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NEUROSURGICAL ANESTHESIA

Electrocerebral Silence After Intracarotid Propofol Injection Is a Function of Transit Time

From the Department of Anesthesiology, College of Physicians and Surgeons of Columbia University, New York, New York.

Address correspondence to: Shailendra Joshi, MD, Department of Anesthesiology, P&S P Box 46, College of Physicians and Surgeons of Columbia University, 630 West 168th St., New York, NY 10032. Address e-mail to sj121{at}columbia.edu.

	Abstract

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BACKGROUND: We hypothesized that the duration of electrocerebral (electroencephalogram, EEG) silence after bolus injection of propofol, a highly lipid soluble anesthetic drug, during transient cerebral hypoperfusion, will be directly related to the time taken by the bolus of drug to transit the cerebral circulation.

METHODS: We randomly divided 24 New Zealand White rabbits into two propofol volume groups: 0.5 and 0.8 mL groups. In each group, 12 animals received two intracarotid injections of 1% propofol: the first injection was made under normal physiological conditions and the second injection during cerebral hypoperfusion produced by bilateral carotid occlusion and IV bolus injection of adenosine and esmolol. We determined the duration of electrocerebral silence and the transit time of propofol emulsion under both cerebral circulation conditions. The transit time was measured by videomicroscopy through an implanted cranial window.

RESULTS: Cerebral hypoperfusion increased transit time with both low (2.3 ± 0.7 to 55.7 ± 21.4 s, n = 12, P < 0.0001) and high (2.2 ± 0.6 to 62.5 ± 31 s, n = 12, P < 0.0001) bolus volumes. The duration of electrocerebral silence during cerebral hypoperfusion was a function of the transit time with low (electrocerebral silence s = 152 + 2.3 x transit time, n = 12, r = 0.73, P = 0.007) and high (electrocerebral silence s = 186 + 3.2 x transit time, n = 12, r = 0.68, P = 0.02) bolus volumes.

CONCLUSION: These results suggest that manipulation of the transit time of highly lipid-soluble drugs profoundly enhances the effect site delivery.

	Introduction

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Local drug delivery to the brain is complicated by the presence of multiple tissue barriers within the brain parenchyma, such as the blood–brain barrier (BBB), extracellular matrix, cell membranes of astrocytes and neurons. Drugs have to diffuse through these barriers to reach their target site of action, which may be the neuronal cell membrane, the cytoplasm, or the nucleus (Fig. 1). Structurally, neurovascular units in the brain are configured in such a way that small molecules are able to diffuse through the intercapillary space within 1 s, which is the normal brain capillary transit time through the cerebral circulation (1). We have shown that the duration of electrocerebral (electroencephalogram, EEG) silence after intracarotid propofol administration is a direct function of the cerebral blood flow (CBF), and that changes in CBF by ventilation, vasodilators, or by transient cerebral hypoperfusion directly affect the duration of electrocerebral silence with intracarotid propofol (2).

View larger version (50K): [in this window] [in a new window]   Figure 1. Factors affecting local drug delivery in the brain parenchyma.

However, intraarterial propofol could alter the CBF and cerebral blood volume (CBV) directly by its vasodilator effects, or indirectly by its central nervous system effects (3). We therefore investigated the effects of altering the cerebral transit time (CTT) and CBF on the duration of electrocerebral silence with intracarotid propofol.

Propofol is a highly lipid-soluble drug with an octanol–water solubility coefficient of approximately 6800. Its high lipid solubility, combined with its low molecular weight (280 Da) permits it to readily diffuse across the BBB and within tissue compartments within the brain parenchyma. In theory, bolus injection of propofol (97% plasma protein bound) avoids or minimizes protein binding, and will generate high free concentrations of the drug to enhance drug delivery to the brain. The effect of bolus propofol injection would be further enhanced if the bolus of the drug were injected during cerebral hypoperfusion, which increases the time required for the transit of propofol bolus through the cerebral circulation so as to augment the diffusion of the drug into the brain tissue. Hence, we hypothesized that the uptake of the drug after bolus injection will be a direct function of the time of contact between the bolus of the drug and the BBB. Increased bolus transit time through the cerebral circulation will permit a greater uptake of the drug, which will directly affect the duration of electrocerebral silence.

