Capnography: waveforms and sudden loss of ETCO2
Stem
A 62-year-old man, 88 kg, with COPD, is having a laparoscopic sigmoid colectomy under general anesthesia with volume-controlled ventilation. Ninety minutes in, the end-tidal CO2, which had been a steady 38 mmHg with a normal waveform, abruptly flattens to zero. The ventilator is still cycling and the pulse oximeter reads 96%.
Question
How do you interpret this capnogram, and what do you do?
Ideal answer
HeadlineSudden loss of the trace is disconnection, extubation or esophageal tube, complete obstruction, or arrest until proven otherwise; I check his pulse first.1
- No pulse: CPR, and I stop looking at the machine.
- Perfused: trace patient to machine, so the dangerous causes are excluded first: tube, connections, sampling line.
- Hand-ventilate with the bag, which tells me at once whether the circuit holds pressure and the chest rises.
- Then read in a fixed order: waveform present, baseline back to zero, upstroke and plateau shape, number against arterial; it separates airway, machine, lungs and circulation.
- Know the other two patterns. A sudden fall to a lower present value is lost pulmonary blood flow; a gradual rise is hypoventilation, laparoscopic CO2, rebreathing or hypermetabolism such as malignant hyperthermia.
Breaking down the answer
- At intubation — the standard requires exhaled CO2 for every tube, then continual analysis with an audible alarm until extubation.2 A trace sustained six breaths confirms the trachea; fewer than six of falling height, the esophagus.3 In arrest, low but present: 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.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, deepen and give albuterol. A curare cleft means breathing against the ventilator: returning neuromuscular function or light anesthesia.4
- Sudden fall with collapse — dead space: pulmonary embolism, air or gas embolism, bone cement implantation syndrome during a cemented arthroplasty, massive hypotension 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 at stable minute ventilation — laparoscopic CO2 absorption, tourniquet release, bicarbonate, fever, thyroid storm, malignant hyperthermia.1
Follow-up question
You find the circuit disconnected at the elbow, reconnect it, and the trace returns. An hour later, as the surgeon re-insufflates the abdomen, the ETCO2 falls from 36 to 18 mmHg over a minute, the blood pressure falls to 70/40 and the SpO2 to 90%.
What is happening, and what do you do?
Follow-up answer
HeadlineA sudden fall in pulmonary blood flow: gas embolism from insufflation, or sudden hemorrhage. I tell the surgeon to stop insufflating and release the pneumoperitoneum.
I go to 100% oxygen and give phenylephrine or epinephrine to support the right ventricle. I confirm the circuit is intact and the tube in place, because a disconnect looks similar without the hemodynamic change. An arterial gas measures the gradient, and TEE looks at right ventricular function and emboli. If he loses a pulse, CPR.
Pitfalls that cost points
- Blaming hyperventilation for a low number while the ventilator settings have not changed.
- Assuming ETCO2 equals PaCO2 in the patient with COPD, shock or one-lung ventilation.
Rapid-fire
Q: Why is ETCO2 normally lower than PaCO2? A: Alveolar dead space dilutes exhaled gas with ventilated, unperfused alveoli: a gradient of a few mmHg (about 2–5) with normal lungs, widening 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 look 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; a partial leak reads low with a normal shape.
Numbers to know
| Item | Value |
|---|---|
| Normal ETCO2 | 35–45 mmHg1 |
| Normal PaCO2–ETCO2 gradient | 2–5 mmHg |
| CPR quality target | ETCO2 ≥10, ideally ≥20 mmHg5,6 |
| 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, ACLS adapted to the operating room, Intraoperative cardiac arrest: care after return of circulation
- Atlas: Monitor atlas: perioperative respiratory events
- ASA Standards for Basic Anesthetic Monitoring; AHA Guidelines for CPR and ECC (2025)
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 9: Adult Advanced Life Support (2025 AHA Guidelines for CPR and ECC) · AHA (Circulation 2025, PMID 41122884), 2025 · cpr.heart.orgReplaces the 2020 Part 3 citation (2026-11-02). Same source and claims as acls-in-the-or (ETCO2 at least 10, ideally 20 mmHg or more; abrupt rise suggests ROSC; epinephrine 1 mg every 3-5 min; manufacturer's biphasic energy). cpr.heart.org returned 403 to this session's fetcher; existence of the 2025 Part 9 confirmed in Europe PMC.
- 6.2025 Korean Guidelines for Cardiopulmonary Resuscitation: Part 4. Adult advanced life support · Korean Association of Cardiopulmonary Resuscitation (Clin Exp Emerg Med 2026), 2026 · europepmc.orgFull text read; corroborates: biphasic 120-200 J (200 J if the manufacturer dose is unknown); ETCO2 below 10 mmHg means improve compressions, target 20 mmHg or more; abrupt rise to 30-40 mmHg prompts a check for ROSC.
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.