546
Relative Importance of Tissue Oxygenation and
Vascular Smooth Muscle Hypoxia in Determining
Arteriolar Responses to Occlusion in the Hamster
Cheek Pouch
JULIAN H.
LOMBARD AND BRIAN R.
DULING
Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017
SUMMARY Cheek pouches of anesthetized hamsters were suffused with solutions with low, medium, and high
oxygen tension, and arteriolar diameter changes were measured during and after occlusions of either single arterioles
or the whole vascular bed. PO2 was measured with microelectrodes on arteriolar walls and at surrounding tissue sites
to indicate oxygen availability to the vascular smooth muscle and parenchymal cells, respectively. In each suffusion
solution, whole pouch occlusion (WPO) and microvessel occlusion (MVO) induced quantitatively similar changes in
tissue and periarteriolar PO 2 ; however, WPO resulted in larger diameter increases and longer recovery times than
MVO. High suffusion solution PO 2 attenuated the reduction in tissue and periarteriolar PO 2 during occlusion (WPO
and MVO), reduced the dilation during occlusion (WPO and MVO), and was associated with faster recovery of
arteriolar diameters following release of WPO. Postocclusion recovery of periarteriolar PO 2 and tissue PO 2 in low
oxygen suffusion was significantly faster than that of arteriolar diameters (WPO and MVO). Following WPO,
periarteriolar PO 2 often had fully recovered while diameter was at its peak value. During intermediate and high PO 2
suffusion, arteriolar dilation occurred despite relatively high periarteriolar oxygen tensions. The evidence suggests
that arteriolar responses to occlusion are determined primarily by indirect mechanisms, i.e., those mediated through
parenchymal cell metabolites, rather than by the direct effects of oxygen deficiency on the vascular smooth muscle
cells of the arteriolar media.
Reactive hyperemia is the period of increased blood flow
following occlusion of the blood supply to a vascular bed.
Two of the major mechanisms proposed to explain this
response involve oxygen either by its effect on vascular
smooth muscle or by an indirect mechanism mediated
through metabolism of the parenchymal cells. The indirect
mechanism involves dilation of the small resistance vessels
induced by the accumulation of vasoactive tissue metabolites during the period of occlusion,1 whereas the direct
effect refers to relaxation of the vascular smooth muscle
induced by low PO2 in the muscle cells.2
Most studies of reactive hyperemia have been performed on whole vascular beds with only indirect indications of the precise behavior of the resistance vessels.
Microcirculatory studies have the distinct advantage of
permitting direct access to the small blood vessels and
observation of their responses to various stimuli. However, relatively few such investigations of reactive hyperemia have been conducted.3"5
Since oxygen availability to tissues and to the microcirFrom the Department of Physiology, University of Virginia School of
Medicine, Charlottesville, Virginia.
Supported by National Institutes of Health Grant HL 12792. This work
was conducted during Dr. Duling's tenure as an Established Investigator of
the American Heart Association.
Dr. Lombard was supported by National Institutes of Health Postdoctoral Fellowship HL 05026.
Address for reprints: Dr. Julian H. Lombard, Department of Physiology, University of Virginia School of Medicine, Charlottesville, Virginia
22901.
Preliminary reports of this work were presented at the First World
Conference of Microcirculation, Toronto, Canada, June 1975; and at the
Microcirculatory Society Meeting, Anaheim, California, April 1976.
Received November 5. 1976; accepted for publication March 1, 1977.
culation can be altered by varying the PO2 of a suffusion
solution covering the tissue, suffused microvessel preparations permit an evaluation of the importance of oxygen in
the vascular response during both occlusion and the ensuing period of reactive hyperemia. Oxygen tensions, measured with microelectrodes on the walls of arterioles and at
various tissue sites, are good indices of oxygen availability
to the vascular smooth muscle and parenchymal cells,
respectively.6 The object of the present study was to use
these methods to determine whether the arteriolar dilation
which occurs in response to vascular occlusion correlates
more closely with oxygen deficiency at the vascular
smooth muscle or with oxygen deficiency of the parenchymal tissue.
