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There may be an electrical potential across a pair of these electrodes equal to the algebraic difference of the two half-cell potentials, called the offset potential. Electrode-related specific problems include excessive offset potential and polarization (buildup of electrical charge at its base plate as a result of current flow). Impedance imbalance between the paired electrodes and movement of the electrodes can significantly distort or eliminate the electrocardiographic signal. Main power line (50/60 Hz), energy radiation from other electrical devices, and electromagnetic and radiofrequency interference can enter, via broken or poorly shielded leads. The output of electrocardiographic electrodes and their leads are amplified, filtered, and displayed by a variety of electronic devices to construct an electrocardiographic display or recording. The performance of an amplifier is defined by its gain (ratio of output signal amplitude to the input signal amplitude), which for routine electrocardiography is 1,000. The frequency range over which the amplifier accurately amplifies (bandwidth) should encompass 0.5– 100 Hz, as required by the American Heart Association standard. The electrocardiographic signals must be amplified without including the many other electrical noise signals in the circuit, so as to minimize the signal-to-noise ratio. This is achieved with a differential amplifier, which detects the difference in potential between the two active electrodes and attenuates those signals common to both electrodes. The common-mode rejection ratio is the ratio of differential voltage gain and the common-mode voltage gain and is a measurement of capability to reject the “noise.” Nevertheless, differences in the electrode impedances and stray currents through the patient, cables, and monitor can transform a common-mode voltage into a false differential signal that cannot be suppressed, even by an infinitely high common-mode rejection ratio. Instrumentation for electrocardiographic recording includes high- and low-frequency electronic filters designed to minimize artifact while preserving the integrity of the signal. “Notch” filters reduce interference due to mains frequency range 50/60 Hz. However, a manufacturing error led to an uncommon occurrence of 60-cycle interference manifesting in several operating rooms. The notch filter was adjusted for a foreign alternating current voltage frequency in cardiac monitors destined for the US market. An analog-to-digital converter allows the digitization of continuous analog signals into binary bits. This permits various digital filtering and pattern recognition algorithms used for subsequent processing, to operate in real time. Time- and temperature-related drift of components should be minimal. Patient and ground leakage current should be as per the national electrical safety standard. The maximum patient leakage current for electrocardiographic monitors (type CF equipment) is 0.01 mA in the normal condition and 0.05 mA for the single-fault condition. For proper display, appropriate setting of gain, display size, and sensitivity controls are needed. Conversely, wrongly set gain and display size may lead to T waves being counted as QRS complexes, leading to erroneous display of heart rate. It is only when all of these factors are carefully examined and realized that reliable interpretation of a consistent and quality electrocardiographic signal allows appropriate clinical decision making.
