Ventricular Performance Modified by Elastic
Properties of Outflow System
By Peter F. Salisbury, M.D., Ph.D., Cecil E. Cross,
and P. Andre Rieben, B.S.
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• In 1834, Ernst Heinrich Weber,1 Carl Ludwig's pi-edeeessor in Leipzig, proposed that
during the systolic phase of cardiac cycles,
the heart accelerated only the ejected blood
volume, which was temporarily stored in the
large arteries and forwarded to the tissues
during diastole. He postulated that the distensibility of the arterial tree enabled it to
receive the ejected stroke volume and that the
stretched arterial walls provided the force
which caused blood to flow not only in systole
but also during diastole. He compared the
elastic storage capacity of the arterial tree
with devices used in 1834 to transform pulsatile into nonpulsatile flow, such as the
"Windkessel" of manually operated fire engines or the compressed-air chambers of pipe
organs that were kept charged from bellows
operated by the organist with his feet. Weber
believed that ejection into an elastic outflow
system would save work because pulsatile flow
in a rigid conduit would require acceleration
of a large mass at the beginning of systole.
Although the idea that a distensible chamber would offer less resistance to ventricular
ejection has been a part of the thinking of
all major schools of cardiovascular physiology,
the literature contains few reports which
describe how one or more directly measured
parameters of cardiac performance were influenced by the elastic properties of the outflow system (i.e., the arterial tree in vivo).
Otto Frank 2 published tracings of atrial and
ventricular pulse contours in frog hearts
Prom the Intensive Treatment Center and Department of Medical Research, St. Joseph Hospital,
Burbank, California.
Supported by Grant H-6766 from the National
Heart Institute, U. S. Public Health Service, and
by a grant from the Tobacco Industry Research
Committee.
Received for publication February 5, 1962.
Circulation Rcecarch, Volume XI, August 1968
before and after substitution of an elastic
ejection tube for a rigid one. De Burgh Daly3
considered the blood circuits of his modified
heart-lung preparation to have '' capacity'' as
well as "resistance" characteristics; he investigated the relationship between effective
circulating blood volume and the pressure in
the atria and ventricles but did not mention
elastic storage capacity as an entity that might
have influenced cardiac function. Knebel4-5
inferred from the shape of aortic pulse curves
in heart-lung preparations that ventricular
performance must have been influenced by the
elasticity of the outflow system; he made no
direct measurements of cardiac function parameters. Analysis of aortic pulse contours
enabled Eemington and Hamilton6 to measure
kinetic and total external work of the left
ventricle from moment to moment during
systole; a relationship of kinetic work and
outflow elasticity is apparent from their data.
Alexander7 and Eemingtons correlated experimental variations of the volume and the
elasticity of the outflow system with simultaneously observed heart size (cardiometer).
Sinn9 and Kenner10 inferred modification of
cardiac performance after changes of outflow
system elasticity; their ingenious and searching mathematical analyses originated from
concepts developed by Otto Prank 11 and by
Wezler and Boger.12 Imperial, Levy, and
Zieske13 recently published left atrial and
"systemic" pressure data from a heart-lung
preparation that permitted intentional variation of elastic storage capacity.
Because the relationship between cardiac
function and outflow system elasticity had
not yet been clarified, we performed the experiments reported here, which investigated
the influence on the heart of this previously
neglected parameter under rigidly controlled
conditions.
319
320
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Methods
In 24 anesthetized (25 mg./Kg. pentobarbital
sodium, I.V.), heparinized (3 mg./Kg.) mongrel
dogs under positive pressure ventilation, the chest
was entered through a bilateral thoracotomy in
the fourth intercostal space. The azygos and two
great veins were ligated. All systemic blood issued
into a "venous" reservoir (gravity drainage
through cannulae inserted via jugulnr and femoral
veins into both cavae). A calibrated pump injected venous blood into both pulmonary arteries,
bypassing the right ventricle. A catheter in the
right heart returned all coronary venous blood to
the venous reservoir and afforded measurements
of total coronary venous flow (timed collection in
graduated cylinder) and of coronary venous oxygen content. In 12 experiments, the ascending
aorta was partially transected about 2 to 3 cm.
