A Review of Electrocardiograph Design

An electrocardiogram (ECG or EKG) is used to measure the electrical signals of the heart muscle over time and display the results in a graphical form. ECG applications range from simple heart rate monitoring to diagnosis of specific heart conditions. The test principle of ECG is the same in any application, but the design details and electronic component requirements vary greatly, from portable devices priced under $200 to desktop devices the size of a fax machine that cost more than $5,000. In some applications, ECGs are even embedded in other instruments, such as patient monitors and automated external defibrillators (AEDs), as shown in Figure 1.

Figure 1 ECG and blood oxygen readings displayed on a patient monitor

All ECGs collect ECG signals through electrodes connected to special parts of the body. The amplitude of ECG signals generated by the body is only a few millivolts. By connecting electrodes to specific locations on the body, ECG activity can be observed from different angles. Each location can be displayed and printed as an output channel of the ECG. Each channel represents the differential voltage between two electrodes or the difference between a certain electrode and the average voltage of several electrodes. Different combinations of electrodes can display more channels than the number of electrodes. These channels are generally called "leads" (or "channels"), and a 12-lead ECG device has 12 independent graphic display channels. Based on different applications, the number of leads can be selected between 1 and 12. The problem is that the wires connecting the electrodes are sometimes also called leads, which can easily cause confusion, because a 12-lead (12-channel) ECG only needs 10 electrodes (10 wires), so it is necessary to carefully judge the "leads" used.

In addition to biological signals, most ECGs detect two artificial signals, of which the implanted cardiac pacemaker (referred to as the "pace" signal) is the most important signal. The pace signal is quite short, ranging from tens of microseconds to several milliseconds, and the amplitude ranges from a few millivolts to nearly 1 V. Usually, the ECG must detect the presence of the pace signal at the same time to prevent interference with other ECG signals. The second artificial signal is used to detect "lead off", that is, poor contact of the electrode. Many ECGs need to issue an alarm indication when the electrode contact is poor. To do this, the ECG equipment generates a signal to measure the impedance between the electrode and the human body to detect whether the lead is off. The measurement signal can be AC ​​or DC, or both. Some ECGs can also detect the respiratory rate by analyzing the impedance while detecting the lead off state. The lead off state should be detected continuously and should not interfere with the accurate measurement of the ECG signal.

Figure 2 shows the overall functional block diagram of an ECG. It is easier to understand the requirements for the electronic components of an ECG if you divide the ECG into the analog front end (AFE) that digitizes the signal and the "rest of the world" that will analyze, display, store, and transmit the data. The AFE generally has the same basic requirements, with differences in the number of leads, signal fidelity, interference rejection, etc. The "rest of the world" of the system varies greatly depending on the specific functional requirements, with typical functions including display, printing hardware copy, wireless (RF) connectivity, and battery charging.

Figure 2 ECG overall functional block diagram

Lead number (derivative)

One of the most notable features is the number of leads. Some ECGs have only one lead, while others have as many as 12. The most commonly used 12-lead ECG requires 10 electrodes, 9 of which are used to collect electrical signals. The 10th electrode is connected to the right leg (RL) and driven by the ECG circuit to reduce the common-mode voltage. The 9 input electrodes are one electrode each for the left arm (LA), right arm (RA), left leg (LL), and 6 electrodes (V1-V6) in the anterior heart (chest) area. Each lead or telecardiogram represents the voltage difference between one electrode and another electrode or group of electrodes. If the electrodes are grouped, the voltage is averaged. The 6 leads derived from the three electrodes RA, LA, and LL are averaged as one side of the differential pair, and V1 to V6 are the other sides of the 6 differential pairs. Three leads are derived from the difference between the mean of RA, LA, and LL and the other two electrodes. The remaining 3 leads are the result of measuring RA, LA, and LL as independent differential pairs. The 6 leads based on RA, LA and LL contain similar information, but are displayed in different ways. Because the information is redundant, it is not necessary to measure all 6 leads. Some channel data can be calculated by analyzing the data of other channels using DSP.

The 12-lead system described here is the most commonly used, but it is not the only option. In addition, the 12-lead ECG can also be used as a 5-lead, 3-lead, or 1-lead system. The key is that when more than 1 lead is needed, a switch array and averaging circuit are required.

