=Acta Nurochrurgica
Acta Neurochir (Wien)(1992) 119:128-133
9 Springer-Verlag 1992 Printed in Austria
Cerebral Blood Flow Autoregulation in Experimental Subarachnoid Haemorrhage in Rat G. Rasmussen, J. Hauerberg, G. Waldemar 1, F. Gjerris, and M. Juhler University Clinics of Neurosurgery and Neurology 1, Rigshospitalet, Copenhagen, Denmark
Summary Haemodynamic instability is of great importance in clinical management of patients with subarachnoid haemorrhage (SAH). The significance of angiographicaUy demonstrable vasospasm for disturbances of cerebral blood flow (CBF) and cerebral autoregulation has not yet been clarified. The present study was designed to describe disturbances of cerebral autoregulation during the timecourse of experimental SAH (eSAH) in rats. A second aim of the study was to relate the results to a reported timecourse of angiographic vasospasm in the same animal model. Previous studies have shown that the timecourse of angiographically visible vasospasm in eSAH is biphasic with maximal spasm at 10 min and 2 days after induction of eSAH. At 5 days, the vasospasms have resolved. CBF was measured using a 133-Xenon intracarotid injection method which allowed serial measurements of mean hemispheric CBF during controlled manipulations of arterial blood pressure. In this way, an autoregulation curve could be constructed. The present study shows that autoregulation is severely disturbed or even totally absent at 2 and 5 days after eSAH. Thus there seems to be no direct correlation between presence of angiographic vasospasm and impairment of autoregulation, or that the impairment of autoregulation is more protracted than the presence of cerebral vasospasm, presuming a correlation exist. Keywords: Experimental subarachnoid haemorrhage; cerebral autoregulation; cerebral blood flow; cerebral vasospasm.
Introduction Cerebral vasospasm is frequently seen following subarachnoid haemorrhage (SAH) from rupture of an intracranial saccular aneurysm 2' 20, 25. Several reports indicate that cerebral autoregulation may be disturbed after SAH in both man and animals is' 19, 21, 25 In man, CBF is kept constant, autoregulated, within wide variations of mean arterial blood pressure (MABP) 23. An ideal autoregulation curve is shown in Fig. 1. This CBF autoregulation is mediated by caliber changes in the small arteries and arterioles. When
CBF
f
f LL
MABP
UL
Fig. 1. An ideal autoregulation curve. L L = lower limit; UL = upper limit
MABP falls, the arterioles dilate to decrease vascular resistance. Below the lower limit of autoregulation CBF decreases because vascular dilatation is almost maximal already, and further dilatation is insufficient to maintain normal CBF 3, 22, 23. Similarly, the increase of vascular resistance by vascular constriction above the upper limit of autoregulation is insufficient to maintain CBF at a normal level2z. The same mechanism is found in the rat. Normal rats have an autoregulation with an upper limit of MABP at 160mmHg and lower limit at 80 m m H g 8. Vasospasm after SAH has its onset in man on day 3 or 4 and reaches a maximum at about seven days after the SAH 27. An angiographic biphasic time course of the spasms is described in animals with experimental SAH (eSAH) 4' ~6. Delgado et al. have shown in the rat that a maximal acute spasm occurs at ten minutes and a maximal late spasm at two days after eSAH 4. After
G. Rasmussen et aI.: Cerebral Blood Flow Autoregulation in Experimental Subarachnoid Haemorrhage in Rat five d a y s a n g i o g r a p h i c e v i d e n c e o f v a s o s p a s m s has disappeared. T h e a p p e a r a n c e o f v a s o s p a s m in p a t i e n t s w i t h S A H o f t e n p r e d i c t s a p o o r o u t c o m e 25. A r e l a t i o n o f a n g i o g r a p h i c a l l y visible v a s o s p a s m to a u t o r e g u l a t i o n h a s n o t yet b e e n e s t a b l i s h e d . T h e a i m o f this s t u d y is to e x a m i n e this r e l a t i o n b y i n v e s t i g a t i n g C B F a u t o r e g u l a t i o n t w o a n d five d a y s a f t e r e S A H in the rat; a n d r e l a t e the f i n d i n g s to t h e r e p o r t e d t i m e c o u r s e o f c e r e b r a l v a s o spasms.
Materials and Methods The experiments were carried out in 28 Sprague-Dawley male rats weighing between 210 and 320 g. Induction of eSAH was carried out by the method described by Delgado et al. 4. An indwelling catheter was placed in the cisterna magna and two days later, 0.07 ml homologous blood was injected through the catheter. The following groups were investigated: Group A (control group, studied 2 days after implantation of the catheter, n = 10), group B (studied 2 days after eSAH, n = 10), and group C (studied five days after eSAH, n = 8). On the day of the autoregulation experiment, the animals were anaesthetized with 70% N20, 30% 02, and 0.5% halothane and connected to a ventilator for controlled ventilation. CBF was recorded using the method described by Hertz et al. 9. Twenty to thirty gl 133-Xenon (370MBq/ml) in isotonic saline was injected into the internal carotid artery and the radioactivity reaching the brain was measured by a single detector placed over the ipsilateral hemisphere. Hemispheric CBF was determined from the clearance curve by analyzing the initial slope of the curve 14. The autoregulation study was performed as described by Waldemar et al. a6. Only the lower part of the autoregnlation curve was investigated, i.e. no attempt was made to reach the upper limit expected at MABP levels above I60mmHg. In the eSAH-groups MABP was elevated to a higher level compared to the control group as baseline MABP was increased in those groups. Two to five resting CBF measurements were performed in each rat. The values were averaged for the determination of baseline CBF level. First MABP was elevated with intravenous norepinephrine and then decreased by controlled bleeding. CBF was measured for every 10 mmHg change in MABP. Following each CBF measurement, PaCO2, PaO2, and pH were analysed in an arterial
blood sample. The blood withdrawn for these measurements was replaced by blood from donor rats of the same strain. The rats were maintained at normocapnia (PaCO> 38-42 mmHg), except at very low values of MABP, where it was impossible to avoid slight hypocapnia in some animals (PaCO2 32-42 mmHg). Body temperature was kept constant about 37.5 ~ The physiological parameters of the animals are shown in Table 1. Statistics
The Student two-sample t-Test for unpaired data was used to compare baseline values in group A vs. B and C. Differences were accepted as significant at p < 0.05. The one-way analyses of variance followed by the Dunnett multiple comparison test were used for statistical comparisons within each group of rats 28. All parameters were compared with the value in the blood pressure range of the baseline MABP, i.e., 70-89 mmHg for the control group and 90-110 mmHg for the eSAH groups. Differences were accepted as significant at p < 0.05. The autoregulation curves are constructed as a function of MABP in intervals of 20 mmHg (Fig. 2). The CBF values are given as % of the baseline CBF. Lower limit of the autoregulation was defined as the lowest MABP range before CBF (percent of baseline) fell significantly below baseline CBF.
