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Am J Physiol Heart Circ Physiol 285: H2027–H2033, 2003.
First published July 17, 2003; 10.1152/ajpheart.00249.2003.
Assessment and consequences of the constant-volume
attribute of the four-chambered heart
Andrew W. Bowman and Sándor J. Kovács
Cardiovascular Biophysics Laboratory and Cardiovascular MR Laboratories, Cardiovascular
Division, Washington University School of Medicine, St. Louis, Missouri 63110
Submitted 20 March 2003; accepted in final form 10 July 2003
FULL UNDERSTANDING of the cardiovascular system at the
organ, tissue, and cellular levels represents a continued challenge (27). To facilitate analysis of physiological data in the context of broad, conceptual ideas, both
computational and theoretical models have been developed (18, 19). We (19) have previously defined and
validated a conceptual and mathematical model that
all ventricles obey when they fill. One beneficial consequence is that if a model is formulated appropriately,
new physiological relationships can be predicted and
validated, and seemingly unrelated physiological or
clinical observations can be related and causally explained (21, 23, 25).
Although physiologically profound, the constant-volume pump attribute of the four-chambered heart is
both conceptually and mathematically simple, yet it
has received insufficient attention. It states that the
total volume of the pericardial sack, containing the
atria, ventricles, myocardium, coronary arteries, and
roots of the aorta and pulmonary artery, remains constant throughout the cardiac cycle.
The constant-volume heart concept was first proposed in 1932 by Hamilton and Rompf (8), who invoked
teleological arguments. In the 1980s, Hoffman and
Ritman (12, 13) performed a series of pioneering experiments using the dynamic spatial reconstructor (DSR).
They found that in dogs, pericardial sack volume varied by ⬃5% between end systole and end diastole. They
also concluded that, independently, the left and right
sides of the heart were relatively constant two-chamber pumps. However, the DSR was unable to validate
the constant-volume attribute in humans due to technological limitations of X-ray imaging in conjunction
with anatomic differences between dogs and humans
(11). Later studies used magnetic resonance (MR) imaging (MRI) to evaluate pericardial volume in humans
at several discrete timepoints (9–11) and in Fontan
procedure candidates (4), but lacked sufficient resolution to measure chamber size accurately and required
long scan times. A detailed human study, using the
most recent MRI technology to determine pericardial
volumes and cardiac chamber volumes simultaneously,
has not been performed.
Mathematical modeling of the constant-volume
pump attribute of the four-chambered heart predicts
specific and testable physiological consequences. Selected examples include 1) a model of left ventricular
(LV) filling that accurately predicts transmitral flow
patterns in normal and pathological states (19, 20); 2)
the reciprocation of atrial and ventricular volumes and
Address for reprint requests and other correspondence: S. J. Kovács, Cardiovascular Biophysics Laboratory, Barnes-Jewish Hospital, Washington Univ. Medical Center, Box 8086, 660 S. Euclid Ave.,
St. Louis, MO 63110 (E-mail: [email protected]).
The costs of publication of this article were defrayed in part by the
payment of page charges. The article must therefore be hereby
marked ‘‘advertisement’’ in accordance with 18 U.S.C. Section 1734
solely to indicate this fact.
cardiac magnetic resonance imaging; mathematical modeling; diastasis; pericardial motion
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0363-6135/03 $5.00 Copyright © 2003 the American Physiological Society
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Bowman, Andrew W., and Sándor J. Kovács. Assessment and consequences of the constant-volume attribute of
the four-chambered heart. Am J Physiol Heart Circ Physiol
285: H2027–H2033, 2003. First published July 17, 2003;
10.1152/ajpheart.00249.2003.—The constant-volume hypothesis regarding the four-chambered heart states that total pericardial volume remains invariant throughout the
cardiac cycle. Previous canine studies have indicated that the
pericardial volume remains constant within 5%; however,
this hypothesis has not been validated in humans using
state-of-the-art technology. The constant-volume hypothesis
has several predictable functional consequences, including a
relationship between atrial ejection fraction and chamber
equilibrium volumes. Using cardiac magnetic resonance
(MR) imaging (MRI), we measured the extent to which the
constant-volume attribute of the heart is valid, and we tested
the accuracy of the predicted relationship between atrial
ejection fraction and chamber equilibrium volumes. Eleven
normal volunteers and one volunteer with congenital absence
of the pericardium were imaged using a 1.5-T MR scanner. A
short-axis cine-loop stack covering the entire heart was acquired. The cardiac cycle was divided into 20 intervals. For
each slice and interval, pericardial volumes were measured.
