A 52-year-old male with hypertension undergoing elective open cholecystectomy develops sudden hypotension (BP 85/50 mmHg) and hypoxia (SpO₂ 88%) 30 minutes after induction with propofol and succinylcholine. Breath sounds are bilaterally absent. Which is the investigation of choice to confirm the suspected diagnosis?

## Clinical Context The patient presents with acute hypotension and hypoxia with absent bilateral breath sounds during general anaesthesia. This constellation suggests **oesophageal intubation** — the endotracheal tube has entered the oesophagus instead of the trachea. ## Capnography: Gold Standard for Tube Placement **Key Point:** Capnography (end-tidal CO₂ measurement) is the **gold standard and most specific investigation** to confirm endotracheal tube position in real time. ### Why Capnography is Superior: - **Presence of CO₂ waveform** → confirms tracheal placement (lungs are the source of exhaled CO₂) - **Absence of CO₂ waveform** → indicates oesophageal intubation (no CO₂ exhaled from stomach) - **Real-time detection** — immediate feedback during intubation - **Sensitivity ~100%, Specificity ~100%** in confirming tracheal placement ## Capnography Waveform in Oesophageal Intubation ``` Normal (Tracheal): Absent/Flat (Oesophageal): CO₂ waveform present ___________________________ /\ (No rise in CO₂) / \ ____/ \___ ``` **High-Yield:** A **persistent absence of CO₂ waveform** after 6 breaths is diagnostic of oesophageal intubation and mandates **immediate tube withdrawal and re-intubation**. ## Why Other Investigations Are Secondary | Investigation | Utility | Limitation | |---|---|---| | **Chest X-ray** | Confirms tube position post-hoc | Delayed; requires time; not real-time | | **ABG** | Assesses gas exchange status | Does not confirm tube location; reflects consequence, not cause | | **TOE** | Assesses cardiac function | Not a tube placement confirmation tool; invasive | **Clinical Pearl:** In the operating theatre, capnography is **non-negotiable** — it is the only investigation that provides **immediate, continuous, real-time confirmation** of tube placement. Chest X-ray is a confirmatory adjunct after correction. ## Management Algorithm ```mermaid flowchart TD A[Acute hypoxia + hypotension + absent breath sounds]:::urgent --> B{Capnography waveform present?}:::decision B -->|No CO₂ waveform| C[Oesophageal intubation diagnosed]:::outcome C --> D[Immediately withdraw tube]:::action D --> E[Re-intubate trachea under direct vision]:::action E --> F[Confirm with capnography]:::decision F -->|CO₂ waveform present| G[Secure tube, proceed with surgery]:::action B -->|CO₂ waveform present| H[Tube is tracheal]:::outcome H --> I[Investigate other causes of hypoxia]:::action ``` **Mnemonic:** **ETCO₂ = Endotracheal Tube Confirmation** — if capnography shows CO₂, the tube is in the trachea.