Counted cycles method to quantify the onset response in high-frequency peripheral nerve block

The clinical use of high frequency alternating current (HFAC) to block nerve conduction in peripheral nerves is limited due to the large volley of nerve activity generated at the initiation of HFAC. This “onset response” must be characterized in order to determine if it is possible to eliminate it. In this study, preliminary experiments were conducted in an in-vivo animal model using counted cycles of HFAC to investigate and quantify the onset response. Using this method, it is possible to show quantitatively that the onset response has two phases with distinct characteristics. Eliminating the onset response is likely to require addressing each phase independently. It was also possible to show that HFAC establishes a complete block of nerve activity in 50–100 ms.