The slices were stacked and summed, and total pericardial
volume as a function of time was determined for each subject.
In the normal subjects, chamber volumes at ventricular end
diastole, end systole, and diastasis were measured. Pericardial volume remained invariant within 5 ⫾ 1% in normal
subjects; maximum variation occurred near end systole. In
the subject with congenital absence of the pericardium, total
heart volume, defined by the epicardial surface, varied by
12%. The predictions of the relationship between atrial ejection fraction and chamber equilibrium volumes were well fit
by MRI data. In normal subjects, the four-chambered heart is
a constant-volume pump within 5 ⫾ 1%, and constant-volume-based modeling accurately predicts previously unreported physiological relationships.
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THE CONSTANT-VOLUME FOUR-CHAMBERED HEART
METHODS
Imaging. After appropriate informed consent was obtained
(consistent with the Washington University Medical Center
Human Studies Committee guidelines), 11 normal subjects (6
men and 5 women) and 1 (male) subject with congenital
absence of the pericardium were scanned with a 1.5-T Philips
Gyroscan MRI system (Release 9.0, Philips Medical Systems;
Best, The Netherlands). Survey images and standard planes
for ventricular four-chamber, short-axis, and LV outflow
tract (LVOT) views were obtained. Once determined, highresolution cine loops of the four-chamber and LVOT views
were obtained during subject breathholds (duration of ⬃10 s
each). These cine loops were divided into 20–27 cardiac
phases triggered from the ECG R wave covering the entire
cardiac cycle.
The initial four-chamber view was used to plan a shortaxis cine-stack protocol of between 18 and 20 slices, 9 mm
thick with zero gap, spanning the apex of the ventricles
through the superior-posterior wall of the LA. Image slices
were obtained during 10-s breathholds per slice, and each
cine loop was divided into 20 cardiac phases triggered from
the ECG R wave covering the entire cardiac cycle. For the
short-axis stack image acquisition, the repetition time, echo
time, and flip angles were 4.0 ms, 1.47 ms, and 50°, respectively. In-plane resolution of 1.41 mm was obtained with a
field of view of 36 cm and a matrix size of 192 ⫻ 256
interpolated to 256 ⫻ 256.
Upon completion of the exam, the data was archived to
4.1-GB magnetooptical disks and transferred to a remote Sun
Microsystems workstation running EasyVision analysis software (Philips Release 5.1). All image analysis was performed
off-line using EasyVision or on a remote personal computer
using eFilm 1.5.3 (eFilm Medical; Toronto, Ontario, Canada),
Paint Shop Pro 7 (Jasc Software; Minnetonka, MN), and
Scion Image (Scion; Frederick, MD).
In all subjects, the pericardial contour was manually
traced in each slice at each phase of the three-dimensional
(3-D) dataset (Fig. 1A), and the corresponding segmental
volume was determined. For each phase, the segmental volumes of the pericardial traces were summed and compared
over the cardiac cycle. To compare hearts of different sizes,
the pericardial volumes were normalized to the volume measured at ventricular end diastole (phase 1) in all subjects.
Volumes of the individual cardiac chambers were measured
in a similar manner (Fig. 1, B and C).
