Basics of Blood Gases

Hannah Kieffer

Measuring Oxygenation 

see also 'Hypoxia and Hypoxemia' for additional details 

  • Systemic O2 Delivery = 13.4*(Cardiac Output)*(Hb)*(O2 Saturation)
  • SpO2 vs PaO2: Before obtaining a blood gas, consider first whether you need laboratory testing to confirm oxygenation
    • SpO2 (pulse oximetry): reports oxygen saturation based on percentage of hemoglobin bound to oxygen; often considered a better reflection of oxygen content in the blood
      • Pros of SpO2: Non-invasive, easier to trend, inexpensive
      • Cons of SpO2: Less accurate in certain disease states as well as patients with darker skin and nail polish
    • PaO2 (partial pressure of oxygen in arterial blood): reports concentration of O2 in plasma
      • Pros of PaO2: provides more precise oxygenation status
      • Cons of PaO2: invasive (requires ABG), expensive (~$100-200)
    • When to get an ABG vs rely on SpO2:
      • Unreliable pulse oximetry wave form (e.g. non-pulsatile flow from ECMO, bad pleth)
      • Poor perfusion in severe shock
      • Severe anemia (SpO2 can be falsely reassuring)
      • Methemoglobinemia – artificially lowers SpO2 without affecting PaO2 and O2 delivery
      • When calculating PaO2/FiO2 ratio for determining hypoxemia severity (determine whether to prone patients)
  • A-a gradient: The difference between the oxygen levels in the alveoli vs arteries; helps determine the etiology of hypoxemia
    • Equation = PAO2 (alveolar O2) – PaO2 (arterial O2)
      • A-a gradient is assessing whether the oxygen being inhaled is getting into the blood
      • To calculate PAO2, MD Calc formula available
      • Acceptable level increases with age; to estimate normal A-a gradient= (Age +10)/4
    • Differential: see 'Hypoxia and Hypoxemia' for more details
      • High: Dysfunction to the alveolar/capillary unit, defect in diffusion, V/Q mismatch, or right-left shunt
      • Normal or Low: Hypoventilation or low FiO2

Measuring Ventilation 

  • Ventilation is typically assessed by measuring the PCO2 (partial pressure of carbon dioxide)
    • PCO2 can be measured arterially (PaCO2; gold standard) or venously (PvO2; approximation of ventilation)

Blood Gases: ABG vs VBG

Blood Gas

Reliable Values

Pros

Cons

Comments
ABG pH, PaO2, PaCO2, HCO3
  • Gold standard for determining oxygenation, ventilation, and acidbase status
  • Invasive
  • Expensive
  • VBG usually adequate for clinical decision making
 Usually obtained by RT, more cumbersome to obtain unless patient has an arterial line
VBG pH, PvCO2*, HCO3*
  • Noninvasive
  • cheaper
  • pH reliably correlates to ABG
  • Cannot assess oxygenation
  • PCO2 and HCO3 are less accurate than ABG especially in certain conditions (shock, hypercapnia)
  • Can be drawn off a peripheral IV or central line
  • A peripheral VBG PvO2 cannot be substituted for a Mixed Venous O2 from a central line
  • Culturally, we use VBGs most often for:
    • Assessing ventilation (pCO2):
      • Concern for COPD exacerbation, to assess CO2 retention
      • Respiratory support adjustments: Assessment of patient’s response to BiPAP or mechanical ventilation / determine whether setting changes are indicated
      • Generalized mental status changes, e.g. lethargy, confusion
      • Hypoventilation -> increased pCO2 and decreased pH -> encephalopathy
    • Assessing Acid/Base status (DKA, renal failure, sepsis, etc)
  • Important to note, as stated above, VBG PCO2 and HCO3 are less accurate in shock and hypercapnia, so interpret with caution

Assessing Respiratory Acid/Base Status

Respiratory Status

pH

PaCO2 (Primary Change)

HCO3 (Compensation)

Etiology/DDx
Normal 7.36- 7.44 36-44 22-26
Respiratory Acidosis ≤7.35 ≥ 45 mmHG Acute: 1 mEq increase per 10 mmHG increase in PCO2
Chronic: 3-4 mEq increase per 10 mmHg increase in PCO2		 Impaired gas exchange, decreased respiratory drive, chest/diaphragm dysfunction, iatrogenic (vent issues)
Respiratory Alkalosis ≥7.45 ≤35 mmHg Acute: 2 mEq decrease per 10 mmHg PCO2
Chronic: 4-5 mEq decrease per 10 mmHg decrease in PCO2		 Increased respiratory drive (pain, fever, anxiety), hypoxia-induced (high altitude, PE, anemia), sepsis, iatrogenic (vent issues)
  • In a compensated respiratory disturbance (i.e. normal pH, you do not need to intervene and try to normalize the PCO2)
  • Further management largely depends on underlying cause
    • In general, avoid correcting respiratory acidosis with sodium bicarbonate, even if not fully compensated; there is a lack of evidence demonstrating clinical benefit and potential risks associated
    • See Nephrology section for further discussion on metabolic acid/base disorders
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