CEREBRAL blood flow (CBF) represents the

Transcription

1 457 Quantitative Measurement of Blood Flow Velocity in Feline Pial Arteries During Hemorrhagic and Hypercapnia Masahiro Kobari, Fumio Gotoh, Yasuo Fukuuchi, Kortaro Tanaka, Norihiro Suzuki, and Daisuke Uematsu Due to methodologic difficulties, few investigations have been made on the blood flow velocity in the cerebral mlcrocirculation. Using a newly developed video camera method, we simultaneously measured the blood flow velocity and diameter of pial arteries during hemorrhagic hypotension, after blood pressure recovery, and during CO 2 inhalation in cats. When the mean arterial blood pressure was lowered from ± 6.6 to 71.5 ± 4.1 mm Hg, the bloodflowvelocity inevitably decreased from 36.6 ± 5.3 to 27.0 ± 3.9 mm/sec (p< 0.001). The calculated bloodflowrate [IT X (diameter/2) 2 x flow velocity] was preserved in cases with concomitant vasodilation. Conversely, the blood flow velocity increased from 25.3 ± 5.1 to 31.0 ± 5.4 mm/sec (p < 0.001) after mean arterial blood pressure recovery from 67.1 ± 3.7 to ± 5.8 mm Hg. The blood flow rate was again preserved in vessels with a vasoconstrictive response. Each pial artery apparently dilated or constricted in proportion to the decrease or Increase in flow velocity during blood pressure changes, maintaining a constant cerebral blood flow. This indicated the importance of the pial arteries in the mechanisms of cerebral blood flow autoregulation. During 5% CO 2 inhalation, the blood flow velocity increased markedly from 25.4 ± 4.6 to 37.2 ± 10.0 nun/sec (p<0.05), while the pial artery diameter (85.0 ± 13.7 fim) increased by 9.6 ± 1.5% (p<0.01). The increased flow velocity might be attributable to preferential dilatation of small arterioles or intraparenchymal vessels during hypercapnia. Not only the pial arterial dilatation but also the increase in bloodflowvelocity may contribute to the cerebral blood flow increase during CO] inhalation. (Stroke 1987;18:457^*63) CEREBRAL blood flow (CBF) represents the total sum of the blood flow in individual vessels, which is determined by two basic components, i.e., the velocity of blood flow and the caliber of the vessel. CBF is therefore affected by changes in either or both of these components. It has been well established that the pial arteries dilate in response to a fall in systemic arterial blood pressure, 1 " 4 while CBF remains constant (autoregulation of CBF). 43 Dilatation of the pial arteries 2 ' 6 -* together with an increase in CBP 10 during CO 2 inhalation has also been well documented. Although important, information concerning the changes in blood flow velocity under such conditions is scarce, probably reflecting the lack of an appropriate method for its quantitative estimation. A few approaches, e.g., a two-slit photometric method for pial and intraparenchymal microvessels" 12 and an ultrasound Doppler method for major cerebral arteries, 1314 have been reported with limited results. We recently developed a video camera method for simultaneous measurement of the blood flow velocity and diameter of pial vessels 1316 that permits repetitive measurement of blood flow velocity over a wide range of pial vessel diameters. The purpose of the present experiments was to elucidate the interrelations among From the Department of Neurology, School of Medicine, Keio University, Tokyo, Japan. Address for reprints: Fumio Gotoh, MD, Department of Neurology, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku-ku, Tokyo 160, Japan. Received August 30, 1986; accepted October 13, systemic blood pressure, diameter, blood flow velocity, and flow rate of pial arteries during various procedures such as reduction and restoration of systemic blood pressure and CO 2 inhalation. The results obtained have implications for the control mechanisms of the CBF. Materials and Methods Experiments were carried out on 25 cats of either sex, weighing between 1.9 and 5.0 kg (average 3.1 kg). All animals were anesthetized by i.p. injection of a-chloralose (50 mg/kg) and urethane (500 mg/kg) supplemented with local application of 0.5% procaine hydrochloride. After endotracheal intubation and immobilization with alcuronium chloride, the cats were ventilated artificially using a respirator (Harvard, model 