Chapter 6: Benefits of the first derivative of the PPG: how it enhances high-quality PPG analysis

Introduction

Previously we already discussed the origin of a photoplethysmogram (PPG), how it’s different from an electrocardiogram (ECG), and how smartphones can be used to record PPG. In this chapter we’ll discuss the characteristics of a high quality PPG-signal, what PPG-derivatives are, and how they can be used to measure changes in the physiology of the patient.

Key messages

✔️ A high-quality PPG-signal is characterized by a strong and consistent waveform with clearly defined peaks and valleys that correspond to each pulse.

✔️ The first derivative of the PPG-signal is a mathematical transformation that provides additional information about the rate of change of the signal over time.

✔️ The first derivative of the PPG-signal supports healthcare providers to accurately detect the heart rate, identify changes in pulse waveform and remove baseline drift, allowing them to make informed decisions.

High-quality PPG-signal

A high-quality PPG-signal is characterized by a strong and consistent waveform with clearly defined peaks and valleys that correspond to each pulse. The waveform of a high-quality PPG-signal typically has a sharp and distinct upstroke, followed by a more gradual downstroke. The peak of the waveform should be well-defined and have a rounded shape. To obtain a high-quality PPG-signal, it is important to be relatively free from noise or interference, with no significant fluctuations or spikes. By meeting these standards, a high-quality PPG-signal will provide healthcare providers with reliable and accurate data to monitor a patient's heart rate and rhythm.

Characteristics and benefits of the first derivative of PPG-signal

The first derivative of the PPG-signal is a mathematical transformation that provides additional information about the rate of change of the signal over time. It's calculated by determining the rate of change of the raw PPG-signal at each point in time, revealing how quickly the amplitude of the signal is changing over time. The analysis of the first derivative is valuable as it provides healthcare providers with additional insights into the pulse waveform and can be used to detect changes in a patient's physiology. 

Measuring a patient’s heart rate

Using the first derivative of the PPG-signal, healthcare providers are able to measure a patient's heart rate more accurately by detecting the time of the peak of each pulse. The peak of the first derivative represents the maximum rate of change in the PPG-signal, which corresponds to the time when the pulse reaches its maximum change in amplitude.

Detecting changes in the pulse waveform

By analyzing the first derivative, healthcare providers can detect and monitor changes in the pulse waveform, which allows them to identify changes in the underlying physiology that may require further investigation.

Importance of removing the baseline drift

The first derivative of the PPG-signal can also be used to remove baseline drift, which is a slow, gradual change in the amplitude of the PPG-signal over time that can obscure important features of the waveform. By calculating the rate of change of the signal at each point in time, the first derivative can help remove this baseline drift and make it easier to detect changes in the waveform. By removing baseline drift, clinicians can more accurately analyze the waveform and make informed decisions about a patient's heart rate and rhythm.

⏩ Learn more about potential artifacts in PPG-signal recordings in the next chapter!