It's not hard to find that in higher-level systems and component systems of patient monitoring equipment, many data acquisition systems follow a typical signal chain, which includes signal acquisition, conditioning, processing, and communication. When you dig deeper, you'll discover various design challenges that must be addressed, such as maintaining signal integrity and understanding the impact of common-mode rejection on the signal. Ensuring patient safety through electrical connections is also critical, but it adds another layer of complexity to the system design. Patients may sometimes require defibrillation, and during such events, it's essential to protect the monitoring system from potential damage. In addition, numerous industry standards and real-world challenges further influence the final design of these systems.
Figure 1 illustrates the typical signal chain of a 12-lead ECG (electrocardiogram) monitoring device. The architecture is highly complex, with multiple nuances and technical challenges involved.
ECG Monitoring
ECG, or electrocardiogram, measures the electrical activity of the heart. Devices used for ECG measurements range from portable dynamic monitors to clinical electrocardiographs and high-channel cardiac mapping systems, all of which are continuously evolving. These systems operate in diverse environments, some of which are illustrated in Figure 2. For instance, hospital settings include operating rooms, intensive care units, and electrophysiology labs. Each environment presents unique design challenges, and the complexity of these systems continues to grow as healthcare moves toward remote patient monitoring. Developers must now address new challenges that demand innovative solutions.
ECG Signal
An ECG signal is a transthoracic representation of the heart's electrical activity over time, captured by skin electrodes and recorded externally. It is a non-invasive record produced by an electrocardiograph. The amplitude typically ranges from 0.25mV to 5mV and consists of various waves, such as the normal sinus rhythm. Any deviation from this pattern may indicate an underlying issue that doctors can use to assess a patient’s condition and take appropriate action.
The ECG waveform includes the P wave, QRS complex, and T wave, each representing different phases of the heart’s electrical cycle. The P wave corresponds to atrial contraction, the QRS complex reflects ventricular depolarization, and the T wave indicates ventricular repolarization. By analyzing the electrical properties of the heart at different levels, cardiologists can detect a wide range of abnormalities, as shown in Figure 3.
ECG Measurement
In ECG measurements, electrodes act as potential sensors placed on the chest and/or limbs. Leads are derived from mathematical combinations of these electrodes. Although some consider this approach outdated due to historical reasons, the 12-lead ECG remains the standard in clinical practice. This system includes three standard limb leads (Einthoven leads), three augmented limb leads (Goldberg leads), and six precordial leads (V1–V6). The precordial leads are often treated as independent vectors, calculated based on the Wilson central terminal. As the number of measured leads increases, the complexity of capturing cardiac bioelectric signals also rises.
As shown in Figure 4, the Einthoven leads can be viewed as independent vectors, with signals coming from leads I, II, and III. The augmented leads (aVL, aVR, aVF) are derived from the same three electrodes. For example, aVL uses the left arm as the positive electrode and the combination of the right arm and left leg as the negative electrode. Meanwhile, the precordial leads (V1–V6) use six electrodes placed on the chest, with the Wilson center serving as the reference point.
Figure 2: Environment that requires ECG monitoring in the healthcare system
Figure 3: ECG signal
Figure 4: AVx lead measurement through the heart
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