Download Myocardial oxygen consumption, oxygen supply/demand

PATHOPHYSIOLOGY AND NATURAL HISTORY
MYOCARDIAL ISCHEMIA
Myocardial oxygen consumption, oxygen
supply/demand heterogeneity, and microvascular
patency in regionally stunned myocardium
LLOYD D. STAHL, M.D.,* HARVEY R. WEISS, PH.D.,
AND
LEWIS C. BECKER, M.D.
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ABSTRACT Although oxygen consumption closely parallels mechanical work in the normal heart,
previous studies have found that stunned myocardium may have normal or even increased oxygen
consumption despite depressed function. In this study we used microspectrophotometry to measure the
oxygen saturations within arteries and veins of less than 100 ,um diameter in quick-frozen biopsy
samples from normal and regionally stunned myocardium of 10 open-chest anesthetized dogs. Regional
myocardial blood flow, measured by radioactive microspheres, was similar in stunned and normal
regions, as was mean arteriolar oxygen saturation. However, mean venous oxygen saturation was lower
in the stunned region (epicardium 38.0% vs 43.8%, p < .02; endocardium 36.2% vs 39.5%, p = .12),
indicating increased oxygen extraction and consumption, despite a marked reduction in mean systolic
segmental shortening from 14.4% to 0.5%. In addition, there was greater vein-to-vein heterogeneity
of oxygen saturation in the stunned region, with an excess of veins having low saturations (statistically
significant in epicardium, nonsignificant trend in endocardium). Microvascular injection studies with
Microfil or drafting ink revealed filling of over 95% of arterioles and 85% of capillaries in the stunned
region, similar to the findings in the normal region. Our results are consistent with an inefficient transfer
of energy into myocyte contraction or an increased use of energy for noncontractile activities in stunned
myocardium. In addition, the finding of increased heterogeneity of oxygen extraction suggests that the
injury to stunned myocardium may not be uniform to all contractile elements, but instead may be focally
and irregularly distributed.
Circulation 77, No. 4, 865-872, 1988.
REPERFUSION after a brief episode of mycardial
ischemia may result in prolonged dysfunction without
necrosis, a phenomenon now known as "myocardial
stunning."''3 Since myocardial oxygen consumption
closely parallels energy production and mechanical
work in the normal heart 4-6 one would anticipate that
stunned myocardium should exhibit reduced oxygen
consumption. Although several abstracts have appeared describing reduced,7 8 normal,9' 10 or even
increased1' oxygen consumption in stunned myocardium, the only published study bearing on this issue
From the Division of Cardiology, Department of Medicine, Johns
Hopkins University School of Medicine, Baltimore, and Department of
Physiology and Biophysics, Heart and Brain Circulation Laboratory,
UMDNJ-Robert Wood Johnson Medical School, Piscataway, NJ.
Supported by a Research Fellowship fronm the American Heart Association, Maryland Affiliate, and by U.S. Public Health Service grants
17655-12 (SCOR in Ischemic Heart Disease) and 26919 from The
National Heart, Lung, and Blood Institute.
Address for correspondence: Lewis C. Becker, M.D., Johns Hopkins
University School of Medicine, 600 N. Wolfe St., Halsted 500, Baltimore, MD 21205.
Received Aug. 17, 1987; revision accepted Dec. 30, 1987.
*Present address: Menorah Medical Center, 4949 Rockhill Rd., Kansas City, MO 64110.
Vol. 77, No. 4, April 1988
indicates that postischemic myocardial oxygen utilization may be increased above normal.'2 Following
reperfusion after 2 hr of hypothermic cardioplegic
ischemia, canine hearts demonstrated normal pressurevolume curves, but a 40% increase in oxygen utilization at matched levels of developed pressure.'2
Although the cause of myocardial stunning is
unknown, it has been suggested that calcium overload
may represent a major factor.13 Intracellular sodium,
which accumul4tes during ischemia, may rapidly
exchange for calcium during reperfusion, as acidosis
(an inhibitor of sodium-calcium exchange) resolves.
