The etiology and clinical significance of early cerebral hyperemia remains obscure. Both neuronal and vascular mechanisms have been implicated in the etiology of early cerebral hyperemia (1,2). Intracarotid infusion of anesthetic drugs can achieve electroencephalographic (EEG) silence at a fraction of IV doses that do not cause significant systemic hypotension (3). We hypothesized that if neuronal activity was primarily responsible for early cerebral hyperemia then electrocerebral silence by intracarotid anesthetics would prevent the hyperemic response. Clinically, small doses of intraarterial anesthetics could alter the outcome of reperfusion injury after intraarterial thrombolysis because the catheter for the delivery of drugs would be located in the region of injury.
After approval of the protocol by the institution’s animal care and use committee, the study was conducted on New Zealand white rabbits (3–4 lb). Rabbits, like primates, have near complete separation of the external carotid and the internal carotid circulations (4). Therefore, these animals are ideally suited for intracarotid drug infusion studies. The rabbits were given full access to food and water before the experiment and sedated with intramuscular ketamine (50 mg/kg). IV access was obtained through an ear-lobe vein. Hydrocortisone 10 mg was given IV to prevent hypotension that often occurs in these animals after surgical interventions. Subsequently, the rabbit received 0.2-mL boluses of IV propofol (Diprivan® 1%, Astra Zeneca Pharmaceutical LP, Wilmington, DE) as required for maintaining adequate depth of anesthesia before tracheotomy. After infiltration of the incision site with 0.25% bupivacaine with 1:200,000 epinephrine, a tracheotomy tube was placed for mechanical ventilation by a Harvard small animal ventilator (Harvard Apparatus Inc, South Natick, MA). End-tidal (ET)CO2 was continuously monitored with a Novametrix Capnomac Monitor (Novametrix Medical Systems Inc, Wallingford, CT). After securing the airway, anesthesia was maintained with a IV infusion of propofol (1–1.5 mL · kg−1 · h−1), fentanyl (1–2 μg · kg−1 · h−1), and vecuronium bromide (10–20 μg · kg−1 · h−1). A femoral arterial line was placed for monitoring mean arterial blood pressure (MAP). By infiltrating the incision sites with local anesthetics, we were able to provide adequate levels of anesthesia (as judged by changes in heart rate and MAP) with relatively small doses of IV sedation and analgesia so as to minimize the effects of systemic drugs on the outcome of the study.
The right common carotid artery was dissected in the neck and cannulated using a 20-cm-long PE-50 tubing (Becton Dickinson and Co, Spark, MD). The catheter tip was located ≈3–5 mm below the putative origin of the internal carotid artery (ICA). We have observed that the ICA occlusion in rabbits tends to decrease the distal cerebral pressure within 80%–90% of the MAP and is clearly more than the normal autoregulatory range. Experiments suggest that although occlusion may decrease hemispheric cerebral blood flow (CBF), unilateral ICA occlusion alone in these rabbits is unlikely to cause injury (5–7). There are profound anatomical variations in the size and origin of the rabbit ICA (8,9). Therefore, rather than attempt retrograde cannulation, our approach was to isolate the ICA by cannulating the common carotid and ligating all branches other than the ICA. Correct identification of the ICA and its isolation was confirmed by the retinal discoloration test (10). Briefly, this test entails intraarterial injection of 0.2 mL of 0.5% indigo-carmine blue. Injection of indigo-carmine blue changes the retinal reflex from red to blue when the ICA is correctly identified.
On the contralateral side, the common carotid artery was dissected, and a sialotic arterial compression device was placed around the vessel. The device permitted controlled occlusion of the common carotid when the rabbit was placed prone on the stereotactic frame. Tympanic temperature probes were used to monitor core temperature (Mon-a-therm, 400H, Mallinckrodt Anesthesia Products, St Louis, MO). The rabbit’s temperature was kept constant between 36°C–37°C using an electrically heated blanket. An IV infusion of fluid was given at 10 mL · kg−1 · h−1 through an IVAC pump (IVAC 599 volumetric pump, IVAC Co, San Diego, CA). The IV infusion consisted of 3 fluids: Ringer’s lactate solution, 5% dextrose, and 5% albumin mixed in a ratio of 3:1:1, respectively.
