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February 28, 2011

Capnography: 5 things EMS responders should know

Capnography is the best tool since the EKG. It not only helps you diagnose and treat your patient, it also helps you see if your treatment is working. It has been around for a while now and is probably one of the most underutilized tools in our industry. Why? Maybe it is because providers really don't understand the benefits of this tool in their practice of medicine.

There are five simple things that I feel EMS responders should know about capnography.

1. Understanding capnography is easy
Capnography is one of the easiest tools to use and interpret compared to all of the other tools we utilize in EMS. Here is my KISP principle (Keep It Simple Puryear): "You breathe in oxygen and you breathe out carbon dioxide.' We want to measure carbon dioxide (Capnos — smoke).
The normal capnography waveform should be nice and square. It should not be "rounded" or "shark finned." Why?

  • Inhalation — When you inhale, there is only atmospheric air crossing the monitoring device at the mouth and the nares. This atmospheric air sample has relatively zero carbon dioxide. So your graph will read "0." This is Phase I (Inhalation).
  • Exhalation — When you exhale, the only gas exchange occurring is in the alveoli, not from the mouth/nares to the bronchioles. So the first split second of exhaled air (dead space volume) has the same amount of carbon dioxide as the atmospheric air and should still be baseline. When the alveolar air hits the sensor, it causes a sharp vertical line to the normal 35-45 mmHg. This is Phase II (Ascending). As long as you exhale and release the alveolar air, it should remain at the normal 35-45 mmHg until you inhale again. This is Phase III (Alveolar Plateau).
  • Inhalation — Now you inhale and draw in atmospheric air and no carbon dioxide is being released from the body. So the wave should create an immediate sharp vertical line back down to zero and remain there until you repeat the process. This is Phase IV (Inhalation).

2. It helps you diagnose your patient
If the waveform is abnormal, then there IS an airway problem. If the waveform is "shark finned," then you know that the body is having some type of bronchoconstriction restricting the ability to release the carbon dioxide immediately. If the waveform is "rounded," then you know that the body is not only having trouble releasing it immediately during exhalation but is also having trouble getting rid of it at the end of exhalation.

If the reading is high, the body is releasing a high amount of carbon dioxide (hypoventilation, increased metabolic rate, acidosis, etc.). If the reading is low, the body is releasing a low amount of carbon dioxide (hyperventilation, decreased metabolic rate, alkalosis, etc.). If the reading immediately disappears, then the patient is either apneic or your endotracheal tube became dislodged.

3. It helps you treat your patient
If the waveform is "shark finned," then you know that this patient requires bronchodilators. If the waveform is "rounded," then you know that your patient requires a BVM or other positive pressure ventilation to take over their respirations. If the carbon dioxide level is low, they may be in a type of alkalosis. If the level is high, then they may be in a type of acidosis. If the waveform is absent, then you need to BVM or ventilate your patient or check your endotracheal tube placement.

4. It helps you know if your treatment is working or not
By comparing your initial capnography reading to the changes during your treatment, you will be able to see if your treatment is working. If your capnography shows "shark finned" waveforms and during the course of your bronchodilators the waveform remains unchanged, then you know that your current treatment may not be effective. If it changes to a normal square waveform, then it may be. If you are ventilating a hypoventilation patient and the capnography level decreases to a normal range then you know your treatment may be working. If it doesn’t, then you may increase your respiratory rate and/or tidal volume.

5. It is your "get out of jail free" card
If you are performing endotracheal intubation, waveform capnography is mandatory in my opinion. Why? If someone is accusing you of not properly managing your advanced airway and that your endotracheal tube became dislodged during treatment, waveform capnography may prove this is a false accusation. Knowing the normal physiology of the respiratory system, if the waveform is square then it most likely is placed in the trachea, which is highlighted in the 2010 AHA guidelines.

Capnography is the best tool since the EKG; it not only helps you diagnose and treat your patient, it also helps you see if your treatment is working.