Sensors & transducers


venting it into the atmosphere. This so-called closed-loop or closed-circuit system demands two key features. It is critical to ensure all the


CO2 is removed from the exhausted gas. The breathing reflex is triggered by CO2 concentration levels in the blood and increasing levels can be extremely uncomfortable for the


patient. Therefore, exhaled CO2 must be removed from the air before it can be re-cycled. To remove CO2 requires a device called a


scrubber. This is a piece of equipment that is


designed to absorb CO2, typically consisting of a material that chemically reacts with the CO2 in an exothermic reaction. The efficacy of this scrubbing


process needs to be measured, both to ensure it is working correctly and to determine when the material needs to be replaced. GSS has a family of sensors ideally suited to


this application. The ExplorIR-W-F is designed to take two measurements per second, which


supports real time analysis of CO2 levels in the exhaust gas. The flow port adaptor attached to the main sensor body helps cycle the exhaust gas for rapid measurements.


waveform shapes can indicate different conditions. This analysis is becoming increasingly sophisticated and is driving the demand for faster and more accurate CO2 sensing devices.


CAPNOGRAPHY Capnograph devices generally fall into two categories, either main-stream or side- stream sensing.


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. The other key advantage of a side-stream position is that the sensor does not need to be integrated into the tubing directly attached to the patient, making it easier to accommodate as well as being less sensitive to power consumption and size restrictions. Whilst side-stream sensing has some


advantages, there are several disadvantages. The expired gas has longer to travel, and therefore the response time will be longer. As described earlier,


Figure 3: ExporIR-W with Flow Port Adaptor 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


saturation and CO2 levels. CO2 monitoring during respiration is called capnography and is usually part of the overall patient monitoring system when used in conjunction with a ventilator. Capnography is used to determine several


patient vital signs. CO2 is a product of the body metabolising carbohydrates, fats, and amino acids,


in a process known as cellular respiration. CO2 is passed from the blood to the lungs in a process


called perfusion and expelled through ventilation. The CO2 expirated from the body is


parameterised in a number of different ways.


End-tidal Carbon Dioxide (ETCO2) is the level of Carbon Dioxide that is released at the end of an exhaled breath. Monitoring of the quantity,


rate, and shape of the ETCO2 waveform is an invaluable tool for patient monitoring. Analysis of


the ETCO2 waveform shape allows you to generate information about the metabolism, ventilation, perfusion and health of the patient. The shape of the waveform should normally be a rectangle with rounded corners. Different


Instrumentation Monthly February 2021


Figure 4: Schematic of Capnography Monitor Continued on page 46...


45


accurate CO2morphology needs an understanding of how the signal changes over time and this signature can be ‘smeared’ when the gas must travel further to reach the sensor. Short term peaks may be partially averaged out due to turbulent flow in the sampling path.


Main-Stream Sensing In a main-stream capnograph, the expirated breathe is sampled directly in the main airway path. All the same problems of condensation and mouth secretions are present in the main-stream


sensing application, but with the added complication of having to mitigate the effects whilst maintaining light weight, low power and small size due to the location close to the patient.


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 sidestream 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 and is especially onerous. Regardless of the architecture, the demands


on the sensor depends on what information needs to be acquired. Increasingly, doctors are looking at the detailed morphology of the


CO2 waveforms to help diagnose patient health. To improve the fidelity of the measurements, there are several factors that need to be considered. Accurate CO2 morphology requires an in-


depth understanding of how the waveform shape


evolves over time. The CO2 waveform must be analysed essentially in real time and with sufficient resolution to identify specific health issue markers. There are several design constraints that must be


considered when designing a CO2 sensor suitable for this requirement. For historical reasons, CO2 levels have also


been reported as a partial pressure, traditionally measured as the height of a column of mercury in mmHg.


A partial pressure of 37mmHg is approximately 4.9 per cent CO2.


SENSOR ACquISITION RATE AND RESOluTION A capnograph is a waveform (as above) that represents the varying CO2 level throughout the


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