Modeling. In the context of the constant-volume hypothesis, we assume steady state; hence, cardiac output, ejection
fraction, and stroke volume remain constant on a beat-tobeat basis. In analogy to the LV ejection fraction (EFLV),
defined as (stroke volume)/(LV end-diastolic volume), the
atrial ejection fraction EFA is defined by
EF A ⫽
AD ⫺ Am
AD
where AD is the (left ⫹ right) atrial volume at diastasis
(immediately before atrial contraction) and Am is the (constant) minimal (left ⫹ right) atrial volume (after atrial contraction). According to the constant-volume attribute of the
heart (ignoring the roots of the aorta and pulmonary artery
for simplicity)
VD ⫹ AD ⫽ C
(2)
where VD is the (left ⫹ right) ventricular volume at diastasis
and C is the (constant) total volume of the LV and RV plus
the LA and RA. Rearranging Eq. 2 and dividing both sides by
AD yields
Fig. 1. Typical balanced fast-field echo magnetic resonance imaging (MRI) images (short-axis view). A: pericardial
trace at midventricular level. B: endocardial traces of the left ventricle (LV) and right ventricle (RV). C: endocardial
traces of the left atrium (LA), right atrium (RA), aorta (Ao), and pulmonary artery (PA). S, superior; I, inferior; A,
anterior, P: posterior; R, right; L, left.
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redirection of blood momentum and flow during the
cardiac cycle (16); 3) the relation between the Doppler
E-to-E⬘ ratio and LV end-diastolic pressure (21); 4)
maximal LV wall thinning-to-Doppler E-wave relation
(3); and 5) the compensatory changes of right ventricular (RV) and LV stroke volumes caused by acute
perturbations in left (LA) or right atrial (RA) volumes
(7). Additionally, the constant-volume attribute model
predicts a specific, MRI-testable relationship between
diastatic chamber volumes (17) and the atrial ejection
fraction (EFA).
Because of its high spatial (⬍1 mm) and temporal
(⬍30 ms) resolution, cardiac MRI has become the gold
standard for LV mass and chamber size (volume) determination (33). The availability of a true steady-state
gradient echo technique [balanced fast-field echo
(bFFE)] (2, 29) makes MRI ideally suited for quantifying cardiac anatomy, function, and flow (1, 6, 33).
Cardiac MRI can image the heart in any plane at any
instant of the cardiac cycle. We are not aware of any
prior MRI study that investigated the functional (dynamic) interrelationships of all cardiac chambers
throughout the cardiac cycle and sought to validate
model-predicted relationships between chamber volumes and function.
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VD
C
⫽
⫺1
AD
AD
(3)
and solving Eq. 3 for AD in terms of VD/AD gives
AD ⫽
C
(4)
VD
1⫹
AD
Substituting Eq. 4 into Eq. 1 yields
C
1⫹
EF A ⫽
VD
AD
C
(5)
VD
AD
which, when rearranged in the y ⫽ mx ⫹ b form, generates
EF A ⫽ ⫺
冉 冊冉 冊 冉
Am
C
VD
AD
⫹
1⫺
Am
C
冊
(6)
Therefore, Eq. 6 predicts that as EFA increases, VD/AD
decreases linearly. This prediction can be tested by measuring the volumes of the atria and ventricles at ventricular end
diastole and diastasis. Chamber volumes were also measured
at ventricular end systole, and 3-D determined LV ejection
fractions were compared with those obtained using the arealength method from the LVOT view to corroborate the 3-D
measurement. Intra- and interobserver variability for measurements were determined using 10 randomly selected images.
Fig. 2. Plot of total pericardial volume versus time. The data are for
11 normal subjects (closed symbols) and one subject with congenital
absence of the pericardium (open circles). Time 0 corresponds to the
peak of the ECG R wave (ventricular end diastole). Total volume is
normalized to the pericardial volume measured at ventricular end
diastole. Maximum variation from end diastole was 5 ⫾ 1% in
normal subjects and 12% in the subject with congenital absence of
the pericardium. Maximum variation occurred at end systole in eight
of the normal subjects and within 75 ms of end systole in the other
three normal subjects; maximum variation occurred at end systole in
the subject with congenital absence of the pericardium. The ECG
displayed is a representative trace to clarify the timing of pericardial
volume variation. See text for details.
