Chapter 4: Although ECG & PPG are based on different technologies, they also have common ground

Introduction

Despite the fact that PPG and ECG signals arise from different origins, they are both directly linked to the cardiac cycle. In chapter 1, we discussed the cardiac conduction system and its relation to the different stages of the cardiac cycle. We also described the different features of the ECG and the PPG waveform in chapter 2 and chapter 3, respectively. Here, we will dive deeper into the similarities and differences between both technologies within the arrhythmia space.

Differences between PPG and ECG

Photoplethysmogram	Electrocardiogram
No additional or specific hardware required, signals can be collected with straightforward technologies such as smartphones	Additional or specific hardware required. Signals can be collected with technologies such as Holter devices and hand-held devices
Heart rate is derived, abnormal heart rhythm can be detected by embedded algorithms	Allows direct analysis of the heart rhythm
Diagnosis requires confirmatory ECG, combined with clinician oversight	Can be diagnostic, clinician oversight is required for rhythm confirmation
Sensor-based (light source & photodetector)	Electrode-based
Represents changes in capillairy blood volume via the arterial blood pressure waveform (mechanical activity)	Represents electrical activity of the heart
Represents ventricular activity	Represents atrial and ventricular activity
Requires only a single contact point to sense the signal	Requires a closed loop between at least 2 electrodes
Spotcheck and long-term continuous monitoring	Spotcheck and short-term continuous monitoring

The common ground of PPG and ECG

Although PPG waveforms solely resemble ventricular activity and do not show atrial activity (i.e. P-waves on ECG), PPG-deriving applications can be utilized to identify cardiac irregularities. One of the first FibriCheck trials demonstrated the capability to use PPG signals in the cardiac space by comparing peak-to-peak intervals and RR intervals of simultaneously recorded PPG and ECG data in patients with sinus and atrial fibrillation (AF) rhythms. They reported a 99.3% correlation between simultaneously recorded intervals, implying that PPG can be used to accurately determine the heart rate, and more importantly, the regularity of the heart rhythm. 

The overlay of both signals reveals that the PPG waveform is delayed when compared to the ECG signal as a result of the pulse-arrival time. This metric is defined as the time for the pressure wave produced by the left ventricular contraction to travel from one arterial site to another arterial site. In the case of smartphone-based PPG measurements via the fingertip, the pulse arrival time is defined as the time for the blood pressure wave to travel from the aorta to the capillary network in the fingertip.

What do the clinical guidelines state?

“When AF is first detected by a screening tool, including mobile or wearable devices, a single-lead ECG tracing of ≥30 s or 12-lead ECG showing AF analyzed by a physician with expertise in ECG rhythm interpretation is necessary to establish a definitive diagnosis of AF. When AF detection is not based on an ECG recording (e.g. with devices using photoplethysmography) or in case of uncertainty in the interpretation of device-provided ECG tracing, a confirmatory ECG diagnosis has to be obtained using additional ECG recording (e.g. 12-lead ECG, Holter monitoring, etc.).“

Source: 2020 ESC guidelines for the diagnosis and management of atrial fibrillation

“Although PPG technology is not diagnostic of AF according to the 2020 European Society of Cardiology (ESC) Guidelines for the diagnosis and management of AF, its widespread accessibility and low cost make it an interesting tool for remote heart rate and rhythm monitoring in patients with known AF.”

Source: EHRA consensus statement on how to use digital devices to detect and manage arrhythmias

Key Messages:

✔️ According to the ESC guidelines, ECG is the golden standard for heart rate and rhythm assessment. PPG is increasingly gaining more evidence and popularity to be used in heart rhythm use cases.

✔️ ECG can be diagnostic, while PPG still requires a confirmatory ECG in patients without a prior AF diagnosis. In any case, clinical oversight is required for rhythm confirmation.

✔️ The overlay of PPG and ECG signals reveals that the PPG wave is delayed when compared to the ECG signal, as a result of the pulse-arrival time.

✔️ Peak-to-peak intervals and RR intervals of simultaneously recorded PPG and ECG data revealed a 99.3% correlation between intervals, implying that PPG can be used to accurately determine the heart rate, and more importantly, the pulse rhythm.