system within its
effective refractory period. Had A-V nodal conduction time been more prolonged, the H-H intervals
as
can
occur
premature ventricular beat
540
DAMATO ET AL.
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Figure 4
Patient 3. (A) Atrial pacing at the same CL as shown in figure 3 results in 1:1 A-V conduction. The first
atrial capture beat results in an Hi-H. interval of 590 msec which is outside the effective refractory period of the His-Purkinje system. (B) Atrial pacing at a CL of 380 msec resulted in 1:1 A-V conduction
with aberration of a LBBB pattern.
would have been greater than the ERP of the HisPurkinje system and 1:1 conduction would have
always resulted. Alternatively, a greater delay in
A-V nodal conduction time may have only resulted
in A-V nodal Wenckebach type of conduction or 2:1
A-V nodal block during abrupt atrial pacing.
The occurrence of a functional 2:1 A-V block
within the His-Purkinje system during abrupt atrial
pacing simulates a type II second-degree A-V
block. 1 2, 7, 8, 11, 12 As seen clinically, patients with
type II block are generally in sinus rhythm.
However, patients who have type II second-degree
A-V block during sinus rhythm almost always show
2:1 A-V block within the His-Purkinje system when
the atrial rate is incrementally increased. In both
groups of patients, the His-Purkinje system has the
longest effective refractory period of the A-V
conducting system. Despite these electrophysiologic
similarities, the two groups can and should be
distinguished. The more frequent clinical use of His
bundle recordings and atrial pacing as a stress test
makes this distinction of more than academic
interest.
Patients who have a spontaneous or true type II
second-degree A-V block generally have symptoms
which can be attributed to the A-V block and
invariably develop complete heart block for which
permanent ventricular pacemaker therapy is indicated. On the other hand, the eight patients who
had functional block with the His-Purkinje system
did not have symptoms which could be related to
A-V block and never demonstrated A-V block
during sinus rhythm prior to or following this study.
A majority of patients with type II A-V block have
an associated right or left bundle-branch block
pattern while most patients in this study did not.
Also, the H-V interval during sinus rhythm is almost
invariably prolonged (>55 msec) in the former
group, while it was found to be within normal limits
in the latter group.
A functional 2:1 block within the His-Purkinje
system can also occur during abrupt spontaneous
increases in atrial rate. This has been documented
to occur in a case of paroxysmal A-V nodal
reentrant tachycardia in an otherwise healthy
Circulation, Volume XLVII, March 1973
A-V BLOCK
541
A
430
H-H
T
B
<390
430
440
2:1
430
430
2:1
PVB
_~~~~~~~0
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~~~~~~~~~~~~~~~~~A
AHv ~
.
iBE
1S
AM
-J1
-
r::rsvrS
F1 S,
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AMY
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A
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430
A
Js,
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430
440
430
440
430
Figure 5
Patient 4. (A) A 2:1 block within the His-Purkinje system is converted to 1:1 conduction following a
sponttaneous premature ventricular beat (PVB) which retrogradely depolarizes the bundle of His and
shortens the His-Purkinje cycle length to <390 rnsec. The
grade His deflection would have appeared had it not been
PVB occurs slightly earlier in the cardiac cycle and fails to
The cycle length of the His-Purkinje system was effectively
patient (fig. 4 of reference 13). In addition, we have
recently observed a case of functional 2:1 block
2
3
HBE
4
400
A
arrow indicates the point at which the ante-
retrogradely depolarized by the PVB. (B) A
retrogradely depolarize the bundle of His.
not changed and 2:1 A-V block continues.
within the His-Purkinje system occurring during the
acute onset of atrial flutter.
A
400 A
A H V
AHVAH
AH
A H ,fA115A
_4VV
V
v~~~~~~~~~~~~~~~~~~~~~vIY
45,~W140
H-H755
100
^
10
451
(
5045
f1~
Figure 6
Functional nature of 2:1 His-Purkinie block. Atrial pacing at a constant cycle length of 400 msec resulted
in continuous 1:1 A-V conduction as shown by the first two beats. The third stimulus fails to capture
the atria, and the ventricular cycle length is suddenly increased from 400 to 755 msec. The associated
increase in the refractory period of the His-Purkinje system results in 2:1 His-Purkinje block during
continuous pacing.
Circulation, Volume XLVII,
March 1973
542A(
DAMATO ET AL.
Finally, figure 6 demonstrates that when 1:1 A-V
conduction suddenly reverts to 2:1 His-Purkinje
block an erroneous diagnosis of a true type II block
can be avoided by carefully monitoring those events
which immediately precede the period of block.
during cardiac pacing. Amer J Cardiol 27: 570,
1971
7. SCHUILNBERG RM, DUBRER D: Observations on
atrioventricular conduction in patients with bilateral
bundle branch block. Circulation 41: 967, 1970
References
BJ, HILDNER FJ: Localization of A-V conduction
defects in man by recording of the His bundle
electrogram. Amer J Cardiol 25: 228, 1970
9. MOE GK, MENDEZ C, HAN J: Aberrant A-V impulse
propagation in the dog heart: A study of functional
bundle-branch block. Circ Res 16: 261, 1965
10. MENDEZ C, GRUHZrr CC, MOE GK: Influence of cycle
length upon refractory period of auricles, ventricles
and A-V node in the dog. Amer J Physiol 184: 287,
1956
11. PEUCH P, GROLLEAU-RAOUX R: Les blocs auriculoventriculares classification basee sur l'enregistrement
de l'activite du tissu de conduction. Coeur Med
Interne 10: 615, 1971
1. DAMATO AN, LAU SH, HELFANT RH, STEIN E, PATTON
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Circglation, Voinme XLVII, March 1973
Functional 2:1 A-V Block within the His-Purkinje System: Simulation of Type II Second-Degree
A-V Block
ANTHONY N. DAMATO, P. JACOB VARGHESE, ANTHONY R. CARACTA, MASOOD
AKHTAR and SUN H. LAU
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Circulation. 1973;47:534-542
doi: 10.1161/01.CIR.47.3.534
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