# Unobtrusive Respiratory Flow Monitoring Using a Thermopile Array: A Feasibility Study
## 4 Discussion  
### Dataset A  
* Distance: Comparison of the performance between different distances proved that the thermopile would lose accuracy at larger distances. In particular, amplitude, SNR, and the number of pixels would decrease with larger distances, and the RMSE would increase. Amplitude decreased from an average of 0.98 \({}^{\circ}\)C to 0.83 \({}^{\circ}\)C. The SNR reduced from 5.66 dB to 2.97 dB, whereas RMSE went from 0 BPM to 0.18 BPM with a standard deviation of 0.45 BPM. The number of pixels did not result in a big change on average, but it can be noticed that the standard deviation for the 20-30 cm case was much higher compared to the 40-50 cm case. This behavior was mainly due to the partial area effect. As already expected, the low resolution combined with the broad field of view limited the distance at which the sensor could be used. * Increasing RR: Breathing with higher RR resulted also in a reduction of the amplitude from 1.30 \({}^{\circ}\)C to 0.82 \({}^{\circ}\)C, and the SNR drastically reduced from 9.34 to 0.71 dB, while the number of pixels from 1.60 to 1.00. The RMSE remained constant. These variations can be considered to be caused by the reduced flow during higher RR. The flow applied to the surroundings would be imposed for less time, due to the higher frequencies, reducing the heat exchanged. * Orientation and oral/nasal respiration: In this dataset, both orientation and nasal/oral respiration were compared. Frontal measurements were expected to produce a reduced variation in temperature due to the small surface for heat exchanges available. Frontal nasal respiration resulted in performing particularly worse compared to the other settings. That is because the heat would be exchanged mostly with the nasal area around the nostrils. Frontal oral respiration performed better compared to the nasal case, but the RMSE was still much higher compared to the side cases. The improved performance in the frontal oral breathing could be due to the increased surface for heat exchanges, i.e., nose and lower lip, and to the higher temperature of oral expired air. Side oral respiration resulted in causing a bigger temperature variation and higher SNRs. This behavior was expected since in the side case, the pillow's textile material would be involved in heat exchanges as well, increasing the surface. This setting involved many more pixels, i.e., on average 2.76 compared to side nasal breathing, which resulted in 1.21 pixels on average; whereas, the variation in amplitude and SNR was not as big. The differences can be explained by considering the differences in expired air temperature as already indicated in Section 2.1 and differences in the flow direction. * Shoulder motion: Measurements were collected in a seated position at 50 and 100 cm when the shoulders were included in the field of view. Figure 5a,b shows the two different situations: in the first case, the signal resulting in the maximum peak magnitude was coming from the shoulder area, whereas in the second case (side view with the subject laying down), the signal was coming from the nose area. This was due to the fact that the temperature variation due to respiratory shoulder motion had a higher amplitude compared to the variation due to the heat exchanged  
Figure 7: Bland–Altman plot and correlation plot for all the subjects and settings in Dataset B. Reference indicates the RR estimated with the impedance pneumography, whereas estimate indicates the one estimated using the thermopile array. In the Bland–Altman plot, the blue horizontal line represents the mean bias, whereas the two horizontal black lines indicate the average plus or minus 1.96. _Standard Deviation_. The black line in the correlation plot indicates the linear relationship \(y=x\). because of respiratory flow in the frontal view at the same distance. Shoulder and head motion due to respiration can be suppressed by performing the measurement with the subject lying down.