Capnography: waveforms and sudden loss of ETCO2
Why examiners ask this
The capnogram is the monitor examiners use to hide a diagnosis: a falling number, a shark fin, a baseline that will not return to zero. They want a systematic reading that separates the airway, the machine, the lungs and the circulation, and they want you to act on the dangerous causes before the benign ones. It tests application of knowledge and organization, and it reappears in the OSCE monitors station.
The 60-second answer
I read the capnogram in a fixed order: is there a waveform at all, does the baseline return to zero, what is the shape of the expiratory upstroke and plateau, and what is the number relative to the arterial value. A sudden loss of the trace means disconnection, extubation or esophageal intubation, complete obstruction, or cardiac arrest until proven otherwise.1 I check the patient first: pulse, blood pressure and chest movement. Then I trace the circuit from the patient to the machine: tube, connections, sampling line. A sudden fall to a lower but present value means a fall in pulmonary blood flow: massive hypotension, pulmonary embolism, air embolism. A gradual rise means hypoventilation, absorbed CO2 from laparoscopy, rebreathing, or hypermetabolism such as malignant hyperthermia.
Decision points and the "why"
- Waveform present at intubation — the monitoring standard requires exhaled CO2 to confirm every tube, then continual end-tidal analysis with an audible alarm until extubation.2 A sustained trace over six breaths confirms tracheal placement; fewer than six waveforms of falling height mean esophageal intubation with gas from the stomach.3 In cardiac arrest the value is low but present, so I confirm with video laryngoscopy if in doubt.
- Baseline above zero — rebreathing: exhausted absorbent, an incompetent expiratory or inspiratory valve, or inadequate fresh gas flow in a Mapleson circuit. Raising fresh gas flow is the immediate fix while I find the cause.1
- Shark fin (sloped upstroke, no plateau) — expiratory obstruction: bronchospasm, COPD, a kinked tube or secretions.1 I listen, check peak pressures, pass a suction catheter and treat bronchospasm with deeper anesthesia and albuterol.
- Curare cleft — a notch in the plateau means the patient is breathing against the ventilator, a sign of returning neuromuscular function or light anesthesia.4
- Sudden fall with hemodynamic collapse — dead space has increased: pulmonary embolism, air embolism or low cardiac output. The arterial-to-end-tidal gradient widens beyond the normal 2–5 mmHg.
- During CPR — the AHA suggests compressing to an ETCO2 of at least 10 mmHg and ideally 20 mmHg or more, because persistently low values predict failure;5 an abrupt, sustained rise, often to 35–40 mmHg, signals return of spontaneous circulation.4
- Rising ETCO2 — with a stable minute ventilation, I think of laparoscopic CO2 absorption, release of a tourniquet, bicarbonate administration, fever, thyroid storm and malignant hyperthermia.1
The complication they will inject
During a total hip arthroplasty in the lateral position, the ETCO2 falls from 36 to 18 mmHg over a minute as the surgeon inserts the cemented femoral component; the blood pressure falls to 70/40 and the SpO2 to 90%. I recognize a sudden fall in pulmonary blood flow and suspect bone cement implantation syndrome or fat or air embolism. I tell the surgeon, increase oxygen to 100%, and give phenylephrine or epinephrine to support the right ventricle. I confirm the circuit is intact and the tube is in place, because a disconnect looks similar but without the hemodynamic change. I send an arterial gas to measure the gradient and ask for TEE to look at right ventricular function and emboli. I start CPR if the patient loses a pulse.
Pitfalls that cost points
- Starting with the machine when the patient has no pulse.
- Calling every low ETCO2 "hyperventilation" without looking at the minute ventilation.
- Forgetting that the sampling line can leak, giving a low value with a normal shape.
- Reading the number without the waveform; the shape tells you where the problem lives.
- Assuming ETCO2 equals PaCO2 in the patient with COPD, shock or one-lung ventilation.
Rapid-why
Q: Why is ETCO2 normally lower than PaCO2? A: Alveolar dead space dilutes the exhaled gas with alveoli that are ventilated but not perfused, giving a gradient of a few mmHg (about 2–5) with normal lungs that widens with V/Q mismatch.1
Q: Why does ETCO2 rise transiently after tourniquet release? A: Washout of CO2 and acid from the ischemic limb; it peaks within a few minutes.
Q: Why can a capnogram be normal during an esophageal intubation? A: After bag-mask ventilation or carbonated drinks, the stomach holds CO2 for a few breaths; it decays quickly, which is why I wait for six.
Q: Why does a sampling line disconnection cause a reading of room air? A: The side-stream analyzer aspirates 150–200 mL/min, so a break draws in room air and the trace flattens.
Numbers to know
| Item | Value |
|---|---|
| Normal ETCO2 | 35–45 mmHg1 |
| Normal PaCO2–ETCO2 gradient | 2–5 mmHg |
| CPR quality target | ETCO2 ≥10, ideally ≥20 mmHg5 |
| ROSC signal | Abrupt, sustained rise, often to 35–40 mmHg |
| Side-stream sampling | 150–200 mL/min |
| Confirmation of intubation | Sustained trace over 6 breaths |
Links
- Anesthesia machine checkout and intraoperative failures, Venous air embolism and the sitting position, Malignant hyperthermia, Intraoperative bronchospasm, Bone cement, tourniquets and hip fracture (coming), Intraoperative cardiac arrest
- Atlas: Monitor atlas: perioperative respiratory events
- ASA Standards for Basic Anesthetic Monitoring; AHA Guidelines for CPR and ECC (2020)
Sources
- 1.Quick Reference Guide: Capnography Waveforms · Masimo · masimo.comManufacturer education sheet; gives the PaCO2–ETCO2 gradient as 1–5 mmHg with normal lungs.
- 2.Standards for Basic Anesthetic Monitoring · ASA, 2025 · asahq.orgLast amended October 15, 2025; section on ventilation.
- 3.Capnography Waveform Interpretation · LITFL · litfl.com
- 4.Interpreting Waveform Capnography: Pearls and Pitfalls · emDocs · emdocs.net
- 5.Part 3: Adult Basic and Advanced Life Support (2020 AHA Guidelines for CPR and ECC) · AHA, 2020 · cpr.heart.org
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.