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Year 2 Med > ABGs > Flashcards

ABGs Flashcards

1
Q

What are the normal blood gas values?

A
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2
Q

What is an acid and a base?

A
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3
Q

What is pH homeostasis?

A
  • Respiratory
    • Δ resp. rate can change PaCO₂ rapidly
    • Lung pathology can change PaCO₂
  • Metabolic
    • Kidneys regulate HCO₃⁻ levels more slowly
    • Many pathologies can change [HCO₃⁻] and other acids/bases
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4
Q

Low CO2 -> high pH…

A

Respiratory alkalosis

Eg. Altitude→hyperventilation

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5
Q

High CO2 -> low pH…

A

Respiratory acidosis

Eg. Trauma→hypoventilation

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6
Q

Low HCO3 -> low pH…

A

Metabolic acidosis

Eg. Diabetic ketoacidosis→gain in fixed acid

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7
Q

High HCO3 -> high pH…

A

Metabolic alkalosis

Eg. Chronic vomiting→loss of fixed acid

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8
Q

What are the steps to interpreting an ABG?

A
  • Step 0: oxygenation
  • Step A: acidosis/alkalosis
  • Step B: buffers/bases (CO₂ & HCO₃⁻)
  • Step C: compensation/chronic
  • Step D: differential diagnoses
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9
Q

What happens in step 0- oxygenation?

A
  • Normal = 10 – 13 kPa
  • PaO₂ <8 kPa = respiratory failure (RF)
  • Type I RF = normal or low PaCO₂ (hypoxaemic)
    • Impaired gas exchange at alveolar-capillary interface; V/Q mismatch
    • O₂ gas exchange more vulnerable than CO₂
    • Solubility: CO₂&raquo_space; O₂ → 20× more rapid diffusion
    • Proportion: In lungs, < 10% CO₂ lost; > 25% O₂ gained
    • See Fick’s Law: O₂ exchange is often compromised before CO₂
  • Type II RF = high PaCO₂ (hypercapnic)
    • Global underventilation
  • Type I & II can occur together
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10
Q

What happens in step A- acidosis or alkalosis?

A
  • Check the pH
  • Normal pH = 7.35 – 7.45
  • If pH < 7.35 → acidosis
  • If pH > 7.45 → alkalosis
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11
Q

What happens in step B- buffers (CO2)

A
  • FIRST: check PaCO₂ (N = 4.7 – 6.0 kPa)
  • Q. Is PaCO₂ contributing to or compensating for pH?
  • If pH < 7.35 (acidotic) then…
    • PaCO₂ > 6.0 → contributing → RESPIRATORY ACIDOSIS
    • PaCO₂ < 4.7 → compensating → METABOLIC ACIDOSIS
  • If pH > 7.45 (alkalotic)
    • PaCO₂ > 6.0 → compensating → METABOLIC ALKALOSIS
    • PaCO₂ < 4.7 → contributing → RESPIRATORY ALKALOSIS
  • SECOND: Check bases (HCO₃⁻ & base excess)
  • HCO₃⁻ has two possible values:
    • Actual – derived from pH and PaCO₂ using HH Equation
    • Estimate of [HCO₃⁻] actually in plasma
    • Used to assess both metabolic and respiratory causes of acid- base disturbances
    • (if there is only 1 reported value, it is likely to be this one)
    • Standard – derived plasma [HCO₃⁻] in blood equilibrated with normal PaCO₂ (5.3 kPa), PaO₂ & temperature (37°C)
    • When PaCO₂ (et al) is normal, aHCO₃⁻ = sHCO₃⁻
    • sHCO₃⁻ is a measure solely of the metabolic component of imbalance
  • Base excess (BE) measures ALL bases not just HCO₃⁻
    • HCO₃⁻ accounts for 75% of total buffering
    • In metabolic acid-base disturbances BE provides similar information to sHCO₃⁻
  • Amount of acid/base causing a deviation from pH 7.40, at normal PaCO₂ & temperature
  • Normal range is easy to interpret (-2 to +2 mEq/L)
    • BE > +2 mEq/L = high HCO₃⁻ = metabolic alkalosis
    • BE < -2 mEq/L = low HCO₃⁻ = metabolic acidosis
  • mEq/L: an ‘equivalent’ is amount required to react with one mole of H⁺
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12
Q

What happens in step C- compensation/chronic?

A
  • If pH and PaCO₂ point to a primarily metabolic cause,
    BE (and sHCO₃⁻) will confirm, i.e.,
    • high in alkalosis and low in acidosis
  • If pH and PaCO₂ point to a 1° respiratory cause, then BE (and sHCO₃⁻) can give insight into duration…
  • In a respiratory acidosis…
    • Normal BE = no compensation yet = acute
    • High BE = compensation = chronic
    • Low BE indicates mixed respiratory and metabolic!
  • In a respiratory alkalosis
    • Normal BE = no compensation yet = acute
    • Low BE = compensation = chronic
    • High BE indicates mixed respiratory and metabolic!
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13
Q

What happens in step D- differential diagnoses?

A
  • How can things go wrong?
  • How can things be compensated?
  • Lungs
    • V/Q mismatch (V = ventilation / Q = perfusion)
    • ↑ or ↓ respiratory rate
  • Metabolic & kidney
    • Gain/loss of acid/base
    • ↑ or ↓ HCO₃⁻ elimination
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14
Q

What is ventilation-perfusion mismatch vs compensation?

A
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15
Q

What is the response to respiratory acidosis by the kidney?

A
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16
Q

What is the response to respiratory alkalosis by the kidney?

A
17
Q

What metabolic factors can disrupt acid/base balance?

A
18
Q

What is the reflex increase in ventilation rate?

A
19
Q

What is the respiratory compensation for chronic metabolic acidosis?

A

Year 2 Med (60 decks)

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