CanadiEM Podcasts: CRACKCast, ClerkCast, CarmsCast, First Year Diaries CRACKCast E005 - Patient Monitoring
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Feb 4, 2016 A lively dive into patient monitoring tools and their limits. Pulse oximetry basics and common situations that give false readings are listed. Practical uses of capnography and differences between qualitative and quantitative monitoring are explored. The shape and phases of the end-tidal CO2 waveform are explained. Clear reasons for placing invasive arterial monitoring are outlined.
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Pulse Oximetry Measures Hemoglobin Saturation Not PaO2
- Pulse oximetry measures percentage of hemoglobin bound to oxygen, not arterial oxygen tension (PaO2).
- Oximeters are reliable mainly between 80–100% saturation and small SpO2 changes can reflect large PaO2 shifts on the steep part of the curve.
Avoid Relying On SpO2 In Dyshemoglobinemia
- Don't trust SpO2 in suspected dyshemoglobinemia like methemoglobinemia or carbon monoxide poisoning because readings can be falsely normal or high.
- Recognize clinical signs (cyanosis, headache, lactic acidosis) and use ABG or co-oximetry when exposure suspected.
Mnemonic To Remember When SpO2 Is Unreliable
- Remember specific causes of unreliable SpO2 using the mnemonic SpO2: S for structural hemoglobin changes, P for post-methylene blue, O2 for nail polish and low perfusion.
- Think methemoglobinemia, CO poisoning, blue nail polish, poor perfusion, dark skin as pitfalls.