Breathing Rate Estimation From the Electrocardiogram and Photoplethysmogram: A Review

Abstract

Breathing rate (BR) is a key physiological parameter used in a range of clinical settings. Despite its diagnostic and prognostic value, it is still widely measured by counting breaths manually. A plethora of algorithms have been proposed to estimate BR from the electrocardiogram (ECG) and pulse oximetry (photoplethysmogram, PPG) signals. These BR algorithms provide opportunity for automated, electronic, and unobtrusive measurement of BR in both healthcare and fitness monitoring. This paper presents a review of the literature on BR estimation from the ECG and PPG. First, the structure of BR algorithms and the mathematical techniques used at each stage are described. Second, the experimental methodologies that have been used to assess the performance of BR algorithms are reviewed, and a methodological framework for the assessment of BR algorithms is presented. Third, we outline the most pressing directions for future research, including the steps required to use BR algorithms in wearable sensors, remote video monitoring, and clinical practice.

Fig. 1. ECG and PPG are subject to three respiratory modulations: baseline wander (BW), amplitude modulation (AM), and frequency modulation (FM).

Source: [33] (CC BY-NC 4.0: http://creativecommons.org/licenses/by-nc/4.0/).

Fig. 2. Stages of a BR algorithm. Dashed stages are optional.

Fig. 3

Extraction of exemplary respiratory signals: ECG (upper plot) and PPG (lower plot) signals and extracted respiratory signals (grey) are shown on the left. The corresponding frequency spectra are shown on the right. The frequency spectra of the raw ECG and PPG signals are dominated by cardiac frequency content at 1.2 Hz. In contrast, the extracted respiratory signals are dominated by respiration at 0.3 Hz, which is approximately the BR provided by a reference respiratory signal (shown by the dashed line).

Fig. 4

Exemplary feature-based techniques for extraction of respiratory signals from ECG (left) and PPG (right) signals: measurements of baseline wander (BW), amplitude modulation (AM), and frequency modulation (FM) have been extracted for each beat from fiducial points (shown as dots). Adapted from [33] (CC BY-NC 4.0).