This is the recording and display of cardiac electrical activity. First performed in 1887.
Potentials from the heart are transmitted through the tissues and can be detected by electrodes which produce an ECG recording. Silver and silver chloride forms a stable electrode combination. They are separated from the skin by a foam pad soaked in conducting gel. The ECG signal is boosted by an amplifier, which also filters out noise. The amplified ECG signal is then displayed on an oscilloscope.
This mode has a frequency response of 0.5-40 Hz. All ECG monitors use filters to narrow the bandwidth in an attempt to reduce environmental artefacts. The high-frequency filters reduce distortions from muscle movement, mains current and electromagnetic interference from other equipment. The low-frequency filters help to provide a stable baseline by reducing respiratory and body movement artefacts.
This mode monitors the ST-segment and there is a greater need for filtering of the signal. Thus, there is a wider frequency response range of 0.05-100 Hz. The high-frequency limit of 100 Hz ensures that tracings allow assessment of QRS morphology and tachyarrhythmias. The low-frequency limit allows representation of P and T wave morphology and ST-segment analysis.
Lead II is best for detecting arrhythmias.
CM5 detects 89% of ST-segment changes due to left ventricular ischaemia. (Right arm electrode on manubrium, left arm electrode on V5 and indifferent lead on left shoulder).
CB5 is useful in thoracic anaesthesia. Right arm electrode over the centre of the right scapula and left arm electrode over V5.
Sources of error
Electrical interference. This is the distortion of a biological signal by capacitance effects or inductance effects. Any electrical device, powered by AC, can act as one plate of a capacitor and the patient acts as the other plate. This may cause a current with AC frequency to flow in the ECG leads. Interference may also result from high frequency diathermy. Shielding of cables and leads, differential amplifiers and filters help to reduce such interference. The shielding consists of woven material which is earthed. Interference currents are induced in the metal screen and not in the monitoring leads. The screening layer may often be covered by a second layer of insulation. Shivering can produce artefacts. Thus, aim to place electrodes over bony prominences.
The differential amplifier measures the difference between the potential from two different sources. Hence, if there is interference common to the input terminals (e.g. mains frequency) it can be eliminated, since it is only the difference between the terminals that is amplified by the differential amplifier. This is known as common mode rejection. The ratio of the output signal amplitude to the input signal amplitude is known as the gain of the amplifier.
[i] Value of the bipolar lead CM5 in electrocardiography.
Quyyumi AA et al.
Br Heart J 1986; 56: 372-6
[ii] What clinicians should know about the QT interval.
JAMA 2003; 289: 2120-7