50Hz Interference : the patient's body acts as can an antenna picking up several volts of 50 Hz noise from the power line. Differential amplifiers are the key separating the 1 mV E.C.G. signal wanted from 50Hz. Further reduction of this interference is possible with the help of high input impedance amplifiers and proper electrode techniques. 2. Isolated inputs : electrodes internal to the body give rise to potential hazards. Isolated inputs prevent dangerous currents from flowing directly through the patient to ground, should he come into contact with 50Hz AC power. Even micro-shocks caused by leakage currents as low as 50 micro-amps are prevented. These can cause ventricular fibrillation when applied directly to the heart. 3. Electrosurgery interference : electrosurgery machines generate high frequency signals with amplitudes of several hundred volts. This interference is minimized by low-pass filtering, shielding of the input circuitry and using proper monitoring and electrosurgery techniques. (Electrodes, cables, monitor and electrosurgery machine positioning). 4. Patient burns protection :isolated inputs do not protect against high frequency eleectrosurgery currents. Burns occur if the normal return path is interrupted and the current flows to ground through the monitor. Burns can be prevented by making sure that the patient is in intimate contact with the return patient plate, by using high frequency blocks (chokes) close to the E.G.C. electrodes sites and by using isolated-output electrosurgery units. 5. Monitor protection against damage from defibrillator pulses and electrosurgery currents : very sensitive input amplifiers must be able to survive up to 6,000 volt levels. 6. Battery powered monitors have a unique problem : Lack of a connection to ground. Whereas this is most often considered a safety advantage, it can be a hazard if the patient or operator becomes the link from the equipment to ground for a dangerously high current. A very expensive cabinet design could eliminate the above hazard and make the battery powered monitor the ideal operating room instrument.
