Introduction to ABGs

• What is the pH in veins?

  7.33 - 7.44
• What is the actual HCO3- in veins?

  24 - 30
• What is the PaCO2 in veins?

  5.3 - 6.7
• What is the PaCO2 in alveolar gas?

  4.8
• What is the pH in arteries?

  7.35 - 7.45
• What is the PaCO2 in arteries?

  4.7 - 6.0
• What is the PaCO2 in arteries?

  10 - 13
• What is the PaO2 in alveolar gas?

  14.2
• What is a conjugate acid?

  When a base gains a H+
• What is the actual HCO3- in arteries?

  22 - 28
• What is the PaO2 in veins?

  4.0 - 5.5
• What is metabolic acidosis?

  When [HCO3-] decreased & pH decreases
• What is Henry's Law?

  Concentration in liquid phase is proportional to partial pressure in gaseous phase
• How are respiratory & metabolic systems involved in pH homeostasis?

  Resp

  • change in resp rate -> can change PaCO2 RAPIDLY
  • lung pathology can also change PaCO2

  Metabolic

  • kidneys regulate HCO3- levels SLOWLY
  • many pathologies can change HCO3- & other acids/bases
• What is Type II respiratory failure?

  Failure in process of breathing (underventilation)

  = low O2 & high CO2
• What are the steps for interpreting an ABG?

  Steps

  1. Look at oxygenation
  2. Look at pH
  3. Look at pH-PaCO2 relationship
  4. Look at bicarb
  5. Base Excess
  6. Look for compensation
• What is respiratory acidosis?

  When CO2 increases & pH decreases
• What is a cinjugate base?

  Particle that remains when an acid has donated a H+
• How can you tell whether a pH is respiratory or metabolic?

  Using PaCO2 (normal= 4.7 - 6 kPa)

  If acidotic then…

  • PaCO2 > 6 = RESPIRATORY acidosis
  • PaCO2 < 4.7 = METABOLIC acidosis

  If alkalotic then…

  • PaCO2 > 6 = METABOLIC alkalosis
  • PaCO2 < 4.7 = RESPIRATORY alkalosis
• What is Type I respiratory failure?

  Faliure of oxygenation (normal/low PaCO2 AND low O2 (PaO2 < 8kPa))

  Impaired gas exchange (V/Q mismatch) leads to increased ventilation -> indicative of something going wrong in the lungs
• What does BE < -2 mEq/L mean?

  Low HCO3-

  = metabolic ACIDOSIS
• Alkalosis

  pH > 7.45
• What is Ka?

  Acid dissociation constant

  How readily acids dissociate into H+ & A-
• Define partial pressure.

  Total pressure an individual gas would exert on its own in the same total volume at the same temperature
• What is classed as resp failure?

  PaO2 < 8kPa
• What is metabolic alkalosis?

  When [HCO3-] increases & pH increases
• Acidosis


  pH < 7.35
• What is respiratory alkalosis?

  When CO2 decreases & pH increases
• What is the difference between actual & standard HCO3-?

  Actual HCO3- is used to assess both metabolic & respiratory causes of acid-base disturbances

  Standard HCO3- is a measure of only the metabolic component
• What happens when there is physiological dead space? How is this compensated?

  High V/Q ratio (decreased perfusion)

  -> reduction in effective alveolar ventilation -> could lead to reduction in PaO2

  Compensation by bronchoconstriction & reduced surfactant -> helps divert air to well-perfused alveoli
• What does a BE > 2 mEq/L mean?

  High HCO3-

  = metabolic ALKALOSIS
• What does the body do in response to respiratory alkalosis?

  Type B intercalated cells in the kidneys secrete HCO3- into lumen -> increased [HCO3-] in urine -> decreases plasma [HCO3-] & decreases plasma pH
• How do you find out if there is any compensation in respiratory acidosis/alkalosis?

  Respiratory ACIDOSIS

  • Normal BE = no compensation (acute)
  • High BE = compensation (chronic)
  • Low BE = mixed resp & metabolic

  Respiratory ALKALOSIS

  • Normal BE = no compensation (acute)
  • Low BE = compensation (chronic)
  • High BE = mixed resp & metabolic
• What does the body do in response to respiratory acidosis?

  Liver produces more glutamine

  Kidney produces glutamate dehydrogemase & PECK to breakdown glutamine -> glutamine breaks down into NH4+ & HCO3-

  = increased plasma [HCO3-] & increases pH
• What happens when there is physiological shunting? How is this compensated?

  Low V/Q ratio (decrease in ventilation)

  -> poorly oxygenated blood mixed with blood from normally ventilated side -> reduction in PaO2

  Compensation by hypoxic vasoconstriction -> helps to divert blood to well-ventilated alveoli
• How is a stable pH maintained?

  Respiratory & metabolic systems act together to maintain stable pH

  NOTE: even for the respiratory system, pH is the priority, not CO2 or O2
• How does hyperventilation affect PaCO2?

  Decreases
• What is the base excess in arteries & veins?

  -2 to +2
• Define acid.

  Proton donor
• Define base.

  Proton acceptor