Download Regional Myocardial Capillary Erythrocyte Transit Time in the

187
Regional Myocardial Capillary Erythrocyte
Transit Time in the Normal Resting Heart
Michael F. Allard, Craig T. Kamimura, Dean R. English, Sarah L. Henning, and Barry R. Wiggs
Downloaded from http://circres.ahajournals.org/ by guest on June 12, 2017
A major determinant of oxygen transport to the myocardium is the time spent by the erythrocytes (red
blood cells [RBCs]) traversing the microcirculation. Although it has been shown that the myocardium has
regional differences in blood volume, blood flow, metabolism, and sensitivity to ischemic injury, the
regional distribution of RBC transit times through the myocardial capillaries has not been previously
measured. The present study was designed to measure the regional myocardial capillary RBC transit time
by a new technique to determine whether there are regional differences in the capillary RBC transit time
in the normal resting heart. Anesthetized open-chest male New Zealand White rabbits (3.0-3.7 kg, n=8)
were studied. Regional myocardial blood volume was determined using chromium-51-labeled RBCs, and
regional blood flow was measured using a reference flow technique and a left atrial injection of
15-gm-radiolabeled (gadolinium-153, 10-20 gCi) microspheres. Capillary blood volume was determined
by multiplying the regional blood volume by the histologically determined fraction of the total blood
volume that was in the capillaries. Capillary RBC transit time was calculated as the quotient of capillary
blood volume and blood flow. The myocardial capillary blood volume was the same in the endocardium
and the epicardium (4.67±0.67 ml/100 g for endocardium versus 4.52±0.70 ml/100 g for epicardium,
p=NS), whereas myocardial blood flow tended to be greater in the endocardium (6.09±0.73 ml/sec per 100
g for endocardium versus 5.47±0.75 ml/sec per 100 g for epicardium), although this was not statistically
significant. Myocardial capillary RBC transit times ranged from 0.22 to 2.58 seconds with a mean of
0.89±0.13 and 0.83+±0.13 seconds and a median of 0.72 and 0.70 seconds in the epicardium and
endocardium, respectively. We conclude that there are no regional differences in the myocardial capillary
RBC transit time in the normal heart at rest. (Circultion Research 1993;72:187-193)
KEY WoRDs * erythrocyte transit time * oxygen transport * myocardium
I ncreases in myocardial oxygen consumption in response to modest levels of work are generally
accomplished by corresponding increases in coronary blood flow.12 At more extreme levels of cardiac
work, increases in coronary flow are insufficient, and
increased extraction of 02 from the blood may contribute up to 40% of the increased myocardial oxygen
consumption.2 A major determinant of the transport of
02 from blood to tissue is the time spent by erythrocytes
(red blood cells [RBCs]) passing through the capillaries,
the capillary RBC transit time.3-9 The capillary transit
time in a given region of myocardium is dependent on
the blood volume and blood flow in that region, both of
which may vary significantly.1,'0"l If the capillary RBC
transit time also shows regional differences, it might be
an important factor contributing to the well-known
regional variability in myocardial susceptibility to ischemia.12 Although the RBC transit time in cardiac muscle has been previously determined by others,47,9"1 no
From the University of British Columbia, Pulmonary Research
Laboratory, St. Paul's Hospital, Vancouver, Canada.
Supported by a British Columbia Health Research Foundation
Research Grant. M.F.A. is a Research Scholar of the British
Columbia Health Research Foundation.
Address for reprints: Michael F. Allard, BSc, MD, Department
of Pathology, University of British Columbia, Pulmonary Research Laboratory, St. Paul's Hospital, 1081 Burrard Street,
Vancouver, B.C., Canada V6Z 1Y6.
Received December 24, 1991; accepted October 2, 1992.
study has yet specifically measured the myocardial RBC
transit time in the capillaries of different regions of the
heart. The present study was designed to measure the
regional myocardial capillary RBC transit time by a new
technique to determine if there are regional differences
in the capillary RBC transit time in the normal resting
heart.
Materials and Methods
Animal Preparation
Male New Zealand White Rabbits (3.0-3.7 kg, n=8)
were anesthetized with intravenous urethane (1 g/kg)
and chloralose (100 mg/kg).'3 A midline tracheotomy
was performed, and the animals were mechanically
ventilated. Catheters were placed in the right carotid
and left femoral arteries for measurement of systemic
blood pressure and the collection of the reference flow
sample and in an ear vein for the administration of
drugs and fluids. The animals were paralyzed by a slow
bolus infusion of succinylcholine (1 mg/kg i.v.) followed
by a continuous infusion (1 mg/kg/hr) into the ear vein.
