CO2 Sensors for Ventilator and Capanography Monitoring
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APPLICATION NOTE AN009: CO2 Sensors for Ventilator and Capnography Monitoring ABSTRACT CO2 sensors are used in a variety of medical applications, ranging from simple pH sensitive paper for detecting dissolved levels of CO2 through to ultra high-speed sensors for capnography for helping diagnose heart and lung function. Gas Sensing Solutions (GSS) is a world-leader in designing CO2 sensors ideal for a range of these medical applications. A ventilator helps the patient to breathe. An airway is connected to the patient either with a mask or through tubes in the mouth or nose. Ventilators blow air into the lungs replicating the normal breathing. Releasing pressure causes the lungs to relax and exhale naturally. It is possible to synchronise the output from the machine with the patient’s own breathing patterns. When people breathe normally, a tightening of the diaphragm and other muscles inhales air into the lungs. Oxygen then diffuses into the bloodstream through the lung walls. CO2 is diffused into the lungs from the blood and exhales when the muscles relax. The level of CO2 exhaled by the patient is a key indicator their health condition. There is an on-going desire to improve the fidelity of CO2 measurements to help diagnose patient condition in real time and with greater certainty. This application note describes some of the sensor attributes and issues that need to be overcome, in order for them to be used in demanding ventilator and capnography equipment. Gas Sensing Solutions Ltd. Page | 1 For regular updates, sign up at https://gassensing.co.uk Revision 1.0, 8 October 2020 Copyright © 2021 Gas Sensing Solutions Ltd.

Gas Sensing Solutions Ltd. Page | 2 For regular updates, sign up at https://gassensing.co.uk Revision 1.0, 8 October 2020 Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTE AN009: CO2 Sensors for Ventilator and Capnography Monitoring INTRODUCTION TO NDIR CO2 SENSORS GSS sensors use a technique called non-dispersive infra-red (NDIR). The sensors work by measuring the amount of infra-red light absorbed by the CO2 gas. The concentration is proportional to the amount of light absorbed as it passes through the gas. GSS has a long track-record of developing and manufacturing commercially advanced mid-infrared LEDs and photodetectors using its state-of-the-art molecular beam epitaxy (MBE) facility in the UK. The mid infra-red LEDs are tuned to emit a...

APPLICATION NOTE AN009: CO2 Sensors for Ventilator and Capnography Monitoring USING CO2 SENSORS IN MECHANICAL VENTILATORS The COVID-19 pandemic highlighted the lack of mechanical ventilators for critically ill patients in many countries including advanced economies. At the beginning of the pandemic, many countries ramped up mass-production of ventilators in the belief the health care system would not have enough capacity to cope. However, a poorly considered part of this strategy was the unexpected impact on the ability of hospital infrastructures to be able to fully utilise an increase in...

APPLICATION NOTE AN009: CO2 Sensors for Ventilator and Capnography Monitoring The gas delivered from a ventilator is a mixture of approximately 21% oxygen, 78% nitrogen, trace amounts of carbon dioxide and the remainder from other inert gases. The air that is exhaled however is different with a composition that depends on the health of the patient. Typically, it is 16% oxygen, 78 % nitrogen, 5% carbon dioxide and about 1% of other gases. About 5% of the oxygen is exchanged by the patient into carbon dioxide, but the remaining oxygen is exhausted. Most ventilators will exhaust into the...

APPLICATION NOTE AN009: CO2 Sensors for Ventilator and Capnography Monitoring In addition, the ExplorIR-W also offers a digital alarm that can be used to drive a visual indicator of CO2 levels. The SprintIR-6S offers similar functionality but with an even faster sampling speed of 20Hz. PATIENT MONITORING The operational parameters of the ventilator are set based on several different vital signs gathered by the patient monitoring system. The patient monitor may be integrated into the ventilator or a separate piece of equipment. Parameters includes blood pressure, heart rate (ECG), oxygen...

APPLICATION NOTE AN009: CO2 Sensors for Ventilator and Capnography Monitoring CAPNOGRAPHY Capnograph devices generally fall into two categories, either main-stream or side-stream sensing. Figure 4: Schematic of Capnography Monitor Gas Sensing Solutions Ltd. Page | 7 For regular updates, sign up at https://gassensing.co.uk Revision 1.0, 8 October 2020 Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTE AN009: CO2 Sensors for Ventilator and Capnography Monitoring SIDE-STREAM SENSING A side-stream configuration is one where the expirated air from the patient is sampled away from the main flow back to the ventilator. The CO2 sensor is placed in the sample flow and this placement mitigates some of the application issues that arise when sampling expired air. High humidity can cause condensation without special precautions, and mouth secretions can become a problem if not dealt with correctly. GSS has written an application note on how to mitigate the impact of condensation...

APPLICATION NOTE AN009: CO2 Sensors for Ventilator and Capnography Monitoring CO2 SENSOR DESIGN CONSIDERATIONS GSS has several cost-effective NDIR based gas sensors capable of being used in capnography applications depending on the configuration and positioning of the CO2 sensor. The ideal sensor will depend to some extent what architecture is being used. In a side stream architecture, the expired gas from the patient is sampled away from the main air flow and is less demanding from a sensor point of view. In a mainstream architecture, the expired gas is measured directly in the main flow...

APPLICATION NOTE AN009: CO2 Sensors for Ventilator and Capnography Monitoring SENSOR ACQUISITION RATE AND RESOLUTION A capnograph is a waveform (as above) that represents the varying CO2 level throughout the breath cycle. The shape of the waveform can change quite dramatically depending on patient condition. Accurately and stable sampling the waveform over time can represent a significant challenge for the CO2 sensing system. The number of breaths is normally in the range of 12-20 per minute so the average sampling rate of the system does not need to be very high. Capturing the ETCO2 level...