The rabbits were then turned prone, and the head was mounted in a stereotactic frame. The skull was exposed through a midline incision. A 5 × 4-mm area of the outer table of skull was shaved using an air-cooled drill just anterior to the bregma and 1 mm lateral to the midline. Skull shaving exposed the inner table to a depth that revealed cortical vessels through a fine layer of the bone (11). The probes were maneuvered to obtain a laser Doppler blood flow (Periflux System 5000, Perimed Inc, Jarfalla, Sweden) reading between 50 and 250 perfusion units (PU). Once the optimum site of placement was identified, the probes were secured within plastic retainers and glued to the skull. Two probes (Probe# 407–1, Perimed Inc) were placed on either hemisphere. Satisfactory probe placement was also judged by an abrupt increase in flow velocity values during intracarotid injection of a small volume of saline (0.2 mL). Laser Doppler blood flow measurement techniques provide a relative measure of blood flow changes in the tissue; therefore, laser Doppler blood flow values were normalized to the baseline value and were expressed as percent-change from the baseline value (12–14). Cerebrovascular resistance (CVR) was calculated in two ways: absolute and relative. Absolute CVR was calculated by dividing MAP by laser Doppler flow (LDF) value (PU). The units of absolute CVR were mm Hg/PU. The relative CVR was calculated by dividing the MAP by percent-change LDF normalized to baseline (mm Hg/percent-change LDF).
Frontoparietal leads were used to monitor the bilateral electrocerebral activity. Electrocerebral activity was monitored using standard stainless steel needle electrodes (impedance is <10 kOhms). The frontal and the parietal needle electrodes were secured to the skull by small stainless steel screws. The neutral electrode was placed behind the ear. Frontoparietal electrocerebral activity was recorded by bioamplifier (ML136, AD Instruments, Grand Junction, CO), with a range of 100 mV and an EEG recording mode having a pass-band 0.3–60 Hz. Analog data were sampled at 100 Hz per channel with an analog-to-digital converter and displayed using the Chart 4.0 program (AD Instruments). Electrocerebral silence was defined operationally using a reference recording obtained with an identical recording technique from a known brain dead preparation after the administration of IV KCl (7). Electrocerebral activity, MAP, inspired and expired CO2 concentrations, and LDF were continuously recorded on a computer using Powerlab software (Chart 4.0, AD Instruments;Fig. 1).
Preliminary studies to determine dose requirements for intracarotid anesthetics for EEG silence were conducted on eight rabbits. Four rabbits each received intracarotid boluses of propofol and sodium thiopental 1% (Pentothal®, Abbot Laboratories, North Chicago, IL). We selected bolus delivery for the continuous infusion of anesthetic drugs because the streaming of drugs results in a markedly uneven distribution of drugs (14).
The rabbits for occlusion-reperfusion studies were randomized into three groups: saline-control, propofol, and thiopental groups. After the baseline measurements, MAP was decreased with IV nitroprusside (20–40 μg) and esmolol (5–10 mg). The dose of esmolol was titrated to keep the heart rate between 150–175 bpm. Nitroprusside (Sodium Nitroprusside, Abbot Laboratories) was given as a 20-μg bolus just before esmolol (Brevibloc, Baxter Healthcare Corporation, Deerfield, IL) to decrease cardiac afterload. Esmolol was withheld if ETCO2 decreased to less than 25 mm Hg. The contralateral common carotid artery was occluded after the MAP had been decreased to approximately 40–50 mm Hg. The hypotensive drug boluses were administered to decrease MAP to 15–30 mm Hg to produce electrocerebral silence lasting 10 min (Fig. 2). After 10 min of electrocerebral silence, the carotid occlusion was released, and the hypotensive drug administration was stopped. The MAP increased to approximate baseline values. IV phenylephrine (10–20 μg) was given if it was required to increase MAP. During reperfusion, as shown in Figure 1, repeated 0.1-mL boluses of intracarotid normal saline, propofol (1%), or thiopental (1%) were injected in each rabbit to maintain electrocerebral silence. We used intracarotid saline injections to provide us with control data because carotid catheters are routinely flushed with saline during clinical use to decrease the risk of thromboembolism. The recovery of physiological data was recorded for different periods of time that ranged from 30 min to 8 h after reperfusion. The rabbits were killed at the end of this period of observation. Rabbits that became hemodynamically unstable within 30 min of reperfusion, i.e., MAP less than 50 mm Hg, were excluded from the protocol.