A midline thoractomy was then performed, and a
catheter was placed in the left atrial appendage and
secured by a suture. The small animal ventilator settings
and the oxygen content of the inspired air were adjusted
to maintain Pao, between 80 and 140 mm Hg and Paco2
between 24 and 28 mm Hg. Periodic positive endexpiratory pressure (10 cm H2O) was administered
throughout the experiment to prevent atelectasis. Nor-
188
Circulation Research Vol 72, No 1 January 1993
mal saline was infused (1 ml/min) into the ear vein, and
arterial blood pressure was monitored throughout.
Downloaded from http://circres.ahajournals.org/ by guest on June 12, 2017
Experimental Protocol
The rabbits were allowed to stabilize for 30 minutes
after completion of the surgery. Myocardial blood flow
was measured by the reference flow technique14 by
means of a left atrial injection of 15-gm microspheres
(gadolinium-153, 10-20 liCi). The microspheres were
injected in 1 ml saline over 15-20 seconds followed by a
5-ml saline flush. The microspheres were vigorously
vortexed for 2 minutes immediately before injection.
Approximately 700,000-1,000,000 microspheres were
injected to obtain greater than 400 microspheres in each
myocardial sample to ensure reliable flow measurements.15 The reference flow was collected at a rate of 4
ml/min into preweighed tubes rotating on a fraction
collector every 10 seconds. Collection began 15 seconds
before the injection of the microspheres and continued
for 2.5 minutes. This technique, using multiple collection tubes, allowed evaluation of the fractions to ensure
complete collection of microspheres in the reference
sample. During injection of the microspheres, the recorder paper speed was increased to detect abnormalities of heart rhythm. Heparinized, washed RBCs obtained from a donor rabbit and resuspended in normal
saline were infused into the ear vein at a rate of 4
ml/min, concurrent with the withdrawal of the reference
sample, to prevent a significant decrease in blood pressure during withdrawal. At the completion of the blood
flow measurement, the rabbits were given RBCs obtained from a donor rabbit and labeled with chromium-51 (500 ,uCi).16 After allowing 5 minutes for the
labeled RBCs to circulate, a reference sample of arterial
blood was taken, and the base of the heart was rapidly
clamped to maintain the myocardial blood volume present during life. An ice and saline slush was immediately
poured into the thoracic cavity to arrest the heart.
Cessation of visible contractile activity usually occurred
within 30 seconds. The heart was then removed from
the thoracic cavity, a tie was secured around the base of
the heart, and the heart was placed in 10% formalin.
Both kidneys were removed in entirety, sectioned,
placed in preweighed scintillation vials, and weighed.
They were subsequently counted with the myocardial
samples to evaluate the adequacy of mixing of
microspheres.15
After fixation in formalin, the heart was processed in
a standardized manner. The heart was transversely
sectioned parallel to the atrioventricular groove into five
slices (Figure 1). The right ventricular free wall was
removed from the left ventricle plus septum of each
slice. The middle three slices of the left ventricle plus
septum were divided into anterior, lateral, posterior,
and septal regions. Each region was then divided into an
inner portion (endocardium [ENDO]) and an outer
portion (epicardium [EPI]) or into right and left portions in the case of the septal pieces. The basal and
apical slices of the left ventricle plus septum were each
cut in half in the sagittal plane. Myocardial samples
were then placed individually in preweighed scintillation vials and weighed. Five milliliters of 10% formalin
was added to each vial before gamma counting.
The gamma-emitting radionuclides were counted in a
gamma scintillation counter (model 8000, Beckman
FIGURE 1. Diagram showing procedure for sectioning the
rabbit heart. Five transverse slices were divided as follows: the
middle three slices of the left ventricle plus septum (LVS) were
divided into septal, anterior, lateral, and posterior regions and
then subdivided into endocardial (En) and epicardial (Ep) or
inner right (IR) and inner left (IL) halves. The basal and
apical slices were sectioned sagitally. The right ventricle (RV)
was separated from the LVS of each slice.
Instruments, Inc., Fullerton, Calif.). The gamma
counter was linked to an IBM computer, with windows
selected to maximize counts for each radionuclide while
minimizing the spillover into other channels. Appropriate corrections were made for background, decay, and
overlap.
