Patient Data: Gathering, Validating, and Interpreting Clinical Information
Collect before you interpret
Every data question starts with the same discipline: confirm the source and quality of the value before reasoning from it. Verify that a blood gas is arterial (PaO2 and PaCO2 relationship is plausible), that a pulse oximeter is tracing a believable plethysmograph, and that a spirometry effort meets acceptability and repeatability criteria before you act on the number.
Core concepts you must own
- Vital signs and expected ranges, including how fever, pain, and anxiety shift heart rate and respiratory rate.
- ABG interpretation in three steps: pH status, primary disorder from PaCO2/HCO3, then compensation. Memorize the quick rules: a 10 mm Hg rise in PaCO2 lowers pH about 0.08 and raises HCO3 about 1 mEq/L acutely.
- Oxygenation versus ventilation: PaO2, SaO2, and SpO2 measure different things, and oxygen content also depends on hemoglobin.
- Capnography: the EtCO2 value is read at the end of phase III, not the peak of the upstroke; a sloping upstroke suggests obstruction.
- Hemodynamics: cardiac output, cardiac index, and SVR, and how they pair with the physical exam.
- Radiographic and laboratory findings you must be able to describe and report, not diagnose.
- Documentation: what you measured, when, with what device, and what you did about it.
Traps
Expect items where the SpO2 looks reassuring but oxygenation is not (carbon monoxide, anemia, poor perfusion, motion artifact, dark nail polish) and items where a "normal" PaCO2 masks chronic compensation. Also expect questions on whose job it is to report abnormal data - almost always yours, promptly.
Worked rule
pH 7.28, PaCO2 60 mm Hg, HCO3 26 mEq/L. The pH is acidemic and the PaCO2 is elevated, so the primary problem is respiratory acidosis. Expected acute HCO3 at that PaCO2 is about 25 mEq/L; 26 is within 1, so this is acute with essentially no compensation. Report it and escalate rather than waiting for the next gas.