Estimation of Physiological Impedance from Neuromuscular Pulse Data.

Introduction: A Conducted Electrical Weapon (CEW) deploys 2, or more, probes to conduct current via the body to induce motor-nerve mediated muscle contractions, but the inter-probe resistances can vary and this can affect charge delivery. For this reason, newer generation CEWs such as the TASER X3, X2 and X26P models have feed-forward control circuits to keep the delivered charge constant regardless of impedance. Our main goal was to explore the load limits for this "charge metering" system. A secondary goal was to evaluate the reliability of the "Pulse Log" stored data to estimate the load resistance.

Methods: We tested 10 units each of the X2 (double shot), X26P, and X26P+ (single-shot) CEW models. We used non-inductive high-voltage resistor assemblies of 50, 200, 400, 600, 1k, 2.5k, 3.5k, 5k, and 10k Ω, a shorted output (nominal 0 Ω), and arcing open-circuits. The Pulse Log data were downloaded to provide the charge value and stimulation and arc voltages for each of the pulses in a 5 s standard discharge cycle.

Results: The average reported raw charge was 65.4 ± 0.2 µC for load resistances < 1 kΩ consistent with specifications for the operation of the feed-forward design. At load resistances ≥ 1 kΩ, the raw charge decreased with increasing load values. Analyses of the Pulse Logs, using a 2-piece multiple regression model, were used to predict all resistances. For the resistance range of 0 - 1 kΩ the average error was 53 Ω; for 1 kΩ - 10 kΩ it was 16%. Muzzle arcing can be detected with a model combining parameter variability and arcing voltage.

Conclusions: The X2, X26P, and X26P+ electrical weapons deliver an average charge of 65 µC with a load resistance < 1 kΩ. For loads ≥ 1 kΩ, the metered charge decreased with increasing loads. The stored pulse-log data for the delivered charge and arc voltage allowed for methodologically-reliable forensic analysis of the load resistance with useful accuracy.