662), and their body temperature was maintained between 37 and 37.5 C with a heating blanket. Polyethylene catheters were inserted into the bilateral femoral arteries, one for monitoring the arterial blood pressure with a pressure transducer (Statham P23Db) and the other for sampling arterial blood for blood gas analysis (Radiometer, BMS3 Mk2 Blood Micro System) and for withdrawal of blood. Another catheter was inserted into a femoral vein for reinfusion of blood. The right lingual artery was cannulated with a thin polyethylene catheter used to inject a small amount of saline into the carotid artery. The head of each animal was fixed in a stereotactic head holder (Narishige, type SN-1). The scalp was reflected at the midline with the aid of electrocautery, and a burr hole 1 cm in diameter was trephined in the

2 458 Stroke Vol 18, No 2, March-April 1987 right parietal region of the skull. The dura was incised and reflected meticulously to expose the surface of the brain. A cranial window made of a stainless steel frame and a glass disk was screwed into the skull. The space beneath the cranial window wasfilledwith artificial cerebrospinal fluid having the following composition: Na meq/1, K meq/1, Cl" meq/1, Ca meq/1, IP 1.7 meq/1, Mg meq/1, HCO meq/1, glucose 0.05%, and ph Measurement of Blood Flow Velocity and Diameter of Pial Vessels The blood flow velocity and diameter of pial vessels were measured simultaneously employing the video camera system developed in our laboratory, described in detail previously. 15 " 17 In brief, the system consisted of an inverted 24-mm or 50-mm lens (Nikon), extension tube (Nikon), video camera (Hamamatsu TV), camera control (Hamamatsu TV), width analyzer (Hamamatsu TV), video densitometer (FOR-A), TV monitor (Dcegami), A/D converter (Adtek System Science), desktop computer (Seiko 8500), and multichannel recorder (Rikadenki, model PG-6). Images of the pial vessels, which were observed through the cranial window, were visualized on the TV monitor at 25- x magnification. Since the responses to various stimuli were basically similar among the vessels, we arbitrarily selected a most suitable artery for the following measurements. Using the width analyzer, the diameter of the vessel was measured and recorded continuously on the multichannel recorder. At the same time, intensities of the video signals derived from 2 targets located at the center-line of a proximal and a distal part of the same vessel were monitored continuously with video densitometers. Their outputs were recorded on the multichannel recorder and were also transmitted to the computer. Indicator time-concentration curves were obtained from the 2 targets by injecting a small amount of saline (0.1 ml) into the right lingual artery. The blood flow velocity in the pial artery was calculated automatically from the time difference between the 2 peaks of the curves and the distance between the 2 targets using the computer. The results were displayed on a cathode ray tube (CRT). The blood flow velocity could be measured every 2-3 seconds at the highest frequency. Each value for blood flow velocity presented in this paper represents the average of 3 or 4 successive measurements since timeto-time fluctuations in flow velocity have been observed in pial arteries. 18 The blood flow rate in a single pial artery was estimated as the product of the crosssectional area of the vessel [IT X (diameter/2) 2 ] and the blood flow velocity. Experimental Protocol The effects of reduction of the systemic arterial blood pressure on the cerebral hemodynamic parameters were investigated in 17 cats. Acute arterial hypotension was induced by withdrawal of blood through the catheter inserted into the femoral artery. The arterial blood pressure was lowered to approximately 50-60% of the initial pressure, to between 50 and 90 mm Hg, which was slightly higher than the known lower limit of autoregulation. The blood pressure was maintained at the reduced level for approximately 1 minute. The blood flow velocity and diameter of the pial artery were measured simultaneously under steady-state conditions and during the induced hypotension. The effects of restoration of the systemic arterial blood pressure on the hemodynamic parameters were observed in 12 