The Pre-brief
End tidal capnography is becoming an increasingly used tool in intensive care units, and more commonly, out in the field when intubating. Like a lot of concepts in critical care, a waveform can tell you quite a bit about a patient. It is most often used to determine whether an endotracheal tube (ETT) is in the trachea, esophagus, or false lumen, but there are many other things the waveform itself can reveal. Let’s break down what capnography is and what some common different waveforms can tell us regarding a patient’s clinical situation.
What is end tidal capnography?
To put it simply, an end tidal capnography is a monitor that measures a patient’s breathing in real time utilizing the amount of carbon dioxide that is present during the breath. In intubated patients, a “mainstream device” is used, which means it is located directly within the path of the gas flow on a ventilator between the tubing and the endotracheal tube.
A normal capnography has four phases:
Phase I: inspiratory phase where there is no carbon dioxide being exhaled.
Phase II: This is the expiratory upstroke, where there is an interface between the anatomic dead space and early alveolar ventilation, as the CO2 from the alveoli is slowly reaching the upper airway.
Phase III: This is the alveolar plateau, where the CO2 has reached its highest concentration, which is normally around 40 mmhg.
Phase IV: Inspiration begins again and there is a decline in the CO2 as it approaches the phase I level.
Common waveforms:
The Shark Fin:
In patients with obstructive lung disease (asthma, COPD), there is a loss of the plateau and an increase in phase 3. The amount of CO2 that is exhaled is higher due to the air trapping and dead space that occurs from the inefficient ventilation. This causes a rapid rise in the CO2 concentration on exhalation.
The Squiggly Flat Line:
This can happen when the EtCO2 approaches zero or becomes zero. If it is decreasing or approaching zero, think about an error in the machine itself. Is there a calibration error? Too much condensation in the tubing? Apnea, pulmonary embolism, cardiac arrest, hyperventilation, or anything that obstructs the airway can cause this as well.
If it suddenly drops to zero, consider a disconnected ETT, esophageal ETT placement, or extubation.
Think of the mnemonic DOPE — dislodgement of the ETT, obstruction, pneumothorax, and equipment failure!
The Hill into Mountain:
If you notice that there is a sudden increase in the end tidal CO2, this usually happens when you achieve return of circulation in cardiac arrest patients who are actively coding. A gradual increase is usually seen in any instance where there is increased effort to breathe, ie, not eliminating CO2 effectively. This is usually a marker of impending respiratory failure and can be a useful tool to predict respiratory arrest. Other causes are over feeding, increased blood pressure or cardiac output, and lastly a ventilator malfunction. More HERE on EtCO2 in cardiac arrest.
The Debrief
Waveform capnography can tell you a lot about a patient, and it is extremely useful for continuous monitoring of intubated patients.
References
- Block FE Jr., ,McDonald JS. Sidestream versus mainstream carbon dioxide analyzers. J Clin Monit 1992;8(2):139–141.
- Rudraraju P, et al. Confirmation of endotracheal tube position: a narrative review. J Intensive Care Med 2009;24(5):283–292.142.
- Walsh, et al. Capnography/Capnometry During Mechanical Ventilation. Respiratory Care April 2011, 56 (4) 503-509