distal to the aortic valve and was entered with
the flared end of a curved polyethylene tube of
12 mm. I.D. In one experiment (25-Kg. greyhound, 280-Gm. heart), a methyl methacrylate elbow tube of 27 mm. I.D. was connected with the
ascending norta (this experiment is not reported
separately because the results did not differ substantially). The stumps of ascending aorta were
fixed to the rigid outflow elbows with several
umbilical tape ligatures. In eight of the 12 experiments, coronary flow and cardiac oxygen consumption were measured (14S observations), in four
experiments, changes of left ventricular circumference (full circle) and changes of length of a
longitudinal meridional segment were followed by
recording tlie resistance of mercury-filled rubber
tubing.14- 15 In 12 other experiments, the brachiocephalio artery was entered with a curved, thinwalled stainless steel cannula of 9 mm. I.D.; pressure in the aortic arch was measured through
the left subelavian artery; the thoracic aorta
could be occluded with a screw clamp just distal
to the. left subelavian artery; the aortic arch
could be excluded by tightening a plastic snare
around the ascending aorta, which obliterated the
space between the outer wall of the stainless steel
cannula and the inner wall of the ascending aorta.
In all experiments, the cannnhi from the ascending aorta was connected with an "arterial" reservoir of variable elevation through a system of two
"T" connectors (stainless steel, 9 mm. I.D.) and
semirigid polyvinyl chloride (Tygon) tubing of
9 mm. I.D. and 15 mm. O.D. When this "rigid"
outflow conduit was occluded, an alternative "distensible" outflow conduit was opened; blood flowed
from the left ventricle to the arterial reservoir,
either through a rigid or else through a distensible channel. The distensible portion of the alternative outflow conduit began at the end of the "Y"
SALISBURY, CROSS, RIEBEN
connector about 15 cm. above the ascending aorta;
44 cm. of 5/8-inch Natural Latex Rubber Penrose Drainage Tubing (Da.vol Rubber Company, Providence, Rhode Island, no. 9791) was
ligated on one of the distal ends of the "Y" connector and rose in a vertical direction. It was followed by 30 cm. of 1-inch latex tubing (Da.vol,
no. 9794), which also rose in a vertical direction.
The distensible rubber tubing was connected with
the second "Y" and with the arterial reservoir by
means of Tygon tubing. The dimensions of the
rubber tubing had been selected to make the
shape of the "systemic pressure curve" resemble
the natural arterial pulse contour. The pressurevolume characteristics of the rigid and the distensible outflow systems are summarized in table 1.
Pressures were recorded on an eight-channel direct-writing Offner oscillograph-recorder at paper
speeds of 0.5 aJid 50 mm./sec, by means of
Statham P23Gb transducers and 10- to 15-em.
lengths of 1.8 mm. O.D. cardiac catheter tubing.
Pressure registration cannulae entered the left
ventricle via the apical dimple, the left atrium
through a segmental pulmonary vein, and the outflow conduit about 10 cm. above the aortic valve
["T" collection (lateral pressure) or gauge 20, P/G
inch needle (end pressure) ; pressures adjusted to
level of aortic valve]. Arterial and mixed coronary
venous oxygen contents were measured as described before18 (cuvette oximcter). Multiple measurements of blood oxygen content from the same
sample showed a, standard error of 0.2 to 0.3
volume per cent. The temperature of blood was
maintained constant by passing it through a heat
exchanger.17 In some experiments, the heart was
paced by atrial or ventricular pacing.18 The
heart rates varied from 116 to 170, Imt were
virtually constant in each observation.
The arterial reservoir had a wide (1.5 cm. diameter) overflow outlet which kept the blood level
at the desired elevation above the heart regardless
of cardiac output. Overflow from the arterial reservoir drained back into the venous reservoir. It
was assumed tha.t durinar steady states, all blood
injected by the calibrated pump into the pulmonary arteries issued into the left atrium and was
then ejected by the left ventricle; the small amount
of blood which might have been diverted through
bronchial veins was not measured. Resistance was
known from the constant level of the arterial
reservoir and from the mean pressure in the outflow tract. All measurements of pressure, coronary
flow, and cardiac oxygen consumption were made
during steady states. Representative tracings are
presented in the figures included here; a typical
experimental protocol is reproduced in table 2.
Circulation Research, Volume XJ, August 196ft
321
VENTRICULAR PERFORMANCE
TABLE
1
Volume-Elastic Properties of the Two Mechanical Outflow Systems Used Here Compared with Human
and Animal Arteries
Pressure Volume Pressure Volume
(mm. Ha) (ml.) (mm. Hgr) (ml.)
