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RAJ test....what we don't know about it ?


The following is a brief description of the Raj test: when nerve stimulation is being used to locate a nerve, a twitch is observed when the needle tip is close to the neural target. Ideally, the twitch is required to persist at a current of 0.5 mA. The clinician then injects a small volume of local anesthetic or normal saline through the needle. If the needle tip is in the correct location, the muscle twitch immediately disappears.

 Until very recently the disappearance of the twitch was thought to be caused by physical displacement of the nerve by the injectate.

 We recently learned that this mechanism is best explained in electrical terms and is not entirely a result of the physical dis- placement of the nerve.In a porcine model, the injection of 0.9% sodium chloride solution (NaCl) abolished the motor response, and a subsequent injection of 5% dextrose reestablished a motor response during peripheral nerve stimulation.

 An accompanying in vitro experiment showed that injections of solutions, such as 0.9% NaCl, cause a change in the electrical field at the needle tissue interface. 

 It was concluded that the injection of electrically conducting solutions (saline or local anesthetic) increases the conductive area surrounding the stimulating needle tip, leading to a decrease in the current density surrounding the target nerve. The current density surround- ing the needle tip is then no longer sufficient to stimulate the desired nerve.This observation suggests that effective nerve stimulation is sensitive to changes that occur at the needle tissue interface, such as the angle of the needle or the injection of the local anesthetic. The net effect of these changes is to alter the current density at the tip of the needle or the path of the electric current, ultimately resulting in a change in the quality of the motor response.

 Next part in the next post....
 
 
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