• Oxygenation status can be assessed using the following values obtained from ABG (VBG is not reliable):
PaO2: oxygen tension (amount of oxygen dissolved) in arterial blood
SaO2: proportion of oxyhemoglobin (oxygen bound to hemoglobin) in arterial blood
PaO2/FiO2 ratio: assess oxygen tension based on varying numbers of FiO2 often seen in ventilated patients
CaO2: oxygen content in arterial blood
PAO2: oxygen tension in alveoli, calculated using the formula: PAO2 = FiO2 x (Patm - PH2O) - PaCO2/0.8
P(A-a)O2 or A-a gradient: reflects oxygen movement from the alveoli to arterial blood, measured and expected values are calculated using the formulas shown below
• In addition, co-oximetry analysis using arterial blood sample can offer useful information regarding oxygenation status:
tHb: total hemoglobin concentration in arterial blood
FCOHb: proportion of carboxyhemoglobin (carbon monoxide bound to hemoglobin) in arterial blood
FMetHb: proportion of methemoglobin (hemoglobin with oxidized iron which cannot bind reversibly to oxygen)
• Hypoxemia is defined as abnormally low PaO2, caused by 5 main mechanisms:
Low FiO2: abnormally low oxygen content in inspired air
Hypoventilation: not breathing in enough air due to any cause interfering with breathing mechanism including the lungs, airway, chest wall, respiratory muscles and nerves, and central respiratory drive in the brain
V/Q mismatch: areas in the lungs where ventilation is normal but perfusion is inadequate, or vice versa (recall- ventilation and perfusion level must match for optimal oxygen movement from outside air to alveoli to pulmonary capillaries)
O2 diffusion impairment: defective alveolar walls preventing O2 from moving across
Right to left shunt: either anatomic (deoxygenated blood goes straight to the left heart bypassing alveolar capillaries) or physiologic (alveoli adequately perfused but not ventilated)
• Hypoxia is defined as inadequate oxygen delivered to tissue, which is different from:
Hypoxemia: low oxygen tension in blood
Ischemia: reduced blood flow to tissues
• Based on the equation for oxygen delivery below, hypoxia can result from either:
Low cardiac output (Q)
Low hemoglobin concentration (Hb)
Low oxygen bound to Hb (SaO2)
Low oxygen tension (PaO2)
• In addition, tissues’ inability to use delivered oxygen, oxygen overuse, or any change in tissue architecture that limits oxygen diffusion.