Hoerter et al. 14 have shown that increased myocellular
calcium content, induced by sodium-calcium exchange
via manipulations of perfusate composition, can result
in reduced systolic function, while myocardial oxygen
consumption is paradoxically increased. The recent
finding by Kusuoka et al.13 that reperfusion with low
calcium-containing perfusate eliminates myocardial
stunning provides further support for the calcium overload hypothesis. Inappropriately high oxygen consumption, if present, could reflect an increased utili865
STAHL et al.
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zation of energy for movement of calcium in stunned
myocardium.
Virtually nothing is known about the homogeneity
of abnormalities within stunned myocardium. It is
unclear whether the dysfunction of a stunned myocardial segment represents uniform depression of all contractile elements, or patchy, focally distributed injury.
Information on this issue is critical for an understanding of the pathophysiology and pathogenesis of stunned
myocardium.
In this study we used the technique of microspectrophotometry to compare oxygen extraction and consumption in stunned and normal myocardium by determining the oxygen saturation of blood in small arteries
and veins less than 100 ,um in diameter in quick-frozen
myocardial biopsy samples taken from regionally
stunned dog hearts. The microscopic nature of this
technique allowed us to measure the vein-to-vein uniformity of oxygen extraction within the stunned and
normal myocardial segments. In addition, we assessed
the patency of the microvasculature in stunned and
normal myocardium by microscopic analysis of hearts
infused in situ with Microfil or drafting ink.
Methods
Animal preparation. Fifteen mongrel dogs of either sex (20
to 25 kg) were anesthetized with intravenous sodium thiamylal
(12.5 mg/kg) and intramuscular cx-chloralose (100 mg/kg) in
urethane, intubated and ventilated with room air at a constant
volume with a piston respirator. After a left thoracotomy, the
lungs were retracted and the heart was exposed through a peri-
cardiotomy.
Pairs of 5 MHz cylindrical piezoelectric crystals (0.06 inches
outside diameter, 0.06 inches in length, 0.015 inches thick;
Vernitron, Columbus, OH) were implanted in the myocardium
served by the anterior descending coronary artery and the anterolateral basal region within the circumflex artery territory. The
crystals were implanted in the midwall of the left ventricle, 10
to 15 mm apart, and were oriented parallel to the minor axis.
Segmental lengths were measured with a pulse transit sonomicrometer (model Sono-1-XB, James Davis Consultants). Left
ventricular pressure, and its first derivative with respect to time,
left ventricular dP/dt, were measured with a catheter-tipped
pressure transducer (Millar) that was inserted through an
implanted silicone rubber left atrial catheter and advanced across
the mitral valve. Aortic pressure was measured by inserting a
catheter-tipped pressure transducer (Millar) into the aortic root
from the right carotid artery. Catheters were placed in the
descending aorta for withdrawal of microsphere reference samples, the left atrium for microsphere injections, and the femoral
vein for infusion of fluids. Aortic and left ventricular pressures
and regional segmental lengths were recorded continuously on
a direct-writing recorder (Gould Brush 200).
Experimental protocol. A pneumatic occluder was placed
around the left anterior descending coronary artery just distal to
the first diagonal branch. Stunning was produced by ten 5 min
occlusions alternating with 10 min reflow periods, followed by
a 90 min recovery period.
Blood flow to stunned and normal myocardium was determined by radioactive microspheres (15 ,im diameter, New
866
England Nuclear Co.) labeled with 141Ce or 13Sn. Three to five
million microspheres were injected into the left atrium after a
3 min mechanical agitation, while reference arterial samples
were withdrawn at a constant rate of 2.16 ml/min by a calibrated
pump. Blood flow was calculated from the radioactivity in the
tissue and reference blood samples determined in a well-type
gamma scintillation counter (Packard model 5986) by standard
methods after correction for overlap of the energy peaks. In each
dog, flows to stunned and normal zones represented measurements in single tissue blocks weighing 1 to 3 g, divided into
endocardial and epicardial halves. Flows were measured at
baseline and 90 min after the last ischemic episode.