cats. Recovery of blood pressure was accomplished byreinfusionof the withdrawn blood through the catheter inserted into the femoral vein. In 14 cats, the effects of CO 2 inhalation were examined by ventilating the animals with 5% CO 2 in air. The blood flow velocity, pial artery diameter, arterial blood gas tensions, and ph were measured before and approximately 3 minutes after the beginning of the inhalation. The values obtained are presented as the mean ± SEM. Statistical analysis was performed using Student's t test and Wilcoxon's signed rank test. Results Figure 1 shows an example of simultaneous recordings of pial artery diameter and indicator time-concentration curves obtained from the proximal and distal parts of the same vessel during reduction and restoration of the systemic arterial blood pressure. Diameter of the pial artery increased as blood pressure was reduced, and the time difference between the 2 timeconcentration curves was prolonged, i.e., computed flow velocity in the pial artery decreased during induced hypotension. Figure 2 presents mean values for the pial artery diameter, flow velocity, flow rate, and mean arterial blood pressure (MABP) before and during hemorrhagic hypotension. The blood flow velocity decreased significantly in all vessels examined (p<0.001), changing from 36.6 ± 5.3 to 27.0 ± 3.9 mm/sec when MABP was lowered from ± 6.6 to 71.5 ± 4.1 mmhg (p<0.001). The changes in pial artery diameter (initial diameter ± 14.6 jtim) were not consistent during hemorrhagic hypotension; therefore the vessels were divided into two groups according to the presence (Group A) or absence (Group B) of a vasodilatory (or autoregulatory) response. In Group A (H=11), the pial arteries dilated by 17.1 ±3.6% lp<0.005) while the calculated blood flow rate remained almost constant (0.8 ± 3.7% increase, Figure 3) during hypotension. In Group B (n = 6) where the pial arteries were constricted by 5.8 ±1.6% \p<0.005), the blood flow rate decreased markedly by 35.5 ± 5.9% (p<0.05). Mean values for the cerebral hemodynamic parameters during hemorrhagic hypotension (control) and after recovery of the blood pressure are given in Figure 4. When MABP was elevated from 67.1 ±3.7 to ± 5.8 mm Hg (p<0.001), the blood flow velocity increased significantly in all vessels from 25.3 ±5.1 to 31.0 ±5.4 mm/sec (/><0.00l). The changes in pial artery diameter (initial diameter

3 Kobari et al Blood Flow Velocity in Pial Arteries 459 Withdrawal of Blood ReinfusJon Pial Artery Diameter Saline Injection Saline Injection 1 Saline InJectJoii 164.8/<m -I Intensity of Video Signals Proxinl Distal S A B P TmmHg - It 1 mln It 1 rain It FIGURE 1. Simultaneous recordings ofpial artery diameter, intensity of video signals obtainedfrom 2 targets located in the proximal and distal parts of the same vessel, and systemic arterial blood pressure (SABP). The pial artery dilated as the blood pressure was reduced and constricted as its level recovered. The upward deflections on the video signal recordings are time-concentration curves obtained following saline injection (0.1 ml) into the carotid artery. The time difference between the peaks of the 2 time-concentration curves obtained from the 2 targets was prolonged during hemorrhagic hypotension. The time difference, flow velocity, and flow rate in the pial artery were calculated on-line using a computer. The downward deflections on the arterial diameter recording are artifacts caused by the saline injection. T, time difference between the two peaks; V, calculated blood flow velocity ± 20.2 yum) during recovery of blood pressure were again variable. In 8 vessels where a vasoconstrictive (or autoregulatory) response was present (Group C), the decrease in pial artery diameter was 9.6 ± 1.9% (/J<0.02), and the calculated blood flow rate increased by only 8.6 ± 14.6% (Figure 3). On the other hand, in 4 arteries where the diameter increased (6.5 ± 2.5%) during recovery of blood pressure (Group D), the blood flow rate increased markedly by 89.0 ±28.7%. Figure 5 summarizes the findings obtained during 5% CO 2 inhalation. Paco^, Pao2, and ph in the control state were 30.6 ±1.1, 92.2 ±4.2, and ± 0.029, respectively, which was in good agreement with previously reported values for awake cats. 19 " 21 During CO 2 inhalation, the PacOj, Pac>2, and ph values were43.2± 1.8,.1 ±4.9, and ±0.015, respectively. All of the 14 vessels examined responded with vasodilatation of 9.6 ± 1.5% (p<0.01) from a diameter of 85.0 ± 13.7 to 93.3 ± 15.2 /im during hypercarbia. This was accompanied by a marked (35.6 ± 14.7%) increase (/?