Results A f t e r i n d u c t i o n o f e S A H the a n i m a l s s u f f e r e d a 5 1 0 % w e i g h t loss. T h e y w e r e l e t h a r g i c b u t w i t h o u t f o c a l n e u r o l o g i c a l deficits. B e f o r e m a n i p u l a t i o n w i t h t h e b l o o d p r e s s u r e , baseline
values
of
CBF
and
MABP
were
measured
( T a b l e 2). T h e r e w e r e n o s i g n i f i c a n t d i f f e r e n c e s in b a s e line C B F b e t w e e n c o n t r o l s a n d t h e e S A H g r o u p s . O n the other hand MABP
w a s s i g n i f i c a n t l y e l e v a t e d in
b o t h e S A H g r o u p s c o m p a r e d to t h e c o n t r o l s . A l t h o u g h t h e r e w a s a slight d i f f e r e n c e in the b a s e l i n e a r t e r i a l PaO2, P a C O 2 , a n d p H ( T a b l e 1) t h e s e p a r a m e t e r s d i d not change
significantly during
the autoregulation
s t u d y , e x c e p t at e x t r e m e h y p o t e n s i o n t o w a r d t h e e n d o f t h e s t u d y ( T a b l e 3). T h e s e a l t e r a t i o n s w e r e to s m a l l
Table 1. Baseline Values
Group A (control) Group B (eSAH-2) Group C (eSAH-5)
129
No.
PaCO2 mmHg
pH
PaO2 mmHg
Bodytemp. ~
10
39.8 • 0.2
7.40 :t= 0.01
165 • 3
37.3 • 0.1
I0
39.9 • 0.2
7.44 • 0.01#
151 • 2#
37.1 • 0.1
8
40.0 • 0.3
7.38 • 0.01
144 • 3#
37.5 • 0.1
Physiological conditions in control rats (group A); 2 and 5 days after experimental subarachnoid haemorrhage (group B and C). All values are presented as mean :t: SEM. Group A vs. B and vs. C by Student two-sample T-test for unpaired data. # p < 0.05.
130
G. R a s m u s s e n et al.: Cerebral Blood Flow Autoregulation in Experimental Subaraehnoid Haemorrhage in Rat
to explain any differences in CBF autoregulation in the groups studied. T h e autoregulation curves for all groups are seen in Fig. 2. CBF, PaCO2, PaO2, and p H values for intervals of MABP are shown in Table 3. In the control animals, values of CBF around 100% were maintained down to the MABP range of 5069 mmHg, defined as the lower limit as described above. When MABP fell to a range of 30-50 mmHg, a significant decrease in CBF was found. Two days after eSAH, a lower limit was found in
the MABP interval from 70-89 mmHg, indicating a disturbed autoregulation with a shift to the right of the autoregulation curve. With further increase of MABP, there was a moderate, but not significant increase of CBF; indicating that the plateau of the autoregulations curve was reached. Five days after eSAH, a significant fall in CBF was found when M A B P was lowered to a range of 7089 mmHg, with the lower limit elevated to a range of 90-109mmHg. CBF was increased to a significant level, when MABP was increased to a range from 130-
CBF % of baseline
Table 2, Baseline Values o f CBF and M A B P
Group A (control) Group B (eSAH-2) Group C (eSAH-5)
No.
CBF ml/100 g/min
MABP mmHg
10
93 4- 4
10
94 4- 4
794- 1# 94 • 4
8
89 4- 4
93 4- 4
lo
100
50
0
p
0
Baseline values of cerebral blood flow (CBF) and mean arterial blood pressure (MABP) in control rats (group A) and 2 and 5 days after experimental subarachnoid haemorrhage (group B and C). All values are presented as m e a n 4- SEM. G r o u p A vs. B and vs. C by Student two-sample T-test for unpaired data. # p < 0.05.
50
p
i
100
150
MABP m m H g CONTROL
"-'+- eSAH-2
~
eSAH-5
Fig. 2. Autoregulation curves in control rats and eSAH rats 2 and 5 days after subarachnoid haemorrhage. C B F is expressed as a percentage of the baseline cerebral values. C B F is given as m e a n 4- SEM
Table 3. Data from an Autoregulation Study for All Variables Observed in Each 20 mmHg Range of M A B P
Group A
(control) Group B
(eSAH-2)
Group C
(eSAH-5)
No.