Hemodynamic and ultrasonic crystal measurements were
made at end-expiration during the baseline state and 90 min after
the final occlusion. In the first group of 10 dogs, after the
stunning protocol, 3 to 5 g transmural biopsy samples were taken
with use of a scalpel from the area of the stunned and normal
regions encompassed by the sonomicrometer crystal pairs and
were immersed immediately in liquid nitrogen. The time from
initiation of the biopsy to immersion in liquid nitrogen was less
than 10 sec in all cases. The samples were coded and stored in
liquid nitrogen for future blinded analysis.
The animals were then killed by intra-atrial injection of potassium chloride. The hearts were cut into five to six short-axis
slices from apex to base, each of which was incubated at 370 C
in triphenyltetrazolium chloride for 20 min and photographed in
color. After this the site of each crystal pair was excised,
weighed, and counted for radioactivity for measurement of
blood flow. The fresh tissue and photographs were inspected to
identify unstained areas that represented myocardial necrosis.
A second group of five dogs underwent the identical instrumentation and repetitive coronary occlusion protocol. Ninety
minutes after stunning was produced the sonomicrometer crystal
pairs were removed and their locations were marked by loosely
tied epicardial purse-string sutures. A 1/4 inch Teflon catheter
was inserted through the left subclavian artery and advanced into
the aortic root just above the aortic valve. The main pulmonary
artery was ligated and the right atrium was vented to a reservoir.
After approximately 3 to 5 sec, the ascending aorta was ligated
proximal to the great vessels. In three dogs 50 ml of heparinized
normal saline followed by 50 ml of Microfil silicone rubber
compound (MV-122 Yellow, Canton Biomedical Products, Inc.,
Boulder, CO) was infused into the aortic root under 120 mm Hg
constant pressure. In two other dogs, 8000 units of intravenous
heparin was given before ligation of pulmonary artery. After
ligation of the ascending aorta, 50 ml of heparinized normal
saline was infused into the aortic root followed by 300 ml of
black drafting ink (Pelikan, Hannover, West Germany) at a
pressure of 120 mm Hg. All hearts continued to beat through
the injection period. The hearts were excised below the atrioventricular ring, frozen immediately in liquid nitrogen, and then
stored at 700 C for future analysis.
Analysis of function. Signals from the ultrasonic crystals,
left ventricular and aortic pressure, and left ventricular dP/dt
were routed to a microcomputer (North-Star Horizon) at selected
times during the experiment, digitized at 150 Hz by a Tecmar
12-bit analog-to-digital converter, and recorded on floppy disk
for later computer analysis. The end-systolic segment length was
measured 20 msec before peak negative left ventricular dP/dt15;
end-diastolic length was measured just before the onset of positive left ventricular dP/dt. Percent systolic segmental shortening was calculated as segmental shortening (end-diastolic minus
end-systolic length) divided by end-diastolic length times 100,
and represented the average of 5 heartbeats.
Analysis of oxygen extractions. The quick-frozen biopsy
samples from the stunned and normal regions of the dogs in the
first group were prepared as follows. Thirty micrometer thick
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PATHOPHYSIOLOGY AND NATURAL HISTORY-MYOCARDIAL ISCHEMIA
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frozen tissue sections were cut on a Slee automated microtomecryostat set at - 350 C in a nitrogen atmosphere. Each section
was then transferred to a precooled slide, covered with degassed
silicone oil, and rapidly transferred to the microspectrophotometer cold stage flushed with nitrogen gas. Arteries and veins less
than 100 Km in diameter were located and absorbances at 560,
523, 506 nm were obtained to give oxygen saturation of the
blood contained within the vessels. 16 Only vessels seen in transverse section were studied, so that the light path was only
through blood. Between seven and 10 arterioles and seven to 10
venules in the endocardial and epicardial segments of the
stunned and normal regions were analyzed for each dog.
Regional oxygen extraction (ml 02/100 ml blood) was calculated as the difference between mean arteriolar and venous oxygen
saturation, multiplied by the arterial hemoglobin concentration,
times the maximal oxygen-combining capacity of 1.36 ml O2/g
hemoglobin. Hemoglobin concentration was determined with a
Fisher hemophotometer (Fisher Scientific Co., Pittsburgh) from
blood samples obtained at the end of the study. The oxygen
consumption for each segment of interest was calculated as the
product of oxygen extraction and regional blood flow.
