Basics of Blood Gases

Hannah Kieffer

Measuring Oxygenation 

See 'Hypoxia and Hypoxemia' for additional details 

  • SpO2 vs PaO2: Before obtaining a blood gas, consider first whether you need laboratory testing to confirm oxygenation 
  • Systemic O2 Delivery = 13.4*(Cardiac Output)*(Hb)*(O2 Saturation) 
  • SpO2 (pulse oximetry): reports O2 saturation based on percentage of hemoglobin bound to oxygen 
  • PaO2 (partial pressure of oxygen in arterial blood): concentration of O2 in plasma

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; assesses whether inhaled O2 gets into the blood 

  • Equation (available on MD calc) = PAO2 (alveolar O2) – PaO2 (arterial O2) 
  • Acceptable level increases with age; to estimate normal A-a gradient= (Age +10)/4 
  • See ‘Hypoxia and Hypoxemia’ for more details

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) 
  • HCO3 off a VBG is calculated, not measured

Blood Gas

Reliable Values

Pros

Cons

Comments
ABG pH,
PaO2,
PaCO2,
HCO3 		- Gold standard for determining oxygenation, ventilation, and acid-base 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* 		- Non-invasive
- Cheaper
- pH reliably correlates to ABG 		- Cannot assess oxygenation
- PCO2 and HCO3 are less accurate than ABG especially in certain conditions (shock, hypercapnia) 		- 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 CO2 retention 
    • Respiratory support adjustments: Assessment of patient’s response to changes in minute ventilation on BiPAP or mechanical ventilation 
    • Generalized mental status changes, e.g. lethargy, confusion 
  • Assessing Acid/Base status (DKA, renal failure, sepsis, etc) 
  • In a compensated respiratory disturbance (i.e. normal pH), DO NOT intervene and try to normalize the PCO2

Assessing Respiratory Acid/Base Status

Respiratory Status

pH

PaCO2 (Primary Change)

HCO3 (Compensation)

Etiology
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,
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 		Hyperventilation,
hypoxia-induced (high altitude, PE, anemia),
sepsis, iatrogenic (vent issues)
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