Coef.
elast.*
(dP/dV)
Modulus
elast.t
[(dP X V)/dV]
Coef. vol.
distensibil.t
[(dV X dP/V]
Structure
studied
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50
100
125
162
37.5 mm.
51,375 dynes
6,000 mm.
10,220,000 dynes
0.0016
0.0000116
Rigid system
50
240
75
310
87 mm.
117,000 dynes
1.17
0.00084
Distensible system
75
310
100
350
0.3 mm.
478 dynes
0.6 mm.
834 dynes
189
265,780
0.52
0.00037
Distensible system
100
350
125
375
1.0 mm.
1,334 dynes
257 mm.
342,500 dynes
0.28
0.00020
Distensible system
0.52 mm.
250,000 dynes
.Human ' ' Windkessel''
(cross section)"
0.9-1.2 mm.
108,000250,000 dynes
Human systemic
arteries (cross
section)"
1.2-3.2 nun.
200,000860,000 dynes
80-120
40
80
120
160
Whole human aorta
various ages2'
150-350 mm. Hg
DOR aortae27
1,500,000 dynes
2-4,000,000
5-8,000,000
9-12,000,000
Dog nortae,
carotids,22
femorals
*Coof. elast: Coefficient of volume elasticity (dP/dV) is the pressure, in mm. Hg (upper number) or in
dynes (lower number) per cm.5, which will increase the volume of a hollow container in the amount of 1.0
ml.11 For artery pressures, given in mm. Hg only.
tModulus olast.: The modulus of volume elasticity [(dP X V)/dV] is the ratio of the pressure increase
following1 a percentage change in volume.28 It characterizes the elastic properties of a distensible liollow
object, under conditions of elastic equilibrium in a manner which makes the increase of pressure independent of the total volume.50
JCocf. vol. distensibil.: The coefficient of volume distensibility [(dV X dP)/V] is the ratio of fractional
change in volume to change in pressure (i.e., the reciprocal of the modulus of volume elasticity). As expressed
above, per cent increase of volume with each 1 mm. Hg (upper number) or 1 dyne (lower number) per cm.'
increase in pressure.30
Results
"SYSTEMIC" PRESSURE INFLUENCED BY
ELASTIC PROPERTIES OF THE
OUTFLOW SYSTEM
Changes of systemic pressure discussed here
were observed when heart rate, left ventricular stroke volume, and stroke work were constant and the characteristics of the outflow
system were intentionally altered. Figure 1
shows a representative record. At a rate of
166 beats per minute and a stroke volume of
5.5 ml./.I.OO Gm. heart weight, the arterial
reservoir was fixed and its level held constant.
In condition A, the systemic circulation was
intact, except for the open cannula in the
brachiocephalic artery connecting with the
Circulation Research, Volume XI, August 19G2
arterial reservoir and for the occlusion of the
left subclavian artery; the ventricle ejected
into the remaining two-thirds of the arterial
tree and also into the arterial reservoir. Figure 1 B shows conditions after the thoracic
aorta had been occluded just distal to the
subclavian artery, so that the entire cardiac
output was ejected into the arterial reservoir
through a distensible conduit, with the arch
of the aorta acting as a "Windkessel." Figure 1 C shows ventricular and other pressures
when blood was ejected into the arterial reservoir through rigid tubing and other conditions
identical with B. Figure 1 demonstrates that,
with decreasing storage capacity of the out-
322
SALISBURY, CROSS, RIEBEN
TABLE 2
Protocol of a Representative Experiment
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HR
sv
LAP
150
149
4.9
5.0
18
15
150
150
4.9
4.9
16
13
151
153
5.9
5.9
13
17
152
148
7.4
7.6
18
15
150
149
7.5
7.5
18
21
146
148
7.7
7.6
17
20
148
.148
7.6
7.6
20
18
147
145
7.6
7.7
27
22
AP
LVP
Qcor
Ao2
A-V02
Vo/100
73
75
20.0
19.8
10.6
13.8
5.8
7.7
97
95
19.6
19.8
10.7
11.0
7.8
7.8
rigid
distensible
rigid
distensible
112
121
19.8
19.3
10.0
10.0
8.4
9.0
distensible
rigid
112
109
19.5
19.5
10.1
10.2
8.4
8.3
rigid
distensible
126
125
19.4
19.5
9.6
10.2
distensible
rigid
112
112
19.4
19.3
10.0
9.9
9.0
9.5
8.4
8.3
112
119
19.3
19.3
9.9
10.0
8.3
9.0
rigid
distensible
16.8
16.7
7.4
6.8
7.6
10.0
rigid
distensible
145/60
120/85
140/55
112/78
113/78
145/45
130/20
103/60
118/75
150/35
105/65
125/18
125/18
107/80
130/2
123/2
125/2
120/2
120/4
137/6
120/3
115/3
130/5
142/7
117/3
120/6
120/6
120/6
130/25
110/80
pulmonary edema
125/15
152
122/12
196
outflow system
distensible
rigid
•Entire cardiac output ejected into arterial reservoir through rigid or alternatively through distensible
conduit. Dog weight 15 Kg.; heart weight 134 Gm.