Microscopic evaluation of the microvasculature. To identify
perfused microvessels (both Microfil and drafting ink), samples
were prepared as follows: 2 pum thick sections were cut from the
hearts on a Slee automated microtome-cryostat set at - 350 C.
Eight to twenty sections from the endocardial and epicardial
segments of both the normal and stunned regions were cut for each
animal. Each section was at least 150 to 200 pum from the previous
one. As described previously,17 19 the slides were photographed
through a Zeiss microscope equipped for automated photography.
A 40 x planapochromat objective was used to photograph the
capillary fields and a 10 x objective was used to photograph the
fields used for observation of arterioles. The slides were then
stained for alkaline phosphatase, as described previously.'9
Briefly, they were fixed in a sucrose-formalin buffer for 1 min,
washed twice in distilled water, and then placed in an incubation
mixture for 30 min at 370 C. The incubation mixture consisted of
3.8 g/liter of Fast blue RR, 0.5 g/liter of cL-napthylphosphate, 3.8
g/liter of sodium metaborate, and 1.7 g/liter of magnesium sulfate.
The slides were postfixed and dried.
Various stereologic determinations were made, counting each
field twice, once for the total and once for the perfused microvasculature. The system used was a Dapple image analyzing
device. We placed the slides that were stained for alkaline
phosphatase under a microscope and the image analyzer evaluated the field for total morphometric indexes of microvascularity. The image was obtained from a Panasonic television
camera attached to the microscope and digitized with the Dapple
image analyzer. The number of microvessels per square millimeter was estimated from the number per unit test area. The
diameter of the microvessels was determined by measurement
of the minimum diameter of any vessel cut in transverse sections
such that the maximum diameter was no more than 1.5 times
the minimum. After the total microvasculature was studied, the
perfused portion was obtained by analysis of photographs of the
field for the presence of either microfil or the carbon particles
of the drafting ink.
Data analysis. Data are presented as the means + SD. Comparisons between the means were by paired t test. Comparisons
of proportions in the stunned and normal regions were performed by chi-square analysis.
Results
Hemodynamic changes after repeated brief coronary
occlusions. Comparing baseline and 90 min after repeated brief coronary occlusions, there were no sigVol. 77, No. 4, April 1988
nificant differences in heart rate (128 ± 20 vs 133 + 26
beats/min), peak left ventricular pressure (124 + 18 vs
131 ± 13 mm Hg), or mean aortic pressure (100 ± 13
vs 105 ± 10 mm Hg).
The effects of repeated coronary occlusions on regional
function. Systolic segmental shortening in the left anterior descending region decreased from 14.4 ± 3.3% at
baseline to 0.4 + 4.6% after the repeated 5 min coronary occlusions (p <.001; table 1). The end-diastolic
segmental length increased from 12.2 ± 2.1 mm at
baseline to 14.6 ± 2.9 mm after stunning (p < .05). The
control region showed no change in systolic segmental
shortening (11.0±2.2% at baseline vs 10.7± 1.7%
after repeated occlusions) or end-diastolic segment
length (11.7 ± 1.9 vs 11.5 ± 2.1 mm).
Myocardial necrosis. No areas of necrosis were
observed on visual inspection of the triphenyl tetrazolium chloride-stained left ventricular slices from any
of the 15 animals.
Regional arterial and venous oxygen saturations and oxy-
gen extraction. The mean arterial and venous oxygen
saturations in the stunned and control regions are shown
in table 2. One hundred fifty-eight arterioles were sampled from the control regions of the 10 dogs (79 in the
endocardial region and 79 in the epicardial region). One
hundred sixty-one arterioles were sampled from the
biopsy specimens from the stunned region (79 in the
endocardial region, 82 in the epicardial region). Values
for mean arterial oxygen saturation were somewhat low,
ranging from 83.4% to 85.7%, probably reflecting the
fact that the dogs were ventilated with room air in the
right lateral decubitus position. However, there were no
significant differences in the mean arterial oxygen saturation or in the frequency distribution of the arterial
saturations in the stunned and control regions.