MABP (mmHg)
CBF (% baseline)
PaCOz (mmHg)
pHa
PaO2 (mmHg)
TP (~
6 9 20 14 10
30-49 50-69 70-89 90-109 110-129
63 80 88 101 104
+ 9* 4- 6 + 4 4- 4 4- 6
37.0 38.7 38.7 39.2 39.5
4- 0.1 4- 0.3 4- 0.5 4- 0.4 4- 0.5
7.34 7.35 7.35 7.40 7.41
4- 0.03 4- 0.02 4- 0.02 4- 0.01 4- 0.01
193 166 172 166 168
4- 5* 4- 8 4- 4 4- 5 4- 4
37.5 37.3 37.3 37.1 37,1
4- 0.2 4- 0.2 4- 0.1 4- 0.1 4- 0,1
4 14 20 21 16 9
30--49 50-69 70-89 90-109 110-129 130-149
52 77 93 114 133 138
4- 9* 4- 5* 4- 3 + 5 4- 8 4- 14
34.2 38.9 39.5 39.5 39.9 39.4
4- 0.6* 4- 0.3 4- 0.2 4- 0.3 4- 0.3 4- 0.3
7.40 7.36 7.41 7.42 7.42 7,44
4- 0.02 4- 0.02* 4- 0,01 4- 0.01 4- 0.01 4- 0.01
191 165 150 148 146 143
4- 9* 4- 4* 4- 1 4- 3 4- 4 4- 3
37.3 37.1 37.2 37.1 37.3 37.0
4- 0,1 4- 0.2 4- 0.1 4- 0.1 4- 0.1 4- 0.1
3 9 12 7 11 8
30-49 50-69 70-89 90-109 110-129 130-149
26 67 84 115 128 147
4- 8* 4- 4* • 6* 4- 9 4- 6 4- 10"
35.4 38.1 39.6 40.0 39.5 39.8
4- 1.4" 4- 0.4 4- 0.4 4- 0.3 4- 0.4 4- 0.5
7.33 7.38 7.38 7.37 7.39 7.36
4- 0.05 4- 0.01 4- 0.02 4- 0.03 4- 0.02 4- 0.01
164 143 137 147 142 139
4- 14" 4- 3 4- 3 4- 4 4- 2 4- 2
37.2 37.5 37.6 37.3 37,4 37.5
4- 0.2 4- 0.1 4- 0.1 4- 0.1 4- 0.1 4- 0,1
The analyses o f variance and the D u n n e t t multiple comparison test were used to compare each range vs. baseline M A B P range. All values are presented as m e a n • SEM. * p < 0.05.
G. Rasmussenet al.: Cerebral BloodFlow Autoregulationin ExperimentalSubarachnoidHaemorrhagein Rat
131
149, which may indicate that the upper limit was reached. However, a further elevation of MABP was not done, because of the risk of breakdown of the blood-brain barrier. Thus it seems that autoregulation is severely disturbed 5 days after eSAH with a narrowing of the plateau to a MABP range from 90129 mmHg where there is no significant change in CBF, compared to the baseline value. The results indicate a severely disturbed autoregulation 2 days after eSAH with no sign of recovery 5 days after eSAH.
(CPP). This compensatory blood pressure increase seems unaffected while the CBF autoregulation is disturbed. Two days after eSAH, CBF autoregulation is severely disturbed. The same observation is made 5 days after eSAH. Thus small arteries and arterioles seem unable to actively regulate and compensate for changes of MABP. This paralysis of the vessels causes CBF to depend mainly on the pressure gradient between the cerebral arteries and veins. This corresponds to the CBF/MABP relationship beneath the lower limit of the autoregulation in normal animals.
Discussion
Autoregulation and Vasospasm
Method
Voldby etal. found that patients in poor clinical condition have the severest disturbances of intracranial vasomotor function. Furthermore, a close correlation was found between the degree of vasospasms and the degree of autoregulatory impairment25. Heilbrun et al. observed no relationship between vasospasm and impaired autoregulation, but reported a correlation between defective autoregulation and hydrocephalus - possibly because of altered perfusion pressure due to increased ICP 7. However, both clinical investigations were performed with only a few MABP/CBF measurement points, which makes firm conclusions on cerebral autoregulation difficult. A recent study in monkeys showed vasospasm and impairment of CBF autoregulation 7 days after eSAH, but the investigation lacks a description of the time course of the changes24. Unfortunately, our study does not include angiographic evidence of the vasospasm, but the manipulations with the rats inside the experimental setup have to be as few as possible. Our conclusion is based on the reported timecourse of vasospasm. Delgado et al. 4 found angiographic vasospasm with a maximum two days after eSAH in exactly the same eSAH model as used in the present study. On day 5, the vasospasms had resolved. However, autoregulation is severely disturbed or even absent at both timepoints indicating that there is no direct correlation between angiographically visible vasospasms and disturbance of autoregulation. The explanation may be, that vasospasm visualized on angiograms is localized to the larger intracranial arteries, whereas autoregulation is mediated by small arteries and arterioles which are not seen on angiograms. Another explanation of our study is, that the presence of cerebral vasospasm affect and impair autoregulation and that the impairment is more protracted than the vasospasm.