Methods
Male golden hamsters (90-130 g) were anesthetized
with an intraperitoneal injection of sodium pentobarbital,
60 mg/kg. A tracheostomy was performed to ensure a
patent airway, and a femoral vein was cannulated. Supplemental anesthesia was administered, iv, as required. Physiologic saline solution was infused at a rate of 0.42 ml/hr to
compensate for evaporative water loss and to maintain
circulating blood volume. Preparation of a single layer
suffused cheek pouch was performed as previously described.7 The blood supply to the whole vascular bed was
occluded by a lifting ligature around the common carotid
artery or by compression of the base of the pouch with a
hydraulic occluder8 or a soft silicone rubber snare. Single
arterioles were occluded by compression with glass micropipettes guided by a Leitz or De Fonbrune micromanipulator.
ROLE OF OXYGEN DURING OCCLUSION/Lombard and Duling
Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017
The hamster was placed on the stage of a Leitz Labolux
II microscope equipped with a 50 x UMK objective and
the preparation was transilluminated with a high intensity
xenon lamp filtered at 540 nm. The cheek pouch was
continuously suffused with a bicarbonate-buffered physiologic saline solution, pH 7.35, with the following millimolar composition: NaCl, 131.9; KC1, 4.7; CaCl2, 2.0;
MgSO4,1.17; and NaHCO 3 ,20.0. The temperature of the
pouch was maintained at 35-36°C and the preparation was
allowed to equilibrate for each experiment until diameters
and oxygen tensions were stable. The presence of tone in
arterioles under observation was verified by observing the
dilation produced by the topical application of a 10~4 M
solution of adenosine or acetylcholine.
The microcirculation was also observed on a Cohu
closed circuit television system having a useful magnification of about 500x. Inside diameters of arterioles were
measured continuously before, during, and after occlusion
with a pair of movable lines generated by a Colorado
Video Analyzer (model 321). The DC voltage output
from the Video Analyzer was recorded by a Brush model
260 chart recorder. The entire system was calibrated
against a Vickers image-shearing eyepiece, and had an
overall accuracy of ±1 ^m.
Oxygen tensions on the walls of arterioles and in the
tissue were measured amperometrically with oxygen microcathodes9 as described previously.6 The tissue sites selected were free of capillaries and thus reflect minimal PO2
in the tissue. The microelectrodes were also used to measure the PO2 of the suffusion solution as it flowed over the
pouch. Electrodes were calibrated in saline equilibrated
with 100% N2 and room air before and after each experiment, and readings were discarded if any significant
change occurred in the calibration currents. Currents,
measured on a Keithley model 602 picoammeter, were in
the range of 0.1 x 10"" A in 0% O2, and the ratio of 0%
O2/21 % O2 currents was 0.05 ± 0.005 (SEM). The tip size
of the microelectrodes used in this study ranged from 1 to
4 /am.
To determine the effect of oxygen supply on the responses of arterioles to occlusion, the oxygen content of
the suffusion solution was altered by equilibrating the
solution in the supply reservoir with gas mixtures containing either 0% O2, 5% O2, or 10% O2, with 5% CO2 and
N2 as a filler gas. The PO2 of the various suffusion solutions, measured over the pouch with microelectrodes, was
7.2 ± 1.3 mm Hg for the 0% O2 solution, 35.0 ±1.1 mm
Hg for the 5% O2 solution, and 74.1+3.3 mm Hg for the
10% O2 solution.
Several parameters of the microvascular response were
evaluated in an effort to develop expressions relevant to
the pressure-flow relations in whole organs. Measurement
of diameter and changes in diameter at the end of an
occlusion period were used as corollaries to changes in
peak flow. Similarly, the time required for recovery of the
arteriolar diameter following occlusion should be an index
of the duration of reactive hyperemia. We have therefore
expressed the data as the half-time for recovery.
Results
ARTERIOLAR DIAMETER CHANGES DURING
OCCLUSION
Table 1 compares the final diameter increases (/*m)
following 60 seconds of whole pouch occlusion or single
vessel occlusion during suffusion with solutions with low,
intermediate, and high oxygen content. In the case of
single vessel occlusions, all measurements were made
downstream from the pipette to ensure that intravascular
pressure decreased during the occlusion. Diameter increases during single arteriole occlusions have also been
described in detail as part of a previous investigation.10
Arteriolar diameters increased during occlusion with all
three suffusion conditions and, in each solution, diameter
increases during whole bed occlusion were significantly
greater than those during single vessel occlusion. During
both single vessel and whole pouch occlusion, less dilation
occurred as the PO2 of the suffusion solution was increased.
