pulse oximetry
Summary
Pulse oximetry is a non-invasive method to measure oxygen saturation (SpO2) of hemoglobin in arterial blood using light absorption principles. Normal values are 95-100%, with readings below 90% indicating hypoxemia requiring immediate attention.
Detail
Pulse oximetry works by emitting red (660 nm) and infrared (940 nm) light through tissue, typically a fingertip or earlobe. Oxygenated hemoglobin (oxyhemoglobin) absorbs more infrared light, while deoxygenated hemoglobin absorbs more red light. The ratio of absorption at these wavelengths allows calculation of oxygen saturation. The device uses photoplethysmography to detect pulsatile arterial flow, distinguishing arterial from venous blood. Limitations include poor accuracy in carbon monoxide poisoning (carboxyhemoglobin appears as oxyhemoglobin), methemoglobinemia, severe anemia (Hb <5 g/dL), nail polish, poor perfusion, and motion artifacts. The oxygen-hemoglobin dissociation curve shows that SpO2 of 90% corresponds to PaO2 of approximately 60 mmHg. Pulse oximetry is essential for monitoring respiratory status, guiding oxygen therapy, and detecting hypoxemia before clinical cyanosis appears (which occurs around 85% saturation).
Sources
- Harrison's Principles of Internal Medicine
- Respiratory Physiology: The Essentials (West)
- Critical Care Medicine: Principles of Diagnosis and Management
- Pathophysiology of Disease (Hammer & McPhee)
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