With respect to venous saturations, a significantly
lower mean epicardial venous oxygen saturation was
TABLE 1
Regional function and blood flow in stunned and control regions
Systolic End-diastolic Endocardial Epicardial
flow
flow
shortening
length
(%)
(mm)
(ml/g/min) (mlg/mi)
Stunned region
Baseline
90 mm after
ischemia
Control region
Baseline
90 mm after
ischemia
12.2±2.1
1.1±+0.2
1.2±0.2
14.6 ± 2.9B
1.0±0.1
1.3±0.2
11.0±2.2
11.7± 1.9
1.0±0.2
1.4±0.3
10.7±1.7
11.5±2.1
0.9±0.1
1.3±0.2
14.4±3.3
0.5 ±4.6A
Ap < .001 compared with baseline value; Bp < .05 compared with
baseline value.
867
STAHL et al.
TABLE 2
Mean oxygen saturations, extraction, and consumption in stunned and control regions
Stunned
Arterial saturation (%)
Venous saturation (%)
02 extraction (ml O2/100 ml blood)
02 consumption (ml]2/zmin/lOO g)
Control
Endo
Epi
Endo
Epi
83.9 ± 3.4
36.2±14.6A
7.1 ± 1.OA
6.9± 1.5
85.7 ± 4.0
38.0± 13.9
7.1 0.9B
8.8+1.5B
83.4± 2.9
39.5+ 15.3
6.6± 1.0
6.4±1.9
85.4 ± 4.2
43.8±11.8
6.3 ± 0.8
7.8±1.3
±
Endo = endocardium; Epi = epicardium.
Ap = .12 compared with control value.
Bp < .05 compared with control value.
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observed in the stunned region compared with control
(38.0 ± 13.9% vs 43.8 ± 11.8%, p < .02, paired t test).
There was also significantly greater vein-to-vein heterogeneity in the epicardium of the stunned region
(figure 1). The proportion of veins with low oxygen
saturations (<20%) was higher in the stunned than
normal epicardium (14.9% vs 4.5%, p<.05).
In the endocardium of the stunned region, the mean
venous oxygen saturation was also lower (36.2 ±
14.6% in the stunned region vs 39.5 + 15.3% in the
control region), although the difference did not reach
statistical significance. The frequency distribution
showed a tendency toward more veins with lower oxygen saturations in the stunned region (figure 2).
Oxygen extraction was higher in the stunned region
(7.1± 1.0 vs 6.6± 1.0 ml 02/100 ml blood in the
endocardium and 7.1±9 vs 6.3±.8 ml 02/100 ml
blood in the epicardium), although the difference
reached statistical significance only in the epicardium
(p<.05; figures 3 and 4, table 2).
Regional myocardial oxygen consumption, calculated from regional oxygen extraction and blood flow,
was about 10% higher in the stunned region (statistically significant in the epicardial segment; figures 3 and
4, table 2). The values for mean oxygen consumption
(6.4 to 8.8 ml 02/min/100 g) were somewhat low, based
on both the low arterial oxygen saturation and the low
arterial hemoglobin concentration (average 11.25
mg/dl), but were within the range previously reported
for open-chest dogs.20
There was no difference in the diameters of the
vessels identified and used to obtain oxygen saturations
in the stunned (mean arterial diameter 29.1 ± 21.2 gm
with 87% of the vessels having a diameter of less than
60 ,um; mean venous diameter 25.2 ± 21.1 gm with
91% less than 60 ,um) and the control regions (mean
arterial diameter 29.4 ± 24.6 ,um with 89% less than 60
,im; mean venous diameter 24.7 + 19.3 ,um with 91%
less than 60 ,im).
