Basics of Blood Gases

Lexi Haugh

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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”