Capnography: The best tool since the EKG
Capnography is standard of care for all intubated patients and should be for all major medical patients
By Jon Puryear
Capnography is one of the best tools for EMS providers to use since the EKG. It not only helps you diagnose your patient, but it also helps you see if your treatment is working or not.
Today, capnography is standard of care for all intubated patients and should be for all major medical patients. In the video below, I explain what is a normal capnograph, why it is normal and the normal physiology of the body that creates the waveform.
Let's compare the body's respiratory system with a motor vehicle. There are three things that a car requires to run properly:
- Fuel: If you don't have fuel then the car will not run. We see our fuel level by looking at the fuel gauge.
- Motor: If you don't have a motor to combust the fuel and make the pistons turn, the car will not run. You can have a full tank of fuel but it is useless if you don't have a motor.
- Exhaust: If you don't have a proper exhaust system to get rid of the combusted fuel, the car won't run property. If you put a potato in the tail pipe, the car won't run.
Now let's compare this to the human body:
- Oxygen: If you don't have oxygen, you will die. We can measure our oxygen level (saturation on the hemoglobin) by looking at the pulse oximeter.
- Perfusion: If you can't turn the oxygen into an energy unit that the body can use, carbon dioxide (exhaust), then you will die. Converting oxygen into energy releases the byproduct gas called carbon dioxide. You can have 100 percent oxygen in your bloodstream, but if you can't "burn" it into carbon dioxide then it is useless.
- Exhalation: If you can't get rid of the carbon dioxide, then you will die. If you have mucous plugs or bronchoconstriction, carbon dioxide backs up in the body and can create dangerous acidosis.
It makes sense then, that being able to measure carbon dioxide during exhalation might give us a good sense of how well the body is performing. Capnography measures the amount of carbon dioxide present at the mouth/nares. During inhalation, there shouldn't be carbon dioxide present at the mouth/nares, so the reading is zero.
Now at exhalation, when the alveolar air passes the sensor, it should immediately climb to the amount of carbon dioxide present. This should create a sharp vertical line up to the amount of carbon dioxide in the breath, and then continue throughout the exhalation.
Then, inhalation occurs again and the amount of carbon dioxide should immediately disappear. This causes a sharp vertical line back down to zero and then repeats at the next exhalation. The resulting waveform, called a capnograph should be a nearly square, with a normal reading between 35-45 mmHg.