H B = H e a r t rate.
SV=Stroke volume per 100 Gm. heart weight, ml.
T_jAP=Mcnn left atrial pressure, mm. Hg.
A P = " Systemic" pressure, mm. Hg, at level of aortic valve; in this experiment true lateral pressure was
not measured.
LVP=Left ventricular pressure, mm. Hg.
Qcur=Coronary flow, ml./min.
Ao 2 =Arterial oxygon content, ml./lOO ml. blood.
A-Vo2=Coronnry arteriovenous oxygen difference, ml./lOO ml. blood.
Vo/100=Myoeardial oxygen consumption, ml./lOO Gm. heart/min.
flow system, the mean systemic pressure remained constant while the systemic pulse
pressure increased, so that the systolic pressure became elevated and the diastolic pressure depressed. Similar changes are evident
in the subsequent figures and from the data
in table 2. The relationship between the elastic
storage capacity of the outflow system and
the amplitude and shape of the "systemic"
pressure pulse was consistent, reproducible,
and reversible (256 paired observations).
When the dimensions of the latex rubber
segments were increased beyond the magnitude already described (under Methods), excessive secondary oscillations were observed in
the "systemic" pressure registration, and the
pulse contour in the outflow system ceased to
resemble the natural arterial pressure pulse.
Data from such "excessively distensible"
outflow systems are not included here.
LEFT VENTRICULAR PRESSURE AND
DIMENSIONS INFLUENCED BY
ELASTIC PROPERTIES OF THE
OUTFLOW SYSTEM
When the blood conduit to the arterial reservoir was distensible, left ventricular systolic
peak pressure was higher than systemic systolic peak pressure. Systemic and left ventricular systolic peak pressures Avere virtually
identical when the outflow system had no
elastic storage capacity and when lateral sysCirculation Research, Volume XI, August 196S
323
VENTRICULAR PERFORMANCE
100-/
••••
50-
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fflfl Hg
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FIGURE 1
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Influence of decreasing outflow elastic storage capacity on "systemic" and left ventricular pulse
contours. "Systemic" pressure (AP), measured
here in aortic arch through left subclavian artery;
left ventricular pressure (LVP). (A) Ventricle
ejecting into arterial tree and arterial reservoir
connected with brachiocephalic artery. (B) Aorta
occluded distal to subclavian artery. Entire cardiac output ejected into arterial reservoir through
distensible conduit. (C) Bigid conduit substituted
for distensible; other conditions as in B. See text
for details.
temic pressure was measured from a " T "
connection.
Contour and magnitude of ventricular pressure pulses were profoundly influenced by the
elastic properties of the outflow system. All
statements made here apply to fresh as well
as deteriorated* hearts, except that the performance of the latter was even more influenced by changed outflow elasticity. Under
otherwise constant conditions, the left ventricular (LV) peak pressure increased with
increasing rigidity of the outflow system (fig,
1). When the ventricular peak pressure increased as a result of changing from the
distensible to the rigid outflow system, the
increment of ventricular pressure was generated during the auxotonic (i.e., ejection)
phase of the cardiac cycle; at the same time,
the magnitude of ventricular pressure generated under isovolumic conditions declined or
remained the same. Left ventricular enddiastolic pressure varied in the same direction
as left ventricular peak systolic pressure, so
that ejection into a rigid outflow system would
*Detorioration refers to the condition of isolated
hearts after prolonged experimentation, when the
hearts had been exposed to multiple insults, usually
exhibited evidence of myocardial edema, and worked
at relatively increased left ventricular end-diastolic
pressure.