	METHODS

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After approval of the protocol by the Institution’s Animal Care and Use Committee, the study was conducted on 24 New Zealand White rabbits weighing approximately 1.5–2 kg. The animals were given full access to food and water before the experiment. The animals were sedated with IM ketamine (50 mg/kg) and anesthesia was maintained with IV infusion of propofol (Diprivan 1%, Astra-Zeneca Pharmaceuticals, Wilmington, DE) 2–3 mL · kg^–1 · h^–1. This level of anesthesia enables monitoring of electrocerebral activity over several hours. The experimental preparation consisted of tracheostomy, femoral arterial cannulation, isolation of the right internal carotid artery, placement of bilateral electroencephalogram leads, and bilateral skull shaving to the inner table, for the placement of laser Doppler probes to monitor CBF. The satisfactory isolation of the internal carotid artery on the side of drug infusion was confirmed by the retinal discoloration test (4). We then placed a silastic loop around the contralateral carotid, which enabled occlusion of the artery while the animal was still placed in the stereotactic frame. All surgical sites were infiltrated with 0.25% bupivacaine with epinephrine, and an esophageal temperature probe monitored the core temperature. The technical details of this preparation have been described in our earlier publications (2,5,6).

We implanted a cranial window, over the right temporal–parietal bone just across the midline so that we could directly observe the delivery of the intraarterial drug. Trephination (diameter 12 mm) was prepared over the right cerebral hemisphere, the dura mater was reflected, and a sterilized metal chamber with inflow and outflow channels was inserted into the cranial defect. The chamber was covered with a glass disk held in place with dental cement. The pial surface was suffused with mock cerebrospinal fluid (CSF) at a rate of 0.1 mL/min, and the outflow was adjusted approximately 3–4 cm above the window. The mock CSF contained sodium 150 mM, potassium 3 mM, calcium 1.4 mM, magnesium 0.8 mM, phosphate 1.0 mM, and chloride 155 mM (Harvard Bioscience, Holliston, MA). The dead space of the window and the connectors was ranged from 1.2 to 1.5 mL. Mock CSF was infused in the cranial window. Videomicroscopy was performed with green light that enhances the contrast, such that blood appeared black. Bolus injection of propofol appeared as a white column (7). Reperfusion appeared as an inflow of dark blood into the arteries and the veins (Fig. 2). Video images were captured by a custom-built microscope and archived with Archos 800 video data recorder at 29 frames/s for subsequent determination of transit time by counting frames by an observer blind to the dose of the drug.

View larger version (53K): [in this window] [in a new window]   Figure 2. Sequence of videomicroscopic images showing the transit of intracarotid propofol (0.5 mL) during cerebral hypoperfusion. A, Baseline image; B, shows the arterial phase; C, as cerebral blood flow returns, the drug is washed out of the artery but can still be seen in the veins; D, complete transit of the drug from the cerebral circulation.

The animals were randomized into two groups: propofol small dose and large dose (n = 12, respectively). For each group, they were further randomized to undergo propofol injection during cerebral hypoperfusion and noncerebral hypoperfusion groups i.e., normal physiological variables. Cerebral hypoperfusion was produced by bilateral carotid occlusion and IV bolus injection of adenosine (30 mg) and esmolol (20–30 mg). For each animal, the data were acquired at three points of time—baseline, propofol, and recovery.

Selection of Bolus Volumes
The intracranial blood volume of a rabbit is 0.2 mL (8). Our videomicroscopic studies have shown that an initial bolus injection of 0.5 mL is sufficient to displace blood in the cerebral arteries. We have measured the dead space of our catheter and the stopcock to be 0.2 mL, the dead space of the extracranial carotid is about 0.1 mL. Thus, a 0.5-mL initial bolus injection is sufficient to displace the intracranial blood volume. We elected to use two test volumes: 0.5 mL which delivered 0.2 mL (2 mg) of the bolus into the distal cerebral circulation and 0.8 mL which delivered a 0.5-mL (5 mg) bolus into the distal cerebral circulation. The bigger volume was used to test if we could increase transit time without manipulating blood flow.