Histological Determination of the Volume
Fraction of Myocardial Capillaries
The myocardial blood volume determined by isotopic
methods alone yields a blood volume measurement that
is distributed among large, intermediate, and small blood
vessels. To determine the volume of blood in capillaries
in a given myocardial sample, the proportion of blood
vessel volume that is capillary was assessed histologically
in each myocardial sample. A representative section of
myocardium from each ENDO and EPI region was
embedded in glycol methacrylate, sectioned at 2 g,in and
stained with methenamine-silver. The methenamine-silver stain highlights the basement membrane zone (Figure 2) and thus allows easier identification and quantification of myocardial blood vessels. The volume fraction
of myocardial blood vessels was determined by point
counting using a multilevel technique similar to that
described by Cruz-Orive and Weibel.17 The point counting, using a 100-point grid, was carried out at three levels
of magnification: low (x40), to distinguish between fat
large vessels (>100-gm diameter) and "myocardium";
medium (x 100), to distinguish between intermediate
vessels (>10- and <100-gm diameter) and "myocardium"; and high (x400), to distinguish between capillaries
(<10-gm diameter) and myocardium. The entire myocardium was counted at the low magnification; random
fields were used at the medium and high magnifications.
A total of 1,500 points were counted per section at these
two higher magnifications, yielding an approximate relative standard error of less than 10%.18 The proportion of
Allard et al Myocardial Capillary Erythrocyte Transit Time
189
FIGURE 2. Photomicrograph
showing representative glycol
methacrylate-embedded 2-pLm
section of myocardium. Arrowheads indicate capillaries. Stain,
methenamine-silver; bar, 50 gm.
Downloaded from http://circres.ahajournals.org/ by guest on June 12, 2017
myocardial blood vessel volume traction occupied by
capillaries and intermediate and large vessels was calculated as the quotient of the area of the particular vessel
size in question and the sum of the area occupied by all
vessels.
19
Calculations
The myocardial blood volume (Vm) was calculated
from the `1Cr activity in each piece, the specific activity
of `1Cr in the arterial blood reference sample, and the
density of blood (1.06 g/ml). The myocardial blood flow
(Qm) was calculated using the formula Qm=QrxCm!Cr,
where Q, is the reference flow (in milliliters per
minute), Cm is counts per minute of '53Gd in the
myocardium, anid C, is counts per minute of '5Gd in the
reference sample. The regional myocardial RBC transit
time was determined as the quotient of blood volume
and flow (Vm/Q).'9
The volume of blood within the different vessel size
categories was determined in each myocardial piece by
multiplying the proportions of the different vessel sizes
and the total blood volume in that piece, e.g.,
PcXVm=Vc, where P, is the proportion of myocardial
blood vessel fraction occupied by capillaries and Vc is
capillary blood volume. The capillary RBC transit time
was then calculated for each myocardial sample using
the ratio Vc/Q,.
In addition, histological techniques were also used to
determine the volume of blood in each myocardial
sample. The histologically derived blood volume in each
myocardial sample was determined from the volume
fraction of the myocardium that was blood vessel and
the volume of each myocardial sample, which was
calculated from the sample weight and tissue density
(1.06 g/ml).
All values were normalized to the blood-free myocardial sample weight and were expressed per 100 g
myocardium.
Statistical Analysis
All data are expressed as mean-SEM except where
otherwise noted. Paired t tests corrected for multiple
comparisons using a sequentially rejective Bonferroni
procedure21' were used to test for differences between
regions.
Results
Hemodynamics and Arterial Blood Gases
The open-chest anesthetized rabbits had mean systolic and diastolic blood pressures of 89+7 and 61+4
mm Hg, respectively, with a mean heart rate of 259±12
sec`l. Arterial blood gas analysis showed a mean Pao2 of
113±7 mmlHg and a mean Paco2 of 26+1 mm Hg.
Cardiac output was determined in two additional openchest anesthetized rabbits by indicator dilution techniques.16 The cardiac outputs of these two rabbits were
581 and 511 ml/min, which were similar to results
obtained previously in our laboratory in a closed-chest
anesthetized rabbit preparation. '6
Myocardial Blood Volume and Blood Flow
The mean left ventricular myocardial blood volume
determined with 51Cr RBCs was 6.30+1.08 ml/100 g.
The myocardial blood volume tended to be greater in
the EPI compared with the ENDO region of the
myocardium (7.01±1.31 versus 5.59+0.93 ml/100 g,
p=NS) (Table 1).
Myocardial blood flow was 5.78+0.73 ml/sec per 100
g, which accounted for approximately 3-5% of the
TABLE 1. Regional Blood Volume, Blood Flow, and
Transit Times
V
TTm
Q
(ml/sec per 100 g) (ml/100 g) (seconds)
EPI
5.47+0.75
7.01 + 1.31 1.38+0.25
ENDO
6.09+0.73
5.59+0.93 0.99+0.19*
Total
5.78+0.73
6.30+1.08 1.19+0.21
TT,
(seconds)
0.89+0.13
0.83+0.13
0.86+0.13
Q, myocardial blood flow; V, myocardial blood volume; TTm,
myocardial erythrocyte transit time; TTC, myocardial capillary
erythrocyte transit time; EPI, epicardial myocardium; ENDO,
endocardial myocardium. Values are mean+SEM.