RESULTS
Demographics for all subjects are listed in Table 1.
The normalized total pericardial volume measurements as a function of time for all subjects are displayed in Fig. 2. Data from the subject with congenital
absence of the pericardium are shown separately (Fig.
2, open circles). In normal subjects, the maximum
change in total pericardial volume over the cardiac
cycle is ⬃45 ml, or 5 ⫾ 1% (mean ⫾ SD), relative to a
total end-diastolic pericardial volume of ⬃800 ml and
⬃25% compared with a combined (right ⫹ left) ventricular stroke volume of ⬃180 ml. In eight subjects, the
maximum pericardial volume change occurred at ventricular end systole. In the three other normal subjects,
the maximal pericardial change occurred within 75 ms
of end systole. In these subjects, however, the total
pericardial volume change during the 75-ms interval
from end systole was ⬍1% of the end-diastolic pericardial volume and essentially negligible (data not
shown). During ventricular systole, the combined
(left ⫹ right) atria filled by ⬃105 ml, and the combined
Table 1. Study population
28 ⫾ 8
174 ⫾ 11
71.6 ⫾ 15.9
64 ⫾ 12
Age, yr
Height, cm
Weight, kg
Heart rate, beats/min
Values are means ⫾ SD; n ⫽ 12 subjects.
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(aortic and pulmonary artery) roots filled by ⬃25 ml. In
the subject with congenital absence of the pericardium,
the maximum change in total epicardial surface heart
volume was 12% of the end-diastolic heart volume and
also occurred at end systole.
For the VD/AD and EFA measurements, the values
obtained for each parameter are listed in Table 2. The
plot of EFA versus VD/AD is shown in Fig. 3A. Linear
regression analysis yielded EFA ⫽ ⫺0.23(VD/AD) ⫹
0.79 (r ⫽ ⫺0.84). Similar analysis of the left and right
sides of the heart individually yielded the plots shown
in Fig. 3, B and C, respectively. For the left side of the
heart, EFLA ⫽ ⫺0.11(LVD/LAD) ⫹ 0.64 (r ⫽ ⫺0.48); for
the right side of the heart, EFRA ⫽ ⫺0.23(RVD/RAD) ⫹
0.67 (r ⫽ ⫺0.79).
Table 2. Calculated MRI parameters
VD/AD
EFA, %
C, ml
Am, ml
Am/C
2.13 ⫾ 0.37
30 ⫾ 10
353 ⫾ 85
81 ⫾ 24
0.23 ⫾ 0.02
Values are means ⫾ SD; n ⫽ 11 subjects. VD, (left ⫹ right
ventricular volume at diastasis; AD, (left ⫹ right) atrial volume at
diastasis; EFA, atrial ejection fraction; C, (constant) total volume of
the left and right ventricles plus the left and right atrial; Am,
(constant) minimal (left ⫹ right) atrial volume. Chamber volumes at
ventricular end diastole were used to calculate C.
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1⫹
⫺ Am
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THE CONSTANT-VOLUME FOUR-CHAMBERED HEART
Intra- and interobserver variability for tracings were
1.3% and 8.7%, respectively.
DISCUSSION
V pericard ⫽ V ventr ⫹ V atria ⫹ V roots ⫹ V tissue
(7)
This rule has predictable consequences concerning normal heart function. Most importantly, when differentiated, it implies conservation of flow over the pericardial boundaries and requires that as the ventricles
empty during systole, the atria and the roots of the
great vessels within the pericardial sack must simultaneously fill (8). Specifically, when differentiated with
respect to time [change in volume over time (dV/dt)],
we obtain
dV/dt pericard ⫽ dV/dt ventr ⫹ dV/dt atria
⫹ dV/dt roots ⫹ dV/dt tissue
Fig. 3. Regression plots for the relationship between the atrial
ejection fraction (EFA) and VD/AD [where VD is the (left ⫹ right)
ventricular volume at diastasis and AD is the (left ⫹ right) atrial
volume at diastasis (immediately before atrial contraction)] in
normal subjects for the left and right sides of the heart combined
(A), the left side of the heart alone (B), and the right side of the
heart alone (C). EFLA and EFRA, LA and RA ejection fraction,
respectively; LVD and LAD, LV and LA volumes at diastatis,
respectively; RVD and RAD, RV and RA volumes at diastatis,
respectively. See text for details.
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(8)
This expression defines the required reciprocation of
volumes of structures below the atrioventricular plane
(ventricles) to those above the plane (atria and great
vessel roots), a feature consistent with the findings of
other recent MRI studies (16, 32).
In the subject with congenital absence of the pericardium, the total heart volume change throughout the
cardiac cycle was 12%, or 7SD above the normal mean.
This suggests that the presence of the normal pericardium itself is required for the heart to maintain its
constant-volume feature within normal limits (see animations 3 and 4; http://ajpheart.physiology.org/cgi/
content/full/00249.2003/DC1). Previous findings have
demonstrated that the intact pericardium directly affects chamber pressures (24) and volumes (7), consistent with the notion that the normal pericardium has a
functional effect on cardiac physiology.
Contrasting the constant (four-chamber heart)-volume concept in the setting of simultaneous ejection and
filling using a quantitative example helps to elucidate
the dynamic nature of the physiology, as illustrated in
animation 1. The combined (left ⫹ right) ventricular
stroke volumes in the normal subjects sum to ⬃180 ml.
This is the total blood pumped out of the four-chambered heart during systole, lasting 0.33 s. This yields
an average ventricular ejection rate of 180/0.33 ⫽ 545
ml/s. The total pericardial volume change during this
time interval is ⬃45 ml, and this 45-ml change is
attained (effectively) by the time ventricular end sys-
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The findings of this study are significant because
they demonstrate in humans that the volume of the
normal four-chambered heart is rather constant (within 5%) throughout the cardiac cycle. As the ventricles
contract, the epicardial surface slides within the pericardial sack and follows its contour (see animations 1
and 2; http://ajpheart.physiology.org/cgi/content/full/
00249.2003/DC1). The individual volumes composed
of the blood-filled cavities [ventricles (Vventr), atria
(Vatria), roots of the outflow tracts (Vroots)], muscle, and
connective tissue (Vtissue) must add up to the total
volume of the pericardial sack (Vpericard) and hence
must obey the following summation rule
THE CONSTANT-VOLUME FOUR-CHAMBERED HEART
tole is reached. Accordingly, the average rate of change
for the entire pericardial volume is ⬃45/0.33 ⫽ 136
ml/s, or one-quarter the rate of change of the (left ⫹
right) ventricular volumes during ejection. The difference between the ventricular ejection rate and total
pericardial volume decrease rate is accounted for by
simultaneous filling of the atria and great vessel roots
by 130 ml, yielding an inflow rate of (105⫹25)/0.33 ⫽
394 ml/s. In the context of Eq. 8, using the sign convention that volume decrease is negative and volume
increase is positive, these rates yield
(⫺45/0.33)pericard ⫽ (⫺180/0.33)ventr ⫹ (105/0.33)atria
(9)
⫹ (25/0.33)roots ⫹ dV/dttissue
⫺136 ml/s ⫽ ⫺545 ml/s ⫹ 318 ml/s
⫹ 78 ml/s ⫹ dV/dt tissue
(10)
and assuming the tissue to be incompressible (dV/
dttissue ⫽ 0) or, at the least, negligible (dV/dttissue ⬍⬍
100 ml/s), we get
⫺136 ml/s ⬵ ⫺149 ml/s
(11)
When expressed in words, Eq. 11 means
Average rate of pericardial
(four-chamber heart) volume decrease
⬵ Difference between ventricular emptying
and simultaneous atrial ⫹ great vessel root filling
Combining the ventricular outflow volume and (atrial ⫹
root) inflow volume yields a net four-chamber heart
volume decrease of 50 ml (from 800 to 750 ml), which
closely approximates the 45-ml change observed when
measuring total pericardial volume rather than integrating over individual chamber volume changes.
