Developing an algorithm for pulse oximetry derived respiratory rate (RR_oxi): a healthy volunteer study

Objective The presence of respiratory information within the pulse oximeter signal (PPG) is a well-documented phenomenon. However, extracting this information for the purpose of continuously monitoring respiratory rate requires: (1) the recognition of the multi-faceted manifestations of respiratory modulation components within the PPG and the complex interactions among them; (2) the implementation of appropriate advanced signal processing techniques to take full advantage of this information; and (3) the post-processing infrastructure to deliver a clinically useful reported respiratory rate to the end user. A holistic algorithmic approach to the problem is therefore required. We have developed the RR_OXI algorithm based on this principle and its performance on healthy subject trial data is described herein.

Methods Finger PPGs were collected from a cohort of 139 healthy adult volunteers monitored during free breathing over an 8-min period. These were subsequently processed using a novel in-house algorithm based on continuous wavelet transform technology within an infrastructure incorporating weighted averaging and logical decision making processes. The computed oximeter respiratory rates (RR_oxi) were then compared to an end-tidal CO₂ reference rate (\( {\text{RR}}_{{{\text{ETCO}}_{ 2} }} \)).

Results \( {\text{RR}}_{{{\text{ETCO}}_{ 2} }} \) ranged from a lowest recorded value of 2.97 breaths per min (br/min) to a highest value of 28.02 br/min. The mean rate was 14.49 br/min with standard deviation of 4.36 br/min. Excellent agreement was found between RR_oxi and \( {\text{RR}}_{{{\text{ETCO}}_{ 2} }} \), with a mean difference of −0.23 br/min and standard deviation of 1.14 br/min. The two measures are tightly spread around the line of agreement with a strong correlation observable between them (R ² = 0.93).

Conclusions These data indicate that RR_oxi represents a viable technology for the measurement of respiratory rate of healthy individuals.