Capnography Waveform Interpretation: ETCO2 vs PaCO2

By Sachi Lord, CRNA Last updated October 26, 2026

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How do you interpret a capnogram and compare ETCO2 with PaCO2?

Capnography displays carbon dioxide in sampled respiratory gas. The waveform reflects carbon dioxide production, blood delivery to the lungs, ventilation and sampling conditions. End-tidal CO2 comes from the end of exhalation. Arterial PaCO2 comes from blood. Compare matched measurements rather than assuming a fixed difference or diagnosing a cause from one waveform shape.

Key takeaways

A capnogram reflects metabolism, circulation, ventilation and the sampled gas.
End-tidal CO2 is the end-expiratory sample, not any isolated tall spike.
Calculate PaCO2 minus ETCO2 from measurements matched in time and pressure units.
A sudden ETCO2 fall requires assessment of the patient and the airway, circulation and sampling together.

In this video

Where does the carbon dioxide on the waveform come from?

Cells produce carbon dioxide, blood transports it to the lungs and alveolar ventilation removes it. The monitor then measures sampled respiratory gas. A change anywhere in this sequence affects the final tracing, so capnography is not a direct measurement of ventilation alone.

A time capnogram plots carbon dioxide against time. A volumetric capnogram plots it against exhaled volume. Check the axes before interpreting slopes or areas. Sidestream sampling also takes time to transport gas to the sensor. Line design, leaks, dilution and oxygen delivery conditions influence what reaches the monitor.

What do the normal phases of a capnogram represent?

Early expiration first samples relatively low-CO2 gas from conducting airways. The upstroke develops as CO2-rich gas mixes into the sample. The alveolar plateau reflects gas from lung regions emptying at different rates and with different ventilation-perfusion relationships. The end-expiratory point supplies the end-tidal measurement.

Inspiration replaces exhaled gas at the sensor and the tracing falls toward the inspired-gas baseline. That baseline is normally low, but is not an unconditional guarantee of zero. Letter labels differ among teaching systems. Read the physiological sequence and signal quality rather than relying on a particular diagram’s lettering.

Why is ETCO2 not interchangeable with arterial PaCO2?

ETCO2 measures sampled exhaled gas, whereas PaCO2 measures arterial blood. Perfusion, dead space, uneven emptying and measurement conditions change their relationship. A fixed add-five rule fails when that relationship changes. Pulse oximetry measures a different variable again, so good saturation does not establish adequate carbon dioxide clearance.

In two hypothetical paired samples, the same ETCO2 of 30 mmHg occurs with PaCO2 of either 40 or 50 mmHg. The gradients are 10 and 20 mmHg. Neither example predicts the patient’s next blood gas. Pairing an old blood gas with a current breath produces arithmetic without a valid current gradient.

How do you calculate and interpret the arterial-to-end-tidal gradient?

Subtract ETCO2 from PaCO2 with the same pressure units and measurements from the same clinical moment. The recording changes from 42 minus 36, a gradient of 6 mmHg, to 40 minus 24, a gradient of 16 mmHg. The gradient widens even though both carbon dioxide values fall.

Reduced delivery of carbon dioxide to ventilated lung contributes to a lower ETCO2, while increased effective ventilation sometimes lowers arterial PaCO2. The two observations therefore do not require opposite movements. The example supports interpreting the entire patient and sample, not diagnosing pulmonary embolism from a gradient or treating one value as a universal cutoff.

What do an abnormal shape or sudden fall require you to consider?

A prolonged upstroke or sloping plateau raises questions about obstructed or uneven emptying and expiratory timing. A raised inspiratory baseline raises questions about inspired carbon dioxide, rebreathing and sampling. None independently identifies one disease or broken component.

An abrupt ETCO2 fall sometimes reflects reduced pulmonary blood flow, changed ventilation, airway problems or altered sampling. Pulmonary embolism is one possible perfusion cause but is not established or excluded by one trace. Assess deterioration urgently while checking equipment, rather than allowing a device investigation to delay care.

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Frequently Asked Questions

Is the normal PaCO2 to ETCO2 difference always 2 to 5 mmHg?

A small difference is a common teaching reference, not a fixed relationship for every patient. Dead space, pulmonary blood flow, uneven emptying and sampling change the gradient. Use matched blood and breath measurements when the arterial value matters.

Does a shark-fin capnogram prove asthma?

An obstructive-looking shape supports questions about slowed or uneven expiration, but does not diagnose asthma. Airway conditions, breath timing and sampling require assessment. A waveform shape alone does not select treatment.

Does a falling ETCO2 mean PaCO2 must rise?

No. ETCO2 and PaCO2 measure different compartments. Reduced pulmonary carbon dioxide delivery sometimes lowers ETCO2 while increased effective ventilation lowers PaCO2. A widening gradient does not require the arterial value to rise.

Does a sudden ETCO2 drop diagnose pulmonary embolism?

Pulmonary embolism is one cause of reduced perfusion to ventilated lung. Airway, ventilation, circulation and sampling changes also produce low ETCO2. One fall or widened gradient does not independently diagnose or exclude embolism.

Does oxygen saturation confirm adequate ventilation?

Pulse oximetry estimates oxygen saturation, not carbon dioxide clearance. Supplemental oxygen sometimes preserves saturation despite inadequate ventilation. Interpret respiratory movement, ventilation assessment and capnography alongside oxygenation.