Patient Monitoring and Physiologic Data Interpretation
Trust the waveform, not the number
Capnography is the most reliable confirmation of tracheal intubation. A normal capnogram has a flat inspiratory phase, a steep upstroke, an alveolar plateau, and a rapid downstroke to near zero. No waveform after intubation means esophageal placement until proven otherwise - do not be talked out of it by chest rise or a single colorimetric reading. A sloping (shark-fin) upstroke suggests obstruction: bronchospasm, a kinked tube, or COPD. A low plateau with normal shape suggests hyperventilation or reduced pulmonary blood flow.
Pulse oximetry and its failure modes
SpO2 lags true desaturation by 30–60 seconds. Methemoglobinemia typically pins the reading near 85% regardless of true saturation; carboxyhemoglobin also reads falsely high. Poor perfusion, motion, nail polish, and ambient light produce dropouts or errors. A reassuring saturation should never override a falling ETCO2 or a worsening examination.
Neuromuscular and hemodynamic data
Train-of-four: at least 2 twitches are needed to reverse with neostigmine, and adequate recovery is a TOF ratio >0.9. An arterial waveform should be square and free of under- or overdamping artifact. CVP and PA waveforms help with rhythm, tamponade, and trend, but should not be read as absolute preload.
Rule of thumb: an ETCO2 falling toward zero means disconnection, obstruction, or cardiac arrest - check the patient and circuit before adjusting a monitor.
Trap: over-reading ETCO2 during CPR; a value persistently below about 10 mmHg is a poor prognostic sign.