Therefore, as the above numerical example illustrates,
whether measuring the total pericardial volume or
integrating over the rate of change of volume throughout the cardiac cycle, one concludes that the fourchambered heart is (within ⬇5%) a constant-volume
pump. The meaning of Eqs. 7–11 underscores this
feature and highlights that the ventricles empty
slightly faster than the atria and roots are able to fill.
This results in a small (⬵5%) end-systolic overall decrease in four-chamber volume that keeps the fourchambered heart at (essentially) constant volume.
Stated another way, in normal subjects, due to motion
and shape changes as a result of ventricular systole,
the inanimate pericardial sack develops a remarkably
low 5% “ejection fraction,” underscoring the nearly
constant-volume pump attribute of the four-chambered
heart! As shown in Fig. 2, the small four-chamber
volume decrease is made up during the next diastolic
interval.
Using the constant-volume attribute of the fourchambered heart, we derived a mathematical relation
predicting that the ratio of ventricular volume to atrial
volume at diastasis decreases as atrial ejection fraction
AJP-Heart Circ Physiol • VOL
increases. In vivo measurements using cardiac MRI
validated this prediction. Note the close agreement
between the value of Am/C in Table 2 and the magnitude of the slope of the linear regression equation.
A benefit of this approach is that it may be applied to
hearts that vary substantially in size: even though the
values for Am and C vary widely among the population
studied, the ratio of Am/C is remarkably fixed, again as
predicted by the constant-volume relationship. This
ratio may have physiological significance in describing
the proper anatomic relationship between atria and
four-chamber heart volume. It is also of interest to note
that the four-chambered heart as a whole obeys the
constant-volume prediction more closely than does either the left of right side of the heart individually. This
underscores the importance of evaluating global fourchamber heart function instead of restricting study to
just the left- or right-sided chambers.
Additionally, the lower correlation coefficient between EFA and VD/AD for the left heart (⫺0.48) compared with the right heart (⫺0.79) and the knowledge
that the maximum deviation from the ideal constantvolume condition occurs at ventricular end systole is of
physiological interest. We note that there appears to be
little change in the longitudinal (apex to base) dimension of the heart throughout the cardiac cycle compared with the change in the radial (short axis) dimension (compare Animations 1 and 2), a distinction that is
particularly striking in the subject with congenital
absence of the pericardium (compare Animations 3 and
4). This observation combined with the substantially
different correlation coefficients of the data presented
in Fig. 3, B and C, implies that the normal small (⬃45
ml) change in total pericardial volume may be due
primarily to systolic contraction and subsequent radial
displacement of the LV epicardial surface directed toward the centroid of the chamber rather than radial
displacement of the RV epicardial surface. Further
studies to address this kinematic issue are in progress.
Limitations. The cine loops obtained in the present
study were obtained during 10-s breathholds, so the
resulting “cardiac cycle” loop represents data obtained
over multiple heartbeats. However, during the exam,
subjects remain in a physiological steady state at rest,
so the resulting cine loop reflects this physiology. Indeed, the fact that identical (breathhold) cine loops
comprise the standard used in cardiac MRI imaging
(28) underscores that beat-to-beat variability is insignificant.
During breathhold scans prospectively gated by the
QRS, it is impossible for the scanner to continuously
scan from R wave to R wave; therefore, the scanner
often does not obtain data during all of the atrial
contraction phase of late diastole. This effect is exacerbated when the actual heart rate and the heart rate
that the scanner “expects” do not match well. Other
scanning techniques were available, such as retrospective gating, where the scanner acquires images continuously throughout the cardiac cycle and reconstructs
them in order after the scan is completed. However,
this technique may substantially increase scan time,
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or
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subjects using state-of-the-art cardiac MRI. We conclude that in normal subjects, the four-chambered
heart defined by the contents of the pericardial sack (of
⬃800 ml) behaves as a constant-volume pump within
5 ⫾ 1% and that an intact pericardium is required for
this degree of constancy to be maintained. We also
showed that mathematical modeling of the constantvolume attribute accurately predicts a novel relationship that simultaneously demonstrates coupling between chamber diastasis volumes and atrial function.