Hemodynamic and CBF variables for each drug were evaluated at three points: (a) at baseline; (b) during ischemia; and (c) during reperfusion when there was a peak increase in CBF. The data are presented as mean ± sd. Data were analyzed by repeated-measures analysis of variance. Bonferroni-Dunn post hoc test to correct for multiple comparisons was undertaken to determine significance. A P value of <0.0167 was considered significant for the 3 repeat measures. Comparisons among the drugs were undertaken by factorial-analysis of variance. P < 0.05 was considered to be significant. Data were analyzed by Statview 5.0® software (SAS Institute Inc, Cary, NC).
The study was conducted on 27 adult New Zealand white rabbits. Eight rabbits were used for the preliminary studies to establish the dose of intracarotid thiopental and propofol. Nineteen rabbits were used for occlusion-reperfusion studies.
Intracarotid infusion of thiopental and propofol were conducted on eight rabbits (four with each drug). The initial dose required to produce electrocerebral silence with propofol was 3 ± 1 mg and with thiopental was 3 ± 2 mg, and the total dose required was 15 ± 3 mg and 14 ± 5 mg, respectively. The number of bolus injections required to maintain electrocerebral silence was 12 ± 3 and 11 ± 5 with propofol and thiopental, respectively. No significant changes in CBF, MAP, and ETCO2 occurred during the injection of anesthetics. Based on these preliminary dose requirements, we selected an initial volume of 0.3 mL and a maintenance dose of 0.1 mL injected every minute to be sufficient to produce EEG silence during occlusion-reperfusion studies. Additional doses of the anesthetic drugs were given if any electro-cerebral activity was evident during reperfusion.
Nineteen rabbits were used in this arm of the study. Data from three rabbits were excluded from the final analysis. One rabbit had sustained hypotension during reperfusion. In the other two, there was a technical failure of the laser Doppler probes caused by hemorrhage under the probe, in one case, and its displacement from the site of measurement in the other. Of the 16 rabbits, during reperfusion, five received intracarotid saline, six received intracarotid propofol, and another five received intracarotid thiopental. In all the three groups, MAP, ETco2, and CBF decreased with occlusion and systemic hypotension. There was no difference among the 3 groups with regard to MAP and CBF at baseline and during cerebral ischemia (Table 1). Compared with the ischemic state, MAP and CBF showed a significant increase during reperfusion (Fig. 3). However, neither the ipsilateral CVR (Table 1) nor percent-change in CBF showed any difference with respect to individual drugs or saline.
The etiology of early cerebral hyperemia remains obscure. Both vascular and neuronal mechanisms have been implicated in its etiology (15,16). We hypothesized that if neuronal activity was predominantly responsible for early postischemic cerebral hyperperfusion, then EEG silence induced by intracarotid anesthetic infusion would attenuate the hyperemic response. However, results of this study suggest that intracarotid injections of both propofol and thiopental fail to attenuate the increase in CBF despite producing electrocerebral silence during reperfusion. Our results suggest that vascular, and not neuronal, mechanisms are probably responsible for the early increase in CBF after transient cerebral ischemia.