<0.05) in the flow velocity from 25.4 ± 4.6 to 37.2 ± 10.0 mm/sec. As a result, the blood flow rate through the pial arteries during CO 2 inhalation increased by 56.8 ± 13.8% (p<0.05) from ± X 10" 3 to ± x 10" 3 mnrvsec. Discussion The details and validity of the video camera method for simultaneous measurement of blood flow velocity and diameter of pial vessels have been described previously This method seems to be the only one that has succeeded in measuring absolute values of the flow velocity in pial arteries over a wide range of arterial diameters. Normal values and the diameter-dependence of the flow velocity and flow rate under steadystate conditions have beenreportedfor cats. l6 With the present method, the center-line red blood cell (RBC) velocity in the pial vessels was measured, and the blood flow rate was calculated as n x (diameter/ 2) 2 x flow velocity. The calculated blood flow rate for the pial vessels may thus deviate from the actual mean flow rate due to the difference between the center-line RBC velocity and the mean blood flow velocity, especially in small vessels. 22 Several equations have been proposed for estimating the mean blood flow velocity from the center-line RBC velocity, but their validity is uncertain since they were based on in vitro experiments Nevertheless, the results obtained by the present method should be sufficiently accurate for making comparisons before and during various procedures. Duringreductionof MABP from to 71.5 mm Hg, the blood flow velocity in the pial arteries decreased markedly from 36.6 to 27.0 mm/sec, or to < VA of the initial value. Russell et al 24 observed a 77% prolongation of the pial arteriovenous transit time of fluorescein during reduction of the arterial blood pressure in rabbits. Using a two-slit photometric method, Koo and Cheng" briefly reported a decrease in flow velocity during hemorrhagic hypotension in rats.

4 460 Stroke Vol 18, No 2, March-April 1987 MABP (n»hg) Pial Artery Diameter P< L -p<o.oi Hemorrtiagic Hemorrhaglc Blood Row Velocity (BIB/1) 80 r 60 Blood Flow Rate (IO-'BBVI) 0 FIGURE 2. Comparison of mean arterial blood pressure (MABP), pial artery diameter, blood flow velocity, and blood flow rate before and during hemorrnagic hypotension. The flow velocity decreased in all vessels during hemorrnagic hypotension K Hemorrhaglc -P<0.05- Hemorrhagic S Cbaige ii Blood How Rate Chang et al 23 described a decrease in red cell velocity during hypotension in feline cerebral arterioles of < 30 /Am diameter. The decrease in blood flow velocity appears to reflect a reduction in the perfusion pressure at the inlet of the pial artery. In fact, Harper et al 12 have demonstrated a linear relation between mean arterial pressure and cerebral arteriolar pressure in rats. The changes observed in pial artery diameter in response to reduction of the flow velocity were not consistent. When a vasodilatory (or autoregulatory) response 3 was present, as in most cases, the blood flow rate was well preserved before and during hypotension (Figure 3, Group A). It is clear, therefore, that each pial artery dilated in proportion to the decrease in perfusion pressure or flow velocity, maintaining a constant flow rate through each pial artery. In cases where the pial artery constricted with the reduction in blood pressure, the decrement of the blood flow rate was prominent (Figure 3, Group B). The inverse held true during recovery of the blood pressure. When MABP was elevated from 67.1 to mm Hg, the blood flow velocity increased slightly but definitely from 25.3 to 31.0 mm/sec. This was compatible with the observations of Rosenblum 26 and Russell et al, 24 who noted a shorten J L r<m»i-> A B Heaorrtogic Hfpotantion I L r«lb J C D Attar Rw^nrery ot Blood Preuirs FIGURE 3. Changes in blood flow rate in pial arteries during hemorrhagic hypotension (A, vasodilatory response [ + ]; B, [ ]) and after recovery of blood pressure (C, vasoconstrictive response [+]; D, [-]).

