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Departments of 1 Neurosurgery and 2 Anesthesiology, Baylor College of Medicine, Houston, Texas 77030
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX
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Cerebrovascular reactivity to CO2 or hypotension was studied in vivo and in vitro [pressurized arteries (~82 µm) and arterioles (~30 µm)] at 1 h after mild controlled cortical impact (CCI) injury in rats. The cortical perfusion response [assessed using laser-Doppler flowmetry (LDF)] to altered CO2 was diminished (up to 81%) after mild CCI injury. The responses to CO2 alterations in arteries and arterioles isolated from the injured cortex were similar to responses in vessels isolated from sham-injured animals. After mild CCI injury, the autoregulatory response to hypotension (measured using LDF) was maintained or even enhanced, depending on the method used to measure the response. Vessels isolated from the injury site showed a response to changes in pressure similar to that in vessels isolated from sham-injured rats. We conclude that mild CCI injury produces complicated alterations in cerebrovascular control. Whereas the autoregulatory response to hypotension was maintained or even enhanced, the in vivo vascular response to CO2 was severely compromised. The altered response to CO2 was not caused by an intrinsic vascular perturbation but rather an altered milieu after mild CCI injury.
autoregulation; carbon dioxide reactivity; rat; secondary injury; traumatic brain injury
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX
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THE DEVELOPMENT AND EXTENT of secondary injury to the brain accounts for a major component of the morbidity and mortality of traumatic brain injury (TBI) (11). This secondary injury, which arises from subsequent complications such as hypotension or hypoxia after TBI, has been estimated to double the mortality rate in the clinical setting (10). Fortunately, the delayed nature of these events provides an opportunity for therapeutic intervention and the possibility of alleviating the ensuing functional deficits that arise after TBI.
In our laboratory we recently developed a rat model of secondary injury after TBI (9). Mild controlled cortical impact (CCI) injury, when followed by bilateral carotid occlusion (BCO), produced tissue damage as measured by contusion volume and neuronal loss. However, neither the mild CCI injury nor the BCO alone produced any overt neuronal loss or tissue injury. Importantly, the timing of the BCO was critical; the increased neuronal loss was maximal when the BCO occurred 1 h after injury, whereas at 24 h damage was minimal. Preliminary studies in our laboratory have demonstrated that cerebral blood flow (CBF) is reduced >50% after mild CCI injury (B. K. Giri, I. K. Krishnappa, R. M. Bryan, Jr., and C. S. Robertson, unpublished observations). This reduction in flow and possibly other alterations in vascular control likely increase the susceptibility of the brain to secondary injury.
The purpose of the present study was to determine whether mild CCI injury altered the lower limit of autoregulation or the cerebrovascular response to changes in CO2. Autoregulation and the CO2 response are important mechanisms for maintaining adequate blood flow to the brain (4). If either of these mechanisms were compromised after CCI injury, then the brain would be more susceptible to secondary injury. The following two hypotheses were tested: 1) mild CCI injury diminishes the capacity of the cerebrovascular system to maintain adequate blood flow when the perfusion pressure is decreased; and 2) mild CCI injury diminishes the cerebrovascular response to changes in CO2. The cerebrovascular response was studied in vivo using laser-Doppler flowmetry (LDF) and in vitro using isolated pressurized arteries and arterioles.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX
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General Surgical Protocol
The Animal Protocol Review Committee at Baylor College of Medicine approved the experimental protocol before experiments were initiated. Adult male Long-Evans rats (310-370 g; n = 117) were maintained on rodent chow and water before surgery. Anesthesia was induced with 5% isoflurane. Animals were tracheotomized (14-gauge catheter) and mechanically ventilated with 2-2.5% isoflurane in room air supplemented with 100% O2 (to maintain PO2 between 80 and 100 mmHg). Both femoral arteries were cannulated with polyethylene tubing (PE-50, Intramedic), one for monitoring mean arterial blood pressure (MABP) and the other for sampling blood to measure gases (using a Ciba Corning 280 Blood Gas System) and for inducing hemorrhagic hypotension. Rectal temperature was maintained at 37°C using a heating pad and a temperature controller (Digi-Sense; Cole-Palmer, Vernon Hills, IL). The head of each rat was secured in a stereotaxic frame. The skull was exposed by a midline incision, and the scalp (including the periosteum) and temporal muscle were reflected. A 10-mm-diameter craniotomy, centered over the right parietal cortex, was then performed using a dental drill. To prevent thermal injury to the cortex, a stream of cool air was directed at the drilling site. In some experiments (n = 10), a burr hole was made over the left frontal lobe for insertion of a 3-F microsensor transducer (Codman and Schertleff, Randolph, MA) to monitor intracranial pressure (ICP). Relative changes in local cortical perfusion were monitored using LDF (Perimed, Stockholm, Sweden). The laser-Doppler probe was positioned 0.5 mm above the dural surface in an area free of vessels. A drop of mineral oil was applied at the probe tip to provide optical coupling between the probe and the tissue.Traumatic Brain Injury
