Basics of Blood Gases¶
Lexi Haugh
Did you order an ABG or VBG?¶
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Need to assess oxygenation ABG
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Need to assess hypoventilation May use VBG. ABG if shock, severe hypercapnia
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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
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ABGs are obtained by respiratory both at VUMC and the VA
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VBGs can be ordered like other labs on the floor. Ran as point of care test (iSTAT) in the ICU
Assessing Oxygenation:¶
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A-a gradient = PAO2 (alveolar O2) – PaO2 (arterial O2)
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PAO2 = FiO2 (Pbarom - PH2O) - PaCO2/R
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FiO2 = 0.21 on room air otherwise obtain from ventilator
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Pbarom = 760, PH2O = 47
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R (respiratory quotient) = 0.8
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PaCO2 = arterial CO2 measured on ABG
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Normal A-a gradient = (Age +10)/4
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See hypoxia section for ddx of increased A-a gradient
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Assessing Acid/Base Status and Ventilation:¶
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Look at the pH (normal = 7.35 – 7.45): pH \< 7.35 = acidosis; pH > 7.45 = alkalosis
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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 |
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Is the primary disorder acute vs. chronic?
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Is the primary disorder appropriately compensated?
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If not appropriately compensated, what additional process is present?
Primary Respiratory Acidosis:¶
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Acute versus chronic
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Acute respiratory acidosis and hypercarbia will often present with somnolence or AMS
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Similarly, if a PCO2 is 80, and the patient is talking to you, it is most likely chronic
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Acute Causes: Decreased respiratory drive (opiates, intoxication), respiratory muscle weakness (i.e., myasthenia gravis)
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Chronic Causes: COPD, sleep apnea
Primary Respiratory Alkalosis
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Acute Causes: Mechanical ventilation, anxiety/panic attack, pain, PE
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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¶
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Appropriate Respiratory Compensation?
- Expected pCO2 = 0.7*(HCO3) + 20 (±5)
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Etiologies: Associated with high aldosterone (either appropriate or inappropriate)
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Chloride Responsive = appropriate hyperaldosteronism, can usually be fixed with Cl administration/ volume resuscitation
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Etiologies: Volume contraction i.e., vomiting, over-diuresis)
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Urine Cl \<20
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Chloride Unresponsive = inappropriate hyperaldosteronism, cannot fix with NaCl administration/ volume resuscitation
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Etiologies: Steroids, Cushing’s, Conn’s, RAS, CHF, cirrhosis
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Urine Cl >20
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Primary Metabolic Acidosis: See nephrology section, “Metabolic Acidosis”