New insight was provided regarding how global fourchamber heart function differs from constant-volume
left or right heart function and the importance of longitudinal (apex to base) versus radial (short-axis) degrees of freedom. Application of this approach to intrapericardial dynamics and selected pathological states
is in progress.
The authors thank Shelton Caruthers for technical direction;
Mary Watkins and Todd Williams for image acquisition; Amr el
Shafei, LaTish McKinney, and Katherine Lehr for subject recruitment; and Mustafa Karamanoglu, Tim Meyer, Ryan Wu, and Alex
Waters for helpful comments regarding manuscript preparation.
DISCLOSURES
This study was supported in part by the Heartland Affiliate of the
American Heart Association (Dallas, TX), the Whitaker Foundation
(Roslyn, VA), National Heart, Lung, and Blood Institute Grants
HL-54179 and HL-04023, the Alan A. and Edith L. Wolff Charitable
Trust (St. Louis, MO), and Philips Medical Systems (Best, The
Netherlands).
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rendering breathholds less feasible. Free-breathing
scans typically exhibit increased motion artifacts,
making tracing of the pericardial and endocardial contours more challenging. Because breathhold images
are substantially less noisy, the breathhold method
was chosen for this study, even if parts of the atrial
contraction phase were omitted. To minimize this
atrial contraction loss, particular care was taken to
input the correct heart rate into the triggering algorithm. Future studies using other imaging techniques
may overcome the current limitations of prospectively
gated cardiac MRI. Specifically, multidetector spiral
computed tomography scanning (5, 26) may not only
cover a larger percentage of the cardiac cycle but also
provide higher resolution of cardiac chamber volumes.
It suffers from its own limitations, however, including
increased radiation exposure to the subject as well as
an intravenous contrast administration, both of which
were avoided by the use of MRI in this study.
Because the volumetric data were acquired using
9-mm-thick slices, there is the possibility of systematic
error in the volumes reported in the present study.
This error is most likely the largest when the chamber
to be measured is the smallest and has the thinnest
walls (i.e., the atria and outflow tract roots). However,
previous MRI studies evaluating atrial size have used
a similar if not larger slice thickness (14, 15, 30, 32).
Manual tracings of the chambers may also raise concerns, but it has been the standard approach for chamber volume determination, and a recently developed
automated edge detection method for volume measurement showed high agreement to segmental volumes
traced manually (32).
It should be noted that the contents of the pericardial
sack includes both the blood (including lymph) and
tissue of the heart. Despite evidence for changes in
bulk myocardial tissue volume over the cardiac cycle
(12, 31), in the absence of changes in cellular volume, it
is most likely that the majority of the volume change in
pericardial sack content is accounted for by the change
in total blood (fluid) volume rather than cellular (tissue) volume. This implies that because of shifts in
coronary arterial, capillary, and venous volumes, the
total chamber volumes in the pericardium may be less
constant throughout the cardiac cycle than the blood
pool content of the entire pericardial sack. This may
explain why the correlation coefficient in Fig. 3A is
⫺0.84 rather than a value closer to unity. Further MRI
studies investigating the accuracy of the constant-volume attribute applied to the blood volumes of individual chambers are currently in progress.
We presume in the subject with congenital absence
of the pericardium that the absence of the pericardium
is the only cardiac anomaly. Frequently, patients with
congenital absence of the pericardium have other genetic defects (22); however, nothing in the clinical history of the patient or MRI dataset indicated that there
were any other cardiac malformations or dysfunction.
In conclusion, this study evaluated the constantvolume attribute of the four-chambered heart throughout the cardiac cycle in a sample of normal human
THE CONSTANT-VOLUME FOUR-CHAMBERED HEART
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