Most studies on CBF autoregulation after eSAH in animals and after SAH in man have been performed by measuring CBF at a few different values of MABP. The method in the present study uses several MABP values for measurement and therefore gives a more complete illustration of CBF autoregulation curve. The intracarotid 133-Xenon method for measurement of CBF is rapid, reproducible and allows repetitive measurement of CBF in individual rats 9. Unfortunately this method lacks regional and bilateral information which would be desirable when comparing focal versus global CBF disturbances following SAH. Norepinephrine used to induce hypertension causes a momentarily reversible acute hypertension and does not per se affect CBF 13. The concentration of 0.5% halothane used in both control and experimental groups has been shown to decrease CBF by about 20% in normal monkeys. However CBF autoregulation was not influenced by this concentration11. There is no evidence to suggest that CBF autoregulation should be affected differently by halothane in eSAH animals. Therefore the method is particularly well suited for repeated CBF measurements for construction of CBF autoregulation curves. Results
There was no significant change in baseline CBF after eSAH. However there was a significant increase in baseline MABP on both experimental days after eSAH. Delgado etal. found decrease in global CBF after eSAH with an unchanged MABP s. Thus a higher MABP seems necessary to maintain CBF unchanged. This may be explained by an increase in ICP after eSAH resulting in a decrease of cerebral perfusion pressure
132
G. Rasmussen etal.: Cerebral Blood Flow Autoregulation in Experimental Subarachnoid Haemorrhage in Rat
M e c h a n i s m s o f Autoregulation
The exact mechanism of autoregulation is unknown. Metabolic, myogenic and neurogenic factors have been proposed 3, 22. A study of autoregulation dynamics in normal humans showed that autoregulatory adjustments of the cerebral vascular resistance take place much quicker at hypocapnic CO2 levels than at normoor hypercapnic levels 1. Furthermore it has been shown that hyperventilation can restore impaired autoregulation in patients with tumours and cerebral ischaemia 15. These findings indicate, that autoregulation may partly depend on arterial PCO2, or perhaps on p H in the arteriolar wall or in brain extracellular space. Patients with SAH often hyperventilate spontaneously< 25. In the present study, the rats were kept normoventilated during the experiment. This may have caused a slight perivascular acidosis, and it is possible that the autoregulation would have been less impaired, if the animals had been hyperventilated.
Clinical Relevance
Patients with SAH from rupture of an intracranial aneurysm often have several different kinds of intracranial vasomotor dysfunction. Fifty percent of the patients develop angiographic vasospasm 17' 20. Disturbed autoregulation or impaired CO2-response are also frequently seen 1~ 19, 25 The present study underscores the importance of a sufficient observation of MABP, and the danger of blood pressure manipulations in patients with SAH. Because of a less effective or maybe even absent cerebral autoregulation, blood pressure changes may directly provoke drastic changes of CBF. There is a risk of inducing hyperperfusion by hypertension resulting in cerebral oedema or haemorrhage. Hypotension is likely to induce hypoperfusion and ischaemia. Studies on the additional influence of ICP and arterial PaCO2 on CBF and autoregulation are needed to further illuminate the complex interactions of these variables. These factors also need to be taken into account when evaluating the benefit and risks of pharmacologic intervention in SAH. Additionally, absence of angiographic vasospasm should not automatically lead to conclusions of haemodynamic stability in patients with SAH. As our study indicates, autoregulation may be disturbed, and thus the patient could be haemodynamically vulnerable even after disappearance of visible vasospasm. The duration of autoregulatory instability and its rate of re-
turn is u n k n o w n - b o t h in experimental animals and in man.
References 1. Aaslid R, Lindegaard KF, Sorteberg W, etal (1989) Cerebral autoregulation dynamics in humans. Stroke 20:45-52 2. Allan GS, Ahn HS, Preziosi TJ etal (1983) Cerebral arterial spasm-a controlled trial ot" nimodipine in patients with subarachnoid haemorrhage. N Engl J Med 308:619-624 3. Busija DW, Heistadt DD (1984) Factor involving in the physiological regulation of the cerebral circulation. Rev Physiol Biochem Pharmacol 101:162-211 4. DelgadoTJ, Arbab MR, Diemar NH, etal (1985) Subarachnoid haemorrhage in the rat: angiography and fluorescence microscopy of the major cerebral arteries. Stroke 16:595-602 5. Delgado T, Arbab MR, Diemer NH, etal (1986) Subarachnoid haemorrhage in the rats: cerebral blood flow and glucose metabolism during the late phase of cerebral vasospasm. J Cereb Blood Flow Metabol 6:590-599 6. Derubach PD, Little JR, Jones SC, etal(1988) Altered cerebral circulation and CO2 reactivity after aneurysmal subarachnoid haemorrhage. Neurosurgery 22:822-826 7. Heilbrun MP, Olesen J, Lassen NA (1972) Regional cerebral blood flow studies in subarachnoid haemorrhage. J Neurosurg 37:36-44 8. Hernandez MJ, Brennan RW, Bowman BS (1987) Cerebral blood flow autoregulation in rat. Stroke 9:150o155 9. Hertz M, Hemmingsen R, BolwigTG (1977) Rapid and repetitive measurements of blood flow and oxygen consumption in the rat brain using intraarterial xenon injection, Acta Physiol Seand 101:501-503 10. Messeter K, Brandt L, LjunggrenB, etal(1987) Prediction and prevention of delayed ischemic dysfunction after aneurysmal subarachnoid haemorrhage and early operation. Neurosurgery 20:548-552 ll. Morita H, Nemoto EM, Bleyaert AL, etal (1977) Brain blood flow autoregulation and metabolism during halothane anesthesia in monkeys. Am J Physiol 233:H6700H676 12. Nagai H, Noda S, Mabe H (1975) Experimental cerebral vasospasm. Part 2: effects of vasoactive drugs and sympathectomy on early and late spasm. J Neurosurg 42:420--428 13. Olesen J (1972) The effect of intracarotid epinephrin-norepinephrine and angiotensin on the regional cerebral blood flow in man. Neurology 22:978-987 14. OlesenJ, Paulsen OB, LassenNA (1971)Regionalcerebral blood flow in man determinated by the initial slope of the clearance of intraarterially injected 133-xenon.Stroke 2:519-540 15. Paulson OB, Olesen J, Christensen MS (1972) Restoration of autoregulation of cerebral blood flow by hypocapnia. Urology 22:286-293 16. Peerlees SJ, Fox AJ, Komatsu K, et al (1982) Angiograplfic study of vasospasmfollowingsubarachnoidhaemorrhagein monkeys. Stroke 13:473-479 17. Petruk KC, West M, Mohr G, et al (1988)Nimodipinetreatment in poor-grade aneurysm patients. J Neurosurg 68:505-517 18. Pickard JD, Boisvert DPJ, Graham DI, etal (1979) Late effects of subarachnoid haemorrhage on the response of the primate cerebral circulation to drug-induced changes in arterial blood pressure. J Neurol Neurosurg Psychiatry42:899-903
