• Blood gases, including ABG (arterial) and VBG (venous), are often obtained from to assess acid-base status, oxygenation, and ventilation.
• Standard blood gas measurements includes pH level, oxygen tension (PaO2), carbon dioxide tension (PaCO2), oxyhemoglobin satulation (SaO2), bicarbonate concentration (HCO3), and base excess (BE).
• Some ABG analyzers also measure electrolytes (Na, K, ionized Ca, and Cl), metabolites (glucose, lactate), oxyten content (CaO2), total hemoglobin (tHb), methemoglobin (FMetHb), and carboxyhemoglobin (FCOHb).
• Based on the input values of oxygen fraction in inspired air (FiO2), PaO2/FiO2 ratio, alveolar oxygen tension (PAO2), alveolar-arterial oxygen gradient P(A-a)O2 are calculated by the analyzer.
• In cases when ABG sample cannot be obtained (e.g. peripheral arterial vasoconstriction), venous blood gas (VBG) is used as an alternative.
• VBG analysis results typically include venous pH level, oxygen tension (PvO2), carbon dioxide tension (PvCO2), oxyhemoglobin satulation (SvO2), bicarbonate concentration (HCO3), and base excess (BE)-- all of which are then converted to arterial values and the conversion depends on site of venous sampling (central vs peripheral):
Generally, central VBG values are better estimates of ABG than peripheral VBG values.
Arterial pH ~ Central venous pH + 0.03-0.05 ~ Peripheral venous pH + 0.03-0.04
PaCO2 ~ Central PvCO2 - 4-5mmHg ~ Peripheral PvCO2 - 3-8mmHg
Arterial HCO3 ~ Central HCO3 ~ Peripheral HCO3 - 1-2 mmol/L
PaO2 cannot be estimated by VBG
• Acid-base, oxygenation, and ventilation status and the corresponding ABG parameters are listed below (click on each box for more details).