Circulation Research, Volume XI, August X96S
>
; \
Hj
•
FIGURE 2
Comparison of distensible and rigid outfloiu conduits. Stroke work constant. Heart rate, 160;
stroke volume, 5.7 ml./'100 Gm. heart. Left atrial
pressure (LAP). "Systemic" pressure recorded
from needle in outflow system. Between A and B,
the rigid outflow system ions substituted for the
distensible.
elevate and ejection into a distensible system
would reduce not only the systolic but also
the diastolic pressures in the left ventricle
(figs. 2 and 3 and table 3).
When the elevation of the arterial reservoir
was adjusted during changes of outflow elasticity, so as to keep the left ventricular peak
pressure constant, ejection into the rigid outflow system was still associated with increased
left ventricular end-diastolic pressures, even
though stroke work had to be reduced and
mean systemic pressure had to be decreased in
order to keep the "rigid" LV peak pressure
at the same magnitude as the "distensible"
LV peak pressure (fig. 4 and table 3 B).
A change from the distensible to the rigid
outflow conduit was always associated with
increased systolic and diastolic circumference
of the left ventricle. The segment of longitudinal left ventricular meridian either did
not change or else increased in length during
the transition from distensible to rigid outflow conditions (fig. 4).
The '' triangular'' shape of left ventricular
pulse contours (figs. 1, 2, and 3) did not depend on the freshness of the preparation; it
was noted as soon as the ascending aorta had
been connected with the mechanical outflow
system. After "triangular" LV pulse contours due to ejection into the reservoir, when
the ascending aorta was disconnected from the
reservoir and reconnected with the systemic
circulation (supplied by a heart-lung machine
in the interim), the LV pulse contour re-
324
SALISBURY, CROSS, RIEBEN
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FIGURE 3
Same heart as in figure 2, deteriorated condition.
Heart rate, 160; stroke volume, 4.6 ml./100 Gm.
heart. Between A and B, the distensible outflow
system ivas substituted for the rigid.
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LEFT ATRIAL PRESSURE AND ELASTIC
PROPERTIES OF THE VENTRICULAR
OUTFLOW SYSTEM
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CORONARY FLOW AND ELASTIC PROPERTIES OF THE VENTRICULAR
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In the experiments reported here, the mean
left atrial pressure as well as the individual
phases of the left atrial pressure tracing were
related to the magnitude of ventricular diastolic pressure. Left atrial pressure at heart
rates above 170 was not reported here because
it appeared to be related to incomplete ventricular relaxation and not to ventricular
diastolic pressure.18
•
02
O
O
roke
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turned to its original "square" shape. When
progressively increasing lengtlis of aorta acted
as elastic storage reservoirs, the isovolumic
and auxotonic phases of the LV pulse contour
became more sharply differentiated, and the
shape became progressively less "triangular."
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11
During consecutive observation periods
which followed each other, coronary vasomotor tonus did not appear to vary, and coronary
flow was related to coronary driving pressure17 when arterial oxygen content was constant and the heart was not injured by overdistention. Coronary flow was not influenced
by the elastic properties of the outflow system.
Overdistention of the ventricle and arterial
hypoxia (which supervened in some experiments after intentional variations of hemodynamie conditions) increased the coronary flow
when other conditions were constant or comparable; in the course of some experiments,
coronary flow steadily increased, reflecting
Circulation Research, Volume XI, August 196S
325
VENTRICULAR PERFORMANCE
3.0r
U.T13.1
'
2.0 .
RUBBER
*
i
0.0
FIGURE 4
Comparison of distensible and rigid outfloiu conduits. Left ventricular peak systolic pressure kept
constant by adjusting arterial reservoir simultaneously with changes of outflozv system. Left ventricular circumference (TJVC), on.) heart rate,
119; stroke volume, 6.2 ml./100 Gin. heart. Rigid
outfloiu system between A and B.
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progressive cardiac injury caused by repeated
overdistention.
MYOCARDIAL OXYGEN CONSUMPTION
AND ELASTIC PROPERTIES OF
THE OUTFLOW SYSTEM
Oxygen consumption per 100 Gm. heart per
minute was of a magnitude similar to that
reported elsewhere.10 When the distensible
was substituted for the rigid outflow conduit,
the oxygen consumption of the heart varied
little, or increased, although the LV pressure
peak was higher during ejections through a
rigid outflow system (table 3). When the arterial reservoir was adjusted during transitions
from rigid to elastic outflow conduits (or vice
versa), so that peak LV pressure remained
nearly constant (fig. 4), cardiac oxygen consumption appeared to follow a pattern which
was related to the stroke volume (fig. 5); cardiac oxygen consumption varied between 0
and 42 per cent, the direction of variation
depending upon the stroke volume. At stroke
volumes below 3 ml./lOO Gm. heart (caused
by rapid heart rates or intentional decrease
of blood volume pumped into the pulmonary
artery), the heart used more oxygen when
ejecting into the rigid system. At stroke volumes above 6 ml./lOO Gm. heart, cardiac
oxygen consumption was significantly higher
when the heart ejected into the distensible
system, even though it performed less work
per beat and contracted from lower LV diastolic pressures and volume.