Evaluation of microvascular patency. To determine
whether the shift in venular oxygen saturations toward
lower values in stunned myocardium was due to focal
microvascular obstruction with heterogeneously impaired perfusion, hearts were injected in situ with either
Microfil or drafting ink and evaluated microscopically
(tables 3 and 4). In three hearts injected with Microfil
IL
-J
J
CL
n
LU
:J
TF
30 -4
E STUNNED
U)2
40 T
U)
U)
40
0 NORMAL
o
LL
30 -
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20
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0 NORMAL
U STUNNED
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a
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z
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a:
LU
a.
0-J
<20J
02 SATURATION (X)
-30
30-40
40-
02 SATURATION t%)
FIGURE 1. Histogram representing frequency of occurrence of different venous 02 saturations in the epicardial segments of stunned and
normal regions. There were significantly more venules with oxygen
saturations of less than 20% in the stunned region.
868
FIGURE 2. Histogram representing the frequency of occurrence of
different venous 02 saturations in the endocardial segments of stunned
and normal regions. There was a trend toward proportionally more
venules with lower oxygen saturations in the stunned region, with 32%
having saturations less than 30% in the stunned region vs 22% in the
normal region (p= .15).
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PATHOPHYSIOLOGY AND NATURAL HISTORY-MYOCARDIAL ISCHEMIA
10 7T
12 --F
10-
z
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STUNNED
*p<.05
E
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STUNNED
NORMAL
NORMAL
FIGURE 3. The mean epicardial oxygen extraction and consumption for each dog, comparing results from the stunned and
normal regions. The overall mean extraction and consumption was higher in the stunned than in the normal epicardial region.
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to visualize arterioles, similar numbers of vessels were
identified in stunned and control myocardium (epicardium, 0.92 vs 1.03 vessels/mm2; endocardium, 1.79
vs 2.10 vessels/mm2). Approximately twice as many
vessels were observed in endocardial samples as in
epicardial samples in both regions. The percent of
vessels patent (i.e., filled with Microfil) was 96% or
more and did not differ in stunned and control myocardium. In two hearts injected with drafting ink to
visualize capillary filling, similar numbers of vessels
were again identified in stunned and control specimens
(epicardium, 3950 vs 4123 vessels/mm2; endocardium, 3411 vs 3455 vessels/mm2). Approximately
85% of vessels were patent (i.e., filled with carbon
particles), without any difference between stunned and
control regions.
Discussion
The major findings of this study were that despite
reduced systolic function, regionally stunned myocardium exhibited higher mean oxygen extraction and
consumption in the presence of normal mean myocardial blood flow. The stunned myocardium also demonstrated a significantly greater vein-to-vein heterogeneity of oxygen saturations, especially in the epicardial
segment, with proportionately more venules demonstrating low oxygen saturations, suggesting that the
stunned region contained microregions of either increased oxygen extraction or reduced flow. Microscopic examination of normal and stunned regions,
however, revealed the microvasculature to be equally
patent without evidence of occlusion or plugging.
The microspectrophotometric analysis of arterial and
127
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8+
z
0
H
z
H
1-4 Cm
qu
cc0
H0
zo
D
6
+
Z O
OE
4-
8-
6-
4-
E
Xn-i
X
0
HO
Z- 0
2
nl
o
-
-
T
STUNNED
2±_
0
NORMAL
1
i
STUNNED
FIGURE 4. The mean endocardial oxygen extraction and consumption for each dog and the overall
normal regions.
Vol. 77, No. 4, April 1988
NORMAL
means
for stunned and
869
STAHL et al.
TABLE 3
Patency of arterioles in stunned and control regions as assessed by
nilcrofil inJection
Vessels
identified
Control
Epicardium
Endocardium
Stunned
Epicardium
Endocardium
Number Percent Vessel concentration
patent patent (vessels/mm2 tissue)
128
282
122
271
95
96
1.03
2.10
122
236
119
234
98
99
0.92
1.79
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venous oxygen saturations has been detailed in previous publications and is accurate to approximately
±3%.21 The biopsy samples were frozen in liquid
nitrogen within 10 sec of removal and stored in a liquid
nitrogen container until microspectrophotometric measures were made. It has previously been shown that
oxygen saturations in small arteries and veins, even
those less than 50 ,u m in diameter, are not altered if the
samples are frozen within 15 to 30 sec, and that the
oxygen saturations are stable for weeks at this
temperature.22' 23 All samples were coded and the
microspectrophotometric analyses were performed
without knowledge of location within the heart.