Analog Front End (AFE)

The main function of AFE is to digitize ECG signals. The processing is very complicated because it needs to suppress strong interference such as RF signal sources, pacing signals, lead-off detection signals, power frequency common-mode signals, and other body signals and electronic noise. In addition, millivolt-level ECG signals may be superimposed on hundreds of millivolts of DC offset voltage, plus the common-mode voltage between channels, which may exceed 1 V. The electrodes connected to the patient's body must not cause electric shock hazards or interfere with other medical instruments connected to the patient. The effective frequency range of ECG is somewhat application-dependent, usually between 0.05 Hz and 100 Hz.

The second function of the AFE is to detect pacing signals, lead-off, respiratory rate, and patient impedance, which are performed simultaneously or nearly simultaneously on several channels. In addition, most ECG devices require fast recovery during defibrillation, but since defibrillation causes saturation of the front-end circuits and charging capacitors, these capacitively coupled circuits will prolong the recovery time.

AFE Architecture

The AFE architecture has a great impact on system performance. The enhanced architecture described below provides high fidelity over a wide frequency range due to the use of a high-precision, high-speed ADC (analog/digital converter). Instead of using capacitive coupling, the DAC (digital/analog converter) is used as the RL drive, allowing the AFE to quickly recover from defibrillation or RF interference. The digitized pacing signal allows the pacing data to be analyzed, thereby reducing false pacing indications and even detecting defects in the pacemaker or connected parts. On the other hand, it is also necessary to consider that the enhanced system requires expensive components and consumes a lot of power. In contrast, the simplified AFE is cheaper and has a longer battery life, and the other characteristics are slightly different.

Enhanced AFE and DSP AFE: A high-performance ADC (as shown in Figure 3) is required to meet ECG test requirements, which can quantize 9 electrode signals simultaneously with a noise-free accuracy of 20 bits at a sampling rate of 200 kS/s. A digital signal processor (DSP) is then used to calculate each lead signal, isolate the pacing signal, lead-off signal, and respiration signal, and filter out interfering frequency signals. The DSP also calculates the signal strength required by the digital-to-analog converter (DAC) to drive the RL electrode. This AFE architecture requires high matching of each channel of the analog-to-digital converter (ADC). In addition, a buffer is required to isolate the ADC sampling capacitor and the high-impedance electrode. Although this solution meets the measurement index requirements, it cannot meet the cost and power requirements of most applications.

Simplified AFE: The low-end AEF series features a single-channel, consumer ECG. The AFE of these devices uses a capacitive coupling circuit to couple the input signal to a low-pass differential amplifier, which is then fed to a 10-bit, 120 S/s sampling rate ADC. The capacitive coupling circuit can remove the DC offset of the input, and the low-pass filter filters out the pacing signal. These devices are usually battery-powered and have only one channel, so there is no common-mode voltage. Typical ECG device AFE: The circuit used in most ECG devices is somewhere in between the above two. The instrumentation amplifier (IA) is often used to suppress the common-mode voltage, eliminate common-mode noise such as power frequency interference, and provide buffering for the sampling capacitor of the ADC. The subsequent filter can filter out the pacing signal and the off-detection signal, and then send it to the ADC for sampling and digital conversion. In some cases, the ECG signal and DC offset are directly converted to digital by a high-precision ADC. In other cases, a high-pass filter or DAC is used to remove the DC offset, so that the amplified ECG signal can be sampled and converted to digital using a typical 12-bit precision ADC, as shown in Figure 4. Each channel can be equipped with an ADC, or multiple channels can share one ADC for digital conversion. ADC multiplexing causes slight time deviations between channels, and the degree of acceptance depends on the specific application. If the pacing signal needs to be detected, it can be extracted with a high-pass filter, amplified, and then amplified and detected with a comparator circuit.

ECG device type Telemetry ECG

Telemetry ECG systems are used for continuous monitoring of mobile patients in clinical environments. They include an ESG with wireless (RF) transceiver function placed at the patient end and a central station that collects and analyzes the patient's monitoring data through wireless reception. Some telemetry systems also provide additional data (such as blood oxygen values) that are used to verify the effectiveness of treatment or adjust treatment plans, and to warn of impending problems.