8p<0c05 vs. EPI value (n =8).
190
Circulation Research Vol 72, No 1 January 1993
measured cardiac output in this animal preparation.
There was a trend toward slightly greater blood flow to
the ENDO compared with the EPI (6.09±0.73 versus
5.47±0.75 ml/sec per 100 g, p=NS) (Table 1). In
addition, both the myocardial blood volume and blood
flow had heterogeneous distributions. The coefficient of
variation or relative dispersion, defined as the quotient
of the standard deviation and the mean, provides a
measure of the heterogeneity. The myocardial blood
volume and blood flow showed relative dispersions
among animals of 0.47 and 0.36, respectively.
The histologically derived mean myocardial blood
volume was 11.71 ml/100 g, which was almost twice as
great as that measured with 51Cr RBCs. The degree of
regional heterogeneity tended to be less, particularly in
the ENDO, than the corresponding values for blood
volume measured with radiolabeled RBCs. Relative
dispersions among animals were 0.20 and 0.09 in the
EPI and ENDO, respectively.
Downloaded from http://circres.ahajournals.org/ by guest on June 12, 2017
Myocardial Capillary Blood Volume
Histological evaluation of the myocardium showed
that the EPI had a significantly greater proportion of
large vessels than the ENDO (22.97+2.78% for EPI
versus 4.79±1.19% for ENDO, p<0.05), whereas the
ENDO had a greater proportion of capillaries
(86.13±2.36% for ENDO versus 68.85±2.80% for EPI,
p<0.05) (Figure 3A). There were no regional differences in the proportion of intermediate-sized blood
vessels. The volume of blood in myocardial capillaries
was not different in the ENDO compared with the EPI,
whereas the volume in large vessels was greater in the
EPI (Figure 3B). The blood volume in intermediate
vessels showed no regional differences.
Capillary RBC Transit Time
The myocardial capillary RBC transit times from all
hearts ranged from 0.22 to 2.58 seconds with medians of
0.72 and 0.70 seconds and means of 0.89±0.13 and
0.83+0.13 seconds in the EPI and the ENDO, respectively (Figure 4, Table 1). The relative dispersion of
capillary RBC transit times among animals was 0.45 in
the EPI and 0.47 in the ENDO. The mean capillary
transit times are in distinct contrast to the mean myocardial transit times, which were determined without
the contribution of blood in large vessels taken into
account; the mean myocardial transit time in the EPI
(1.38±0.26 seconds) was significantly greater than that
in the ENDO (0.99±0.19 seconds, p<0.05).
Discussion
Myocardial oxygen consumption and blood flow are
closely matched at low to intermediate workloads, and
the extraction of the oxygen from the blood by the heart
remains relatively constant.'2 Higher workloads, however, such as those seen with maximal exercise, are
associated with a substantial increase in the extraction
of oxygen2 that may account for up to 40% of the
increased oxygen consumption. These findings suggest
that alterations take place within the microcirculation
and result in the near complete extraction of oxygen
delivered to the myocardium. Parameters of the microcirculation whose alteration may contribute to the increased extraction of oxygen include the functional
capillary surface area, the oxygen content and hemato-
100
A
o0
60
a00
U1
40
20
0
5-
T
B
o
0
4-
0
e
0a
0
01
.C
A
D
0
large
epicardial
Intermediate
Vessel type
capillary
ondocardial
FIGURE 3. Bar graphs showing distribution of myocardial
blood volume. Panel A: Histological determination of the
proportion of large, intermediate, and capillary-sized vessels.
Panel B: Regional myocardial blood volume in large, intermediate, and capillary-sized vessels. *p <0.05 vs. epicardial
value.
crit, the oxygen concentration gradient, the time for
oxygen release, and the capillary RBC transit time.3,21
The time required by RBCs to pass through the capillaries has been shown to be a major determinant of the
transport of oxygen in striated muscle.3-9 The RBC
transit time in any vascular bed is dependent on the
blood flow to that bed and the volume of blood through
which it passes,19 such that, if blood flow increased
without an accompanying increase in blood volume, the
transit time would decrease. The myocardium has been
shown to have significant regional differences in both
blood flow,"" and vascularity.10"11,22-24 Under resting
conditions in normal hearts, the blood flow, measured
by the reference flow technique with radiolabeled microspheres,' and the microvascular blood volume22-24
tend to be greater in the inner myocardium. Since blood
volume and blood flow are the major determinants of
the RBC transit time, discordant regional alterations in
either may result in significant regional differences in
the capillary RBC transit time. If regional differences in
the RBC transit time do occur, these differences may
contribute to the regional variability in sensitivity of the
myocardium to ischemia and ischemic injury.12 It is
likely that these regional disparities, if present, will be
exaggerated and become significant only during hemodynamic stress in normal and, particularly, in diseased
hearts, such as those with pressure-overload hypertro-
Allard et al Myocardial Capillary Erythrocyte Transit Time
14 b
0
12
-
A
0
10-
6
4-
6- 0
*
2-
-
0
0
-
.