The accuracy of our determination of regional oxygen consumption depends not only on the arterial and
venous saturations, but also on the measurements of
regional blood flow. We used mean blood flow values
in endocardial and epicardial samples weighing about
1 g each. Based on the number of spheres injected, the
estimated measurement error was less than 5%.24 In
addition, the calculation of mean regional oxygen consumption from the individual venular and arteriolar
oxygen saturation data involves the implicit assumption that perfusion is homogeneously distributed. If
flow were diminished to microregions of high oxygen
extraction and replaced by an equal increase in flow to
regions of low extraction, mean oxygen extraction
would actually be lower than the result obtained by
TABLE 4
Patenty of capillaries in stunned and control regions as assessed by
injection of drafting ink
Total
identifiedA
PerfusedA
%
perfused
4123±423
3455 ±t416
3533±365
2894±+ 372
86
84
3950±382
3411±281
3306±310
2872±269
84
84
Control
Epicardium
Endocardium
Stunned
Epicardium
Endocardium
ANumber of vessels/mM2 tissue ± SD.
870
simple averaging of all of the individual extraction
values. In previous studies,'16 25 the validity of the use
of averaged microscopic values to obtain mean oxygen
extraction has been established both for the heart and
gracilis muscle by demonstrating comparability of the
microspectrophotometric method and mean extraction
obtained from measurements in large arteries and
veins. Unfortunately, the distribution of perfusion cannot be examined directly, since methods are not available to measure flow with sufficiently high spatial resolution in the beating heart.
Our finding that oxygen extraction and consumption
were higher in stunned myocardium could theoretically
be incorrect if the microvascular distribution of perfusion were grossly different in stunned and normal
regions. However, the differences involved would have
to be extreme. For example, we found a 13% increase
in oxygen extraction and consumption in stunned epicardium. To lower this excess oxygen consumption to
normal, flow would have to be absent from veins with
the lowest saturation (< 20%) and replaced by flow to
veins with the highest saturation (>50%; figure 1).
Furthermore, if flow in normal myocardium were not
completely homogeneous and also had a tendency to be
distributed more to veins with higher saturations, the
maldistribution in stunned myocardium would have to
be even greater. Preliminary results from Dean et al.,9
who used regional coronary venous oxygen sampling,
support the notion that oxygen consumption in stunned
myocardium is at least equal to normal.
There are a number of questions that arise from the
results of this study. First, how does one interpret the
finding in stunned myocardium of increased oxygen
consumption and normal regional blood flow in the face
of markedly depressed regional systolic shortening?
Given what appears to be decreased mechanical work
being performed by the stunned segment, the measurements of regional oxygen consumption are in
marked excess of the metabolic demands attributable
to muscle contraction. This suggests an impairment of
aerobic metabolic efficiency, or a defect in the conversion of aerobicly derived metabolic energy into contractile work.
Other investigators have recently reported normal or
increased myocardial oxygen consumption in
regionally9' 10 or globally8' 11 12 stunned models, consistent with an impairment of aerobic efficiency. Possible explanations for this phenomenon include uncoupling of oxidative phosphorylation, resulting in
inefficient mitochondrial ATP production, or an increase in the cellular energy requirement for processes
other than contraction. Mitochondrial uncoupling is
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unlikely, since Edoute et al.26 have shown that mitochondrial oxidative dysfunction occurs only after a
relatively long ischemic exposure, well after the
appearance of mitochondrial morphologic changes or
postischemic mechanical dysfunction. An increase in
noncontractile energy requirements seems a more
likely possibility, perhaps related to an increase in ion
pumping (i.e., extrusion of calcium from the mitochondria in calcium overloaded myocytes) or an
increase in the activity of the calcium ATPase to maintain normal sequestration of calcium by the sarcoplasmic reticulum. Krause and Hess27 have demonstrated
that after only 15 min of normothermic global ischemia
in the dog, there is significant depression of sarcoplasmic reticulum function, with reduced calcium transport
and depressed Ca + ++
Mg++ -ATPase activity. Importantly, the coupling between calcium transport and
ATPase activity is also impaired, consistent with a need
for greater utilization of ATP to move calcium into the
sarcoplasmic reticulum.
