Basic Science (25)

Abstract

Motor blockade by brachial plexus block in the sheep. (Université de Rennes, Rennes, France) Anesthesiology 2000;93:292–294.This report aimed to evaluate the approach of the brachial plexus block in a sheep model using bupivacaine. Blocks were performed with electrical stimulation using a 50mm insulated needle. When muscle twitches were elicited with 0.5‐mA impulses, drug solutions were injected over a 1‐min period. Nerve stimulation and injection of methylene blue were performed in two sheep that were killed 2 h later for postmortem anatomic control of setting injection. Twenty‐three nerve stimulations were achieved; the brachial plexus was reached in less than 5 min and within 5‐cm depth (range, 1.5–4.5 cm). Sheep cadaver dissection showed no visible vital structure in close proximity to the site of injection. Dissection records confirmed that the brachial plexus anatomy of sheep was close to that of humans. An optimal distribution of contrast medium was obtained with 30‐mL volume. Usually 1–2 min was sufficient to recover from anesthesia and to allow motor evaluation. No apparent sign of cardiac or central nervous system toxicities were recorded with any dose (0.5–4.3 mg/kg). All animals demonstrated a complete recovery, and no sequelae were recorded. Conclude that the brachial plexus block was performed easily in sheep and produced a motor block that varied in a dose‐related fashion. This model may be useful for assessing regional anesthesia of new drugs and/or new drug delivery. Comment by James E. Heavner, DVM, PhD.Two questions ran through my mind as I read this manuscript. One, is there a need for an animal model for brachial plexus block, the other, is the sheep the model of choice? The investigators state that “laboratory animal studies are necessary models for the assessment of new drugs and/or drug delivery systems used in regional anesthesia.” They indicate that with the exception of sciatic nerve block in rats, there are few data in the literature. This does not directly answer the question of whether or not there is a need for an animal model for brachial plexus block evaluation. To be positive, assume there is a need and then move to the question of whether or not the sheep would be the appropriate model. The investigators describe variations between the sheep and humans with respect of the origin of the brachial plexus and the phrenic nerves. What is not stated is that the gross anatomical morphology of humans and sheep are quite different, which may or may not be significant. Again to be positive, assume that the difference is not significant. If we accept that there is a need for an animal model and that the sheep is a suitable model, then we must ask what is it we want to do with the model? It would have been helpful if the investigators provided some suggestions for future studies. While recognizing the importance of animal models in advancing the science and art of regional anesthesia, I believe there is limited need for an animal model to evaluate brachial plexus block and have reservation regarding the utility of sheep as a model for such studies. I believe that most inquiries regarding brachial plexus block can be conducted safely on humans.