Embedded bioimpedance devices

The optimal output characteristic is linked with the very variable load resistance. Tissue resistivity increases when coagulated. Fat has higher resistivity than muscles and blood, and the contact geometry is very dependent on the electrode chosen and the way it is held by the operator. If constant amplitude current is chosen, power would be proportional to load resistance, and tissue would quickly be carbonized in high resistance situations. If constant amplitude voltage were chosen, power would be inversely proportional to load resistance, and when tissue layers around the electrode coagulate, current stops flowing. Modern instrumentation therefore measures both 

The current path between the active electrode and the neutral plate will depend on the tissue conductivity volume distribution. The site of the surgical intervention is given, but the site of the neutral electrode can be more freely chosen for optimum safe delivery. 

Controlling Critical Coagulation Processes in Vessel Sealing (LigaSure) 

Safety Monitoring of the Neutral Return Plate Function 

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Thus the bioimpedance participation concerns both functions and safety. It illustrates a typical feature often found with bioimpedance bioimpedance devices are embedded in larger instrumentation as rather anonymous parts. This is especially true because use of RF high power directly to patient tissue implies high risk factors imposing strict safety requirements. 

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