Show the model answerAttempt it first — that is what makes it stick
4 marks
Outline the principles of measurement of CO₂ in the anaesthetic breathing system
What earns the marksprinciples
| Infrared absorption | The principle, not the sampling method |
|---|---|
| Other methods, briefly | Mass spectrometry, Raman, colorimetric |
- Carbon dioxide absorbs infrared because its vibrations change its dipole moment. Peak absorption is at 4.26 µm; analysers use a narrow band of 4.2 to 4.3 µm. Oxygen and nitrogen are symmetrical, have no dipole moment and cannot be measured this way.
- Infrared is passed through a sample chamber of fixed path length with sapphire windows, since glass absorbs infrared, and the transmitted intensity is measured by a photodetector against a reference path.
- The Beer–Lambert relationship converts absorbance to concentration.
- The result is scaled against a known standard, with the analyser zeroed against carbon dioxide-free gas and calibrated against a known mixture.
Add the principal source of error, which is worth stating in a measurement question: collision broadening by nitrous oxide and oxygen widens the carbon dioxide absorption peak so the analyser over-reads, and nitrous oxide additionally absorbs close to the carbon dioxide band. Modern analysers correct for both automatically.
2 marks
List the types of capnography available and compare between them
What earns the markscompare
| Sidestream and mainstream | Both, with a real comparison |
|---|---|
| Compare on axes | Response time, dead space, water trap, weight, cost |
| Sidestream | Mainstream | |
|---|---|---|
| Where the gas is analysed | Aspirated away to a remote analyser | In the breathing system at the airway |
| Sampling | 50–200 mL/min through fine-bore tubing | No gas removed from the circuit |
| Delay | Transit delay of about 2–4 s plus analyser rise time | Effectively real time |
| Water and secretions | Water trap and filter needed; line may block or kink | Cuvette heated to about 40 °C against condensation; windows may be soiled |
| Weight and dead space | Light; negligible added dead space | Heavier at the airway; adds apparatus dead space, which matters in neonates |
| Other gases | The same sample gives O₂, N₂O and volatile agent | Usually carbon dioxide alone |
| Non-intubated patient | Usable via nasal cannulae or mask | Needs an airway device |
Commonly lost: Compare this with April 2015: there, describing sidestream and mainstream sampling earned nothing. Here it is the whole of part (b). Same topic, opposite instruction — read the wording, do not recognise the subject.
4 marks
Describe the role of capnography in anaesthetic practice
What earns the marks4 marks
| The part most could do | Examiners noted most would have failed badly without it |
|---|---|
| Do not leave it last | Four of the marks are here |
| Role | What it does |
|---|---|
| Confirming tracheal intubation | A sustained waveform over six breaths confirms tracheal placement. The single most important use, and the standard of care at every intubation. |
| Continuous airway monitoring | Detects disconnection, extubation, tube obstruction and circuit failure faster than any other monitor, including pulse oximetry. |
| Adequacy of ventilation | Continuous trend of hypo- and hyperventilation between arterial blood gases, and a guide to setting minute ventilation. |
| Adequacy of circulation | A fall with unchanged ventilation reflects reduced pulmonary blood flow — embolism, low cardiac output, haemorrhage or arrest. |
| Cardiopulmonary resuscitation | Confirms tube placement without interrupting compressions, indicates compression quality, and signals return of spontaneous circulation. |
| Diagnosing the breathing system | A raised baseline identifies rebreathing; a shark-fin trace identifies obstruction to expiratory flow and tracks the response to a bronchodilator. |
| Metabolic monitoring | A progressive rise resistant to increased ventilation is the earliest and most sensitive sign of malignant hyperthermia. |
| Patient effort and depth of block | A curare cleft warns that spontaneous respiratory effort is returning during controlled ventilation. |
| Sedation, recovery and transfer | Nasal capnography detects apnoea and airway obstruction in the non-intubated patient earlier than desaturation does. |
Commonly lost: Quite a number did not mention capnography for confirming correct tracheal tube placement. In any question about the role of capnography, that is the opening line.
Taking it further
Viva prompts
You are anaesthetising a 2 kg neonate. Which type of capnography would you choose, and why?
Answer
The trade-off runs both ways. Sidestream removes 50–200 mL/min from a circuit whose total minute ventilation may be under 1 L/min, and its transit delay against a short expiratory time means the plateau may never be reached, so end-tidal carbon dioxide is under-read. Mainstream avoids both but adds apparatus dead space and weight at the airway, which matter disproportionately at this size. Low-dead-space neonatal mainstream cuvettes exist and are generally preferred; if sidestream is used, reduce the sample flow and shorten the line.
You intubate and see no waveform at all. Walk me through what you do.
Answer
Assume oesophageal intubation until proven otherwise. Call for help, maintain oxygenation, and look: direct or video laryngoscopy to see the tube between the cords.
The differential also includes complete tube obstruction, a disconnected or blocked sampling line, ventilator failure and cardiac arrest — a patient with no cardiac output produces no waveform even through a correctly placed tube. But the safe default is if in doubt, take it out.
You see two or three small waveforms and then nothing. What does that mean?
Answer
Carbon dioxide that entered the stomach during bag-mask ventilation, or from a recent carbonated drink, being washed out over the first few breaths — an oesophageal intubation. Each waveform is small, misshapen and smaller than the last, and the trace then flatlines. This is exactly why confirmation requires a sustained waveform over six breaths rather than the presence of any trace at all.
Why does capnography detect a disconnection before pulse oximetry does?
Answer
Capnography reports gas movement directly and changes on the very next breath. Pulse oximetry reports arterial saturation, which is defended by the oxygen stored in the functional residual capacity and by the flat upper part of the oxyhaemoglobin dissociation curve — so after preoxygenation there may be minutes of delay before the saturation falls, and by then the patient is already hypoxaemic. Capnography warns; oximetry reports the consequence.
End-tidal carbon dioxide climbs during a laparoscopy. When is that expected, and when would you worry?
Answer
Expected: a gradual rise from peritoneal absorption of insufflated carbon dioxide, plateauing after roughly 15–30 minutes and usually correctable by increasing minute ventilation. It is worse with extraperitoneal insufflation and with subcutaneous emphysema.
Worrying: a rise that continues despite increased ventilation, which raises malignant hyperthermia. And the opposite pattern — a sudden fall with hypotension and desaturation — which suggests carbon dioxide embolism. Stop insufflation, release the pneumoperitoneum, give 100% oxygen and place the patient head-down in the left lateral position.