Photoplethysmographic determination of the respiratory rate in acutely ill patients: validation of a new algorithm and implementation into a biomedical device

Abstract

Background: Respiratory rate is among the first vital signs to change in deteriorating patients. The aims of this study were to evaluate the accuracy of respiratory rate measurements using a specifically dedicated reflection-mode photoplethysmographic signal analysis in a pathological condition (PPG-RR) and to validate its implementation within medical devices.

Methods: This study is derived from a data mining project, including all consecutive patients admitted to our ICU (ReaSTOC study, ClinicalTrials.gov identifier: NCT02893462). During the evaluation phase of the algorithm, PPG-RR calculations were retrospectively performed on PPG waveforms extracted from the data warehouse and compared with RR reference values. During the prospective phase, PPG-RR calculations were automatically and continuously performed using a dedicated device (FreeO2, Oxynov, Québec, QC, Canada). In all phases, reference RR was measured continuously using electrical thoracic impedance and chronometric evaluation (Manual-RR) over a 30-s period.

Results: In total, 201 ICU patients' recordings (SAPS II 51.7 ± 34.6) were analysed during the retrospective evaluation phase, most of them being admitted for a respiratory failure and requiring invasive mechanical ventilation. PPG-RR determination was available in 95.5% cases, similar to reference (22 ± 4 vs. 22 ± 5 c/min, respectively; p = 1), and well correlated with reference values (R = 0.952; p < 0.0001), with a low bias (0.1 b/min) and deviation (± 3.5 b/min). Prospective estimation of the PPG-RR on 30 ICU patients' recordings was well correlated with the reference method (Manual-RR; r = 0.78; p < 0.001). Comparison of the methods depicted a low bias (0.5 b/min) and acceptable deviation (< ± 5.5 b/min).

Conclusion: According to our results, PPG-RR is an interesting approach for ventilation monitoring, as this technique would make simultaneous monitoring of respiratory rate and arterial oxygen saturation possible, thus minimizing the number of sensors attached to the patient. Trial registry number ClinicalTrials.gov identifier NCT02893462.

Keywords: Pulse oximetry; Respiration rate; Respiratory failure.

Figures

Fig. 1

Correlation and Bland–Altman plot for reference and estimated respiratory rate during the retrospective evaluation phase. Reference respiratory rate (RR) values are provided as the chronometric measurement measured over 30 s under spontaneous ventilation or as the ventilatory rate for patients under ventilatory assistance. Estimated RR-PPG was performed using the plethysmographic curve analysis. On the 201 ICU patients’ recordings, retrospective estimation of the RR-PPG was highly correlated with the reference method (r = 0.95; p < 0.0001). Comparison of the methods using the Bland–Altman method depicted a low bias (0.1 c/min) and deviation (< ± 3.5 c/min)

Fig. 2

Correlation and Bland–Altman plot for reference and estimated respiratory rate during the prospective evaluation phase. Reference respiratory rate (RR) values are provided as the chronometric measurement measured over 30 s under spontaneous ventilation or as the ventilatory rate for patients under ventilatory assistance. Estimated RR was performed using the plethysmographic curve analysis (RR-PPG). On the 30 ICU patients’ recordings, prospective estimation of the RR-PPG was well correlated with the reference method (r = 0.78; p < 0.001). Comparison of the methods using the Bland–Altman method depicted a low bias (0.5 c/min) and acceptable deviation (< ± 5.5 c/min). Most of the estimation errors were related to difficult PPG signal acquisition, mostly due to artefacts and/or bad signals

Fig. 3

Three principal types of respiratory modulation of the plethysmogram signal (PPG) that are used for respiratory rate estimation. a Unmodulated PPG waveform. b Baseline modulation [BM]; the red dashed line depicts the baseline modulation related to the respiratory rate component. c Amplitude modulation (AM); the two red dashed lines depict the signal envelope related to the respiratory rate component. d Frequency modulation (FM); this modulation of the heart rate, also called “respiratory sinus arrythmia”, is synchronized with the respiratory rate