Comparative toxicity of glucose and lidocaine administered intrathecally in the rat.

BACKGROUND AND OBJECTIVES: Glucose is a common component of anesthetic solutions used for spinal anesthesia. However, its possible contribution to recent injuries occurring with spinal anesthesia has not been adequately addressed. Accordingly, the present studies compare the functional and morphologic effects of intrathecally administered glucose with those of lidocaine. METHODS: Twenty rats, implanted with intrathecal catheters, were divided into three groups to receive a 1-hour infusion of 5% lidocaine (n = 6), 10% glucose (n = 7), or normal saline (n = 7). Four days after infusion, animals were evaluated for persistent sensory impairment using the tail-flick test. Three days later, the animals were sacrificed, and the spinal cord and nerve roots were examined by a neuropathologist blinded to the solution received and the results of sensory testing. RESULTS: Lidocaine-treated animals exhibited persistent sensory impairment, whereas glucose- and saline-treated animals did not. Neuropathologic evaluation revealed moderate to severe nerve root injury in lidocaine-treated animals. Histologic changes in glucose- and saline-treated animals were minimal, similar, and restricted to the area adjacent to the catheter. Morphologic damage associated with lidocaine preferentially affected the nerve roots, with relative sparing of the spinal cord and dorsal root ganglia. CONCLUSIONS: These results suggest that, at clinically relevant concentrations, glucose does not induce neurologic injury, providing indirect evidence that recent clinical injuries occurring after spinal anesthesia resulted from a neurotoxic effect of the local anesthetic. Additionally, the present studies suggest that deficits resulting from neurotoxicity of intrathecally administered anesthetic result from injury to the axon.

Intrathecal catheterization in the rat. Improved technique for morphologic analysis of drug-induced injury.

BACKGROUND: The authors previously described an in vivo model suitable for investigation of functional impairment induced by intrathecally injected local anesthetic. However, meaningful histologic analysis could not be performed because catheterization, per se, induced morphologic changes in control animals. In the current experiments, the authors sought to identify an alternative, less reactive, catheterization technique for intrathecal drug administration. METHODS: Twenty-five rats received an intrathecal infusion of normal saline through a catheter composed of either 28-gauge polyurethane, 32-gauge polyimide, 32-gauge polyurethane, PE-10 polyethylene, or PE-10 polyethylene that had been stretched to twice its original length. Seven days after infusion, sensory function was assessed using the tail-flick test, and the spinal cord and nerve roots were prepared for neuropathologic evaluation. RESULTS: There was no significant difference in sensory function among groups. Animals in which 28-gauge polyurethane, 32-gauge polyimide, PE-10, and double-stretched PE-10 had been implanted had moderate to severe nerve injury in 11%, 14%, 23%, and 8% of fascicles, respectively, whereas none of the animals in which 32-gauge polyurethane was implanted had any evidence of moderate or severe damage. CONCLUSIONS: Morphologic changes induced by intrathecal catheterization in the rat can be minimized by the use of 32-gauge polyurethane tubing.

Cellular uptake of albumin from lungs of anesthetized rabbits.

Resolution of alveolar edema depends on clearance of serum protein, as well as liquid from the alveolar space. Protein clearance is slower than liquid clearance and may take days to weeks. Our earlier studies presented evidence for the importance of paracellular removal of soluble protein from the air spaces. However, long-term protein clearance may also depend on uptake by alveolar epithelial cells or macrophages. This study examined cellular uptake of soluble human albumin and insoluble colloidal gold-albumin from the lungs of anesthetized rabbits. Native albumin was endocytosed by both alveolar type I and type II cells and appeared in vesicles and endosomes. Neither cell type took up colloidal gold-albumin over periods as long as 8 h. Alveolar macrophages took up native albumin and colloidal gold-albumin to a greater extent and more rapidly than alveolar epithelial cells. The tracer proteins were found in vesicles, endosomes, and phagolysosomes. Similarly, cultured alveolar macrophages took up native albumin more rapidly than cultured type II cells. Thus macrophages may be important in clearing precipitated protein from the air spaces, and they may have a role in completing the clearance of soluble protein. The potential for transepithelial transport of soluble alveolar protein exists, but based on this work and our prior studies, it appears to be a low-capacity pathway.

Local anesthetic neurotoxicity does not result from blockade of voltage-gated sodium channels.

To investigate whether local anesthetic neurotoxicity results from sodium channel blockade, we compared the effects of intrathecally administered lidocaine, bupivacaine, and tetrodotoxin (TTX), the latter a highly selective sodium channel blocker, on sensory function and spinal cord morphology in a rat model. First, to determine relative anesthetic potency, 25 rats implanted with intrathecal catheters were subjected to infusions of lidocaine (n = 8), bupivacaine (n = 8), or TTX (n = 9). The three drugs produced parallel dose-effect curves that differed significantly from one another: the EC50 values for lidocaine, bupivacaine, and TTX were 28.2 mM (0.66%), 6.6 mM (0.19%), and 462 nM, respectively. Twenty-five additional rats were then given intrathecal lidocaine (n = 8), bupivacaine (n = 8), or TTX (n = 9) at concentrations 10 times the calculated EC50 for sensory block. Lidocaine and bupivacaine induced persistent sensory impairment, whereas TTX did not. Finally, 28 rats were given either intrathecal bupivacaine (n = 10) or TTX (n = 9) at 10 times the EC50, or normal saline (n = 9). Significant sensory impairment again occurred after infusion of bupivacaine, but not after infusion of TTX or saline. Neuropathologic evaluation revealed moderate to severe nerve root injury in bupivacaine-treated animals; histologic changes in TTX- and saline-treated animals were minimal, similar, and restricted to the area adjacent to the catheter. These results indicate that local anesthetic neurotoxicity does not result from blockade of the sodium channel, and suggest that development of a safer anesthetic is a realistic goal.

