When the ECG was taken with the 12-lead automated machine, the electrodes, which were approximately 1-inch square, were placed on the patients limbs significantly removed from the entry point of the CAVHD tubing. In fact, the piezoelectric signal could be regarded as common mode interference considering its relative distance from any but the V-lead chest electrodes. This would allow the high common mode rejection ratio (CMRR) of the ECG amplifier to reject this interference. The large, square area of the electrodes used by the12-lead ECG machine also assured a lower impedance connection to the patient than was possible with the monitoring electrodes. This situation would also
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tend to reduce the apparent amplitude of the interfering signal. After much judicious skin prep and repositioning of the ECG electrode, we were able to safely monitor the patient.