5 Kobari et al Blood Flow Velocity in Pial Arteries 461 MABP (nnhg) Pial Artery Diameter P<«.OOI ' Blood Row Velocity ( n/i) 60 () Blood Pressure () Blood Pressure Blood Flow Rate FIGURE 4. Mean arterial blood pressure (MABP), pial artery diameter, blood flow velocity, and blood flow rate during hemorrhagic hypotension (control) and after recovery of blood pressure. The flow velocity showed a significant increase after the recovery of blood pressure P<O,COI ' () Blood Pressure ing of the arteriovenous transit time of fluorescein in pial vessels during systemic hypertension despite the difference in the vascular tree observed. The blood flow rate was again preserved in arteries with a vasoconstrictive (or autoregulatory) response 3 but not in cases with pressure-dependent vasodilatation (Figure 3). The above data suggest a major role for the pial arteries in maintaining the blood flow rate in individual pial arteries as well as the total CBF (autoregulation of CBF) during blood pressure changes/ 3 In the present experiments, we observed a 56.8% increase in blood flow rate during 5% CO 2 inhalation, which agreed well with the results of previous studies. 910 This increment was accomplished by a 9.6% increase in pial artery diameter and, more importantly, by a 35.6% increase in blood flow velocity. Russell 27 observed a decrease in arteriovenous transit time, and Tomita et al 28 demonstrated a reduction of the mean transit time in the pial circulation during CO 2 inhalation. Using their Doppler method, Busija et al 13 reported that the pial blood flow velocity increased during hypercapnia in dogs and cats, although the absolute values were not described. Markwalder et al 14 showed 10 () Blood Pressure a positive correlation between the end-tidal PCO2 and blood flow velocity in the human middle cerebral artery by an ultrasound Doppler study. The observed slight increase in MABP during CO 2 inhalation was too small to account for the large increase in flow velocity in the pial arteries, suggesting rather that small arterioles or intraparenchymal vessels dilated dominantly, leading to a decrease in the resistance of these vessels. This resulted in an increased perfusion pressure, and so a significant increase in the blood flow velocity in the pial arteries. Preferential dilatation of relatively small pial arteries during CO 2 inhalation has been repeatedly described. 78 Therefore, not only dilatation of the cerebral vessels but also an increase in flow velocity appear to contribute to the increase in CBF during hypercapnia. In conclusion: We investigated the blood flow velocity changes occurring during hemorrhagic hypotension and CO 2 inhalation. The changes in blood flow velocity and pial artery diameter are not necessarily parallel or opposite, and the relation between them may be variable depending on the particular stress given to the vessels.

6 462 Stroke Vol 18, No 2, March-April 1987 MABP (tirihg) Plal Artery Diameter 150 -P<0.005 ' -P<0.0l ' CO2 Inhalation CO? Inhalation Blood Row Velocity (nil) Blood Flow Rate (nr'tii FIGURE 5. Changes in mean arterial blood pressure (MABP), pial artery diameter, blood flow velocity, and blood flow rate before and during 5% CO 2 inhalation. Increases in both the arterial diameter and the flow velocity contributed to a marked increment of the flow rate p<o«- CO; Inhalation C0 t Inhalation References Fog M: Cerebral circulation: The reaction of the pial arteries to a fall in blood pressure. Arch Neurol Psychiat 1937;37: Gotoh F, Muramatsu F, Fukuuchi Y, Amano T: Dual control of cerebral circulation: Separate sites of action in vascular tree in autoregulation and chemical control, in Langfitt TW, 9. McHenry LC Jr, Reivich M, Wollman H (eds): Cerebral Circulation and Metabolism. New York, Springer-Verlag, 1975, 10. pp Kontos HA, Wei EP, Navari RM, Levasseur JE, Rosenblum WI, Patterson JL Jr Responses of cerebral arteries and arterioles to acute hypotension and hypertension. Am J Physiol ;234:H371-H MacKenzie ET, Farrar