Data Analysis
Unless stated otherwise, the data are presented as mean ± sd, in the text and in tables. The hemodynamic and laser Doppler flow data were recorded at three time points (baseline, drug injection, and before harvest). Data were analyzed by repeated measures of analysis of variance, a post hoc Bonoferroni-Dunn test, to correct multiple comparisons (three stages, treatments, or groups). Because of the correction for multiple comparisons, a P value of 0.0167 was considered significant for repeat measurements. The correlation between the transit time and electrocerebral silence was determined by simple regression.

	RESULTS

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All of the 24 animals enrolled for the experiments completed the protocol. The mean weight of the animals was not different in the low and the high bolus volume groups. The hemodynamic variables were comparable between the groups and within the groups with the two drug challenges, Tables 1 and 2. The electrocerebral recordings during electrocerebral silence after propofol administration were identical to those observed after death, i.e., a loss of detectable electrocerebral signals and a background noise level <10 µV (Fig. 3).

View larger version (14K): [in this window] [in a new window]   Figure 3. Onset of drug effect (electrocerebral silence) is virtually instantaneous after intraarterial injection, and thus does not affect the duration of silence with each group.

Small-Dose Group
Baseline data were comparable between the cerebral hypoperfusion and normal perfusion propofol challenge. Hemodynamic variables did not significantly change during the normal perfusion propofol challenges. However, when compared with baseline, transient cerebral hypoperfusion decreased mean arterial blood pressure (from 88 ± 13 to 21 ± 5 mm Hg, n = 12, P < 0.0001), heart rate (from 243 ± 26 to 95 ± 44 bpm, n = 12, P < 0.0001), and end-tidal carbon dioxide concentrations (from 38 ± 7 to 20 ± 7 mm Hg, P < 0.0001). CBF also was significantly decreased to about 27% ± 20% of the baseline. The transit time was increased from 2.3 ± 0.7 to 55.7 ± 21.4 s (P < 0.0001, n = 12). The duration of electrocerebral silence was also significantly increased from 52 ± 25 to 282 ± 68 s (P < 0.0001, n = 12) and electrocerebral silence was a linear function of the transit time, electrocerebral silence (s) = 152 + 2.3 x transit time, n = 12, r = 73, P = 0.007.

Large-Dose Group
Like the small-dose group, baseline data were comparable between the cerebral hypoperfusion and normal perfusion propofol challenge. Hemodynamic variables did not significantly change during the normal perfusion propofol challenges. However, when compared with baseline, transient cerebral hypoperfusion decreased mean arterial blood pressure (from 86 ± 10 to 23 ± 9 mm Hg, n = 12, P < 0.0001), heart rate (from 249 ± 17 to 64 ± 28 bpm, n = 12, P < 0.0001), and end-tidal carbon dioxide concentrations (from 35 ± 4 to 15 ± 6 mm Hg, P < 0.0001). CBF also was significantly decreased to about 26% ± 24% of the baseline. The transit time was increased from 2.2 ± 0.6 to 62.5 ± 31 s (P < 0.0001, n = 12). The duration of electrocerebral silence was also significantly increased from 68 ± 75 to 384 ± 143 s (P < 0.0001, n = 12), and electrocerebral silence was a linear function of the transit time, electrocerebral silence (s) = 186 + 3.2 x transit time, n = 12, r = 68, P = 0.02 (Fig. 4).

View larger version (10K): [in this window] [in a new window]   Figure 4. Bivariate scattergrams showing the correlation of transit time and the duration of electrocerebral silence with two doses of propofol injected during cerebral hypoperfusion.

