Photoplethysmographic derivation of respiratory rate: a review of relevant physiology

Abstract

An abnormal respiratory rate is often the earliest sign of critical illness. A reliable estimate of respiratory rate is vital in the application of remote telemonitoring systems, which may facilitate early supported discharge from hospital or prompt recognition of physiological deterioration in high-risk patient groups. Traditional approaches use analysis of respiratory sinus arrhythmia from the electrocardiogram (ECG), but this phenomenon is predominantly limited to the young and healthy. Analysis of the photoplethysmogram (PPG) waveform offers an alternative means of non-invasive respiratory rate monitoring, but further development is required to enable reliable estimates. This review conceptualizes the challenge by discussing the effect of respiration on the PPG waveform and the key physiological mechanisms that underpin the derivation of respiratory rate from the PPG.

Keywords::

• Photoplethysmography
• Respiratory rate
• Physiology
• Remote monitoring

Declaration of interest: The Centre of Excellence in Personalised Healthcare, funded by the Wellcome Trust and EPSRC under grant number WT 088877/Z/09/Z, and the NIHR Biomedical Research Centre Programme supported this work.

Notes

¹The majority of arterial blood passes through a dense capillary network before entering the venous system.

²Multiple scattering by the red blood cells prevents the Beer-Lambert law, which is valid for purely absorptive media from being used to derive SpO2.

³This is rarely made explicit by the manufactures of pulse oximeters or PPG instruments.

⁴This simple explanations assumes an infinitely sharp transition between the pass-band and stop-band of the filters; in pratice both the low pass and high-pass filters will have gradual roll-offs in their frequency response.