Early cerebral hyperemia is characterized by an increase in blood flow within 10–15 minutes of cerebral reperfusion (17,18). During early reperfusion, CBF is increased by as much as 400% (19). The magnitude of response seems to be a function of both severity and duration of ischemia (17,20). Trigeminal gangliotomy attenuates early cerebral hyperemia in cats, which suggests that neuronal activity, probably an axonal reflex, is partly responsible for early cerebral hyperemia (11,21). However, there is evidence to support that vascular mechanisms are responsible for early cerebral hyperemia. For example, inhibition nitric oxide synthase blunts the hyperemic response (15). The concentration of guanylate cyclase (a marker of nitric oxide synthase activity) increases in the smooth muscle cells of cerebral arteries that are subjected to transient cerebral ischemia (2). However, stable cyclic adenosine monophosphate analogs seem to have a less profound effect in prolonging reperfusion (22,23).
Most studies suggest that, during electrocerebral silence, anesthetic drugs such as propofol significantly decrease CBF when given by the IV route (24). These studies demonstrated that the decrease in CBF was directly proportional to the decrease in cerebral metabolic rate. In contrast, in continuing studies we have observed that despite EEG silence, CBF is maintained with an intracarotid injection of anesthetic drugs (25). CBF measurements during the intracarotid injection of drugs have to be interpreted with caution. CBF in such situations can be influenced by a number of factors. These include: (a) the effect of ICA occlusion, (b) suppression of brain metabolism, (c) direct vasodilator effects of the drug, (d) the method of measuring CBF, (e) change in hematocrit during drug intracarotid injection, and (f) biomechanical effects of drug injection. However, in the present study, a change in LDF measurements revealed rapid a transient increase in flow during the intracarotid injection of saline. In addition, ICA occlusion and hypotension resulted in a significant decrease in CBF in all 3 groups. Thus, the failure to observe an attenuation of early postischemic hyperemic response is not due to a lack of sensitivity of CBF measurements.
One intriguing explanation for the failure of intra-carotid anesthetics to attenuate postischemic hyperemia is that after cerebral ischemia, flow and metabolism may be uncoupled (26). An overwhelming effect of ischemia on CVR could, in theory, prevent any anesthetic-mediated decrease in CBF. There is evidence to suggest that electrocerebral silence by intra-carotid anesthetics does not significantly decrease CBF (3). This raises the possibility that intracarotid anesthetics may themselves uncouple CBF and metabolism. Anesthetic drugs are known to have a direct effect on vascular endothelium and smooth muscle cells (27,28). The possibility of intracarotid anesthetics uncoupling flow metabolism in the brain merits further scrutiny.
The clinical significance of early postischemic hyperemia is controversial (29,30). In the setting of intraarterial thrombolysis, such an increase in blood flow could increase the risk of hemorrhagic transformation of an ischemic infarct. The administration of intracarotid anesthetics is particularly attractive in the setting of intraarterial thrombolysis because the catheter for drug delivery is already in place (31). There is already realization that additional pharmacological interventions will be required to optimize the benefits of restoring perfusion after thrombolysis (32). Compared with systemic administration, intraarterial injections have the advantage to rapidly deliver therapeutic drugs into the region of reperfusion injury. However, this study shows that intracarotid anesthetics do not attenuate early hyperemic response and thus, may not decrease the risk of hemorrhagic transformation.
We conclude that an intracarotid injection of anesthetic drugs, in doses that are sufficient to produce electrocerebral silence, do not obtund early cerebral hyperemia after transient cerebral ischemia. This suggests that vascular, and not neuronal, mechanisms are primarily responsible for early postischemic cerebral hyperperfusion. Acknowledgments
Acknowledgments are due to the following staff members of Columbia University: Jodi Wagman, Administrative Assistant in the Department of Anesthesiology, for her help in preparing the manuscript, and Richard Arrington, BA, Cert. AT, Manager of Anesthesiology Services, for helping with technical aspects of the experiments.
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© 2004 International Anesthesia Research Society
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