Intraoperative Monitoring
What the exam tests
Monitoring questions test waveform interpretation, limitations, and when advanced monitors change decisions.
Core rules
- Minimum standards: ECG, SpO2, NIBP, capnography for intubation and general anesthesia, temperature for prolonged cases.
- Capnography phases: inspiratory baseline, rapid upstroke, alveolar plateau, inspiratory downstroke. A sudden zero suggests disconnection, obstruction, or cardiac arrest; a gradual rise suggests hypoventilation, fever, or malignant hyperthermia.
- Pulse oximetry lags and fails with poor perfusion, motion, dyshemoglobinemia, and severe anemia. It does not measure ventilation.
- Arterial lines provide beat-to-beat pressure and sampling; damped waveforms overestimate systolic and underestimate diastolic pressure.
- CVP is a poor predictor of fluid responsiveness; dynamic indices and passive leg raise are better.
- BIS and NIRS are adjuncts, not standalone awareness or perfusion monitors.
Worked example
During laparoscopy, ETCO2 falls abruptly from 38 to 0 mmHg and the waveform disappears. Check tube position, circuit connections, and blood pressure immediately. Reasoning: a sudden zero with no plateau means no CO2 reaching the sensor-disconnection, obstruction, or arrest-not hyperventilation. Hyperventilation lowers ETCO2 gradually and preserves a waveform.
Traps
Using SpO2 as a ventilation monitor; interpreting a cupped capnogram as normal; trusting CVP as preload; ignoring a rising ETCO2 with tachycardia and fever; treating artifact as physiology.