So we have covered the physiology of respiration, now we are going to cover
the waveforms and what each part means.
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
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.
Phase I
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.
Phase II
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
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:
Next time we will talk about normal pressure ranges and the appearance of the waveform in different respiratory conditions as well as it’s usefulness in CPR.




Why It’s Important To Be Nice
I was scrolling Facebook and came across an article about an ambulance service in Maine leaving a widow on the side of the road while they were transporting her husband, who was in cardiac arrest. You can find the article here.
From what I can gather from the article, the family was from Nova Scotia and were skiing in Maine when her husband smashed into a tree. The ski patrol rapidly moved the husband (the patient) off the mountain and to a clinic, where the facility called for an ambulance while they attempted to stabilize him. According to the article, the ski clinic did not perform any assessment, and according to the patient’s wife, neither did the paramedics. She insisted that they listen to his chest, start an IV, and take his blood pressure while they were packing him for transport and splinting his elbow. She says that they spent 30 minutes performing these actions.
Beyond poor care, the widow’s other complaint is that she was told to sit up front. Which if there is a lot of activity in the back, I could understand because I don’t want potentially hysterical or intrusive family members in the back while I’m working something critical. To add insult to injury, she was let out of the ambulance and subsequently left on the side of the road. I don’t know the reasoning behind this, maybe she got hysterical or argumentative, I don’t know but there better be a good reason for that.
Clinical assumptions aside (and I did raise my eyebrow a few times), I bet the crew was less than considerate to the family, and perhaps in response to her insistence of being in the middle of things which could have perceived as being in their way. But had the situation been handled with a little more tact, I bet this would barely have generated a complaint at all.
You as a provider have a fiduciary responsibility, that is, having the characteristics of trust. If you display behavior that is not consistent of being worthy of trust, then the patients will not trust you to deliver the best care and certainly not for their family. This is important in all cases, but even more so in cases that involve poor outcomes, because poor outcomes could equal lawsuits even if you delivered stellar care, simply on the assumption that because you acted poorly that you performed equally as poor.
Remember that family members, unless they are physically in your way, often are merely trying to help. If you explain that their help is appreciated but not needed, they would be more than happy to step aside and let you do your job. You should up the level of insistence as they up their belligerence. My lieutenant has pulled many family members aside while we were working and explained that we have a job to do, and handled them with tact even when the outcome was poor. We often hear back about how well we did even though the job was botched or simply didn’t end well.
It’s as much about protecting yourself as it is your reputation. Appear to be trustworthy, and people will trust you despite how bad of a job you did, and will forgive you for your mistakes.