G. Rasmussen et al.: Cerebral Blood Flow Autoregulation in Experimental Subarachnoid Haemorrhage in Rat 19. Pickard JD, Matheson M, Patterson J, etal (1980) Prediction of late ischemic complications after cerebral aneurysm surgery by the intraoperative measurement of cerebral blood flow. J Neurosurg 53:305-308 20. Rosenorn J, Eskesen V, Schmidt K, etal(1987) Clinical features and outcome in 1076 patients with ruptured intracranial saccular aneurysms: a prospective consecutive study. Br J Neurosurg 1: 33-46 21. Sahlin C, Brismar J, Delgado T, et al (1987) Cerbrovascular and metabolic changes during the delayed vasospasms following experimental subarachnoid haemorrhage in baboons, and treatment with a calcium antagonist. Brain Res 405:313-332 22. Strandgaard S (1978) Autoregulation of cerebral circulation in hypertension. Aeta Neurol Scand [Suppl 66] 57:1-82 23. Strandgaard S, Paulson OB (I984) Cerebral autoregulation. Stroke 15:413-416 24. Takeuchi H, Handa Y, Kobayashi H, etal (1991) Impairment of cerebral autoregtdation during the development of chronic
25.
26.
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cerebral vasospasm after subarachnoid haemorrhage in primates. Neurosurgery 28:41-48 Voldby B, Enevoldsen E, Jensen FT (1985) Cerebrovascular reactivity in patients with ruptured intracranial aneurysms. J Neurosurg 62:59-67 Waldemar G (1990) Acute sympathetic denervation does not eliminate the effect of angiotensin converting enzyme inhibition on CBF autoregulation in spontaneously hypertensive rats. J Cereb Blood Flow Metabol 10:43-47 Weir B, Grace M, Hansen J, eta1 (1978) Time course of vasospasms in man. J Neurosurg 48:173-178 Zar JH (1984) Biostatistical analyses, 2nd Ed. Prentice Hall, Englewood Cliffs, CA, pp 194-195
Correspondence and Reprints: Gitte Rasmussen, M.D., University Clinic of Neurosurgery, Aarhus Kommunehospital, 8000 Aarhus C, Denmark.
Acta Neurochir (Wien)(1992) 119:128-133
9 Springer-Verlag 1992 Printed in Austria
Cerebral Blood Flow Autoregulation in Experimental Subarachnoid Haemorrhage in Rat G. Rasmussen, J. Hauerberg, G. Waldemar 1, F. Gjerris, and M. Juhler University Clinics of Neurosurgery and Neurology 1, Rigshospitalet, Copenhagen, Denmark
Summary Haemodynamic instability is of great importance in clinical management of patients with subarachnoid haemorrhage (SAH). The significance of angiographicaUy demonstrable vasospasm for disturbances of cerebral blood flow (CBF) and cerebral autoregulation has not yet been clarified. The present study was designed to describe disturbances of cerebral autoregulation during the timecourse of experimental SAH (eSAH) in rats. A second aim of the study was to relate the results to a reported timecourse of angiographic vasospasm in the same animal model. Previous studies have shown that the timecourse of angiographically visible vasospasm in eSAH is biphasic with maximal spasm at 10 min and 2 days after induction of eSAH. At 5 days, the vasospasms have resolved. CBF was measured using a 133-Xenon intracarotid injection method which allowed serial measurements of mean hemispheric CBF during controlled manipulations of arterial blood pressure. In this way, an autoregulation curve could be constructed. The present study shows that autoregulation is severely disturbed or even totally absent at 2 and 5 days after eSAH. Thus there seems to be no direct correlation between presence of angiographic vasospasm and impairment of autoregulation, or that the impairment of autoregulation is more protracted than the presence of cerebral vasospasm, presuming a correlation exist. Keywords: Experimental subarachnoid haemorrhage; cerebral autoregulation; cerebral blood flow; cerebral vasospasm.
Introduction Cerebral vasospasm is frequently seen following subarachnoid haemorrhage (SAH) from rupture of an intracranial saccular aneurysm 2' 20, 25. Several reports indicate that cerebral autoregulation may be disturbed after SAH in both man and animals is' 19, 21, 25 In man, CBF is kept constant, autoregulated, within wide variations of mean arterial blood pressure (MABP) 23. An ideal autoregulation curve is shown in Fig. 1. This CBF autoregulation is mediated by caliber changes in the small arteries and arterioles. When
CBF
f
f LL
MABP
UL
Fig. 1. An ideal autoregulation curve. L L = lower limit; UL = upper limit
MABP falls, the arterioles dilate to decrease vascular resistance. Below the lower limit of autoregulation CBF decreases because vascular dilatation is almost maximal already, and further dilatation is insufficient to maintain normal CBF 3, 22, 23. Similarly, the increase of vascular resistance by vascular constriction above the upper limit of autoregulation is insufficient to maintain CBF at a normal level2z. The same mechanism is found in the rat. Normal rats have an autoregulation with an upper limit of MABP at 160mmHg and lower limit at 80 m m H g 8. Vasospasm after SAH has its onset in man on day 3 or 4 and reaches a maximum at about seven days after the SAH 27. An angiographic biphasic time course of the spasms is described in animals with experimental SAH (eSAH) 4' ~6. Delgado et al. have shown in the rat that a maximal acute spasm occurs at ten minutes and a maximal late spasm at two days after eSAH 4. After
G. Rasmussen et aI.: Cerebral Blood Flow Autoregulation in Experimental Subarachnoid Haemorrhage in Rat five d a y s a n g i o g r a p h i c e v i d e n c e o f v a s o s p a s m s has disappeared. T h e a p p e a r a n c e o f v a s o s p a s m in p a t i e n t s w i t h S A H o f t e n p r e d i c t s a p o o r o u t c o m e 25. A r e l a t i o n o f a n g i o g r a p h i c a l l y visible v a s o s p a s m to a u t o r e g u l a t i o n h a s n o t yet b e e n e s t a b l i s h e d . T h e a i m o f this s t u d y is to e x a m i n e this r e l a t i o n b y i n v e s t i g a t i n g C B F a u t o r e g u l a t i o n t w o a n d five d a y s a f t e r e S A H in the rat; a n d r e l a t e the f i n d i n g s to t h e r e p o r t e d t i m e c o u r s e o f c e r e b r a l v a s o spasms.
