286
Enhancement of Cerebrovascular Effect
of CO2 by Hypoxia
STEPHEN R. QUINT, P H . D . , OSCAR U. SCREMIN, M.D.,
RALPH R. SONNENSCHEIN, M.D.,
ANO EDUARDO H. RUBINSTEIN, M.D.,
PH.D.,
PH.D.
SUMMARY Internal carotid blood flow, taken as an index of cerebral blood flow (CBF); arterial pressure;
and respfrfffffry 0 , and CO, concentrations, were measured in halothane (I%)-anestbetized, paralyzed and
mecJiantcaJly ventilated rabbits. CBF was determined at end-tfdal CO, of 4% (normocapnla) and 8%, as Inspired 0 , ([O,]i) was "varied sfepwise over ttte range of 6.5 to 92%. Noraocapnic CBF was constant over the
range of If to 92% fO,],, but ft increased significantly to 240% and 380% of control when [O,], was at 10% and
6.5%, respectively. Cerebrovascular response to CO, was constant orer the range of [0,], tested, except for a
significant elevation to 180% of control at fO,], of 10%.
Stroke, Vol 11, No 3, 1980
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CEREBRAL BLOOD FLOW (CBF) is well known to
increase under the influence of hypercapnia and
hypoxia.1"* A continuous, roughly linear relationship
exists over a wide range between CBF and arterial
Pcoj,5 while the effect of lowered arterial Po8 is seen
only at some threshold level7 below which CBF is inversely related to Pao,.
The combined effect of hypoxia and hypercapnia
has been given much less attention than their separate
actions. Lennox and Gibbs1 examined the effect of
different combinations of inspired gas concentrations
of O3 and CO, on CBF, but their results hare to be interpreted with caution since they estimated CBF from
the arteriovenous difference of Oa while assuming a
constant cerebral O2 consumption. More recently, the
questidn has been reexamined by Shapiro et al." who
studied the effects of combined hypoxia and hypercapnia on CBF in man. Although these authors did not
provide control values of CBF in their subjects under
5% CO2 without hypoxia, they did find higher values
for CBF under 10% Oj-5% CO2 than had been
reported by other authors for 5% CO2 alone. Furthermore, Flohr et al." have reported that the action of
hypoxia on CBF in cats was enhanced by hypercapnia.
While it thus appears that an interaction may exist
between the effects on CBF of hypoxia and hypercapnia, the quantitative aspects of the interaction between
0 , and CO, over a broad range, including hyperoxic
levels, have not been studied. Moreover, while it is
generally, but not unanimously,10 accepted that increase in PaOj from 50 mm Hg to one atmosphere has
negligible effect on CBF at normocapnia, the possible
effect of hyperoxia on CBF reactivity to CO, has not
been explored.
Materials and Methods
Adult male rabbits, 2.5-3.0 kg, were anesthetized
with 4% haiothane, paralyzed with pancuronium (0.6
mg/hr) and mechanically ventilated. Following induction, animals were maintained on 1% haiothane in
40% Or-60% N9 throughout each experiment. A
From the Department of Physiology and the Brain Research
Institute, UCLA School of Medicine, Los Angeles, CA 90024.
femoral artery was cannulated for recording of mean
arterial pressure (MAP) and for withdrawal of blood
samples for analysis. The external carotid artery and
any extracranial branches of the internal carotid
artery were ligated, and a cuff-type electromagnetic
flow probe (Biotronex) was placed on the common
carotid artery for measurement of internal carotid
blood flow (ICBF). A hydraulic occluder11 was applied
to the artery, distal to the flow probe, to allow determination of zero flow. Body temperature was
monitored with a rectal thermometer and maintained
at 38°C with radiant heat. An isotonic solution of
sodium bicarbonate, pH 7.44 (1.26 mg/100 ml), was
injected i.v. at 6.5 ml/kg/hr to maintain pH and
replace fluid loss. Respiratory Oa and CO2 were
monitored at the tracheal airway by Beckman OM-11
and LB-2 analyzers, respectively, and continuously
recorded. Arterial Po,, Pco2 and pH were determined
on a Radiometer 3M5 MK2 blood gas analyzer.
In each experiment ICBF, MAP, and respiratory
O2 and CO, were continuously monitored and expressed as percent. Regulation of the composition of
inspired gas was accomplished by adjusting the
proportions of O2, N, and CO2 delivered through
calibrated flowmeters, with total gas flow maintained
constant. Initially the animal was ventilated so as to
achieve constant "normocapnic" end-tidal CO,
([CO2]ET) of approximately 4% (range: 3.8-4.4%;
average Pacos = 33 ± 1 (SE) mm Hg; average pH =
7.39) and inspired O, concentration ([0,10 of 40%
(Pao, = 147 ± 6 (SE) mm Hg). After stabilization,
[CO,]ET was elevated to 8% (range: 8.0-8.8%; average
Paco, = 52 ± 3 (SE) mm Hg; average pH = 7.23) until ICBF reached a new steady level (3-4 min) and
[CO2]ET was then returned to its original level; [O,]i
was maintained constant during the change in
[CO2]ET. The same procedure was repeated at [O2]x of
6.5, 10, 15, 20 and 92%, corresponding to Pao, (X ±
SE) 24 ± 2, 30 ± 3, 43 ± 5; 63 ± 4, 147 ± 6, 356 ± 15
mm Hg, respectively). At each level of [O2]i and
[CO,]ET, an arterial blood sample was obtained for
analysis.