The finding of normal rather than depressed flow in
stunned myocardium is of interest, since one might
have anticipated that flow would autoregulate downward in concert with an assumed reduction in metabolic
demands. However, our finding of increased oxygen
extraction in stunned myocardium suggests that the
metabolic demands are actually increased rather than
decreased, although the energy produced is not translated into contractile work. Viewed in this light, normal
levels of flow in stunned myocardium may actually
represent an appropriate level of autoregulation.
A second important question concerns the interpretation of our finding of greater heterogeneity of
venous oxygen saturations in stunned myocardium,
with proportionately more venules having very low
oxygen saturations. In principle, this finding should
reflect foci of increased oxygen extraction due to
either focally impaired microvascular perfusion or
microregions of increased oxygen consumption. In
either case, our observation suggests that stunned
myocardium may not be affected uniformly. Instead,
dysfunction of an entire segment may be related to
injury of focal microscopic regions. If stunned myocardium is caused by generation of oxygen free radicals upon reperfusion, as has been suggested by the
beneficial effects of free radical scavengers, 28-30 our
data suggest that radical formation may be patchy,
possibly related to nonuniform generation within the
microvasculature. Based on Engler and Covell's
finding that leukocyte depletion results in an attenuation of myocardial stunning,31 a patchy distribution
of leukocytes within the microvasculature may be
Vol. 77, No. 4, April 1988
responsible for a heterogeneous pattern of free radical
production and myocyte injury.
The vascular injection studies were performed to
gain further insight into the finding of a greater proportion of venules with very low oxygen saturations in
stunned myocardium. Our results appear to exclude
anatomic vascular obstruction at either the arteriolar or
capillary levels as an important mechanism. Because
the microfil or drafting ink was injected under physiological pressures in vivo while the heart was still
beating, it is unlikely that the injection process per se
caused a significant artifactual reopening of previously
obstructed vessels. In addition, microvascular patency
is supported by our previous finding of normal maximal
flow reserve in stunned myocardium.32 Hori et al.33
demonstrated that during progressive occlusion of the
microvasculature with microspheres, resting flow initially remained normal due to compensatory focal vasodilation, but the maximal vasodilating capacity of the
bed fell progressively. Although anatomic obstructions
were not identified, our methods were not designed to
exclude functional heterogeneity of perfusion. Although the injections of drafting ink were made
"semiphysiologically" and carbon particles were seen
in nearly all capillaries, the actual numbers of carbon
particles present in each vessel were not quantified. In
addition, because the ink was injected over several
minutes, even vessels with very slow flow should have
been filled.
In conclusion, our results provide new data concerning postischemic regionally stunned myocardium
in the open-chest anesthetized dog. We found a significant increase in mean oxygen extraction and consumption despite an apparently marked decrease in
contractile function. Although mean perfusion of the
stunned myocardium was normal and the microvasculature was normally patent, microspectrophotometry
revealed increased heterogeneity with an excess of very
low venous oxygen saturations, implying either focally
impaired perfusion or increased metabolic activity.
These results suggest that stunned myocardium is
injured in a nonuniform, patchy fashion, although the
precise type of injury incurred and the mechanisms
responsible remain to be determined.
We gratefully acknowledge the assistance of the technicians
in our research laboratories, Anthony F. DiPaula, Jr., Alexander
(Skins) Wright, and Elizabeth Rodriguez, and of Christine G.
Holzmueller in the preparation of this manuscript.
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CIRCULATION
Myocardial oxygen consumption, oxygen supply/demand heterogeneity, and
microvascular patency in regionally stunned myocardium.
L D Stahl, H R Weiss and L C Becker
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Circulation. 1988;77:865-872
doi: 10.1161/01.CIR.77.4.865
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