We know that capnography is the measurement of exhaled CO2, so how does the monitor interpret the action of cellular respiration and ventilation into something we can use?
The capnography probe on many popular monitors used in EMS and transport services works by one of two principles, but how they measure the sample is basically the same. They both use infrared measuring of the exhaled gas and measure the absorption of the light by the gas molecules. This is called molecular coorelation spectroscopy. I’m going to expect you to understand it, I’m not even sure I understand it totally. So, on to how it measures a sample.
One method is “in line” capnography, meaning that the probe is attached directly onto the source and measured there. If you have ever seen a funny looking clip with a red light on an ET tube that leads back to the monitor, you’ve seen in line capnography. Many flight services like this because it supposedly gives a more accurate reading because it’s measuring at the source. Also it is less likely to be contaminated and is reusable because it is not in the way of being contaminated.
The other is “side stream” capnography. the probe is placed directly into the path of air travel and a small pump inside the monitor draws a sample in to the infrared sensor. This is a pretty popular monitoring method for EMS, and even now in hospital settings because the sampling devices are cheap and disposable, where as the in line devices are extremely expensive. I had a capnography probe interlinked with my pain pump during my last surgery to monitor for respiratory depression in case I accidentally overdosed myself.
So anyway, now that we know basically how the CO2 is measured and how it gets into the monitor, we need to talk about how it is displayed, and what each part of the displayed waveform means. The waveform has 4 phases, and once you understand these phases you will understand how to quickly interpret, diagnose, and treat respiratory emergencies.
Phase 0 is inspiration. This is where the amount of CO2 being inhaled is at 0 mmHg, which means that none is being exhaled. This is the sudden sharp drop and flat line of the waveform. Anatomical dead space is created here, where air inhaled does not come into contact with the alveoli for gas exchange. This includes gas in the mouth and trachea, anywhere but in the alveoli. The baroreceptors in the chest signal the brain that they are reaching their limit and inspiration ceases.
This is where the expiration occurs. The pressure of exhaled CO2 rises as the respiration cycle continues as the pressure in the chest begins to drop.
This is where the pressure of exhaled CO2 plateaus and begins to level off as expiration reaches the end of it’s cycle. The baroceptors in the chest signal the brain to stop exhalation and
Phase III is where the CO2 level peaks and the brain’s baroceptors in the chest as well as the chemoreceptors in the brain signal that the oxygen concentration is low and to begin another inspiration cycle.
The computer in the monitor displays this in a waveform: