**Core Concept**
A capnograph waveform reflects the concentration of carbon dioxide in exhaled air. The shape and characteristics of the waveform, especially the presence of a rising and falling phase with a clear end-tidal CO₂ (ETCO₂) value, indicate the status of ventilation and airway patency.

**Why the Correct Answer is Right**
After a period of apnea or mechanical ventilation, a stable, reproducible capnogram with a distinct rising phase and a clear plateau at end-tidal CO₂ signifies that the patient is now breathing spontaneously. This waveform pattern reflects normal alveolar ventilation and gas exchange, indicating return of spontaneous ventilation. The presence of a normal, consistent waveform with a steady ETCO₂ value (typically 35–45 mmHg) confirms that the patient is breathing independently, which is a key milestone in anesthesia management.

**Why Each Wrong Option is Incorrect**
Option B: Airway obstruction causes a flat or absent capnogram due to lack of exhaled CO₂, not a rising waveform.
Option C: Hyperventilation leads to a lower ETCO₂ and a more rapid decay of the waveform, not a stable, rising pattern.
Option D: Oesophageal intubation results in a flat or absent capnogram because CO₂ is not exhaled from the esophagus, and the waveform is absent or minimal.

**Clinical Pearl / High-Yield Fact**
In anesthesia, a sudden return of a normal capnogram after apnea is a reliable sign of spontaneous ventilation. This is a key indicator for extubation or transition from controlled to spontaneous breathing — always confirm with clinical signs (e.g., chest rise, patient effort) and auscultation.

✓ Correct Answer: A. Return of spontaneous ventilation