| Description |
Amplitude and shape of the respiratory carbon dioxide (CO2) signal (capnogram) are not always a true reflection of the patient's respiratory status, especially during delivery of supplemental oxygen (O2), and can result in false alarms. The goal of the project was to provide a more reliable source of patient monitoring and diagnosis by using the respiratory pressure signal to detect breaths in the presence of supplemental O2. The Respironics LoFlo sidestream capnometer was used in the project to record the two signals. In preliminary tests, it was observed that the sampling pressure obtained from the capnometer was analogous to respiratory pressure and was hence used in analysis as its representative. The sampling flow signal was also acquired from the LoFlo system and studied. The aforementioned signals were recorded from 4 human volunteers who had 100% O2 or air delivered through a nasal cannula. The end-tidal CO2 (etCO2) was 2 ± 0.4 % lower, the sampling pressure and the sampling flow were 3 ± 0.9 times and 4.7 ± 1.2 times greater in amplitude, respectively, with O2, than their corresponding values with air. Another finding in the presence of O2 was the occurrence of double peaks in the pressure signal during expiration. It was hypothesized that O2, being more viscous than air, caused the capnometer to behave differently in its presence. This was confirmed by a series of bench experiments. Clinical data were collected from patients undergoing colonoscopy with supplemental O2 delivered at 6 liters per minute (LPM). The first step in analysis was detection of O2 from the pressure signal so that any observed double peak is accounted for and the amplitude of the signal is not misinterpreted. Breath rate was then calculated from the CO2 and pressure signals and episodes of apnea were detected. The pressure and CO2 signals detected 99% and 98% of apneic events, respectively, with a respective specificity of 100% and 99%. When considered individually, both signals had a sensitivity of 96%. The pressure signal resulted in 60% fewer false positive results than the CO2 signal, thus greatly reducing the rate of false alarms. The pressure signal, being instantaneous, also detected respiratory effort before the capnogram. It is concluded that the sampling pressure signal can provide more reliable and accurate respiratory information than the capnogram. |