RECOVERY TIMES FOLLOWING WHOLE POUCH AND
SINGLE VESSEL OCCLUSIONS
Table 2 compares the recovery times of arteriolar diameter following release of whole bed and single vessel occlusion during 0% O2, 5% O2, and 10% O2 suffusion. The
time necessary for 50% recovery from the maximum dilation attained during the occlusion was measured as an
index of the recovery rate. During all suffusions, the half-
TABLE 1 Peak Diameter Increase during 1-Minute Occlusions of Single Arterioles or the
Whole Vascular Bed
Single vessel occlusion
Whole pouch occlusion
Diameter increase
Suffusion
0%O 2
5% Oj
10% O,
Control
(jim)
20.7 ± 1.4
4.5 ± 0.6
(24) [17]
17.7 ± 1.5
3.3 ± 0.6
(12) [9]
15.1 ± 1.7
2.9 ± 0.5t
(15) [11]
547
Diameter increase
Control
(/im)
21.7 ± 1.7
10.0 ± 1.5*
(24) [15]
18.6 ± 1.5
6.8 ± 1.0*
(8) [5]
18.8 ± 1.8
6.0 ± 1.2't
(13) [7]
All values are expressed as mean ± SEM.
Parentheses indicate numbers of arterioles observed; brackets indicate the number of animals.
* P < 0.05 vs. single vessel occlusion at same suffusion solution PO2.
t P < 0.05 vs. same type of occlusion during 0% O2 suffusion.
CIRCULATION RESEARCH
548
TABLE 2 Half-Times of Arteriolar Diameter Recovery
following 1-Minute Occlusions of Single Arterioles or the
Whole Vascular Bed
Time (sec)
Suffusion
0%O2
5% O2
10% O2
Single vessel occlusion
11.5 ± 1.4
(27) [23]
6.3 ± 1.4
(6) [6]
10.7 ± 2.6
(12) [7]
Whole bed occlusion
33.0 ± 3.8*
(36) [24]
18.6 ± 2.5*t
(16) [8]
17.6 ± 2.6*t
(23) [14]
Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017
All values are expressed as the time (mean ± SEM) from release
of occlusion until 50% recovery from the maximum diameter
attained during 1-minute occlusions of either single arterioles or
the whole vascular bed.
Parentheses indicate numbers of arterioles observed; brackets
indicate the number of animals.
* P < 0.05 vs. single vessel occlusion at same suffusion solution
PO2.
t P < 0.05 vs. whole bed occlusion in 0% O2 suffusion.
time of recovery following whole pouch occlusion was
significantly longer than that following single vessel occlusion.
Recovery times following 1-minute whole pouch occlusions were reduced as the PO2 of the suffusion solution
was increased. Recovery times following 1-minute occlusions of single arterioles were not significantly influenced
by the oxygen content of the suffusion solution. Following
3-minute single vessel occlusions, recovery of arteriolar
diameters were slower in low oxygen suffusion. The halftime of recovery following 3-minute single vessel occlusions in 0% O2 suffusion averaged 22.2 ± 4.1 seconds (n
= 19), compared with half-times of 11.5 ± 1.4 seconds for
1-minute occlusions in 0% O2 suffusion (n = 27) (P <
0.05) and 11.8 ± 3.3 seconds for 3-minute occlusions in
10% O2 suffusion (n = 10) (P < 0.05).
PERIARTERIOLAR PO2 DURING SINGLE VESSEL AND
WHOLE BED OCCLUSION
Table 3 compares changes in periarteriolar PO2 during
whole bed occlusion and during single vessel occlusion
VOL.
41, No.'4,
TISSUE PO2 DURING SINGLE VESSEL AND WHOLE
BED OCCLUSION
Our measurements of tissue PO2 under conditions of
unrestricted blood flow are in good agreement with those
previously described by others.6' "~15 The changes in tissue
PO2 during occlusion on 0%, 5%, and 10% O2 suffusion
are compared in Table 4. Tissue PO2 changes were similar
in response to single vessel and whole bed occlusion.
