Rising End-Tidal CO2 During a Low-Flow Sevoflurane Case

Inspired CO2 above zero means the patient is inhaling CO2 - this is rebreathing, not hypoventilation. Normal inhaled CO2 is 0 mmHg. A square plateau with an elevated inspiratory baseline localizes the problem to the circuit or the absorbent, not to the patient's drive.

This matters because the differentials look alike on the end-tidal number but not on the waveform:

• Hypoventilation (opioid, residual blockade): elevated EtCO2 with a zero inspired CO2, and it responds to increased minute ventilation. This did not.
• Absorbent failure (exhausted, channeled, or bypassed canister): rebreathing, elevated inspired CO2, problem unmasks at low fresh gas flow, often on a later case of the day.
• Valve incompetence or misassembly: a stuck-open inspiratory unidirectional valve lets exhaled gas flow backward into the inspiratory limb, so the next breath carries CO2.
• Malignant hyperthermia: would explain a rising EtCO2 unresponsive to ventilation, but normally with rising temperature, tachycardia, and a mixed acidosis. Temperature 36.6°C and HR 78 make this very unlikely right now - but it must stay on the list until the circuit is ruled out.

Increasing minute ventilation cannot fix rebreathing, because each breath now contains a fixed fraction of CO2.

1. Turn the fresh gas flow to 10 L/min. This is both treatment and test. At high flow, a rebreathed fraction is washed out; if inspired CO2 disappears and EtCO2 falls toward baseline, the fault is absorbent exhaustion/channeling or a modest valve leak. If inspired CO2 persists at high flow, the circuit is genuinely incompetent.
2. Exchange the CO2 absorbent canister and inspect the seals and the direction of the gas flow. Do not rely on the color indicator alone - it can be misleading, and many canisters are opaque.
3. Check the unidirectional valves with a test lung: disconnect the patient, ventilate a reservoir bag through the Y-piece, and look for the inspiratory disc seating and the expiratory disc lifting.
4. Draw an arterial blood gas. EtCO2 is unreliable when dead space changes, and you want the actual PaCO2 as your endpoint.
5. Recheck temperature and heart rate, and keep MH in view if the CO2 load does not correct with a proven-good circuit.

Until the fault is identified, ventilate at high flow and keep the sevoflurane delivered by a route you trust.

Persistent inspired CO2 at high fresh gas flow is evidence against simple absorbent failure and points to a mechanically incompetent circuit - most often a unidirectional valve that is stuck open, reversed, or not seated after cleaning, or a canister seated improperly with a bypassed seal.

Practical response:

• Replace the machine or the breathing system, not just the absorbent. A self-inflating bag with a non-rebreathing setup, or a second circuit and test lung, gets you off the faulty apparatus quickly.
• Confirm with a documented valve check while the patient is on the substitute circuit, so the faulty machine is tagged and removed from service rather than reused by the next provider.
• Reassess the patient with an ABG. If PaCO2 is disproportionately higher than EtCO2, dead space has increased - consider hypovolemia, pulmonary embolism, or developing sepsis. If PaCO2 is only modestly elevated and the gradient is normal, the problem really was apparatus.
• Reconsider MH only if temperature is climbing, tachycardia is new, K+ and CK are rising, and the acidosis is mixed - none of which is present here.

Document the event, the machine number, and notify biomedical engineering.