Physics and Monitoring in Anesthesia
Concepts to know
Physics questions test gas laws and monitoring principles. Boyle's law (pressure–volume at constant temperature), Charles's law (volume–temperature), Dalton's law (partial pressures), Henry's law (solubility and partial pressure - explains why nitrous oxide diffuses into air-filled spaces), and the Bernoulli/Venturi principle (venturi masks, jet ventilation). Understand laminar vs turbulent flow (Poiseuille vs density-dependent), which explains resistance in narrow tubes.
Monitoring: pulse oximetry (absorption of red and infrared light, affected by methaemoglobin, carboxyhaemoglobin, poor perfusion, motion), capnography (waveform phases, end-tidal value, shape changes), ECG (ST analysis, artefact), invasive and non-invasive blood pressure, and neuromuscular monitoring (TOF, post-tetanic count). Understand that pulse oximetry has a delay and does not measure ventilation, and that capnography is the standard for confirming tracheal placement.
Traps
- Confusing carboxyhaemoglobin (falsely high SpO2, normal-looking) with methaemoglobin (SpO2 plateaus around 85%).
- Forgetting that nitrous oxide expands pneumothorax, bowel gas, and middle-ear spaces (Henry's law).
- Assuming a normal SpO2 excludes hypoventilation - it does not.
- Misreading a capnography waveform: a sloping upstroke suggests obstruction, not a leak.
Rule of thumb
When a monitor disagrees with the patient, trust the patient and check the monitor. For gas questions, identify which law links the variable in the stem to the one asked.
Worked example
A patient with a pneumothorax receives nitrous oxide; the pneumothorax enlarges because N2O is more soluble than nitrogen and diffuses in faster than nitrogen leaves - avoid N2O in any closed gas space.