TBI was induced with the use of a CCI device as described in detail by Dixon et al. (13). Briefly, a cylinder with a 9.5-mm-diameter impactor tip was pneumatically driven into the brain (dura intact) at the site of the craniotomy. To induce a mild level of injury, the CCI device was adjusted such that the cylinder velocity was 3 m/s with a 50-ms duration and a 2.5-mm deformation (8). Isoflurane was maintained at 2% when injury was induced. Sham-injured rats underwent surgical manipulations identical to those of the CCI-injured rats with the exception of injury.Study 1: [14C]Iodoantipyrine Assessment of Regional CBF After Mild CCI Injury
Male Long-Evans rats (300-350 g; n = 17) were prepared as described in General Surgical Protocol with the following exceptions and/or additions. The left femoral artery and right femoral vein were catheterized (PE-50 tubing). Arterial blood was sampled from the femoral artery, and 4-iodo[N-methyl-14C]antipyrine ([14C]IAP; ARC, St. Louis, MO), the tracer used to quantitate CBF, was infused into the right femoral vein. The tail artery was cannulated (22-gauge catheter) for monitoring MABP. Animals were randomly assigned into one of two groups: 1) sham CCI injury (n = 10) or 2) mild CCI injury (n = 7). CBF was measured 1 h after either sham CCI injury or mild CCI injury was induced as described in Traumatic Brain Injury. Before CBF was measured, 200 U of heparin were administered via the femoral vein.Regional CBF (rCBF) was measured using [14C]IAP. [14C]IAP (30 µCi) was dissolved in 0.3 ml of physiological saline and infused over a 30-s time period. During this time, nine blood samples from the femoral artery were obtained at different time intervals. At the end of this time, rats were decapitated. The brain was immediately removed, and a tissue sample (~30 mg) of the gray matter was obtained from the injured or sham-injured cortex (ipsilateral). Brain and blood (20 µl) samples were weighed, solubilized, and counted in a liquid scintillation counter (Packard Instruments, Downers Grove, IL). rCBF was calculated according to the method of Sakurada and colleagues (5a, 31).
Study 2: Cerebrovascular Reactivity In Vivo
Assessment of CO2 reactivity in vivo. At 30 min after either sham CCI injury or mild CCI injury, CO2 reactivity was measured by LDF during either hypercapnia (by increasing the inspired CO2 to 10% for 5 min; n = 16) or hypocapnia (by increasing the ventilation rate for 5 min; n = 8). After CO2 inhalation was discontinued, 25 min were allowed for baseline conditions to be reestablished before the cerebrovascular response to hypotension was assessed.
Assessment of the hypotensive response in vivo. At 1 h after either sham CCI injury or mild CCI injury, the cerebrovascular response to hypotension was measured by LDF at the impact site in injured rats and in a corresponding location for sham-injured rats. During this time, the isoflurane concentration was maintained at 1.5% (1.0 minimum alveolar anesthetic concentration) because it has been shown that autoregulation is intact at this concentration (21). In addition, perfusion was only studied in the vicinity of the CCI injury, where disturbances of the cerebrovasculature, if present, would be expected to be maximal. In the present studies, the upper end of the autoregulatory response was not assessed. The cerebrovascular response to hypotension was measured by LDF during stepwise hypotension in 10-mmHg increments by two different means, either 1) controlled withdrawal of blood from the femoral artery (hemorrhagic hypotension) or 2) by applying lower body negative pressure, thereby causing venous pooling in the lower body portions (hypobaric hypotension) (12). For the hypobaric hypotension model, the lower body portion of the rat was placed in a custom-built polyvinyl chloride chamber (inner diameter 7.5 cm) that was connected to a household vacuum cleaner. The chamber was sealed around the animal with a latex glove and secured, with care taken not to impede breathing. Port holes allowed catheters to pass to the outside of the chamber. The desired level of hypotension was manually controlled with the use of a variable transformer connected to the vacuum cleaner. The decreased blood pressure was maintained for 2 min, and the LDF value was averaged during the last 30 s. Animals were randomly assigned into four experimental groups: 1) sham CCI injury, hypobaric hypotension (n = 6); 2) mild CCI injury, hypobaric hypotension (n = 7); 3) sham CCI injury, hemorrhagic hypotension (n = 8); and 4) mild CCI injury, hemorrhagic hypotension (n = 8). In those animals subjected to hypobaric hypotension, MABP was only reduced to 40 mmHg because pilot studies indicated that further reductions in MABP resulted in the death of some animals.