0I
0
0
0
0
2n
0
u
0
14
0
12
B.
0
1
10
8
b-
6
10
4
0
0
Z
2
Downloaded from http://circres.ahajournals.org/ by guest on June 12, 2017
0
*~~~~~
3
., | i | | i i ' '
0
, ! ' |~~
2
0
0
E
1
0
op on op on op en
l
Rabbit
1
ep en op
l
2
3
4
en
ep
L1
1
5
6
en
ep
7
en
ep
en
8
FIGURE 4. Plots showing individual data points for each
heart by region. ep, Epicardium; en, endocardium. Panel A:
Myocardial capillary blood volume. Panel B: Myocardial
blood flow. Panel C: Myocardial capillary erythrocyte transit
time.
phy. The myocardial RBC transit time has been previously determined in other studies by means of indicator
dilution techniques,7,925 by calculations based on estimated capillary pathway length and measured erythrocyte velocities in myocardial capillaries,4 and by techniques similar to those described in the present study."
No study to date, however, has specifically determined
the regional distribution of myocardial capillary RBC
transit times. The present study was designed to measure the regional myocardial capillary RBC transit time
to determine if there are regional differences in capillary RBC transit time in the myocardium of normal
resting hearts.
The myocardial blood flow in this preparation accounted for approximately 3-5% of the cardiac output
and showed the well-described trend for greater blood
flow toward the inner myocardium' (Table 1). Blood
flow to the myocardium was also found to be regionally
heterogeneous. The relative dispersion of blood flows
among animals was 0.35, a value comparable to that
previously reported in anesthetized rabbits" and conscious baboons.26
Myocardial blood volumes reported in the literature
vary substantially."122'24,27 Our results compare favor-
191
ably with blood volumes reported in the dog24 and the
rat.2227 The myocardial blood volumes in the present
study likely represent an average of systolic and diastolic
blood volumes. The blood volumes are intermediate to
those reported in the rat, in which the effects of
barium-induced contracture and potassium chlorideinduced arrest on intramyocardial blood volume were
determined.27 The barium-induced contracture was felt
to recapitulate systole, whereas potassium chlorideinduced arrest was felt to reproduce diastole. A recent
study by Gonzalez and Bassingthwaighte"l reported
blood volumes in rabbit myocardium higher than those
in the present study. The differences between these two
studies may be partially explained by differences in
anesthesia, possible anesthesia-related changes in hemodynamics, and techniques of measurement. In addition, the calculation of blood volume in the present
study did not take into account the decreased hematocrit of blood in the myocardium compared with systemic
vessels.11 As a result, the myocardial blood volume
measurements slightly underestimate the true myocardial blood volume. However, the degree of underestimation, assuming a myocardial hematocrit of approximately 75% of large vessel hematocrit,11 is insufficient to
entirely explain the differences between the data of
Gonzalez and Bassingthwaighte and our own.
Histological analysis of the myocardium (Figure 3A)
showed that capillaries represented 86.1±2.4% of all
vessels in the ENDO and 68.9±2.8% of all vessels in the
EPI (p<0.05). These values compare favorably with
those reported by others, who found that 85 - 86% of the
myocardial vascular volume was located in the capillaries28 or the terminal vascular bed22 in normal dog hearts.
The myocardial capillary blood volume, determined
from the proportion of all blood vessels that are capillaries and the measured blood volume in each myocardial sample, showed no differences between the EPI
and ENDO (Figure 3B). The tendency for the mean
myocardial blood volumes to be greater in the EPI
region (Table 1) can be accounted for by the preponderance of larger coronary vessels in this location
(Figure 3).
The techniques used in the histological analysis of the
myocardium measure the entire anatomic compartment
of myocardial blood vessels29 and, therefore, the theoretical maximal attainable myocardial blood volume.