Materials and Methods The experiments were carried out in 28 Sprague-Dawley male rats weighing between 210 and 320 g. Induction of eSAH was carried out by the method described by Delgado et al. 4. An indwelling catheter was placed in the cisterna magna and two days later, 0.07 ml homologous blood was injected through the catheter. The following groups were investigated: Group A (control group, studied 2 days after implantation of the catheter, n = 10), group B (studied 2 days after eSAH, n = 10), and group C (studied five days after eSAH, n = 8). On the day of the autoregulation experiment, the animals were anaesthetized with 70% N20, 30% 02, and 0.5% halothane and connected to a ventilator for controlled ventilation. CBF was recorded using the method described by Hertz et al. 9. Twenty to thirty gl 133-Xenon (370MBq/ml) in isotonic saline was injected into the internal carotid artery and the radioactivity reaching the brain was measured by a single detector placed over the ipsilateral hemisphere. Hemispheric CBF was determined from the clearance curve by analyzing the initial slope of the curve 14. The autoregulation study was performed as described by Waldemar et al. a6. Only the lower part of the autoregnlation curve was investigated, i.e. no attempt was made to reach the upper limit expected at MABP levels above I60mmHg. In the eSAH-groups MABP was elevated to a higher level compared to the control group as baseline MABP was increased in those groups. Two to five resting CBF measurements were performed in each rat. The values were averaged for the determination of baseline CBF level. First MABP was elevated with intravenous norepinephrine and then decreased by controlled bleeding. CBF was measured for every 10 mmHg change in MABP. Following each CBF measurement, PaCO2, PaO2, and pH were analysed in an arterial
blood sample. The blood withdrawn for these measurements was replaced by blood from donor rats of the same strain. The rats were maintained at normocapnia (PaCO> 38-42 mmHg), except at very low values of MABP, where it was impossible to avoid slight hypocapnia in some animals (PaCO2 32-42 mmHg). Body temperature was kept constant about 37.5 ~ The physiological parameters of the animals are shown in Table 1. Statistics
The Student two-sample t-Test for unpaired data was used to compare baseline values in group A vs. B and C. Differences were accepted as significant at p < 0.05. The one-way analyses of variance followed by the Dunnett multiple comparison test were used for statistical comparisons within each group of rats 28. All parameters were compared with the value in the blood pressure range of the baseline MABP, i.e., 70-89 mmHg for the control group and 90-110 mmHg for the eSAH groups. Differences were accepted as significant at p < 0.05. The autoregulation curves are constructed as a function of MABP in intervals of 20 mmHg (Fig. 2). The CBF values are given as % of the baseline CBF. Lower limit of the autoregulation was defined as the lowest MABP range before CBF (percent of baseline) fell significantly below baseline CBF.
Results A f t e r i n d u c t i o n o f e S A H the a n i m a l s s u f f e r e d a 5 1 0 % w e i g h t loss. T h e y w e r e l e t h a r g i c b u t w i t h o u t f o c a l n e u r o l o g i c a l deficits. B e f o r e m a n i p u l a t i o n w i t h t h e b l o o d p r e s s u r e , baseline
values
of
CBF
and
MABP
were
measured
( T a b l e 2). T h e r e w e r e n o s i g n i f i c a n t d i f f e r e n c e s in b a s e line C B F b e t w e e n c o n t r o l s a n d t h e e S A H g r o u p s . O n the other hand MABP
w a s s i g n i f i c a n t l y e l e v a t e d in
b o t h e S A H g r o u p s c o m p a r e d to t h e c o n t r o l s . A l t h o u g h t h e r e w a s a slight d i f f e r e n c e in the b a s e l i n e a r t e r i a l PaO2, P a C O 2 , a n d p H ( T a b l e 1) t h e s e p a r a m e t e r s d i d not change
significantly during
the autoregulation
s t u d y , e x c e p t at e x t r e m e h y p o t e n s i o n t o w a r d t h e e n d o f t h e s t u d y ( T a b l e 3). T h e s e a l t e r a t i o n s w e r e to s m a l l
Table 1. Baseline Values
Group A (control) Group B (eSAH-2) Group C (eSAH-5)
129
No.
PaCO2 mmHg
pH
PaO2 mmHg
Bodytemp. ~
10
39.8 • 0.2
7.40 :t= 0.01
165 • 3
37.3 • 0.1
I0
39.9 • 0.2
7.44 • 0.01#
151 • 2#
37.1 • 0.1
8
40.0 • 0.3
7.38 • 0.01
144 • 3#
37.5 • 0.1
Physiological conditions in control rats (group A); 2 and 5 days after experimental subarachnoid haemorrhage (group B and C). All values are presented as mean :t: SEM. Group A vs. B and vs. C by Student two-sample T-test for unpaired data. # p < 0.05.