JK, Fitch W, Graham DI, Gregory PC, Harper AM: Effects of hemorrhagic hypotension on the cere- 12. bral circulation. I. Cerebral blood flow and pial artenolar caliber. Stroke 1979;10: Harper AM: Autoregulation of cerebral blood flow: Influence 13. of the arterial blood pressure on the blood flow through the cerebral cortex. J Neurol Neuroswg Psychiatry 1966;29: Wolff HG, Lennox WG: Cerebral circulation. Xn. The effect on pial vessels of variations in the oxygen and carbon dioxide content of the blood. Arch Neurol Psychiat 1930;23: Auer L: Pial arterial reactions to hyper- and hypocapnia: A dynamic experimental study in cats. Eur Neurol 1978;17: Wei EP, Kontos HA, Patterson JL Jr Dependence of pial arteriolar response to hypercapnia on vessel size. Am J Physiol 1980;238:H697-H703 Reivich M: Arterial Po>2 and cerebral hemodynamics. Am J Physiol 1964;206:25-35 Harper AM, Glass HI: Effect of alterations in the arterial carbon dioxide tension on the blood flow through the cerebral cortex at normal and low arterial blood pressures. J Neurol Neurosurg Psychiatry 1965;28: Koo A, Cheng KK: Cerebral microvascular volume flow: Its measurement andresponsesto hemorrhagic hypotension in the rat. Microvasc Res 1974;8: Harper SL, Bohlen HG, Rubin AMI: Arterial and microvascular contributions to cerebral cortical autoregulation in rats. Am J Physiol 1984;246:H17-H24 Busija DW, Heistad DD, Marcus ML: Continuous measurement of cerebral bloodflowin anesthetized cats and dogs. Am J Physiol 1981;241:H228-H234 Markwalder T-H, Grolimund P, Seiler RW, Roth F, Aaslid R: Dependency of blood flow velocity in the middle cerebral artery on end-tidal carbon dioxide partial pressure: A transcranial ultrasound Doppler study. J Cereb Blood Flow Metab 1984;4:

7 Kobari et al Blood Flow Velocity in Pial Arteries Gotoh F, Muramatsu F, Fukuuchi Y, Okayasu H, Tanaka K, Suzuki N, Kobari M: Video camera method for simultaneous measurement of blood flow velocity and pial vessel diameter. J Cereb Blood Flow Metab 1982;2: Kobari M, Gotoh F, Fukuuchi Y, Tanaka K, Suzuki N, Uematsu D: Blood flow velocity in the pial arteries of cats, with particular reference to the vessel diameter. J Cereb Blood Flow Metab 1984;4: Okayasu H, Gotoh F, Muramatsu F, Fukuuchi Y, Amano T, Tanaka K: A method for continuous measurement of pial vessel diameter by means of a vidicon camera system. Ada Neurol Scand 1979;60(suppl 72): Rosenblum WI: Erythrocyte velocity and a velocity pulse in minute blood vessels on the surface of the mouse brain. Circ Res 1969;24: Fink BR, Schoolman M: Arterial blood acid-base balance in unrestrained waking cats (abstract). Fed Proc 1962;21: S0rensen SC: Arterial PCO2 in awake cats calculated from gas tensions in subcutaneous pockets. Resp Physiol 1967;3: Herbert DA, Mitchell RA: Blood gas tensions and acid-base balance in awake cats. J Appl Physiol 1971;30: Gaehtgens P, Meiselman HJ, Wayland H: Velocity profiles of human blood at normal and reduced hematocrit in glass tubes up to 130 fi diameter. Microvasc Res 1970;2: Intaglietta M, Richardson DR, Tompkins WR: Blood pressure, flow, and elastic properties in microvessels of cat omentum. Am J Physiol 1971;221: Russell RWR, Simcock JP, Wilkinson IMS, Frears CC: The effect of blood pressure changes on the leptomeningeal circulation of the rabbit. Brain 1970;93: Chang BL, Santillan G, Bing RJ: Red cell velocity and autoregulation in the cerebral cortex of the cat. Brain Res 1984;308: Rosenblum WI: Effects of blood pressure and blood viscosity on fluorescein transit time in the cerebral microcirculation in the mouse. Circ Res 1970;27: Russell RWR: The reactivity of the pial circulation of the rabbit to hypercapnia and the effect of vascular occlusion. Brain 1971;94: Tomita M, Gotoh F, Sato T, Amano T, Tanahashi N, Tanaka K, Yamamoto M: Photoelectric method for estimating hemodynamic changes in regional cerebral tissue. Am J Physiol 1978;235:H56-H63 KEY WORDS autoregulan'on of CBF bloodflowvelocity hemorrhagic hypotension hypercapnia pial artery