The addition of 7.5% glucose does not alter the neurotoxicity of 5% lidocaine administered intrathecally in the rat.

BACKGROUND: Recent reports of major and minor neurologic sequelae after spinal anesthesia have generated concern regarding the safety of some currently used intrathecal agents. The role of glucose, if any, in neurotoxic injury associated with spinal anesthesia is not known. The current experiments sought to determine whether the presence of 7.5% glucose alters the neurotoxicity of intrathecally administered 5% lidocaine. METHODS: Two experiments were performed. First, 48 rats were implanted with an intrathecal catheter and randomly divided into eight equal groups. Each animal received a single intrathecal infusion of 5% lidocaine (groups P1-P4) or 5% lidocaine with 7.5% glucose (G1-G4) for 0.5, 1, 2, or 4 h at a rate of 1 microliter/min. Sensory function was assessed using the tail-flick test; a deficit was defined as a complete lack of response to the heat stimulus at the proximal, mid or distal portion of the tail persisting 4 days after the infusion. In the second experiment, 60 rats were randomly divided into two groups to receive a 1-h intrathecal infusion of 5% lidocaine or 5% lidocaine with 7.5% glucose. Animals were evaluated for increase in the latency of the tail-flick reflex 4 days after infusion. RESULTS: In the first experiment, the two lidocaine solutions produced similar dose-dependent loss of sensory function. In the second experiment, the two solutions induced similar alterations in tail-flick latency. CONCLUSIONS: The presence of 7.5% glucose does not affect the potential of intrathecally administered 5% lidocaine to induce sensory impairment. These findings provide further support for the hypothesis that recent injuries after spinal anesthesia resulted from a direct neurotoxic effect of the local anesthetic.

Effect of endocytosis inhibitors on alveolar clearance of albumin, immunoglobulin G, and SP-A in rabbits.

Protein in the alveolar space may be cleared by endocytosis and degradation inside alveolar epithelial cells, by transcytosis across the alveolar epithelium, or by restricted diffusion through the epithelium. The relative contributions of these three pathways to clearance of large quantities of protein from the air spaces is not known. This study investigated the effects of monensin and nocodazole, agents which inhibit endocytosis in cell culture, on alveolar epithelial protein transport in anesthetized rabbits. There was evidence that monensin and nocodazole inhibited endocytosis by the alveolar epithelium in vivo. Nocodazole increased the number of vesicles in the alveolar epithelium and capillary endothelium. Monensin increased vesicle density in the endothelium. These results suggested that the inhibitors disrupted microtubules or interrupted cellular membrane traffic in the lung. Both inhibitors decreased lung parenchymal uptake of immunoreactive human albumin from the air spaces. Monensin and nocodazole inhibited albumin uptake in cultured alveolar type II cells. Monensin increased the amount of 125I-labeled surfactant protein A associated with the lungs, compared with the quantity remaining in the air space 2 h after instillation. Although the drugs decreased alveolar epithelial protein uptake, they did not decrease alveolar clearance of 125I-labeled immunoglobulin G or 131I-labeled albumin in anesthetized rabbits. Thus monensin- and nocodazole-sensitive protein-uptake pathways do not account for most alveolar protein clearance when the distal air spaces are filled with a protein solution.

Persistent sacral sensory deficit induced by intrathecal local anesthetic infusion in the rat.

BACKGROUND: Several cases of cauda equina syndrome after continuous spinal anesthesia have been recently reported. One possible etiology is toxic exposure of the sacral roots resulting from intrathecal maldistribution of a relatively large dose of local anesthetic. The current experiments sought to determine whether a local anesthetic solution, injected intrathecally to produce a restricted distribution of anesthesia, could result in a sacral deficit. In addition, we sought to test the hypothesis that, when equal volumes are administered intrathecally, significant differences exist in the potential to three commonly used anesthetic solutions to induce sensory impairment. METHODS: Thirty-two rats were implanted with intrathecal catheters to permit repetitive infusion of local anesthetic. Animals were randomly assigned to four groups of eight to receive either 5% lidocaine with 7.5% dextrose; 0.75% bupivacaine with 8.25% dextrose; 0.5% tetracaine with 5% dextrose; or normal saline. Each rat received, in sequence, a 1-h (60 microliters), a 2-h (120 microliters), and a 4-h (240 microliters) infusion; the infusions were separated by a 4-day rest period. Sensory function was assessed using the tail-flick test, which was performed immediately before each infusion and 6 days after the last infusion by an investigator blinded to the solution infused. RESULTS: There was no significant difference in baseline tail-flick latencies for the four groups. Tail-flick latency for the lidocaine group was significantly prolonged when compared with the bupivacaine, tetracaine, and saline groups. This difference was apparent after the first infusion and persisted throughout the study. CONCLUSIONS: In the rat, restricted anesthetic distribution can be achieved, and sensory impairment may result. These findings further support an etiology of local anesthetic neurotoxicity for recent clinical injuries after continuous spinal anesthesia. The functional model described appears to be suitable for in vivo study of local anesthetic neurotoxicity.