Study 3: Cerebrovascular Reactivity In Vitro
Harvesting and mounting cerebral vessels. One hour after sham CCI injury or mild CCI injury, rats were anesthetized with isoflurane and decapitated. The brain was immediately removed from the skull and placed in cold physiological saline solution (PSS). Penetrating arterioles or branches of the middle cerebral artery (MCA) were isolated from the ipsilateral side (within the impact site in CCI-injured animals), cannulated with micropipettes in an arteriograph (Living Systems, Burlington, VT), and pressurized (17, 18). Transmural pressure was adjusted to 60 mmHg and allowed to equilibrate for 1 h. During this time, the vessels developed spontaneous tone by constricting from their fully dilated diameters at initial pressurization.
Assessment of CO2 reactivity in vitro. Penetrating arterioles (n = 12) and MCA branches (n = 12) were exposed to hypocapnia for 20 min by gassing the PSS with 2.5% CO2. Vessel diameter was recorded 20 min after each CO2 change. Normocapnia was then restored by gassing the PSS with 5% CO2 for 20 min. Finally, hypercapnia was induced (20 min) by gassing the PSS with 10% CO2.
Assessment of the myogenic response in vitro. The myogenic response experiment was initiated by setting the intraluminal pressure to 20 mmHg and increasing the pressure to 80 mmHg in 20-mmHg steps. Vessel diameter was recorded 10 min after each pressure change. The luminal and abluminal baths were then replaced and washed with calcium-free PSS containing 1 mM EGTA. The relationship between vessel diameter and pressure was reassessed in the calcium-free PSS.
Reagents and Drugs
The PSS consisted of 119 mM NaCl, 24 mM NaHCO3, 4.7 mM KCl, 1.18 mM KH2PO4, 1.17 mM MgSO4,1.6 mM CaCl2, 5.5 mM glucose, and 0.026 mM EDTA (6). Calcium-free PSS was prepared identically except that CaCl2 was omitted and 1 mM EGTA, obtained from Sigma (St. Louis, MO), was added.Definitions and Data Analysis
All data are expressed as means ± SE. Comparisons of MABP after sham CCI injury and mild CCI injury were made using a t-test. Absolute rCBF comparisons were made using a t-test. Cortical perfusion, as measured by LDF, was expressed as a percentage of the resting flow before an experimental manipulation (change in arterial CO2 or blood pressure). Changes in LDF value that followed changes in arterial CO2 were compared using a one-way ANOVA followed by a Student-Newman-Keuls post hoc test. CO2 reactivity (% per mmHg) was calculated according to Eq. 1 (15) as
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(1) |
The lower limit of autoregulation was defined as the MABP at which a
subsequent 10-mmHg decrease in MABP resulted in at least a 10%
decrease in the LDF value. Comparisons of the lower limit between sham-
and CCI-injured groups were made using a
t-test. Comparisons of percent changes
in LDF value with hypotension in vivo were made using random effect
model analysis (SAS Proc Mixed) so that the overall relationship
between experimental type (sham or CCI injury) and MABP could be
adjusted for individual differences among the animals. The data were
fit to a third-degree polynomial according to the following general
equation: %
LDF = k1 + k2MABP + k3MABP2 + k4MABP3.
Such an approach adds sensitivity to the analysis whereby each component of the polynomial (linear, quadratic, cubic) can be tested
for significance (see APPENDIX for
further details).