With 0% O2 suffusion, tissue PO2 reached 0 mm Hg in
Periarteriolar PO, changes (mm Hg)
Single vessel occlusion
0% O2
5% O2
10% O2
Conlrol
Minimum
28.7 ± 2.0
9.0 ± 2.1
(13)[8]
33.1 ± 2.4
18.4 ± 1.9*
(11) [51
32.6 ± 2.4
27.9 ± 4.2't
(7) [5]
1977
under the various suffusion conditions. With the low oxygen suffusion, periarteriolar PO2 reached similar minimum
values of 9.0 ± 2.1 mm Hg during single vessel occlusion
and 7.5 ± 1.9 mm Hg during whole bed occlusion. PO2 on
the arteriolar wall reached 0 mm Hg during two of 13
single vessel occlusions and during two of 10 whole bed
occlusions. Periarteriolar PO2 also fell below 2 mm Hg
during two other single vessel occlusions and one other
whole bed occlusion. When the occlusion was released,
oxygen tension on the arteriolar wall rapidly recovered to
a value almost identical to the preocclusion PO2 (29.5 ±
1.8 mm Hg following single vessel occlusion and 26.1 ±
2.4 mm Hg following whole pouch occlusion).
During single arteriole occlusion in 5% O2 suffusion,
periarteriolar PO2 again declined markedly, but reached a
plateau at a minimum value of 18.4 ± 1.9 mm Hg. This is
significantly higher than the minimum value observed during 0% O2 suffusion (P < 0.01). PO2 on the arteriolar
wall failed to fall below 8 mm Hg during any occlusion.
Periarteriolar PO2 changes in response to occlusion of the
whole vascular bed were similar to those observed during
single arteriole occlusion.
During 10% O2 suffusion, periarteriolar PO2 tended to
decrease slightly during both single arteriole and whole
bed occlusion. When evaluated by Student's /-test for
paired samples, this decrease was significant (P < 0.02)
only during whole bed occlusion. The average minimum
periarteriolar PO2 of 28.4 ± 4 . 3 mm Hg (single arteriole
occlusion) and of 27.7 ± 2 . 2 mm Hg (whole bed occlusion) were not significantly different from each other, but
were significantly higher than those occurring during single vessel and whole bed occlusions with 0% O2 suffusion
(P < 0.001).
TABLE 3 Periarteriolar P02 Changes in Response to 1-Minute Occlusions of Single
Arterioles or the Whole Vascular Bed
Suffusion
OCTOBER
Whole pouch occlusion
Control
Minimum
26.3 ± 2.3
7.5 ± 1.9
(10) [7]
32.8 ± 1.8
13.8 ± 1.4*
(8) [4]
32.0 ± 1.9
27.3 ± 2.3*
(8) [6]
All values are expressed as mean ± SEM. Control values represent PO2 immediately prior to
occlusion, and minimum values refer to the lowest PO, reached during the occlusion.
Parentheses indicate number of arterioles observed; brackets indicate the number of animals.
* P < 0.05 vs. same type of occlusion in 0% O2 suffusion.
t P < 0.05 vs. same type of occlusion in 5% O2 suffusion.
ROLE OF OXYGEN DURING OCCLUSION/Lombard and Duling
549
With 10% O2 suffusion, tissue PO2 exhibited a small but
significant decrease (Student's /-test for paired samples) in
response to both single vessel and whole pouch occlusion.
The minimum tissue PO2 reached during whole bed and
that during single vessel occlusion were not significantly
different from each other, but were significantly higher
than those during 0% and 5% O2 suffusion. Preocclusion
values of tissue PO2 during 10% O2 suffusion were also
significantly greater than those during 0% O2 suffusion (P
< 0.001) and 5% O2 suffusion (P < 0.05).
A-Periarteriolar POj
B-Tissue PO2
C- Diameter
p<01
p<J>01
TIME COURSE OF DIAMETER AND PO2 RECOVERY
FOLLOWING OCCLUSION (0% O2 SUFFUSION)
B
A
(19)
«)
(12)
Whole Pouch Occlusion
Single Arteriole Occlusion
Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017
FIGURE 1 Comparison of recovery rates of periarteriolar PO2
(A), tissue PO2 (B), and arteriolar diameter (C) following 1minute occlusions of single arterioles or the whole vascular bed
during 0% O 2 suffusion. Data are plotted as the elapsed time in
seconds (mean ± SEM) from release of occlusion until 50% recovery. Significance levels refer to comparison of recovery times of
periarteriolar PO2 (A) or tissue PO2 (B) with the recovery time for
ateriolar diameter (C). Numbers refer to the number of arterioles
observed.
