Hazards in the use of low frequencies for the measurement of physiological events by impedance

Experiments were conducted in man and the dog to determine the safe levels of sinusoidal alternating current which can be passed through the thorax. The sensation current threshold was determined over a frequency range of 20 Hz to 20 kHz in 8 normal adult males for two different electrode arrangements. The thresholds for vagal stimulation and ventricular fibrillation in the frequency range of 20 Hz to 3 kHz were determined in the dog for the same electrode arrangements, and the data obtained for fibrillation was extrapolated to estimate the comparable levels for man. The results indicate that the magnitudes of the current thresholds to produce these phenomena increase with an increase in frequency. Currents higher than 5 kHz in frequency are recommended for the measurement of physiological events by electrical impedance techniques.