Circulation Research, Volume XI, August 1962
O
o
i.O >
<
2.0 .
3.0
TYGON
4
6
6
10
12
SV ( m l / 1 0 0 Gm. HW)
FIGURE 5
Difference of cardiac oxygen consumption in rigid
and distensible outflow systems related to stroke
volume. Left ventricular peak pressure kept constant as in figure 4. Stroke volume (SV), ml./100
Gm. heart; difference between myocardial oxygen
consumption in rigid outfloiu system and myocardial oxygen consumption in distensible outflow
system (Vo), ml.Os/100 Gm. heart weight/min.
Each point represents one paired observation from
seven experiments in which the entire systemic flow
was directed into the arterial reservoir. Fiducial
limits (dotted line), 5 per cent level of significance
of oxygen measurements; points above and below
dotted lines significant.
Discussion
CRITIQUE OF THE METHOD
19
Pressures, cardiac output, stroke volume,
stroke work, coronary flow, and cardiac oxygen consumption were measured here with a
degree of accuracy sufficient for purposes of
the experiment. When the heart ejected into
the arterial system, the animal's head was excluded from the circulation, and the mechanical properties of the arterial tree were influenced by humoral and nervous effects of
cerebral asphyxia. However, the circulation
to the head was arrested 10 to 20 minutes before the first observations; the full effect of
cerebral asplryxia must have been present at
that time and was not believed to vary arterial elasticity in subsequent observations. The
arterial reservoir was elevated to yield mean
326
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"systemic" pressures in the normotensive or
hypotensive range. The elastic modulus of
the rigid Tygon tube was much greater than
that of any naturally occurring arterial system, but was comparable with conditions in
all published heart-lung or isolated heart
preparations for the study of cardiac performance. The elastic properties of the distensible outflow system were of a magnitude
similar to that found in mammalian arteries,0' 20~22 and yielded pressure tracings which
resembled the "natural" systemic arterial
pulse contour. Aortic regurgitation in excess
of the (unmeasured) physiological quantity
was not evident from our systemic or left ventricular pressure tracings. Atrial pressures
were not determined by incomplete ventricular relaxation due to excessive heart rate. The
hearts beat with their own spontaneous sinus
rhythm, which did not vary when the outflow
elasticity was altered. Atrial or ventricular
pacing had no noticeable effect on the results;
paced and unpaeed observations are not separated in our data. The measured changes of
epicardial dimensions exceeded inherent errors of measurement.
"SYSTEMIC" PRESSURE AND
OUTFLOW ELASTICITY
The constancy of mean '' systemic'' pressure
and the variation of "systemic" arterial pulse
shape and magnitude were a function of
outflow system elasticity, as described before.12' 23' 24 At slow heart rates, the diastolic
pressure in rigid outflow systems reflected the
actual elevation of the arterial reservoir.
When more distensible systems connected the
ventricle with a reservoir of the same elevation, higher diastolic pressures were observed,
which must have been a function of elastic
force applied in diastole to the blood column
by the stretched rubber tubing.
VENTRICULAR FUNCTION AND
OUTFLOW ELASTICITY
When other factors were constant, decreases
of the outflow elastic storage capacity forced
the ventricle to generate greater amounts of
contractile tension with each beat. Such increments of contractile tension were generated during the ejection periods and not dur-
SALISBURY, CROSS, RIEBEN
ing the isovolumic phases of cardiac cycles.