Comparisons of diameter changes in vitro were made using a two-way repeated-measures ANOVA followed by a Student-Newman-Keuls post hoc test. For all statistical tests a value of P < 0.05 was accepted as statistically significant, except for an interaction test in which significance was set at P < 0.1.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX
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Effect of Mild CCI Injury on MABP and ICP
Immediately after mild CCI injury, MABP transiently decreased. However, 1 h after CCI injury there was no significant difference in MABP between CCI-injured (MABP = 89 ± 4 mmHg) and sham-injured animals (MABP = 95 ± 5 mmHg) (P = 0.33; t-test). Preinjury ICP was 9.3 ± 0.1 mmHg (n = 10). Mild CCI injury induced a transient increase in ICP of 61.8 ± 7.6 mmHg (n = 10) for the duration of the impact (50 ms) and immediately returned to preinjury levels for the remaining 1-h monitoring period.Study 1: [14C]IAP Assessment of rCBF After Mild CCI Injury
At 1 h after sham CCI injury, ipsilateral rCBF was 79 ± 11 ml · 100 g
1 · min1
(n = 10). At 1 h after mild CCI
injury, however, ipsilateral rCBF was significantly reduced to 39 ± 4 ml · 100 g1 · min1
(n = 7;
P < 0.05 compared with sham;
t-test).
Study 2: Cerebrovascular Reactivity In Vivo
Effect of mild CCI injury on
PCO2 reactivity in vivo.
Mean PCO2 in normocapnic animals was
40 ± 1 mmHg (n = 12). Mean
PCO2 in hypercapnic animals (5-min
ventilation with 10% CO2) was
66 ± 1 mmHg (n = 12). As shown in
Fig. 1, hypercapnia induced a 68 ± 7%
increase in LDF value in sham-injured animals (n = 8), whereas at 30 min after CCI
injury, the hypercapnia-induced increase in LDF value
(n = 8) was significantly suppressed
to 13 ± 3% (P < 0.0002; 1-way
ANOVA on ranks). CO2 reactivity
(see Eq. 1) in sham-injured animals
was 2.7% per mmHg. In contrast, CO2 reactivity after mild CCI
injury was only 0.5% per mmHg.
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Effect of mild CCI injury on the hypotensive response in vivo.
The percent changes in LDF value, plotted as a function of MABP during
hemorrhagic hypotension for individual sham- and CCI-injured animals,
are presented in Fig. 2,
A and
B, respectively. Statistical analysis
(see APPENDIX, Table 1) revealed a
significant pressure effect (P = 0.0016) and an overall treatment effect between groups (sham vs. CCI
injury, P = 0.0316). In several sham- and CCI-injured rats
the LDF value actually increased as MABP decreased (see Fig. 2,
A and
B). This enhanced or "super"
autoregulation has been previously reported (7, 19, 21a) and was
especially prominent in one of the CCI-injured rats in which the LDF
value increased 250% when MABP was decreased from 100 to 40 mmHg. The
data were reanalyzed after data were omitted for the one CCI-injured
rat with a prominent "super" autoregulation. After these data
were removed, there remained a significant difference between sham- and
CCI-injured animals. The lower limit of autoregulation (see Definitions and Data Analysis in
METHODS) was estimated to be 45 ± 4 and 33 ± 3 mmHg for sham- and CCI-injured animals,
respectively. Although there was a tendency for the lower limit of
autoregulation to be shifted to the left in CCI-injured animals, this
did not reach statistical significance
(P = 0.0650; Mann-Whitney rank sum
test).
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Study 3: Cerebrovascular Reactivity In Vitro
Effect of mild CCI injury on
PCO2 reactivity in vitro.
Under conditions of normocapnia, mean
PCO2 was 36.3 ± 0.4 mmHg (n = 10). After 20 min of
hypocapnia, PCO2 was 20.8 ± 0.3 mmHg (n = 10), and after hypercapnia,
PCO2 was 67.7 ± 0.6 mmHg
(n = 10). Figure
4A shows
the diameter changes of penetrating arterioles isolated from both sham-
and CCI-injured animals. Both vessels significantly constricted from
their maximal diameters during 1 h of equilibration
(P < 0.05; 2-way
repeated-measures ANOVA). After hypocapnia was induced, sham- and
CCI-injured vessels further constricted by ~15%
[P = not significant (NS)
between the 2 treatment groups; 2-way repeated-measures ANOVA].