two of 10 single vessel occlusions and in five of nine whole
bed occlusions. Tissue PO2 also fell below 2 mm Hg during
seven of the single vessel occlusions and during three of
the whole bed occlusions. During whole bed occlusion,
tissue PO2 tended to reach lower minimum values than
during single vessel occlusion, but this difference was not
significant. Upon release of occlusion, tissue PO2 recovered rapidly with a half-time of 3.6 ± 0.4 seconds
following single arteriole occlusion (n = 11) and 10.3 ±
2.2 seconds following whole bed occlusion (n = 8). The
half-time of recovery following single vessel occlusion was
significantly faster than that following whole bed occlusion
(P < 0.005).
In 5% O2 suffusion, tissue PO2 also declined sharply
during occlusion, reaching minimum values which were
intermediate to those reached during occlusion in the 0%
O2 and 10% O2 suffusions. Upon release of occlusion,
tissue PO2 again returned rapidly to control values.
Figure 1 compares the half-times of recovery of arteriolar diameter, periarteriolar PO2, and tissue PO2 following
single arteriole occlusion and whole pouch occlusion in
low oxygen suffusion. Periarteriolar PO2 recovered significantly faster than arteriolar diameter, and, following
whole bed occlusion, vascular diameters were often at
their peak value for some time after periarteriolar PO2 had
fully recovered. After release of both single arteriole occlusion and whole pouch occlusion, tissue PO2 recovered
more slowly than periarteriolar PO2, but significantly
faster than arteriolar diameter.
Discussion
The present experiments indicate that the magnitude
and duration of arteriolar dilation in response to occlusion
are not under direct and continuous control by oxygen
deficiency at the level of the vascular smooth muscle cells.
After release of both whole bed and single vessel occlusion, recovery of oxygen tension on the arteriolar wall was
significantly faster than recovery of arteriolar diameters
(P < 0.001). Following whole pouch occlusion in low
oxygen suffusion, diameters often remained at their peak
value for some time after complete recovery of periarteriolar PO2. Thus, our measurements of periarteriolar PO2
provide direct evidence that hypoxia of the resistance
vessels is not present following release of the occlusion.
This finding supports the conclusions of other investigators 16 ' 17 who reported substantial hyperemic responses
even when vessels were probably exposed to fully saturated arterial blood (and thus argued that the maintenance
TABLE 4 Tissue PO2 Changes in Response to 1 -Minute Occlusions of Single Arterioles or the
Whole Vascular Bed
Tissue POj changes (mm Hg)
Single vessel occlusion
Suffusion
0% O 2
Control
Minimum
11.0 ± 1.0
2.9 ± 0.6
Whole pouch occlusion
Control
11.3 ± 2 . 3
(19) [10]
5% O 2
14.8 ± 2.3
8.2 ± 1.8*
22.8 ± 2.4
17.9 ± 1.8
9.3 ± 1.9*
(8) [4]
19.7 ± 2.2*t
(9) [6]
1.2 ± 0 . 8
(9) [6]
(12) [4]
10% O,
Minimum
20.8 ± 2.0
16.1 ± 1.7't
(11) [6]
All values are expressed as mean ± SEM. Control values represent PO, immediately prior to occlusion, and minimum
values refer to the lowest PO, reached during the occlusion.
Parentheses indicate number of arterioles observed; brackets indicate the number of animals.
' P < 0.05 vs. same type of occlusion in 0% O, sufusion.
t P < 0.05 vs. same type of occlusion in 5% O, suffusion.
550
CIRCULATION RESEARCH
Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017
of reactive hyperemia was not due to the direct effects of
oxygen deficiency on the vascular smooth muscle).
