Chapter 3: Photoplethysmography (PPG), the new kid on the block

Introduction

Now that we've discussed the ECG, we'd like to give you some more information about PPG-technology as well. In this article, we'll go through the basics about PPG and the valuable information that it can provide.

Key messages

✔️ PPG is an optical technique that measures light absorption of tissues.

✔️ The systolic phase of the cardiac cycle results in a cyclic arterial blood pressure waveform traveling throughout the arterial and capillary system.

✔️ The systolic phase results in the expansion of the capillary system and increased blood volume, leading to increased absorbance of light.

✔️ The diastolic phase results in the relaxation of the capillary system and decreased blood volume, leading to reduced absorbance of light.

✔️ When light is emitted at a constant intensity and the absorbance of light is monitored over time, then the difference in absorption of light by pulsatile arterial blood can be measured which results in the photoplethysmogram.

What is photoplethysmography or PPG?

A photoplethysmogram (PPG) graphically presents blood volume changes in the capillary system by using a light source. This non-invasive, optical technique can be applied in clinical practice to collect information related to the cardiac system. While an ECG waveform visualizes the electrical activity of the heart, the PPG waveform reflects the effect of the mechanical pumping activity of the ventricles. More specifically, a PPG is characterised by the volumetric and pressure-based output of the heart, reflected in the arterial blood pressure (ABP) waveform.

PPG has found its way into routine clinical care as it’s used to monitor the oxygenation of the blood through pulse oximetry. This non-invasive technology also enables individuals to measure their heart rate using everyday consumer electronics such as smartwatches. Other application areas and use cases for this technology are rapidly emerging due to its widespread availability.

A spot check pulse oximetry sensor using a red and infra-red wavelength	A smartwatch with a PPG sensor to record the heart rate

How does PPG work?

PPG-based devices require a light source, typically an LED to illuminate the skin, and a photodetector to measure the amount of reflected (figure a) or transmitted light (figure b).

When illuminating the skin, light beams are partially absorbed by the skin and the underlying tissues. There are two components responsible for light absorption:

• The non-pulsatile component: this component represents the absorption of light by non-pulsating tissues such as venous blood, bloodless tissue, muscles, bone tissue, etc.

• The pulsatile component: this component represents the absorption of light caused by cyclic expansion and relaxation of the capillaries due to pulsatile arterial blood pressure waveforms. 

When light is emitted at a constant intensity, and the amount of light absorption is monitored over time, this results in the pulsatile component of the PPG-signal (figure c).

When analyzing the amount of absorbed light, and thereby plotting the pulsatile component over time, PPG waveforms provide insights into the cardiac cycle. More specifically, each heartbeat is reflected by a spike in the PPG. The frequency of the spikes over time and the time interval between consecutive spikes provide insights into the heart rate and the regularity of the heart rhythm, respectively.

Which information is available in the PPG waveform?

Although the PPG waveform doesn’t include information regarding the electrical activity of the cardiac conduction system, there are multiple features included in this signal that provide clinical insights into the cardiac cycle.

The PPG-signal measures the result or output of a cardiac cycle as it indirectly captures the arterial blood pressure waveform. The rising edge of the waveform is primarily related to the systolic phase and the dicrotic notch is considered to be a marker of the end of the aortic systole and the beginning of the diastolic phase. However, the detailed mechanisms behind the dicrotic notch are still not well-understood.

During the diastolic phase, the heart muscle relaxes, the capillaries relax to their initial state, and the blood volume in the capillary system decreases. This leads to diminished absorbance of light by the pulsatile component. The falling edge of the PPG waveform or catacrotic phase is therefore associated with the diastolic phase of the cardiac cycle.

The most commonly understood features are the systolic amplitude (height of the peak) which resembles a volumetric component linked to the arterial blood pressure waves, and the peak-to-peak intervals between systolic peaks (PPI_systolic) representing the frequency and regularity of the heartbeats. Other waveform-related features are under continuous investigation to better understand their clinical relevance and usability in clinical practice.

⏩ Learn more about the differences and similarities between PPG- and ECG-signals in the next chapter!