130
G. R a s m u s s e n et al.: Cerebral Blood Flow Autoregulation in Experimental Subaraehnoid Haemorrhage in Rat
to explain any differences in CBF autoregulation in the groups studied. T h e autoregulation curves for all groups are seen in Fig. 2. CBF, PaCO2, PaO2, and p H values for intervals of MABP are shown in Table 3. In the control animals, values of CBF around 100% were maintained down to the MABP range of 5069 mmHg, defined as the lower limit as described above. When MABP fell to a range of 30-50 mmHg, a significant decrease in CBF was found. Two days after eSAH, a lower limit was found in
the MABP interval from 70-89 mmHg, indicating a disturbed autoregulation with a shift to the right of the autoregulation curve. With further increase of MABP, there was a moderate, but not significant increase of CBF; indicating that the plateau of the autoregulations curve was reached. Five days after eSAH, a significant fall in CBF was found when M A B P was lowered to a range of 7089 mmHg, with the lower limit elevated to a range of 90-109mmHg. CBF was increased to a significant level, when MABP was increased to a range from 130-
CBF % of baseline
Table 2, Baseline Values o f CBF and M A B P
Group A (control) Group B (eSAH-2) Group C (eSAH-5)
No.
CBF ml/100 g/min
MABP mmHg
10
93 4- 4
10
94 4- 4
794- 1# 94 • 4
8
89 4- 4
93 4- 4
lo
100
50
0
p
0
Baseline values of cerebral blood flow (CBF) and mean arterial blood pressure (MABP) in control rats (group A) and 2 and 5 days after experimental subarachnoid haemorrhage (group B and C). All values are presented as m e a n 4- SEM. G r o u p A vs. B and vs. C by Student two-sample T-test for unpaired data. # p < 0.05.
50
p
i
100
150
MABP m m H g CONTROL
"-'+- eSAH-2
~
eSAH-5
Fig. 2. Autoregulation curves in control rats and eSAH rats 2 and 5 days after subarachnoid haemorrhage. C B F is expressed as a percentage of the baseline cerebral values. C B F is given as m e a n 4- SEM
Table 3. Data from an Autoregulation Study for All Variables Observed in Each 20 mmHg Range of M A B P
Group A
(control) Group B
(eSAH-2)
Group C
(eSAH-5)
No.
MABP (mmHg)
CBF (% baseline)
PaCOz (mmHg)
pHa
PaO2 (mmHg)
TP (~
6 9 20 14 10
30-49 50-69 70-89 90-109 110-129
63 80 88 101 104
+ 9* 4- 6 + 4 4- 4 4- 6
37.0 38.7 38.7 39.2 39.5
4- 0.1 4- 0.3 4- 0.5 4- 0.4 4- 0.5
7.34 7.35 7.35 7.40 7.41
4- 0.03 4- 0.02 4- 0.02 4- 0.01 4- 0.01
193 166 172 166 168
4- 5* 4- 8 4- 4 4- 5 4- 4
37.5 37.3 37.3 37.1 37,1
4- 0.2 4- 0.2 4- 0.1 4- 0.1 4- 0,1
4 14 20 21 16 9
30--49 50-69 70-89 90-109 110-129 130-149
52 77 93 114 133 138
4- 9* 4- 5* 4- 3 + 5 4- 8 4- 14
34.2 38.9 39.5 39.5 39.9 39.4
4- 0.6* 4- 0.3 4- 0.2 4- 0.3 4- 0.3 4- 0.3
7.40 7.36 7.41 7.42 7.42 7,44
4- 0.02 4- 0.02* 4- 0,01 4- 0.01 4- 0.01 4- 0.01
191 165 150 148 146 143
4- 9* 4- 4* 4- 1 4- 3 4- 4 4- 3
37.3 37.1 37.2 37.1 37.3 37.0
4- 0,1 4- 0.2 4- 0.1 4- 0.1 4- 0.1 4- 0.1
3 9 12 7 11 8
30-49 50-69 70-89 90-109 110-129 130-149
26 67 84 115 128 147
4- 8* 4- 4* • 6* 4- 9 4- 6 4- 10"
35.4 38.1 39.6 40.0 39.5 39.8
4- 1.4" 4- 0.4 4- 0.4 4- 0.3 4- 0.4 4- 0.5
7.33 7.38 7.38 7.37 7.39 7.36
4- 0.05 4- 0.01 4- 0.02 4- 0.03 4- 0.02 4- 0.01
164 143 137 147 142 139
4- 14" 4- 3 4- 3 4- 4 4- 2 4- 2
37.2 37.5 37.6 37.3 37,4 37.5
4- 0.2 4- 0.1 4- 0.1 4- 0.1 4- 0.1 4- 0,1
The analyses o f variance and the D u n n e t t multiple comparison test were used to compare each range vs. baseline M A B P range. All values are presented as m e a n • SEM. * p < 0.05.
G. Rasmussenet al.: Cerebral BloodFlow Autoregulationin ExperimentalSubarachnoidHaemorrhagein Rat
131
149, which may indicate that the upper limit was reached. However, a further elevation of MABP was not done, because of the risk of breakdown of the blood-brain barrier. Thus it seems that autoregulation is severely disturbed 5 days after eSAH with a narrowing of the plateau to a MABP range from 90129 mmHg where there is no significant change in CBF, compared to the baseline value. The results indicate a severely disturbed autoregulation 2 days after eSAH with no sign of recovery 5 days after eSAH.
(CPP). This compensatory blood pressure increase seems unaffected while the CBF autoregulation is disturbed. Two days after eSAH, CBF autoregulation is severely disturbed. The same observation is made 5 days after eSAH. Thus small arteries and arterioles seem unable to actively regulate and compensate for changes of MABP. This paralysis of the vessels causes CBF to depend mainly on the pressure gradient between the cerebral arteries and veins. This corresponds to the CBF/MABP relationship beneath the lower limit of the autoregulation in normal animals.
Discussion
Autoregulation and Vasospasm
Method
Voldby etal. found that patients in poor clinical condition have the severest disturbances of intracranial vasomotor function. Furthermore, a close correlation was found between the degree of vasospasms and the degree of autoregulatory impairment25. Heilbrun et al. observed no relationship between vasospasm and impaired autoregulation, but reported a correlation between defective autoregulation and hydrocephalus - possibly because of altered perfusion pressure due to increased ICP 7. However, both clinical investigations were performed with only a few MABP/CBF measurement points, which makes firm conclusions on cerebral autoregulation difficult. A recent study in monkeys showed vasospasm and impairment of CBF autoregulation 7 days after eSAH, but the investigation lacks a description of the time course of the changes24. Unfortunately, our study does not include angiographic evidence of the vasospasm, but the manipulations with the rats inside the experimental setup have to be as few as possible. Our conclusion is based on the reported timecourse of vasospasm. Delgado et al. 4 found angiographic vasospasm with a maximum two days after eSAH in exactly the same eSAH model as used in the present study. On day 5, the vasospasms had resolved. However, autoregulation is severely disturbed or even absent at both timepoints indicating that there is no direct correlation between angiographically visible vasospasms and disturbance of autoregulation. The explanation may be, that vasospasm visualized on angiograms is localized to the larger intracranial arteries, whereas autoregulation is mediated by small arteries and arterioles which are not seen on angiograms. Another explanation of our study is, that the presence of cerebral vasospasm affect and impair autoregulation and that the impairment is more protracted than the vasospasm.
Most studies on CBF autoregulation after eSAH in animals and after SAH in man have been performed by measuring CBF at a few different values of MABP. The method in the present study uses several MABP values for measurement and therefore gives a more complete illustration of CBF autoregulation curve. The intracarotid 133-Xenon method for measurement of CBF is rapid, reproducible and allows repetitive measurement of CBF in individual rats 9. Unfortunately this method lacks regional and bilateral information which would be desirable when comparing focal versus global CBF disturbances following SAH. Norepinephrine used to induce hypertension causes a momentarily reversible acute hypertension and does not per se affect CBF 13. The concentration of 0.5% halothane used in both control and experimental groups has been shown to decrease CBF by about 20% in normal monkeys. However CBF autoregulation was not influenced by this concentration11. There is no evidence to suggest that CBF autoregulation should be affected differently by halothane in eSAH animals. Therefore the method is particularly well suited for repeated CBF measurements for construction of CBF autoregulation curves. Results
There was no significant change in baseline CBF after eSAH. However there was a significant increase in baseline MABP on both experimental days after eSAH. Delgado etal. found decrease in global CBF after eSAH with an unchanged MABP s. Thus a higher MABP seems necessary to maintain CBF unchanged. This may be explained by an increase in ICP after eSAH resulting in a decrease of cerebral perfusion pressure
132
G. Rasmussen etal.: Cerebral Blood Flow Autoregulation in Experimental Subarachnoid Haemorrhage in Rat
M e c h a n i s m s o f Autoregulation
The exact mechanism of autoregulation is unknown. Metabolic, myogenic and neurogenic factors have been proposed 3, 22. A study of autoregulation dynamics in normal humans showed that autoregulatory adjustments of the cerebral vascular resistance take place much quicker at hypocapnic CO2 levels than at normoor hypercapnic levels 1. Furthermore it has been shown that hyperventilation can restore impaired autoregulation in patients with tumours and cerebral ischaemia 15. These findings indicate, that autoregulation may partly depend on arterial PCO2, or perhaps on p H in the arteriolar wall or in brain extracellular space. Patients with SAH often hyperventilate spontaneously< 25. In the present study, the rats were kept normoventilated during the experiment. This may have caused a slight perivascular acidosis, and it is possible that the autoregulation would have been less impaired, if the animals had been hyperventilated.
Clinical Relevance
Patients with SAH from rupture of an intracranial aneurysm often have several different kinds of intracranial vasomotor dysfunction. Fifty percent of the patients develop angiographic vasospasm 17' 20. Disturbed autoregulation or impaired CO2-response are also frequently seen 1~ 19, 25 The present study underscores the importance of a sufficient observation of MABP, and the danger of blood pressure manipulations in patients with SAH. Because of a less effective or maybe even absent cerebral autoregulation, blood pressure changes may directly provoke drastic changes of CBF. There is a risk of inducing hyperperfusion by hypertension resulting in cerebral oedema or haemorrhage. Hypotension is likely to induce hypoperfusion and ischaemia. Studies on the additional influence of ICP and arterial PaCO2 on CBF and autoregulation are needed to further illuminate the complex interactions of these variables. These factors also need to be taken into account when evaluating the benefit and risks of pharmacologic intervention in SAH. Additionally, absence of angiographic vasospasm should not automatically lead to conclusions of haemodynamic stability in patients with SAH. As our study indicates, autoregulation may be disturbed, and thus the patient could be haemodynamically vulnerable even after disappearance of visible vasospasm. The duration of autoregulatory instability and its rate of re-
turn is u n k n o w n - b o t h in experimental animals and in man.
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Correspondence and Reprints: Gitte Rasmussen, M.D., University Clinic of Neurosurgery, Aarhus Kommunehospital, 8000 Aarhus C, Denmark.