While Olsson18 has suggested that some time may elapse
before the vascular smooth muscle cells recover from hypoxia (which could explain their relaxation in spite of an
abundant oxygen supply in the postocclusion period),
comparison of the changes in arteriolar diameter and periarteriolar PO2 in response to single vessel occlusion and
whole bed occlusion during 0% O2 suffusion suggests that
this is unlikely. The minimum periarteriolar PO2 and the
recovery rate of periarteriolar PO2 are not greatly different for single vessel and whole pouch occlusion, but the
half-time for recovery of arteriolar diameters is substantially longer following release of whole bed occlusion (Tables 2 and 3). Furthermore, there is good evidence that
the arterioles of the cheek pouch are not particularly
sensitive to changes in PO2 which are restricted to the
environment of the smooth muscle.19
Some studies20'21 have suggested that vascular smooth
muscle is sensitive to oxygen deficiency at a fairly high
PO 2 . However, oxidative phosphorylation of hamster
mesenteric arterioles appears to be highly resistant to
hypoxia, and limitation of oxygen consumption does not
begin to appear until an environmental PO2 of 2-5 mm Hg
is reached.22 Data from isolated vascular strips exhibit a
size dependence for the critical PO2 of arterial smooth
muscle and, based on an average arteriolar wall thickness
of 10 /j.m, 2 mm Hg has been estimated to be the PO2 at
which the vascular smooth muscle should relax.23 These
findings indicate that the vascular smooth muscle is not
limited directly by oxygen availability during occlusion in
the 5% O2 and 10% O2 suffusions, since periarteriolar
PO2 did not fall below the value of 2-5 mm Hg. Moreover,
the minimum periarteriolar PO2 during occlusion was
close to or greater than the preocclusion value of tissue
PO2 during 0% O2 and 5% O2 suffusion. Thus, if dilation
of the arterioles were the direct result of oxygen deficiency
during 5% O2 and 10% O2 suffusion, the vascular smooth
muscle would have to be more sensitive to oxygen limitation than the parenchymal cells.
During several occlusions in 0% O2 suffusion, however,
periarteriolar PO2 approached 0 mm Hg. In such cases,
arteriolar dilation could be at least partially due to the
direct effect of oxygen deficiency on the vascular smooth
muscle.
The direct relationship between the minimum tissue
PO2 during occlusion and the PO2 of the suffusion solution
demonstrates that oxygen supply to the parenchymal cells
can be increased by raising the oxygen content of the
suffusion solution. Oxygen supply from the suffusate
would undoubtedly become even more important for parenchymal cell function when blood flow is interrupted by
occlusion. The increased oxygen supply to the tissue would
then presumably result in decreased production of vasoactive metabolites during occlusion and, in turn, decreased
arteriolar dilation. The decreased vasodilation we observed as the PO2 of the suffusion solution was increased is
consistent with this hypothesis.
Our direct measurements of tissue PO2 also confirm that
hypoxia is not present in most of the tissue following
release of the occlusion. The present investigation thus
VOL. 41, No. 4, OCTOBER
1977
indicates that the maintenance of the hyperemic response
is not due to the persistence of tissue hypoxia following
release of the occlusion. This is consistent with the conclusions of both McNeill16 and Eikens and Wilcken,17 who
observed reactive hyperemia under conditions in which
tissue hypoxia should have been reduced or eliminated.
These measurements of tissue PO2 cannot completely
eliminate the possibility of hypoxic foci, but the sites of
measurement were chosen to lie in areas free of capillaries
and, therefore, presumably represent the least oxygenated
areas of the tissue.
The present study suggests that oxygen-independent
mechanisms can contribute as much as 60-70% to the
dilation during occlusion (as estimated by comparison of
the peak dilation during single vessel or whole bed occlusion in low and high oxygen suffusion). While the precise
mechanisms causing arteriolar dilation during well oxygenated conditions cannot be determined from these experiments, several possibilities might be considered.
1. Arteriolar dilation during suffusion with the high
oxygen solutions might reflect a myogenic response triggered by a decreased intravascular pressure during occlusion ,5-24-27 However, the myogenic contribution to the
steady state diameter changes during arteriolar occlusion
in the hamster cheek pouch is apparently quite small.10
2. Dilation during occlusion in high oxygen suffusion
could be due to the action of aerobic metabolites. This
hypothesis is supported by the results of several other
studies25'28t 29 which have indicated that such metabolites
might contribute to the hyperemic response.
3. The generation of vasodilator metabolites might be
PO2-dependent, even at the high oxygen tensions we observed in the 5% O2 and 10% O2 solutions.
These results are all consistent with an increasing body
of evidence suggesting that regulation of oxygen availability by the circulation occurs at oxygen tensions substantially higher than the limiting PO2 of the cytochrome
oxidase system in isolated mitochondria.30'31
Acknowledgments
We thank Elizabeth Staples and David Damon for their skillful technical
assistance and Betty Haigh for an excellent job in typing the manuscript.