The technique does not, however, account for the
possibility that all vessels in that compartment may not
be perfused under resting conditions.29 The myocardial
blood volume measured by `Cr-labeled erythrocytes
was 56.4±7.6% of the histologically derived theoretical
maximal value. These findings are consistent with data
reported by others in rabbits in which approximately
50-60% of myocardial capillaries and arterioles were
perfused under resting conditions.29 The myocardial
blood volume determined with `Cr RBCs also showed
greater heterogeneity than that derived from histological methods. The increased heterogeneity of blood
volume when measured with labeled erythrocytes is
likely a reflection of different degrees of perfusion of
the various myocardial regions. The proportion of myocardial capillaries and arterioles perfused has been
shown to be similar under resting conditions in hearts
from anesthetized open-chest rabbits.29 Since the heterogeneity of perfusion in that study29 was likely com-
192
Circulation Research Vol 72, No 1 January 1993
TABLE 2. Myocardial Capillary Blood Volume, Blood Flow, and Capillary Transit Time for All Experiments
Q
TT,
(mi/sec per 100 g)
(seconds)
EPI
ENDO
EPI
ENDO
ENDO
EPI
Rabbit
0.50±0.17
0.49±0.12
4.16±1.48
4.34±1.12
8.79±0.77
8.34+0.48
1
7.08±0.70
0.73±0.32
0.51±0.10
3.53±0.45
5.56±0.94
3.86±1.01
2
4.05±0.41
0.73+0.16
0.66±0.15
3.23+0.79
4.91+0.30
2.90±0.51
3
0.92±0.34
1.02±0.17
2.96±0.26
3.01±0.55
3.04±0.50
2.67±0.75
4
0.71±0.18
5.15±0.70
0.55+0.10
4.07+1.23
2.82±0.59
2.80+0.83
5
6.78±0.98
5.46±0.57
6.10±0.98
1.53±0.31
1.14+0.26
8.40±2.15
6
0.71±0.10
8.90+1.40
0.62+0.16
5.31±0.77
6.27+0.97
8.91+2.05
7
1.43±0.52
1.61 +0.53
7.87+2.68
4.37+0.67
4.86+0.37
6.06±2.07
8
V,, myocardial capillary blood volume; Q, myocardial blood flow; TT,, myocardial capillary erythrocyte transit time;
EPI, epicardial myocardium; ENDO, endocardial myocardium. Values are mean+ SD.
V,
(ml/100 g)
Downloaded from http://circres.ahajournals.org/ by guest on June 12, 2017
parable to that in our preparation, it was assumed that
the proportion of vessels perfused in the present study
was not different among the three different categories of
vessel size. Thus, the proportion of different-sized vessels determined histologically was used to calculate the
corresponding blood volumes. However, subsequent
studies in which hearts are examined under nonresting
conditions must specifically evaluate the proportions of
the various-sized vessels perfused because, according to
previous studies, the distribution of perfusion throughout the myocardium may not be uniform during hemodynamic stress.3031
The myocardial capillary RBC transit time was determined in different regions of the heart for the first time
in the present study by measuring regional capillary
blood volume and blood flow. These parameters showed
substantial variability among as well as within individual
hearts (Figure 4, Table 2). Although the heterogeneity
varied up to fivefold among hearts, within a given heart
the heterogeneity was, interestingly, generally comparable between ENDO and EPI regions (Figure 4). By
combining the RBC capillary transit time data from all
hearts (Figure 5), a representation of all possible transit
times under the conditions of study may be obtained.
The myocardial capillary transit times from all hearts
ranged from 0.22 to 2.58 seconds (Figure 5) with
medians of 0.72 and 0.70 seconds and means of
0.89+0.13 and 0.83 ±0.13 seconds in the EPI and the
ENDO, respectively. The relative dispersion of capillary
transit times among the different animals was comparable to that of myocardial blood volume and flow and
measured 0.45 in the EPI and 0.47 in the ENDO.
Within individual hearts, the relative dispersion of
capillary transit times ranged from 0.13 to 0.33 in the
ENDO and from 0.20 to 0.43 in EPI (Figure 4).
Although the mean myocardial capillary RBC transit
times measured in the present study (Table 1) are
somewhat shorter than those reported from studies
using indicator dilution techniques,9'25 they are comparable to RBC transit times estimated by others to occur
in the myocardium under resting conditions.36 Of particular interest with regard to oxygen transport was the
finding that the vast majority of myocardial capillary
RBC transit times were greater than the estimated
minimum time for oxygen to be released from the
erythrocytes to the resting myocardium.6 Although the
data are not strictly comparable, this finding suggests
that the majority of capillary RBC transit times are
sufficiently long to allow for adequate 02 release in the
normal resting heart and that 02 transport to the
myocardium under these conditions is not likely limited
by the capillary RBC transit time.
The techniques used in the present study to determine the capillary RBC transit time in small myocardial
samples and composite regions (ENDO and EPI) of the
heart do have some limitations. The capillary RBC
transit times in the myocardial samples represent mean
values of transit times through the individual capillaries
in a given piece of myocardium. It is these individual
A
0
0
a
0
cr
01
1.1
0.40
0.30
b
a0
cm
0.20-
11
0.10-
0.00
0.5
1.0
1.5
2.0
transit time (a)
2.5
FIGURE 5. Bar graphs showing frequency distribution of
myocardial capillary erythrocyte transit times in the epicardial
(panel A) (n= 72) and endocardial (panel B) (n= 72) myocardium.
