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Basics of Blood Gases

Lexi Haugh


Did you order an ABG or VBG?

  • Need to assess oxygenation ABG

  • Need to assess hypoventilation May use VBG. ABG if shock, severe hypercapnia

  • pH VBG usually correlates with an ABG (venous pH is ~0.04 lower than arterial pH)

    • Less accurate in shock or severe acid-base disturbance
  • ABGs are obtained by respiratory both at VUMC and the VA

  • VBGs can be ordered like other labs on the floor. Ran as point of care test (iSTAT) in the ICU

Assessing Oxygenation:

  • A-a gradient = PAO2 (alveolar O2) – PaO2 (arterial O2)

    • PAO2 = FiO2 (Pbarom - PH2O) - PaCO2/R

      • FiO2 = 0.21 on room air otherwise obtain from ventilator

      • Pbarom = 760, PH2O = 47

      • R (respiratory quotient) = 0.8

      • PaCO2 = arterial CO2 measured on ABG

      • Normal A-a gradient = (Age +10)/4

    • See hypoxia section for ddx of increased A-a gradient

Assessing Acid/Base Status and Ventilation:

  • Look at the pH (normal = 7.35 – 7.45): pH \< 7.35 = acidosis; pH > 7.45 = alkalosis

  • Look at the pCO2 (normal = 35-45 mmHg)

Primary Disorder pH pCO2
Respiratory Acidosis \< 7.35 >45
Respiratory Alkalosis > 7.45 \<35
Metabolic Acidosis \< 7.35 \<45
Metabolic Alkalosis > 7.45 >35
  • Is the primary disorder acute vs. chronic?

  • Is the primary disorder appropriately compensated?

  • If not appropriately compensated, what additional process is present?

Primary Respiratory Acidosis:

  • Acute versus chronic

    • Acute respiratory acidosis and hypercarbia will often present with somnolence or AMS

    • Similarly, if a PCO2 is 80, and the patient is talking to you, it is most likely chronic

  • Acute Causes: Decreased respiratory drive (opiates, intoxication), respiratory muscle weakness (i.e., myasthenia gravis)

  • Chronic Causes: COPD, sleep apnea

Primary Respiratory Alkalosis

  • Acute Causes: Mechanical ventilation, anxiety/panic attack, pain, PE

  • Chronic Causes: Pregnancy, CNS disorder, hormones (thyroid, progesterone)

Compensation for primary respiratory disorders

Respiratory Acidosis Respiratory alkalosis
Acute 10:1 10:2
Chronic 10:3.5 10:4
For every ↑of 10 in pCO2, HCO3 ↑ by 1 or 3.5 For every ↓ of 10 in pCO2 HCO3 ↓ by 2 or 4

Primary Metabolic Alkalosis

  • Appropriate Respiratory Compensation?

    • Expected pCO2 = 0.7*(HCO3) + 20 (±5)
  • Etiologies: Associated with high aldosterone (either appropriate or inappropriate)

    • Chloride Responsive = appropriate hyperaldosteronism, can usually be fixed with Cl administration/ volume resuscitation

      • Etiologies: Volume contraction i.e., vomiting, over-diuresis)

      • Urine Cl \<20

    • Chloride Unresponsive = inappropriate hyperaldosteronism, cannot fix with NaCl administration/ volume resuscitation

      • Etiologies: Steroids, Cushing’s, Conn’s, RAS, CHF, cirrhosis

      • Urine Cl >20

Primary Metabolic Acidosis: See nephrology section, “Metabolic Acidosis”