References
1. Lewis T, Grant R: Observations upon reactive hyperaemia in man.
Heart 12: 73-120, 1926
2. Fairchild HM, Ross J, Guyton AC: Failure of recovery from reactive
hyperemia in the absence of oxygen. Am J Physiol 210: 490-492,
1966
3. Burton KS, Johnson PC: Reactive hyperemia in individual capillaries
of skeletal muscle. Am J Physiol 223: 517-524, 1972
4. Gentry RM. Johnson PC: Reactive hyperemia in arterioles and capillaries of frog skeletal muscle following microocclusion. Circ Res 31:
953-965, 1972
5. Johnson PC, Burton KS, Henrich H, Henrich U: Effect of occlusion
duration on reactive hyperemia in sartorius muscle capillaries. Am J
Physiol 230: 715-719, 1976
6. Duling BR, Berne RM: Longitudinal gradients in periarteriolar oxygen tension; a possible mechanism for the participation of oxygen in
local regulation of blood flow. Circ Res 27: 669-678, 1970
7. Duling BR: The preparation and use of the hamster cheek pouch for
studies of the microcirculation. Microvasc Res 5: 423-429, 1973
8. Jacobson EG, Swan KG: Hydraulic occluder for chronic electromagnetic blood flow determination. J Appl Physiol 21: 1400-1402, 1966
9. Whalen WJ, Riley J, Nair P: A microelectrode for measuring intracellular PO,. J Appl Physiol 23: 798-801, 1967
10. Lombard JH, Duling BR: Relative contribution of passive and myogenic factors to diameter changes during single arteriole occlusion in
TWO-DIPOLE RANGING TECHNIQUE/M/rvis et al.
Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017
the hamster cheek pouch. Ore Res 41: 365-373, 1977
11. Whalen WJ, Nair P: Intracellular PO, and its regulation in resting
skeletal muscle of the guinea pig. Circ Res 21: 251-261, 1976.