Allard et al Myocardial Capillary Erythrocyte Transit Time
Downloaded from http://circres.ahajournals.org/ by guest on June 12, 2017
RBC transit times through the myocardial microcirculation and, in particular, their heterogeneity that are the
values most relevant to oxygen transport in the myocardium. The heterogeneity of RBC transit times observed
at the level of the myocardial samples in the present
study does not, however, necessarily reflect that of the
transit time through individual capillaries. Currently,
there are no techniques available that can directly
measure individual myocardial capillary transit times.
However, the distribution of capillary transit times has
been determined in skeletal muscle either directly, by
measurement of the transit time of fluorescent-labeled
RBCs across a capillary network,32 or indirectly, by
calculations based on an estimated distribution of capillary path lengths and velocity measurements in superficially located microvessels.5 Although velocity measurements of RBCs in epicardial capillaries are
available, the distribution of the lengths of the individual pathways taken by the RBCs through the myocardial
microcirculation is not yet known. Perhaps, in the
future, computer models, based on detailed morphological and hemodynamic analyses of the myocardial microcirculation, will shed light on the important RBC
transit time distributions through the individual myocardial capillaries.
In summary, the regional myocardial capillary RBC
transit time, a major determinant of oxygen transport to
the myocardium, was measured for the first time by a
new technique. We demonstrate that there are no
significant regional differences in myocardial capillary
RBC transit time under resting conditions. Whether
significant alterations occur in the myocardial capillary
RBC transit time in normal and diseased hearts with
hemodynamic stress and how these possible alterations
may affect 02 transport remain to be determined.
Acknowledgments
The authors thank Dr. James C. Hogg for critical review of
this manuscript and for his helpful guidance in the development of the techniques. The authors thank Stuart Greene for
photography, Andrd MacKenzie for assistance with the illustrations, Kent Webb for typing the manuscript, and Joe
Comeau for help with statistical analysis.
References
1. Feigl EO: Coronary physiology. Physiol Rev 1983;63:1-205
2. Von Restoroff W, Holtz J, Bassenge E: Exercise-induced augmentation of myocardial oxygen extraction in spite of normal coronary
dilatory capacity in dogs. Pflugers Arch 1972;372:181-189
3. Honig CR, Gayeski TEJ: Comparison of intracellular P02 and
conditions for blood-tissue 02 transport in heart and working red
skeletal muscle. Adv Exp Med Biol 1987;215:309-321
4. Honig CR, Gayeski TEJ, Federspiel W, Clark A Jr, Clark P:
Muscle 02 gradients from hemoglobin to cytochrome: New concepts, new complexities. Adv Exp Med Biol 1984;169:23-38
5. Honig CR, Feldstein M, Frierson JL: Capillary lengths, anastamoses, and estimated capillary transit times in skeletal muscle. Am
J Physiol 1977;233:H122-H129
6. Gayeski TEJ, Federspiel WJ, Honig CR: A graphical analysis of
the influence of red cell transit time, carrier-free layer thickness
and intracellular P02 on blood-tissue 02 transport. Adv Exp Med
Biol 1988;222:25-35
7. Mildenberger RR, L'Abbate A, Zborowska-Sluis DT, Klassen GA:
The relationship between oxygen extraction and extravascular
mean transit time in the canine heart. Can J Physiol Pharmacol
1977;55:478-481
193
8. Honig CR, Odoroff CL: Calculated dispersion of capillary transit
times: Significance for oxygen exchange. Am J Physiol 1981;240:
H199-H208
9. Rose CP, Goresky CA: Vasomotor control of capillary transit time
heterogeneity in the canine coronary circulation. Circ Res 1976;39:
541-554
10. Crystal GJ, Downey HF, Bashour FA: Small vessel and total
coronary blood volume during intracoronary adenosine. Am J
Physiol 1981;241:H194-H201
11. Gonzalez F, Bassingthwaighte JB: Heterogeneities in regional volumes of distribution and flows in rabbit heart. Am J Physiol 1990;
258:H1012-H1024
12. Lowe JE, Cummings RG, Adams DH, Hull-Ryde EA: Evidence
that ischemic cell death begins in the subendocardium independent of variations in collateral flow or wall tension. Circulation
1983;68:190-202
13. Rankin AJ, Kelpin BG, Courneya CA, Wilson N, Ledsome JR:
Plasma vasopressin response to haemorrhage in the anesthetized
rabbit. Can J Physiol Pharmacol 1986;64:904-908
14. Heymann MA, Payne BD, Hoffman JIE, Rudolph AM: Blood flow
measurements with radionuclide-labelled particles. Prog Cardiovasc Dis 1977;20:55-79