After normocapnia was reestablished, hypercapnia was induced, dilating the sham- and CCI-injured vessels by ~16%
(P = NS between the 2 treatment
groups; 2-way repeated-measures ANOVA). Calcium-free PSS maximally
dilated the sham arterioles to 43 ± 3 µm and the CCI-injured
arterioles to 40 ± 2 µm (P = NS
between the 2 treatment groups; 2-way repeated-measures ANOVA). The
diameter responses of the MCA branches to changes in
CO2 tension were similar to those
of the arterioles, as shown in Fig.
4B. Again, there was no difference
between the responses of the MCA branches isolated from sham- and
CCI-injured animals (P = NS between
the 2 treatment groups; 2-way repeated-measures ANOVA).
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Effect of mild CCI injury on the myogenic response in vitro.
The effect of intraluminal pressure on the diameter of penetrating
arterioles isolated both from sham- and CCI-injured animals is shown in
Fig. 5A.
In the presence of calcium (1.6 mM), both sham- and CCI-injured
arterioles constricted with increasing intraluminal pressure, thereby
showing that the myogenic response was intact. Statistical analysis
revealed a significant pressure effect
(P < 0.0001) but no treatment effect
(P = NS; 2-way repeated-measures ANOVA). Both sham- and CCI-injured arterioles demonstrated a passive dilation in the absence of calcium. Again, there was a pressure effect
(P = 0.001) but no treatment effect
(P = NS; 2-way repeated-measures ANOVA). The removal of calcium provides the passive properties of the
vessel to changing pressure, whereby the diameter at any given pressure reflects the maximal possible dilation of the vessel. Figure 5B shows the effect of
increasing intraluminal pressure on the diameter of MCA branches
isolated from both sham- and CCI-injured animals. Similar to the
response of the arterioles, there was a significant pressure effect in
both the presence (P < 0.0001) and
absence (P < 0.0001) of calcium but
no significant group effect (P = NS;
2-way repeated-measures ANOVA).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX
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In the present study, we have demonstrated that at 1 h after mild CCI injury 1) CBF decreased by ~50% at the site of injury, 2) cerebrovascular reactivity to hypotension (i.e., lower limit of autoregulation) was maintained, and 3) cerebrovascular reactivity to CO2 was reduced in vivo, but the direct effects of CO2 on isolated resistance arteries and arterioles were not altered. On the basis of these observations, we have reached the following two conclusions. 1) After mild CCI injury, increased susceptibility of the brain to secondary injury was not caused by the loss of autoregulation or an upward shift of the lower limit of autoregulation. 2) The loss of CO2 reactivity after mild CCI injury was not caused by injury of the cerebral vessels directly but rather a change in the environmental conditions of the vessels in vivo. The diminished cerebrovascular reactivity to CO2 may be at least partially responsible for the increased susceptibility of the brain to secondary injury after mild CCI injury.
Conclusion 1
A diminished autoregulatory response to hypotension has been reported for different types of TBI and in different species, including humans (for review, see Ref. 17a). However, the presence or absence of an altered autoregulatory response after TBI is complex and can vary depending on a number of factors, including the type and severity of injury. To our knowledge, the present paper represents the first study investigating autoregulation after CCI injury, with the possible exception of a previous paper from our laboratory in which the myogenic response, a component of autoregulation, was studied after a severe level of CCI injury (16).In this study the autoregulatory response to hypotension was preserved 1 h after mild CCI injury. This statement is amplified by the fact that two different methods of decreasing MABP were used (hypobaric and hemorrhagic hypotension). With the use of hemorrhage (see Fig. 2, A and B), the autoregulatory response was not only preserved in CCI-injured rats but was actually enhanced. The random effects model analysis (see APPENDIX, Table 1) demonstrated a significant group effect for percent change in LDF value (sham vs. CCI injury, P = 0.0316). Although the estimated lower limit of autoregulation during hemorrhagic hypotension decreased from 45 mmHg in sham rats to 33 mmHg in CCI-injured rats (i.e., better autoregulation), it approached (P = 0.065) but did not reach the criteria for statistical significance (P < 0.05). Regardless of whether there was any enhanced autoregulation, the fact remains that autoregulation was not diminished, or the lower limit was not shifted upward, after mild CCI injury.
The autoregulatory responses to hypotension presented in Figs. 2 and 3
are expressed in perfusion rates relative to the resting perfusion
(%change in LDF value) of the cerebral cortex in sham- and CCI-injured
rats. It must be remembered that the absolute rate of flow in cortex
from CCI-injured rats was ~50% of that of sham-injured rats (from
[14C]IAP studies).
