Pulse Oximetry: Oxygenation Saturation Measurement

 

Oxygenation: Diffusion and Perfusion

 

  • oxygenation can be measured by blood oxygen saturation
  • a pulse oximeter measures perfusion and diffusion in the capillaries

 

Oxygenation Pathway

  1. oxygen diffuses from alveoli in the lungs
  2. oxygen attach to hemoglobin in red blood cells
  3. oxygen is then transported to the tissues out to the peripheral capillaries
  4. SaO2 = the percent of oxygen bound to hemoglobin in the arteries (typically 95-100%)
  5. SaO2 is affected by factors affecting ventilation, perfusion and diffusion
  6. SvO2 = is the oxygen saturation of the venous blood (typically 70%)
  7. SvO2 is affected by factors affecting tissue oxygen demands

 

Hypoxemia: low blood oxygen can be present before recognizable signs of respiratory distress

  • tachycardia
  • tachypnea
  • cyanosis
  • agitation
  • lethargy

 

The Pulse Oximeter

  • Measures arterial oxygen saturation
  • a probe with a light emitting diode emits light at different wavelengths
  • Red and Infrared light pass through the patient’s blood
  • different wavelengths are absorbed differently by the oxygenated and deoxygenated hemoglobin molecules
  • the amount of light received by the detector on the other side indicates the amount of oxygen that is bound to hemoglobin
  • Oxyhemoglobin absorbs more infrared light than red light
  • Deoxygenated hemoglobin absorbs more red light than infrared light
  • a photo detector then calculates the differences in absorbed light and presents the pulse oxygen saturation
  • light is directed through a pulsing capillary bed site (the finger, earlobe, forehead or toe)
  • SpO2 is a reliable estimate of SaO2 when arterial hemoglobin oxygenation is > 70%

 

Indications for intermittent oxygen saturation monitoring

  1. Patient on supplemental oxygen
  2. Patient with tracheostomy on long term mechanical ventilation for stable chronic respiratory failure

 

Indications for continuous oxygen saturation monitoring:

  1. Critical Airway
  2. Unstable Airway
  3. Lung dysfunction
  4. diagnostic procedure underway which may trigger airway compromise or hypoxia

 

Causes of Erroneous readings

  1. Carbon monoxide poisoning (CO binds Hb with greater affinity than O2 with lower dissociation rates)
  2. Diagnostic Contrast Dyes – interfere with IR and red light absorbtion
  3. Anemia – adequate oxygenation limited number of Hb molecules present in the blood

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