An ECG (or EKG) amplifier is a multi-input amplifier with from three to 12 inputs. Each input of the amplifier corresponds to an electrode that is placed on the skin to detect the electrical activity of the heart. One of these electrodes will act as the signal-ground reference point (usually the right leg electrode). A patient monitor will use either three or five electrode inputs, while an ECG recorder will use five or 10 electrode inputs.
Terminology regarding inputs, which have leads, and data outputs, which are also called leads, can get confusing. There are also limb leads and modified limb leads. Limb leads have the electrodes placed on the limbs, typically on the wrists and just above the ankles, and generally are used with diagnostic ECG recordings. With modified limb leads the electrodes are placed on the shoulders and just above the patients waist. They are used for monitoring in an ICU setting. To further confuse, there are augmented leads, which use resistors that are switched into the inputs of two of the physical limb leads. The resulting voltage is then amplified.
The augmented leads, aVR, aVL and aVF, are used for both diagnostic and comparison purposes by cardiologists.
The V or C lead obtains the voltage from the heart at six specific points on the chest, these locations are clearly shown in most manuals for ECG recorders.
All inputs are isolated from the power supply of the amplifier, usually with an isolation transformer. This prevents any power-supply fault from sending voltage directly to the electrodes, potentially giving a fatal electrical shock to the patient. Each input has a diode, resistor, or spark-gap circuit that will short any high-voltage/high-current pulses to ground. This prevents the amplifier from being damaged. These pulses come from defibrillators and electrosurgical devices used on the patient being monitored. The input impedance of an ECG amplifier is most often listed as ohms, typically 100 MegOhms.
Amplifiers, both those used for monitoring and those used for recording, have a switching mechanism that selects the waveforms (lead) to be displayed. The switch may be rotary, push button, or a flat-panel button. On some recorders there is an automatic button that switches the output through all five, 12, or 14 leads depending upon the mode. On some units there is a calibrate position on the switch, which has to be selected if the 1 mV signal is to be displayed. The 1 mV calibration signal is used to confirm the gain of the amplifier and is a good way to do a quick check to see if the amplifier is functioning properly. If the output shows the 1 mV signal then the amplifier is working properly. This signal can also be used to check the communications between a bedside and central monitor in an ICU without connecting a simulator to the amplifier.
The standard voltage gain of an ECG amplifier is 1,000. The typical ECG input signal is 1 mV, which means that the output is 1 volt. This translates to a 1 cm defection on the screen or chart. Remember that this is ideal, and very few conditions in a human body are ideal. The amplifier may have an automatic gain-control circuit to assure that 1 mV becomes 1 cm. There may also be a switch where the gain can be manually selected. The switch will generally have settings of 0.25 (250 V gain), 0.5 (500 V gain), 1.0 (1,000 V gain, the standard), 3.0 (3,000 V gain), and 5.0 (5,000 V gain).
As with any amplifier, saturation can become a problem, even at the lowest gain setting. This is common when the patient is small and has a very strong heart. Neonates and thin athletic adults are typical examples. A distorted waveform, usually with some squared-off peaks or valleys will be seen when an amplifier is saturated.
There also can be a voltage offset caused by the electrodes placed on the patient. This offset voltage can move the baseline up or down and can cause saturation of the amplifier. To correct the problem the electrode with the high offset voltage has to be located and repositioned or replaced. This is not a major problem now that single-use electrodes have replaced the old reuseable plates and cups.
Basically the electrodes act like a battery putting more DC voltage into the amplifier input. Stainless-steel electrodes have high offset voltages and should be avoided.
ECG amplifiers have two selectable frequency responses, monitor and diagnostic. The monitor frequency response is for long-term observation of a patients ECG, as in an ICU setting, and is typically 0.5 Hz to 35 Hz. This number may vary between manufacturers and may be as high as 50 Hz. The monitor frequency response is not to be used for diagnosing a patients cardiac activity. The diagnostic frequency response is from 0.1 Hz to 100 Hz, with a notch filter that prevents noise from the power lines (60 Hz in the Americas and 50 Hz in the rest of the world, with some exceptions) from being amplified and possibly distorting the waveform. The diagnostic frequency response is used on ECG recorders but will sometimes be included in an ICU monitor.
The upper number in the frequency response (35 or 100) is also called the 3 dB point of an amplifier, which is the frequency at which the gain of an amplifier is reduced by half. (In a stereo system the 3 dB point is generally 20,000 Hz.)
Electrodes are placed on or in the patient to detect the electrical voltages generated by the heart as it goes though a cardiac cycle. These electrodes range from short- to long-term contact with the patient. Problems can occur when the wrong electrodes are used for a particular application. Also, the type of metal or conductive material used in an electrode can react with certain patients and create noise or offset voltage problems.
Types of electrodes
Monitoring electrodes are single-use electrodes constructed with a central column of electrically conductive material surrounded by a plastic foam or paper tape to hold the conductive column in place. At the top of the conductive column is a snap to which the lead wire is attached. The lead wire is connected via the patient cable to an input of the ECG amplifier. If the conductive column dries out, the trace from that electrode will be noisy and the electrode will need to be replaced. Electrodes used with stress units should have a strong adhesive for stability during extensive patient movement during the exercise.
Single-use recording electrodes come in two styles: one that is constructed similarly to the monitoring electrode with a conductive gel center; and one that is constructed with conductive adhesive. The conductive adhesive electrode uses an alligator clip to connect the silver-plated backing of the adhesive to the patient cable. Not all clips are manufactured identically, and sometimes the use of the wrong alligator clip will result in a bad connection. On patients who are perspiring, these conductive adhesive electrodes may not stick well and may cause noise.
The correct positioning or placement of the electrodes on the patient is a critical step. If not done properly, it can compromise the quality of the diagnostic information. Often this results in the clinician incorrectly believing that the equipment has malfunctioned. Points to remember:
- Follow the manufacturers guidelines for electrode placements.
- Electrodes should not be place on scar tissue.
- Electrodes should not be placed over a lot of body hair. It affects the information and is painful when the electrodes are removed.
- Electrodes placed closer than 2 in from one another might cross talk.
- If electrodes from more than one device are required it is best to have each device in a different lead configuration, such as leads I, II, and III. This will reduce the potential of cross-talk between electrodes. The clinical need for the various electrodes should be challenged. Explain the probability of cross talk between the devices and how it could affect the clinical outcome of the procedure.
After the electrode, the weakest link in an ECG acquisition system is the lead wire connection between the electrode and patient cable. While this is a multiuse wire, the attachment system to the electrode can become loose or corroded, which may introduce noise into the system. Sometimes the connection to the patient cable is also affected. If a lead wire is found defective it should be discarded. Before discarding the lead wire be sure to first pull off one end of the wire so it will not be accidentally used again.
David Harrington, PhD, is director of staff development and training at Technology in Medicine Inc in Holliston, Mass. Contributing to this article is Robert L. Moorey, CRES, TiM regional director in Virginia.