Therefore, the autoregulatory response, determined using LDF, would be
relative to 79 ml · 100 g1 · min1
for sham-injured and 39 ml · 100 g1 · min1
for CCI-injured rats. Figure 6 presents the
autoregulatory curves, determined using hemorrhagic hypotension and
LDF, superimposed on the absolute rates of flow quantitatively
determined using [14C]IAP. Note in Fig.
6 that at pressures 
40 mmHg the absolute rates of flow are very
similar for the two groups of rats. From the standpoint of flow alone,
mild CCI injury produced no distinct disadvantage at the lowest
pressures (10-40 mmHg).
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Consistent with the in vivo studies of cortical perfusion, branches of the MCA and penetrating arterioles isolated from the injured cortex had a myogenic response similar to that of vessels isolated from sham-injured rats (see Fig. 5, A and B). The myogenic response is defined as the contraction of a blood vessel with increasing intraluminal pressure (26). The branches of the MCA and penetrating arterioles used in our study represent all vessels supplying blood within the injured cortex (or corresponding cortex in sham-injured rats) that have smooth muscle. Thus all vessels important in the control of CBF in the cortex of interest had a prominent myogenic response. Although the myogenic response is only one component mediating the autoregulatory response (for review, see Ref. 30), these data do corroborate our in vivo findings.
Our laboratory has previously reported that mild CCI injury increases the vulnerability of the brain to cerebral hypotension (see introduction and Ref. 9). One possible explanation for the increased vulnerability is that the autoregulatory response to hypotension is attenuated or abolished, or the lower limit of autoregulation is shifted upward, as a result of mild CCI injury. However, from the in vivo and in vitro results of the present study, we conclude that an increased susceptibility of the brain to secondary injury after mild CCI injury is not caused by a diminished autoregulatory response when pressure is decreased.
Conclusion 2
The cerebrovascular response to changes in PCO2 (or pH) appears to be a fundamental physiological mechanism involved with maintaining adequate blood flow to the brain. At a local level, the mechanism is thought to be involved with the coupling of CBF to energy metabolism (22). Thus disruption of this coupling mechanism could lead to underperfused tissue and could ultimately result in tissue damage. The cerebrovascular response to CO2 has been observed using a variety of techniques in a number of species (1, 28, 34). Although the exact mechanism of action of CO2 has not yet been fully established, it is known that the effect of CO2 on cerebrovascular diameter is mediated through changes in extracellular pH (23, 33). Nitric oxide, prostanoids, cyclic nucleotides, potassium channels, and intracellular calcium have all been implicated in mediating the pH effect (see Ref. 3 for review).In the present study, sham-injured animals had a CO2 reactivity of ~3% per mmHg (2.7% per mmHg for hypercapnia and 3.1% per mmHg for hypocapnia). This value agrees with previous observations in humans (29) and rats (15, 32). CO2 reactivity in the injured cortex after mild CCI injury was significantly reduced to 0.5% per mmHg in response to hypercapnia and 0.2% per mmHg in response to hypocapnia. The latter results reflect hypocapnia alone, because our blood PO2 data indicate that hypoxia during the hyperventilation was not an issue. To the best of our knowledge, these observations are the first to be reported in the acute stages after mild CCI injury.
The LDF data presented in Fig. 1 are expressed in perfusion rates
relative to the resting perfusion in the cerebral cortex in sham- and
CCI-injured rats. Given that the absolute flow rates in cortex from
sham- and CCI-injured rats were 79 and 39 ml · 100 g1 · min1,
respectively (from
[14C]IAP studies), and
the relative CO2 reactivities were
2.7 and 0.5% per mmHg (from hypercapnic response), respectively, the
absolute CO2 reactivities were 2.1 and 0.2 ml · 100 g1 · min1 · mmHg1, respectively. In
relative terms the CO2 reactivity
was suppressed by 81% [(2.7 
0.5)/2.7], and in
absolute terms the CO2 reactivity was suppressed by 90% [(2.1 0.2)/2.1].