15. Buckberg GD, Luck JC, Payne DB, Hoffman JIE, Archie JP, Fixler
DE: Some sources of error in measuring regional blood flow with
radioactive microspheres. JAppl Physiol 1971;31:598-604
16. Allard MF, Doerschuk CM, Brumwell ML, Belzberg A, Hogg JC:
Oleic acid-induced lung injury in rabbits: Effect of fibrinogen
depletion with Arvin. JAppl Physiol 1988;46:920-928
17. Cruz-Orive L-M, Weibel ER: Sampling designs for stereology.
J Microsc 1981;122:235-257
18. Aherne WA, Dunnill MS: Morphometry. London, Edward Arnold,
1982, pp 19-31
19. Hogg JC, Martin BA, Lee S, McLean T: Regional differences in
erythrocyte transit in normal lungs. J Appl Physiol 1985;59:
1266-1271
20. Holland BS, Copenhauer MD: An improved sequentially rejective
Bonferroni test procedure. Biometrics 1987;43:417-423
21. Honig CR, Frierson JL, Gayeski TEJ: Anatomical determinants of
02 flux density at coronary capillaries. Am J Physiol 1989;256:
H375-H382
22. Rakusan K, Braasch W, du Mensil de Rochemont W: The total
vascular capacity of the terminal vascular bed in a normal heart
muscle of a dog. Physiol Bohemoslov 1969;18:7-10
23. Kleinert HD, Scales JL, Weiss HR: Effects of carbon monoxide or
low oxygen gas mixture inhalation on regional oxygenation, blood
flow, and small vessel blood content of the rabbit heart. Pflugers
Arch 1980;383:105-111
24. Myers WW, Honig CR: Number and distribution of capillaries as
determinants of myocardial oxygen tension. Am J Physiol 1964;207:
653-660
25. Vargas FF, Blackshear GL: Vascular resistance and transit time of
sickle red blood cells. Blood Cells 1982;8:139-145
26. King RB, Bassingthwaighte JB, Hales JRS, Rowell LB: Stability of
heterogeneity of myocardial blood flow in normal awake baboons.
Circ Res 1985;57:285-295
27. Judd RM, Levy BI: Effects of barium-induced cardiac contracture
on large- and small-vessel intramyocardial blood volume. Circ Res
1991;68:217-225
28. O'Keefe DD, Hoffman JIE, Cheitlin R, O'Neill MJ, Allard JR,
Shapkin E: Coronary blood flow in experimental canine left ventricular hypertrophy. Circ Res 1978;43:43-51
29. Weiss HR, Conway RS: Morphometric study of the total and
perfused arteriolar and capillary network of the rabbit left ventricle. Cardiovasc Res 1985;19:343-354
30. Eigler NL, Schfihlen H, Whiting JS, Pfaff JM, Zeiher A, Gu S:
Digital angiographic impulse response analysis of regional myocardial perfusion: Estimation of coronary flow, flow reserve, and
distribution of volume by compartmental transit time measurement
in a canine model. Circ Res 1991;68:870-880
31. Chilian WM, Layne SM, Klausner EC, Eastham CL, Marcus ML:
Redistribution of coronary microvascular resistance produced by
dipyridamole. Am J Physiol 1989;256:H383-H390
32. Sarelius IH, Duling BR: Direct measurement of microvessel
hematocrit, red cell flux, velocity and transit time. Am J Physiol
1982;243:H1018-H1026
Regional myocardial capillary erythrocyte transit time in the normal resting heart.
M F Allard, C T Kamimura, D R English, S L Henning and B R Wiggs
Downloaded from http://circres.ahajournals.org/ by guest on June 12, 2017
Circ Res. 1993;72:187-193
doi: 10.1161/01.RES.72.1.187
Circulation Research is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1993 American Heart Association, Inc. All rights reserved.
Print ISSN: 0009-7330. Online ISSN: 1524-4571
The online version of this article, along with updated information and services, is located on the
World Wide Web at:
http://circres.ahajournals.org/content/72/1/187
Permissions: Requests for permissions to reproduce figures, tables, or portions of articles originally published
in Circulation Research can be obtained via RightsLink, a service of the Copyright Clearance Center, not the
Editorial Office. Once the online version of the published article for which permission is being requested is
located, click Request Permissions in the middle column of the Web page under Services. Further information
about this process is available in the Permissions and Rights Question and Answer document.
Reprints: Information about reprints can be found online at:
http://www.lww.com/reprints
Subscriptions: Information about subscribing to Circulation Research is online at:
http://circres.ahajournals.org//subscriptions/