Defibrillators often use an ac measurement to ensure safe electrode placement. Some defibrillators also utilize impedance measurements for ventilation monitoring. This paper investigates how such measurements can be optimized for high respiration sensitivity and finds that it is beneficial to add two extra electrodes in addition to the two defibrillator electrodes. This four electrode setup increases sensitivity and also allows respiration measurements at lower frequencies than the two electrode setup.

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Abstract. This paper reports the design improvement of a circuit for monitoring the quality of contact of un-gelled stainless steel electrodes with a patient's skin in the measurement of the ECG. The circuit is intended to be incorporated into a heart rate monitor used prior to and during childbirth to measure the mother's heart rate. The revised design uses commercially available low-power technology to implement the circuit with a reduction in the power consumption by a factor of 20 over a previous design [1]. The circuit measures the quality of contact of electrode impedances up to 100kΩ and gives a visual indication when the contact impedance of either electrode is above this level. The impedance is measured using a signal injected through the electrodes via the input impedance of the amplifier at a frequency of 5kHz. The resulting signal present at the amplifier input at this frequency is then extracted and processed before threshold detection of the signal level which is used to indicate whether or not the quality of contact of the patient's fingers with the electrodes is acceptable for the purposes of heart rate measurement.

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