	DISCUSSION

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The results of our experiment show that cerebral hypoperfusion increased the transit times with both of the intracarotid bolus volumes, 0.5 and 0.8 mL. The duration of electrocerebral silence during cerebral hypoperfusion was a function of the transit time in each case. These results suggest that manipulation of transit time of highly lipid-soluble drugs profoundly affects site delivery.

This study is the first attempt to track drug delivery to the brain by direct microscopy. However, it should be noted that videomicroscopy reveals the transit time of the propofol emulsion through the cerebral circulation, and not that of the active drug. Diisopropyl alcohol, the active drug in propofol, is a clear oil soluble compound that is invisible under the microscope. Therefore, we could estimate the transit time of the propofol emulsion only. Although, in theory, the rate of propofol release from its emulsion could also affect the local drug uptake (9), to our best knowledge, there is no data that describes the rate of release of propofol from its lipid formulation or whether the extent of such a release occurs in a single pass through the cerebral circulation after intraarterial bolus injection (10,11). Regarding the experimental design, the release of propofol from bolus injection would be common to both the cerebral hypoperfusion and the normal perfusion challenges.

Our previous experiments show how electrocerebral silence effects of anesthetic drugs can be substantially augmented by altering blood flow (2,12). We now show that the increase in the duration of electrocerebral silence during cerebral hypoperfusion is a function of CTT. This supports our hypothesis that the increased duration of contact between the drug and the brain tissue will result in higher tissue concentration because of greater diffusion of the drug. CTT is a function of CBF and CBV and is given by the equation, CTT = CBV/CBF (12). Cerebral hypoperfusion decreases CBF and increases the CTT by increasing CBV via autoregulatory vasodilation. The key to the understanding of intraarterial drug delivery will depend on suitable stable animal models and the ability to measure drug concentrations in real time. Our previous studies with this animal model suggest that, even when the animals are observed for several hours after drug administration under a hypoperfusion protocol, there is no evidence of any brain injury. The preparation is able to withstand the hypotensive challenge well (6).

The redistributive half-life of many brain-selective drugs is often in the 2–5 min range, and therefore, intraarterial drug kinetics are beyond the time resolution of microdialysis (13). Tissue sampling provides only postmortem data. Multiple biopsies, though feasible, do not provide site-specific time histories. Isotopic studies, such as positron emission tomography, sometimes lack spatial resolution and are not feasible in the usual laboratory setting. The latter limitation also applies to magnetic resonance imaging-based assessment of drug kinetics. Our approach, therefore, focuses on the use of optical technology for assessing drug concentration measurements. Videomicroscopy provides only a crude assessment of tissue concentrations. However, emerging technologies, such as elastic scatter spectroscopy or optical pharmacokinetics, could be applied to the brain to precisely determine the tissue concentrations (14).

The electrocerebral silence effects of anesthetic drugs have been studied over the last 30 yr in the clinical setting (15). Some drugs such as propofol and etomidate, that have biphasic effects on electrocerebral activity, demonstrate a poor correlation between clinical signs and brain tissue concentrations (15). However, in our rabbit model, the propofol-induced burst suppression consists of a unique spiking pattern. This spiking pattern and the intermitted electrocerebral quiescence had a good correlation with propofol dose (2,4,5), which provided us an alternative tool to monitor effect-site concentrations.

In conclusion, we observed that the duration of electrocerebral silence after intracarotid injection of propofol during cerebral hypoperfusion was a function of transit time. Reduction of CBF by transient cerebral hypoperfusion considerably augments transit time to enhance intraarterial drug delivery to the brain.

	Footnotes

 
Accepted for publication March 6, 2007.

Supported in part by the National Institute of Health K08 00698 grant and the Irving Clinical Research Career Award (to S.J.).

Presented at the annual meeting of American Society of Anesthesiologist, and the Society for Neurosurgical Anesthesiology, Chicago, IL, October 18, and 13, 2006, respectively.

Reprints will not be available from the author.

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This Article Abstract Full Text (PDF) Videos Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Wang, M. Articles by Joshi, S. Search for Related Content PubMed Articles by Wang, M. Articles by Joshi, S. Related Collections Neuroanesthesia Video Clip Pharmacology