A 64-year-old man is intubated and mechanically ventilated in the Emergency Department after an out-of-hospital cardiac arrest. Initial rhythm was pulseless electrical activity. High-quality CPR is ongoing. The airway had been confirmed earlier with a persistent square-wave capnography trace.

During the next rhythm check interval, the capnography waveform suddenly changes. The trace, which had previously shown low but consistent ETCO₂ values, now shows a sustained abrupt rise in ETCO₂ with preserved waveform shape. There has been no change in ventilator settings, no bicarbonate has been administered, and the endotracheal tube position remains unchanged at the teeth.

What is the single best interpretation of this capnography change?