Capnography is a medical monitoring technology that measures carbon dioxide in exhaled breath. It helps clinicians see how well a patient is ventilating, which means how effectively air is moving in and out of the lungs. The measurement is especially important during anesthesia, emergency care, intensive care, and CPR.
A capnography monitor gives rapid feedback because CO₂ levels change breath by breath.
Key Facts
- Capnography measures CO₂ in exhaled gas and displays it as a capnogram waveform.
- End-tidal CO₂, or ETCO₂, is the CO₂ level at the end of exhalation.
- Normal ETCO₂ in many adults is about 35 to 45 mmHg.
- Minute ventilation = tidal volume x respiratory rate.
- Higher ventilation usually lowers ETCO₂, while lower ventilation usually raises ETCO₂.
- CO₂ production, blood flow to the lungs, and ventilation all affect the capnogram.
Vocabulary
- Capnography
- Capnography is the continuous measurement and display of carbon dioxide concentration in exhaled breath.
- Capnogram
- A capnogram is the waveform that shows how exhaled CO₂ changes during each breath.
- End-tidal CO₂
- End-tidal CO₂ is the carbon dioxide value measured at the end of exhalation, when alveolar gas is sampled.
- Ventilation
- Ventilation is the movement of air into and out of the lungs to exchange oxygen and carbon dioxide.
- Airway circuit
- An airway circuit is the tubing and connectors that carry gases between the patient and breathing or monitoring equipment.
Common Mistakes to Avoid
- Confusing capnography with pulse oximetry is wrong because capnography measures ventilation through CO₂, while pulse oximetry estimates blood oxygen saturation.
- Assuming a normal oxygen reading means normal ventilation is wrong because a patient can retain CO₂ while oxygen saturation remains acceptable for a time.
- Reading only the ETCO₂ number without checking the waveform is wrong because waveform shape can reveal obstruction, poor sampling, disconnection, or rebreathing.
- Treating every low ETCO₂ value as hyperventilation is wrong because low values can also result from poor blood flow to the lungs, cardiac arrest, or equipment leaks.
Practice Questions
- 1 A patient has an ETCO₂ of 55 mmHg. If the usual adult reference range is 35 to 45 mmHg, is this value above, within, or below the range, and by how many mmHg from the nearest limit?
- 2 A ventilated patient has a tidal volume of 0.50 L and a respiratory rate of 14 breaths per minute. Calculate the minute ventilation using minute ventilation = tidal volume x respiratory rate.
- 3 During monitoring, the capnogram suddenly becomes a flat line near zero after showing normal waves. Explain two possible causes and why checking the airway circuit is important.