Definition of "ABG"

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ABG (Arterial blood gas) is a blood test performed on arterial blood. It is useful in pulmonology and critical care to determine gas exchange, reflecting gas exchange across the alveolar-capillary membrane. It is thus one of the most common tests performed in ICU's.

VBG (Venous blood gas) is analogous to an ABG, but taken from a vein.

Patient information

What's the difference between an ABG and VBG?
ABG is done on an artery, VBG on a vein.

Method
  • Obtaining blood from either:
    • Puncturing an artery (most commonly, the radial artery at the wrist, but sometimes the femoral artery in the groin, or even other other sites) with a thin needle, syrindge, and draw blood
    • An existing arterial catheter
Interpretation
  • pH (acidity), which if low is acidemic, and if high is alkalemic
  • Carbon dioxide related, including:
    • PaCO2 (partial pressure of carbon dioxide), which indicates CO2 production and elimination, with a high PaCO2 indicating retention (hypoventilation, or respiratory acidosis), and a low PaCO2 indicating hyperventilation (or respiratory alkalosis). PaCO2 relates to respiratory, because hypoventilation and hyperventilation respectively alter CO2 in blood
    • HCO3- (bicarbonate level), which can indicate a metabolic problem, with a high HCO3- indicating metabolic alkalosis (surplus of HCO3- causing alkalosis), or a low HCO3- indicating metabolic acidosis (lack of HCO3- causing acidosis). HCO3- relates to metabolic, because the kidneys form HCO3- to compensate for imbalances
    • If both values are abnormal in the same direction, it is a "simple" acid-base disorder. Otherwise, suspect a mixed disorder
    • Distinguish the initial change from the compensatory response. The initial change is the abnormal value that correlates with the abnormal pH:
      • For alkalosis (retaining CO2), the 
      • If PaCO2 is the initial change, the process is respiratory. If HCO3- is the initial change, the process is metabolic
  • PaO2 (partial pressure of arterial oxygen), which if low, the Pt is hypoxemic. [NB: a low PaO2 is not required to be hypoxic, which is where a region of the body is oxygen deprived.] If <60, supplemental O2 should be administered. <26 is at risk of death
  • Many blood gas analyzers will also report concentrations of:
    • SaO2 (Arterial oxyhemoglobin saturation)
    • Lactate
    • Hemoglobin (Hg)
    • Several electrolytes
    • Oxyhemoglobin (Hb-O2)
    • Carboxyhemoglobin (CO-Hb)
    • Methemoglobin (Hb-Fe3+)

BE is shorthand for Base Excess

Patient information

What do the numbers for pH mean?
It's talking about how acidic blood is.

Carbon dioxide and bicarbonate, what's the difference?
CO2 is acid, and bicarbonate is basic. CO2 represents breathing, because you breathe CO2 out, so breathing out CO2 makes blood more basic. Bicarbonate represents the kidneys, and because it creates bicarbonate, the kidneys make blood more basic. So both breathing and the kidneys make blood more basic.

Reference ranges depend on whether gases is of arterial or venous blood:

  Arterial Venous
pH 7.35-7.45 7.32-7.42
PaO2 80-100 mmHg 28-48 mmHg
HCO3 22-26 mEq/L 19-25 mEq/L
PaCO2 35-45 mmHg 38-52 mmHg
BE -2 to +2 mEq/L  
SaO2 95%-100% 50-70%

Source: Global RPH

For example:

  • pH<7.35, PaCO2 >45, HCO3- normal: pH is acidemic. Acidemia can be caused by lack of ventilatory compensation (CO2-) or lack of renal compensation (HCO3-). In this case, it is caused by CO2, so it's lack of ventilatory compensation, so a respiratory acidosis
  • pH<7.35, PaCO2>45, HCO3>26: pH is acidemic. Acidemic (per above). In this case, the initial response is the only movement in the correct direction, which if causing acidemia, is due to too little kidney, or too little breathing. CO2 is high, but HCO3 is not low. Thus it is initial respiratory acidosis. The compensation is not full though, because pH is still acidemic, so partly compensated respiratory acidosis
  • pH>7.45, PaCO2<35, HCO3- normal: pH is alkalemic. Alkalemia can be caused by hypoventilation (PaCO2 --) or renal overdrive (HCO3- ++). In this case, it is caused by hypoventilating, so a respiratory alkalosis
  • pH>7.45, PaCO2<35, HCO3- normal: pH is alkalemic. Alkalemia (per above). In this case, the initial response is the only movement in the correction, which is hypoventilation (PaCO2--). Thus it is initial respiratory alkalosis. The compensation is not fully though, because pH is still alkalosis, so partly compensated respiratory alkalosis
  • pH<7.35, PaCO2 normal, HCO3- <22: pH is acidemic. Acidemia can be caused by hyperventilation (PaCO2 ++) or lack of renal compensation (HCO3- --). In this case, it is caused by lack of renal compensation, so a metabolic acidosis
  • pH<7.35, PaCO2<35, HCO3- <22: pH is acidemic. Acidemic (per above). In this case, the initial response is the only movement in the correct direction, which is lack of renal compensation (HCO3-). Thus it is initial metabolic acidosis. The compensation is not fully though, because the pH is still acidemic, so partly compensated metabolic acidosis
  • pH>7.45, PaCO2 normal, HCO3- >26: pH is is alkalemic. Alkalemia can be caused by hypoventilation (PaCO2 --) or renal overdrive (HCO3- ++). In this case, it is caused by renal overdrive, so it is metabolic alkalosis
  • pH>7.45, PaCO2>45, HCO3- >26: pH is alkalemic. Alkemia (per above). In this case, the initial response is the only movement in the correct direction, which is renal overdrive (HCO3- ++). Thus it is initial metabolic alkalosis. The compensation is not fully though, because the pH is still alkalemic, so partly compensated metabolic alkalosis

Source: UCHC (Also covers Acute/chronic, Anion gap, Delta-Ratio, Urine anion gap)

See also
  • Pulse oximetry, which can obtain similar information
  • ICU
  • Metabolic acidosis
  • Metabolic alkalosis
  • Respiratory acidosis
  • Respiratory alkalosis

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Definition of ABG | Autoprac


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