One to three measurements of ICBF were made at
each level of [CO2]ET and [O,]i. These data were used
to calculate the regression equation of ICBF on
[CO2]ET by the least-squares method11 for each
HYPOXIA AND CO, EFFECT ON CBF/Quint et al.
animal. CO 3 reactivity (CO 2 R) was defined as the
coefficient (slope) of that equation. To cancel out
variations between animals, ICBF and CO,R were
normalized. Normalized ICBF (ICBFJ was expressed as a percentage of the value of ICBF at
[COJJET 4% and [O,], 40%. Normalized CO,R
(CO,R n ) was expressed as a percentage of the value of
CO 2 R at [O2]i 40%. An average of ICBF n at normocapnia ( [COJ]ET 4%) and an average of CO,R n obtained from all animals were calculated for each [ O ^
level. Analysis of variance with repeated measures11
was performed to test for differences between means.
Results
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Figure 1 illustrates a representative experiment in
which an increase in [CO,] ET from 4% to 8%, at [ O ^
40%, resulted in an increase in ICBF which reached a
new level within approximately 3 min. In this case
ICBF increased by 40%, yielding a calculated CO,
reactivity of 10, i.e. 10% increase in ICBF for each
percent change in [CO 2 ] E T- Figure 2 depicts the increase in ICBF accompanying reduction of [O,]i from
40 to 10%, in the same experiment as figure 1. ICBF
increased promptly, reaching 230% of control in 3
min. During this period of hypoxia, an increase in
[COj]ET from 4 to 8% resulted in a further, considerably faster, increase of 80% of control ICBF to a
final value of 310%. This yielded a CO, reactivity of
20, i.e. 200% of the reactivity at [O,], 40%; this value
was close to the group mean of 180% for CO a R n as
shown in figure 3b, which presents the combined
results of all nine experiments. ICBF n remained essentially constant over the range of [Of]i 15-92% (fig. 3a).
At [O3]i 10% and 6.5%, however, ICBF n rose to 230
and 380% respectively. On the other hand, only at
[O2]i 10% did CO,R n change significantly, with an increase to 180%.
Changes in MAP that accompanied variations in
[CO,] ET or [O2]r were generally negligible, as in
figures 1 and 2.
287
MAP100
(mmHg)
3OO
ICBFn
(%) 100
0-
t
t
' '2m*
FIGURE 2. Changes in ICBF accompanying a decrease in
[OJ, from 40% to 10%. followed by a superimposed increase
in fCOJsrfrom 4% to 8%. (Designations as in fig. 1.)
Discussion
The results confirm the observation of others1"* that
CBF under normocapnia increases markedly as Pao8
decreases below approximately 50 mm Hg ([O,]i
15%). The maximum CBF attained, approximately
A.
400-
£300
c
m
200-
100
B.
200-
s*
1OO
(
RESPIR.
C0 2
4
(%)
O
40
RESPIR.
02
2
<*)
o
o
6.5 10 15 20
1OO
MAP
(mmHg)
ICBFn'
t
FIGURE 1. Increase in normalized internal carotid blood
flow (ICBFJ in response to a step change in end-tidal CO,
{[COTJER) from 4% to 8%, while inspired O, concentration
(fOJi) was maintained constant. Sharp downward deflections in ICBFn tracing represent determinations of zero flow
by occlusion of the carotid artery.
[o z ] i
40
92
FIGURE 3. A. Normalized internal carotid blood flow
(ICBFJ as a function of inspired oxygen concentration
[OJ,. Each point represented is the mean for 9 rabbits.
Analysis of variance with repeated measures on a single factor was used to determine significance between [OJ, levels.
CBF values for [OJ, 15% to 92% did not differ significantly
from that at [OJ, 40%, while flows at [OJ, 6.5% and 10%
differed significantly (p < 0.05) from each other and from
CBF at [OJ, 40%. B. Normalized CO, reactivity (COJtJ
as a function of inspired oxygen concentration [OJ,. Each
point represents the mean for 9 animals. Values of COJia
were compared using analysis of variance on a single factor,
[OJ,. The value of CO Jin at [OJ, 10% was significantly
different (p < 0.05) from all other values, which did not
differ significantly from the control value ofCOJin at [
40%.