12. Whalen WJ, Nair P: Skeletal muscle PO,; effect of inhaled and
topically applied O, and CO,. Am J Physiol 218: 973-980, 1970
13. Coburn RF, Mayers LB: Myoglobin O, tension determined from
measurements of carboxymyoglobin in skeletal muscle. Am J Physiol
220: 66-74, 1971
14. Duling BR: Microvascular responses to alterations in oxygen tension.
Circ Res 31: 481-489, 1972
15. Whalen WJ, Nair P, Buerk D, Thuning CA: Tissue PO, in normal and
denervated cat skeletal muscle. Am J Physiol 227: 1221-1225, 1974
16. McNeil! TA: Venous oxygen saturation and blood flow during reactive
hyperemia in the human forearm. J Physiol (Lond) 134: 195-201,
1956
17. Eikens E, Wilcken DEL: Reactive hyperemia in the dog heart; effects
of temporarily restricting arterial inflow and of coronary occlusions
lasting one and two cardiac cycles. Circ Res 35: 702-712, 1974
18. Olsson RA: Myocardial reactive hyperemia. Circ Res 37: 263-270,
1975
19. Duling BR: Oxygen sensitivity of vascular smooth muscle. II. In vivo
studies. Am J Physiol 227: 42-49, 1974
20. Detar R, Bohr DF: Oxygen and vascular smooth muscle contraction.
Am J Physiol 214: 241-244, 1968
21. Carrier OC Jr, Walker JR, Guyton AC: Role of oxygen in autoregulation of blood Dow in isolated vessels. Am J Physiol 206: 951-954,
1964
551
22. Howard RO, Richardson DW, Smith MH, Patterson JL Jr: Oxygen
consumption of arterioles and venules as studied in the Cartesian
diver. Circ Res 16: 187-196, 1965
23. Pittman RN, Duling BR: Oxygen sensitivity of vascular smooth muscle. I. In vitro studies. Microvasc Res 6: 202-211, 1973
24. Dornhorst AC, Whalen RF: The blood flow in muscle following
exercise and circulatory arrest; the influence of reduction in effective
local blood pressure, of arterial hypoxia, and of adrenaline. Clin Sci
12: 33-40, 1953
25. Olsson RA: Kinetics of myocardial reactive hyperemia blood flow in
the unanesthetized dog. Circ Res 14/15 (suppl I): 81-86, 1964
26. Patterson GC: The role of intravascular pressure in the causation of
reactive hyperaemia in the human forearm. Clin Sci 15: 17-25, 1955
27. Hilton SM: Experiments on the post-contraction hyperaemia of skeletal muscle. J Physiol (Lond) 120: 230-245, 1953
28. Kontos HA, Patterson JL Jr: Carbon dioxide as a major factor in the
production of reactive hyperaemia in the human forearm. Clin Sci 27:
143-154, 1965
29. Kontos HA, Mauck HP Jr, Patterson JL: Mechanism of reactive
hyperemia in limbs of anesthetized dogs. Am J Physiol 209: 1106—
1114, 1965
30. Rubio R, Wiedmeier VT, Berne RM: Relationship between coronary
flow and adenosine production and release. J Mol Cell Cardiol 6: 561566, 1974
31. Reivich M, Coburn R, Lahiri S, Chance B, (eds): Proceedings of the
Symposium on Tissue Hypoxia and Ischemia, Philadelphia, August
1976. New York, Plenum, 1977
Detection and Localization of Multiple Epicardial
Electrical Generators by a Two-Dipole Ranging
Technique
DAVID M . MIRVIS, FRANCIS W. KELLER, RAYMOND E . IDEKER, JOHN W . C O X , J R . ,
ROBERT F . D O W D I E , A N D D A V I D G . ZETTERGREN
SUMMARY The ability of a numerical procedure to detect and to localize two experimentally induced, epicardial
dipolar generators was tested in 24 isolated, perfused rabbit heart preparations suspended in an electrolyte-filled
spherical tank. Electrocardiograms were recorded from 32 electrodes on the surface of the test chamber before and
after placement of each of two epicardial burns. The second lesion was located either 180°, 90°, or 45° from the first.
Signals were processed by iterative routines that computed the location of one or two independent dipoles that best
reconstructed the observed surface potentials. The computed single dipole accounting for 99.68% of root mean
square (RMS) surface potential recorded after the first bum was located 0.26 ± 0.10 cm from the centroid of the
lesion. Potentials recorded after the second lesion were fit with two dipoles that accounted for 99.36 ± 1.51% of RMS
surface potentials and that were located 0.42 ± 0.26 cm and 0.57 ± 0.49 cm from the centers of the corresponding
burn. Seventy-one percent of computed dipoles were located within the visible perimeter of the burn. Thus, two
simultaneously active dipolar sources can be detected and accurately localized by rigorous study of the generated
electrical field.
CLINICAL electrocardiography strives to semiquantitatively define the physiological state of the heart from the
electrical potentials it generates. The concept of an equivalent cardiac generator has been useful in this effort. An
From the Section of Medical Physics, Department of Medicine, University of Tennessee Center for the Health Sciences, Memphis, Tennessee.
Supported by Grants HL-01362, HL-09495, and HL-20597 from the
National Heart, Lung and Blood Institute, National Institutes of Health,
U.S. Public Health Service. Dr. Mirvis was supported in part by National
Service Award HL-05323 from the National Heart, Lung and Blood
Institute, National Institutes of Health.
Dr. Ideker's current address is the Department of Pathology, Duke
University, Durham, North Carolina.
Address for reprints: David M. Mirvis, M.D., 951 Court Ave., Room
339M, Memphis, Tennessee 38163.
Received November 30, 1976; accepted for publication March 25,
1977.
equivalent cardiac generator may be defined1 as a distribution of electrical sources in a specified volume conductor
which generates potential distributions identical to, or
"equivalent" to, those generated by the natural electrical
generator, i.e., the heart. Many generator models have
been proposed and tested in the search for a truly equivalent cardiac generator. The earliest and simplest generator
was the fixed, single dipole model. Waller2 idealized the
relationships between the electromotive force of the heart
and surface leads by considering the heart to be a lumped
point source and point sink of current located within the
cardiac region of the torso. Einthoven et al. 3 brought the
source and sink pair close together to form an electrical
doublet, or dipole.
Relative importance of tissue oxygenation and vascular smooth muscle hypoxia in
determining arteriolar responses to occlusion in the hamster cheek pouch.
J H Lombard and B R Duling
Downloaded from http://circres.ahajournals.org/ by guest on June 16, 2017
Circ Res. 1977;41:546-551
doi: 10.1161/01.RES.41.4.546
Circulation Research is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1977 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/41/4/546.citation
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/