ECG Sensor vs. PPG Sensor: A Detailed Comparison

This article compares ECG (Electrocardiogram) and PPG (photoplethysmogram) sensors, outlining the differences between these two technologies used for heart rate monitoring.

Introduction:

With the recent rise in COVID-19 and other viruses, the risk of heart-related issues has unfortunately increased. People are now more susceptible to cardiovascular problems and strokes. Therefore, self-diagnosis, including heart-related tests, has become vital, especially when experiencing mild symptoms like fever, fatigue, cough, or cold. ECG and PPG are two common methods used to measure heart rate parameters for diagnostic purposes.

What is an ECG Sensor?

ECG (Electrocardiogram) measures the bio-potential generated by the heart’s electrical signals using electrodes attached to the patient’s skin. Small metal electrode patches are placed on the chest, arms, and legs. This technique represents the expansion and contraction of the heart chambers. ECG tests detect and record the heart’s electrical activity, including the heart rate and rhythm.

ECG sensors capture a wide range of heart metrics, including HR (heart rate) and HRV (heart rate variability).

An example of an ECG sensor is the AD8232 IC module. It uses three electrodes attached to the right arm (RA), left arm (LA), and right leg (RL). It generates an analog output signal representing the heartbeat, which can be read by a microcontroller (like an Arduino Uno) or displayed on an oscilloscope. The AD8232 can be powered using a 3.3V LDO Voltage Regulator or directly from an Arduino Uno board.

A complete ECG typically uses 10 electrodes to capture 12 leads (signals), providing a comprehensive picture of the heart’s activity. While 12 leads are necessary for an accurate diagnosis, a single lead is often sufficient for an initial assessment.

The ECG curve consists of five states: P, Q, R, S, and T, each corresponding to different phases of the heart’s activity. By analyzing changes in these patterns, abnormal heart rates (too fast, too slow, or irregular) can be detected.

What is a PPG Sensor?

PPG (photoplethysmogram) is a non-invasive, light-based technology. It measures volumetric variations in blood circulation during heart pumping action using a light source and a photodetector placed on the skin. An LED and a photodetector are used to sense blood flow rate and derive the heart rate.

The figure illustrates how a PPG sensor is attached to a finger and its working principle. A PPG sensor consists of an LED on one side of the tissue and a photodetector on the other. It measures changes in blood volume based on the obstruction and absorption of incident light. When the LED and photodetector are placed adjacent to each other, the incident light is reflected off the tissue surface.

The PPG signal waveform provides information about the person being diagnosed. The systolic amplitude reflects AC variation in blood volume around the measurement site. The PPI interval (the interval between two systolic peaks) in the PPG curve represents the cardiac beat-to-beat interval. Further information can be derived by estimating the first and second derivatives of the original PPG signal. The second derivative indicates the acceleration of blood.

ECG Sensor vs. PPG Sensor: Tabular Difference

The following table summarizes the key differences between ECG and PPG sensors: