Intraoperative Monitoring
Standards
Know the ASA standard monitors: pulse oximetry, capnography, ECG, blood pressure, temperature, and inspired oxygen analyzer. Understand that capnography is mandatory for all general anesthetics and for moderate sedation.
Modalities
Understand the principles and limitations of pulse oximetry (poor perfusion, dyshemoglobinemias, motion artifact), capnography (the shape of the waveform and the meaning of the end-tidal CO2), and invasive blood pressure monitoring (indications, damping, and the leveling of the transducer at the phlebostatic axis). Know the indications for central venous pressure, pulmonary artery catheter, and cardiac output monitoring.
Neuromuscular Monitoring
Understand train-of-four, tetanic stimulation, and the post-tetanic count. Know that residual neuromuscular blockade is a major cause of postoperative complications and that a train-of-four ratio >0.9 indicates adequate recovery.
Neuromonitoring
Know the role of processed EEG (BIS) in preventing awareness, and the principles of motor and sensory evoked potentials and their sensitivity to anesthetics.
Traps
Questions often test the interpretation of a capnography waveform: an abrupt fall suggests disconnection or esophageal intubation, a gradual rise suggests hypoventilation or hyperthermia. Another trap is the belief that pulse oximetry detects hypoventilation; it does not, and capnography is required. Also know that an arterial line reading can be falsely low with a damped system.
Worked Example
During laparoscopy, end-tidal CO2 rises gradually with a normal waveform. This suggests absorption of insufflated CO2 or hypoventilation; increase minute ventilation and check the airway. An abrupt loss of the waveform suggests disconnection or circuit leak, requiring immediate check of the circuit and patient.