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Monitoring in anaesthesia — oximetry, capnography and depth of anaesthesia
Basic monitoring in every anaesthetic covers oxygenation, ventilation, circulation and temperature — and whoever can read the capnography trace and knows the limits of pulse oximetry catches most mishaps before they become disasters.
Why it matters in the exam
Questions show a capnography waveform or a sudden change in EtCO2 and ask for the cause: disconnection, oesophageal intubation, embolism, bronchospasm, rebreathing or malignant hyperthermia. Others test when pulse oximetry is wrong (CO poisoning, methaemoglobinaemia, dyes), the target BIS value, and what happens to the arterial-line pressure when the transducer is at the wrong height.
Under the ASA standards, every general anaesthetic measures inspired oxygen, pulse oximetry, continuous capnography, a continuous ECG and blood pressure at least every 5 minutes, and temperature when changes are expected. EtCO2 is usually 2–5 mmHg below PaCO2; the gap widens when dead space grows — in pulmonary embolism, low cardiac output and lung disease. A saturation of 90% corresponds roughly to a PaO2 of 60 mmHg, and below that point the curve is steep.
In exam questions remember the order in time: capnography responds at once to a ventilation problem, while pulse oximetry responds late, especially after preoxygenation. So a disconnection or an oesophageal tube is detected first on capnography. Consciousness is monitored too: awareness under anaesthesia is more common with intravenous anaesthesia plus paralysis, at caesarean section and in a hypotensive trauma patient, and depth-of-anaesthesia monitoring helps in these settings.
Key points
- Basic monitoring: inspired oxygen, pulse oximetry, capnography, ECG, blood pressure every 5 minutes, temperature.
- Capnography confirms the tube and detects disconnection, embolism and rebreathing.
- Pulse oximetry is wrong in CO poisoning (falsely high) and methaemoglobinaemia (around 85%).
- BIS in the 40–60 range in general anaesthesia.
- The arterial-line transducer is zeroed at the level of the right atrium.
Summary table
| Finding | Likely meaning | Exam point |
|---|---|---|
| EtCO2 drops to zero at once | Disconnection, oesophageal intubation, complete obstruction, cardiac arrest | "No trace = tube not in place" until proven otherwise |
| EtCO2 drops sharply but not to zero | Pulmonary or air embolism, sudden fall in cardiac output | The PaCO2–EtCO2 gap widens |
| EtCO2 rises gradually | Hypoventilation, CO2 absorption in laparoscopy, fever, malignant hyperthermia | An unexplained rise with tachycardia → malignant hyperthermia |
| Baseline does not return to zero | Rebreathing: exhausted CO2 absorbent or a faulty valve | Change the absorbent, check the valves |
| Shark-fin waveform | Bronchospasm or obstruction to expiratory flow | Upsloping phase 3 |
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Sources
- ASA Standards for Basic Anesthetic Monitoring, last amended 2020 — Oxygenation, ventilation, circulation and temperature
- Miller's Anesthesia, 9th edition (2020) — Respiratory Monitoring; Cardiovascular Monitoring; Monitoring the State of the Brain
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.