Ejections into rigid conduits were characterized by greater LV systolic and diastolic
pressures, circumferences, and higher left
atrial pressures, as compared with the same
performance in distensible systems. When ventricular pressure peaks were purposely kept
constant during changes of outflow elasticity,
so that LV stroke work was reduced when the
Tygon conduit was substituted for the latex
rubber conduit, ejection into rigid outflow
systems was still associated with higher left
ventricular diastolic pressures and circumferences. The data appear to demonstrate the
effect predicted in 1834 by B. H. Weber that
absence of elastic storage capacity imposes a.
greater cardiac effort because the ventricle is
compelled to accelerate large volumes of blood
at the beginning of each systole. In vivo, the
kiuetic energy imparted to one stroke volume
of blood is considered to be 1 to 3 per cent of
the external Avork of a ventricle25; kinetic
energy must be a far more important factor
in systems such as the one used here, where
the ventricle was forced to overcome the inertia
of the entire column of blood distal to the
aortic valve (10 to 30 times greater than the
stroke volume) at the beginning of each systole. The data presented here furnish direct
evidence that the magnitude of cardiac effort
and the manner of ventricular performance
are influenced hy the elasticity of the outflow
systems; decreasing elastic storage capacity
compels the ventricle to generate increasing
amounts of contractile tension, which is refleeted in the increased left ventricular peak
pressure during the ejection period, and
which is developed from proportionally increased diastolic pressures and circumferences.
CARDIAC OXYGEN CONSUMPTION
AND OUTFLOW ELASTICITY
We expected but did not find a considerable
decrease of myocardial oxygen consumption
when the ventricle ejected into the distensible
S3rstem. The unchanged or even decreased oxygen consumption of heart attached to rigid
outflow conduits is of considerable interest,
Circulation Research, Volume XI, August J90S
327
VENTRICULAR PERFORMANCE
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because such preparations expend more mechanical effort from higher diastolie pressures
and greater circumferences at equal or diminished oxygen cost. The preparation described
here did not permit measurement of total energy transformation, and it is possible that
varying fractions of total cardiac metabolism
may have proceeded through anaerobic pathways; for this reason, the oxygen consumption
measured here may not have permitted assessment of "efficiency" of the heart and comparison of the efficiency of its performance in
rigid and distensible outflow systems. At
present, we cannot explain the significant decreases of myocardial oxygen consumption
which were associated with decreased outflow
elastic storage capacity.
SIGNIFICANCE OF DATA PRESENTED HERE
In the course of life from infancy to old
age, the elastic properties of the arterial tree
undergo slow, progressive changes, which
afford the heart ample time for appropriate
adaptation. From calculations12 and direct
measurements,21*20 it is known that the volume of large arteries increases with age, so
that the elastic storage capacity of the arterial
system is not markedly diminished in aged
individuals, in spite of its increased modulus
ot: elasticity. In the experiments reported
here, the elastic storage capacity of the left
ventricular outflow system was changed suddenly and drastically Avhile other parameters
known to determine cardiac performance were
kept constant. An analogous event would not
occur in vivo, except perhaps under conditions
of cardiovascular surgery. Our data cannot
explain changes of cardiac performance in
aged or arteriosclerotic persons. However,
for purposes of designing experiments or evaluating published reports, the results presented
here are significant. Experimental preparations for the study of cardiac performance,
such as isolated hearts or heart-lung preparations, must include a distensible left ventricular outflow system, because rigid arterial
blood conduits will distort cardiac function
to an extent which precludes comparison with
conditions in vivo.
Circulation Research, Volume XI, August 1962
Summary
Left ventricular and atrial pressures, left
ventricular circumference and longitudinal
meridional segment length, coronaryflow,and
cardiac oxygen consumption were measured
under conditions which permitted intentional
variation of the elasticity of the left ventricular outflow system and comparison of cardiac
performance during steady states in which
heart rate, stroke work, and stroke volume
were identical, but the outflow elastic storage
capacity was varied. When ejecting through
a rigid (as compared with a distensible) system, the left ventricle generated a larger fraction of the contractile tension under auxotonic
conditions (i.e., during the ejection phase),
and left ventricular systolic peak pressure
was increased. Ejections through rigid tubes
proceeded from higher diastolic ventricular
pressures and larger external dimensions. The
effort of hearts whicli ejected through rigid
outflow conduits was associated with a reduced
myocardial oxygen consumption. Studies of
left ventricular performance in isolated hearts
are physiologically significant only when the
experimental arrangement comprises an outflow system with distensibility characteristics
similar to that of the arterial tree.
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Circulation Research, Volume XI, August 1962
Ventricular Performance Modified by Elastic Properties of Outflow System
Peter F. Salisbury, Cecil E. Cross and P. Andre Rieben
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Circ Res. 1962;11:319-328
doi: 10.1161/01.RES.11.2.319
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