Investigation of Sternal Photoplethysmography – Design of a Vital Sign Patch

It is important to obtain reliable measures of vital sign parameters, e.g. heart rate, respiratory rate (RR), and the oxygen saturation of the blood (SpO2). In current clinical practice the RR is assessed by manual count of the chest movements and pulse oximetry probes have been limited to the fingertip or the earlobe.

These methods are documented to be time consuming or inaccurate when monitoring patients with low blood flow or decreased perfusion at the extremities (e.g. patients with obstructive lung diseases, diabetes, or heart failure). Furthermore, they are obtrusive for long-term recordings.  The main focus of this project is to investigate the clinical applicability of photoplethysmography (PPG) measured at the chest bone (sternum).

PPG is an optical method from which the RR and the SpO2 level can be obtained. For this purpose we have developed a prototype of a wearable PPG sensor and conducted three clinical studies both on healthy participants and on patients with obstructive lung diseases. The first clinical study was a controlled desaturation study.

All 14 subjects were exposed to hypoxia by breathing an air mixture of reduced oxygen concentration in a closed system. We obtained an average root mean squared error (Arms) of 1.75 % compared to invasive measures of the oxygen saturation in the radial artery (SaO2) which is within the clinically and commercially accepted range.

Furthermore, this study served as a calibration of the PPG sensor in a clinical relevant range (100 % - 70 %). In the second study, sternal PPG recording was conducted from 30 admitted patients with either asthma or Chronic Obstructive Pulmonary Disease (COPD). We compared the SpO2 levels simultaneously obtained from the sternal PPG recording and a conventional finger pulse oximeter.

The Pearson correlation between the SpO2 levels estimated from the two body locations was found to be 0.89 (p < 0:05) and the mean system bias was only 0.052 % with upper and lower limits of agreement of 2.5 % and -2.4 %, respectively. The RR was also obtained from sternal PPG and compared to conventional capnography.

In a range of 11 to 36 breaths/min the Pearson correlation was 0.93 (p < 0:001) and the system mean bias was 0.6 breaths/min. The upper and lower limits of agreement was found to be -2.8 to 4 breaths/min. In the last clinical study, the focus was to investigate the clinical reliability of long-term PPG recordings from the sternum.

Fifteen admitted patients were included in the study and were monitored with the sternal PPG sensor for approximately 20 hours. On average it was found that clinically reliable SpO2 and RR estimates could be calculated for 58 % of the recording time. Furthermore, the average longest period of time with unreliable data in terms of the SpO2 level and the RR was only 23.6 minutes and 20 minutes, respectively.

The results of this project show that it is possible to obtain reliable quasi-continuous recording of the SpO2 level and the RR from sternal PPG in many different clinical applications in the future.