288
STROKE
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400 percent of control, was reached at the lowest [ O ^
tested, i.e. 6.5%. The magnitude of response was lower
than in some reports10-1S and higher than in others,2' u
but the possible effects of species characteristics, type
and depth of anesthesia, extent of control of ventilation, and differences in methodology are too complex
to allow specific explanation of these variations. As
far as we are aware, no observations of this sort have
been made in rabbits. Throughout the normoxic and
hyperoxic range, [O,], of 20-92% (Pao, 63-356 mm
Hg), CBF was constant, again coinciding with
previous reports.1- *• "• 7
At any given [0,^, imposition of hypercapnia
([COJ]ET 8%) significantly increased blood flow by a
mean of 40% of the normoxic normocapnic CBF, except at [OJ]I 10% (Pao, = 30 mm Hg), where the increase amounted to 80% (figs. 1 and 2). This doubling
of the effect of COa is expressed by the corresponding
increase in normalized CO, reactivity (CO,Rn) at
[OJ]I 10%, with a tendency toward increase at [O,]i
15%, as shown in figure 3b. No changes in CO,R occurred in the hyperoxic range. In other words, the
effects of hypoxia and hypercapnia were additive, except at [Oi]i 10% where a greater than additive
response occurred, showing that the two effects potentiate each other. This confirms, in general, the findings
of Lennox and Gibbs,1 Shapiro et al.' and Flohr et al.,B
but with extension to the combined effect of hypercapnia and several levels of hypoxia and hyperoxia.
The reduction of CO,Rn back to control level at
[O2], 6.5% in the face of a continuing rise in ICBFn
may reflect the achievement of maximum CBF at this
level of PaOj. Alternatively, it may result from an interference by deep hypoxia with the basic mechanism
mediating the cerebral vasodilatation induced by
hypercapnia.
The rabbit has an advantage over most laboratory
animals for the study of CBF because the internal
carotid artery in this species supplies only cerebral
tissue in its intracranial course.16"" Thus, measurement of flow in this artery allows a continuous estimate of CBF. Since the level of anesthesia can
significantly affect both CBF and CO, reactivity,18 use
of an inhalation agent, such as halothane in our experiments, allows precise control of effective concentration of the anesthetic, particularly when ventilation
in maintained constant. The ventilatory control also
permits study of the vascular effects of hypoxia and
hypercapnia, independent of indirect actions via
alterations of ventilation.
Although the cerebrovascular effect of asphyxia was
already known a century ago, we have not advanced in
its understanding beyond a characterization of the
separate effects of hypoxia and hypercapnia.* The
mechanisms of these responses remain obscure. The
interpretation prevalent for many years,18- ro that a
decrease in brain extracellular pH due to lactic
acidosis determines the hypoxic vasodilatation, has
been contested by recent reports.21 The role of pH as
the sole determinant of hypercapnic cerebral
vasodilatation also has been challenged by the findings that brain stem lesions18- M- " and atropine17-18- M
VOL 11, No 3, MAY-JUNE 1980
can block this response, which is considerably
enhanced by physostigmine,17-18- ** a cholinesterase inhibitor. These observations have suggested the involvement of neurogenic mechanisms in the
cerebrovascular effects of blood O, and CO,. A comparison of the effects of O, and CO, on CBF with
those on ventilation, an obviously neurally mediated
response, might be pertinent. As has been reported
previously7 and as is confirmed here, CBF does not
change when Pao, is in the range of around 700 to 50
mm Hg, but increases very steeply in inverse proportion to Pao, when the latter is below 50 mm Hg (fig.
3). The response of ventilation to Pao, closely
parallels this, showing a "threshold" at 50-60 mm Hg
Pao, and a negative relation with Pao, below this."
The present study reveals that CBF reactivity to
CO, is affected only when Pao, is reduced below 50
mm Hg. The same has been observed for the effect of
Pao, on the relationship of ventilation to Paco,.37 In
both cases, this increased response to CO, suggests an
effect of Pao, on the basic mechanism that relates
Paco, to ventilation or to CBF, as opposed to a simple
additive effect.
Aside from its resemblance to the ventilatory
response, the enhancement of the cerebrovascular
effect of CO, induced by hypoxia is a phenomenon
that deserves further exploration. It would be interesting to assess the role of peripheral chemoreceptors and decerebration in this response, as well as
the effects of cholinergic blocking drugs, all of which
have been shown, as stated above, to affect CO, reactivity under normoxia. Understanding of the nature of
the interaction of these basic cerebrovascular reactions may ultimately help in the management of
clinical situations in which hypoxia may coexist with
either hyper- or hypocapnia.
Acknowledgment
We thank Kathryn High of the Department of Anesthesiology for
her generous assistance in performing blood gas analyses. This work
was supported by grants from NIH (HL 17903) and American
Heart Association — Greater Los Angeles Affiliate (437IG).
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S R Quint, O U Scremin, R R Sonnenschein and E H Rubinstein
Stroke. 1980;11:286-289
doi: 10.1161/01.STR.11.3.286
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