Our results agree with those of previous studies (24, 32, 35) demonstrating diminished CO2 reactivity after mild fluid-percussion injury. Whereas defective CO2 reactivity has been reported after acute severe head injury in humans (5, 27, 14), no studies to date have investigated this phenomenon after mild TBI in the clinical setting. It should be mentioned that there is some acceptance in the clinical literature that CO2 reactivity is more difficult to disturb than pressure autoregulation (2). However, there are two issues at odds when the clinical studies are compared with the present experimental studies. First, the timing of these clinical observations is at a later point (1-2 days rather than within the first hour after the injury). Bouma and Muizelaar (2) do concede that CO2 reactivity is depressed early after injury with restoration to normal only after 24 h. Second, and probably more importantly, the majority of clinical observations occur after a severe level of injury. Indeed, there is only one study that has investigated pressure autoregulation after mild TBI (22), and it was found that the majority of patients autoregulate (72%), although some do not (28%). Whereas the prevailing concept is that pressure autoregulation is absent or greatly impaired after TBI, the reality may tend toward a mixed population, particularly dependent on the injury severity.
In contrast to the reduced CO2 reactivity seen in vivo after CCI injury (see Fig. 1), our in vitro experiments demonstrated that there was no significant alteration in cerebrovascular reactivity to CO2 in small arteries and arterioles in the damaged cortex 1 h after mild CCI injury (see Fig. 4, A and B). The combination of the in vivo studies of cortical perfusion and the in vitro studies of cerebral vessels has led us to an important conclusion involving the fundamental nature of cerebrovascular dysfunction to changes in CO2 after mild CCI injury; that is, the loss of CO2 reactivity after mild CCI injury was not caused by injury of the cerebral vessels directly but rather a change in the environment of the vessels. Our reasoning for this conclusion is based on the following. First, a change in the contractile state of arteries and arterioles alters cortical perfusion (CBF) in response to changes in CO2. Therefore, the diminished response of the LDF value in injured cortex during hypercapnia, for example, had to be caused by proportionally smaller dilations of the vessels supplying the injured cortex than those supplying the uninjured cortex. Within the cortex of interest, the resistance vessels supplying blood consist of only the third- and fourth-order branches of the MCA and penetrating arterioles (Ref. 20; unpublished observations). However, when these very vessels were isolated, and thus removed from the influence of their traumatized environment, branches of the MCA and penetrating arterioles from injured cortex were indistinguishable from those isolated from the uninjured cortex. Thus the inability of the vessels to dilate appropriately to changes in CO2 in vivo could not have been caused by the direct effects on the vessels within the injured cortex. Therefore, the branches and penetrating arterioles must be receiving inappropriate signals from the environment in vivo. These inappropriate signals would likely result from changes in ions, neurotransmitters, hormones, and/or other vasoactive compounds of the traumatized milieu (25).
In conclusion, the present study has demonstrated for the first time that in the acute stages that follow mild CCI injury in the rat, the cerebrovasculature has a reduced CO2 reactivity, whereas the response to hypotension is maintained. Moreover, we have shown that loss of CO2 reactivity after mild CCI injury is not caused by an intrinsic vascular perturbation but rather the traumatized milieu. Our results suggest that vasoparalysis is not apparent directly within the impact site, an area of the brain that has been shown to be vulnerable to secondary insults (9). Indeed, altered vascular reactivity after mild CCI injury may be specific to CO2. Finally, our findings suggest that mild CCI injury alone does not compromise the cerebrovascular response to hypotension and, therefore, to hypotension and, therefore, does not account for the increased susceptibility of the brain to secondary insults.
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APPENDIX |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX
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Comparisons of percent changes in LDF value with hypotension in vivo
were made using random effect model analysis (SAS Proc Mixed). The
random effects model was fit separately for the two experimental
methods (hemorrhagic and hypobaric hypotension). For each method, two
equations were derived, one for the sham-injured animals and one for
the CCI-injured animals. The MABP was transformed as
m = (MABP 56.5)/100 to allow
for more stable numeric solutions. The equations derived from the
hemorrhagic hypotension experiment were
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(A1) |
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(A2) |
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ACKNOWLEDGEMENTS |
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We thank Dr. Ulrich Dirnagl for advice in constructing the hypobaric hypotension chamber.
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FOOTNOTES |
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This work was supported by National Institute of Neurological Disorders and Stroke Grant P01-NS-27616.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: E. M. Golding, Department of Neurosurgery, Baylor College of Medicine, 6560 Fannin St., Suite 944, Houston, TX 77030 (E-mail: egolding{at}bcm.tmc.edu).
Received 16 February 1999; accepted in final form 20 April 1999.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
APPENDIX
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