J2i: Interpret normal and abnormal arterial blood gases and differentiate arterial from venous blood

Arterial blood gas

  • A blood test that is used to provide information on gas exchange, ventilation and acid base status from an arterial blood supply

Normal values

pH

Hydrogen

7.35-7.45

PaO2

Oxygen (tension)

70-100 mmHg

PaCO2

Carbon dioxide

35-45 mmHg

HCO3-

Bicarbonate

22-26 mmol/L

Measured

  • pH: Sanz electrode
  • PaO2: Clarke electrode
  • PaCO2: Severinghaus electrode

Derived

HCO3, base excess

Interpretation

Step wise approach

pH

  • Increase = alkalosis
  • Decreased = acidosis

PaCO2

  • Increased
    • Primary respiratory acidosis (if pH < 7.35)
    • Respiratory compensation for metabolic alkolosis (if pH > 7.45)
  • Decreased
    • Primary respiratory alkalosis (if pH > 7.45)
    • Respiratory compensation for metabolic acidosis (if pH < 7.35)

HCO3

  • Increased
    • Primary metabolic alkalosis (if pH > 7.45)
  • Decreased
    • Primary metabolic acidosis (if pH < 7.35)

Compensation rules

Full compensation

  • pH is within the normal range

Partial compensation

  • Changes in PaCO2 + HCO3 occurs however pH not within normal physiological range

Expected compensation

  • Expected HCO3 change due to respiratory issues
    • Respiratory acidosis
      • Acute: 24 + (PaCO2 – 40)/10 x1
      • Chronic: 24 + (PaCO2 – 40)/10 x 4
    • Respiratory alkalosis
      • Acute: 24 + (PaCO2 – 40)/10 x 2
      • Chronic: 24 + (PaCO2 – 40)/10 x 5
    • Expected CO2 change due to metabolic issues
      • Metabolic acidosis: 1.5 x [HCO3] + 8
      • Metabolic alkalosis: 0.7 x [HCO3] + 20

Other useful information

A-a gradient

  • PAO2 – PaO2
    • PAO2 = (713 – FiO2) – (paCO2 x1.25)

PaO2 / FiO2 ratio

  • 300-200: mild ARDS
  • 200-100: moderate ARDS
  • <100: severe ARDS (45% mortality)

Consideration

Clinical content

  • The number on an ABG itself is not enough to determine “normal”
  • Rather depends on clinical content
  • Patients on supplemental oxygen with a “normal” range PaO2 is overall an abnormal result suggesting patient is hypoxic without flow

Temperature

  • ABG provides temperature corrected values
  • Hypothermia most clinical relevant – affects pH and gas solubility

Differentiation of arterial and venous blood samples

Colour

  • Dependent on oxygenation
    • Arterial: typically bright red
    • Venous: typically dark red

Location

  • Arterial: taken from artery
  • Venous: taken from vein
  • Note: it is important to be aware if taking blood from an upstream vessel that no other drug is being infused as this can influence measurements on samples

Correlation of information

  • pH
    • Good correlation
    • Venous gas: H+ concentration 2-4nmol/L higher
      • Therefore pH will be 0.02-0.04 lower than arterial sample
  • PaO2
    • PaO2 cannot be correlated between arterial and venous blood sample
    • Necessary for accurate assessment of oxygenation
  • PaCO2
    • Good correlation however depending on clinical content, may require increase specificity and therefore ABG may be more critical in decision making than VBG
    • Venous gas: PaCO2 3-8mmHg higher
  • HCO3
    • Good correlation
    • Venous gas: HCO3 0.5-1.5 mmol/L higher

Other

  • Lactate: Good correlation. 0.02-0.08 difference
  • Base deficit: Good correlation. Temperature dependent
  • Electrolytes: depending on flow of blood, influence lysis of RBC – can influence potassium concentration

Author: Suzanne Luong