Many telemetry systems only have 5 leads, and if the full 12 leads are used, it is difficult to cope with the mobility of patients. Often, patients will use the device for several days in a row, so these devices often use disposable batteries. Other ECGs can also add telemetry capabilities, but "telemetry ECG" specifically refers to mobile units that can be carried around the hospital and send data to a local receiving station. For the design of this system, low power consumption, low noise and small size are key considerations.

Holter monitor

Dr. Newman Holt invented the mobile monitor to collect data and upload it to other systems for analysis. Unlike telemetry devices, these monitors do not require a central receiving station and can be used at home, outdoors, or anywhere else. For Holter ECG monitors, because 12-lead monitors are inconvenient to move, the number of leads will not exceed 5 in most cases. Generally, a memory card is used to transfer data from the monitor. Of course, a USB drive or other methods can also be used. Most patients only need to be monitored for 1 to 2 days. When patients are required to participate in certain pharmacological studies, special long-term monitors are used, and patients may need to use them for a year or even longer. The main requirements for the design of Holter ECG monitors are also low power consumption, low noise, and small size.

Consumer ECG

This type of low-end ECG can be easily fixed on the arm, so people can do ECG examinations at home. These devices can save data and display it on a built-in screen. The data can also be transmitted to a computer or transmitted to a rehabilitation center via a telephone line. Some devices have multiple electrodes attached, while others only have two electrodes installed on the casing. The built-in electrodes can be pressed against the chest, or the hands can be placed on the two electrodes respectively. The resulting ECG may not be of very good quality, but it provides an effective way for people to monitor their own conditions and collect ECG data when abnormalities occur. The design of consumer ECGs mainly focuses on low cost and small size.

Automated External Defibrillator (AED)

In order to deal with some emergencies in public places, AED devices are installed in most public places (such as large shopping malls, gyms, and offices). These devices can be used immediately when a heart attack occurs to release a high-energy electrical pulse to the chest, pace the heart and restore it to a normal heart rate. If used at the wrong time, the pulse shock will be life-threatening, so the ECG must be functionally able to prevent such accidents. AED generally has only one lead, and its electrodes are used to release high-voltage pulses and collect ECG signals. The principle block diagram of the AED device is shown in Figure 5.

AEDs may sit unused for months or years, and are often used by untrained personnel who have no way of knowing if there is a problem with the device. When an AED is needed, it is turned on, a series of self-tests are performed to confirm that it is functioning properly, and then it is run for a short period of time. All ECG data and defibrillation information need to be recorded for later analysis. Using an AED with problems can do more harm than good, so reliability and self-diagnostic capabilities are the first considerations in AED design.

Diagnostic ECG

Diagnostic ECG devices are used in hospitals and doctors' offices to provide high-quality ECG testing, can test a full 12-lead ECG, and create hard copy output. These devices use high-performance AFEs, which can usually improve the quality of ECG testing by adjusting gain and selecting appropriate filters. Due to their large size and seldom being moved, these devices have room to implement more features, such as built-in printers, various communication interfaces, and large display screens. They generally use AC power and usually have rechargeable batteries for backup. The key to designing a diagnostic ECG is low noise, high interference immunity, and flexibility.

Patient Monitor

Patient monitors monitor vital signs (pulse, respiratory rate, blood pressure, and temperature). They also have ECG functions and can monitor blood oxygen and carbon dioxide levels. Integrating these functions into one device can make the operating room simpler and more convenient to use. The AFE of the patient monitor is similar to the diagnostic ECG, but it must meet the radio frequency (RF) suppression requirements because it will be subject to high-intensity RF interference from electronic knives and argon plasma coagulation (APC) equipment during surgery. In addition, the ability to quickly recover from cardiac defibrillation operations is also a basic requirement for this type of AFE. Since the patient monitor is AC-powered and also has a backup battery, power consumption is also an important indicator. The housing must be splash-proof and easy to clean, which of course affects the cooling channel, so heat dissipation must also be considered. In addition to power consumption and heat dissipation, the key to designing a patient monitor lies in RF suppression and low noise indicators.