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Renal · USMLE Step 1

USMLE Renal: Acid-Base Disorders

Primary metabolic and respiratory acid-base disorders and their expected compensation, distinct from the sodium and potassium focus of the fluid and electrolyte deck.

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What is the primary defect in metabolic acidosis?
A primary decrease in plasma bicarbonate (HCO3-).
What is the primary defect in metabolic alkalosis?
A primary increase in plasma bicarbonate (HCO3-).
What is the primary defect in respiratory acidosis?
A primary increase in PaCO2 caused by hypoventilation.
What is the primary defect in respiratory alkalosis?
A primary decrease in PaCO2 caused by hyperventilation.
In metabolic acidosis, the expected compensatory response is ____.
hyperventilation to lower PaCO2
In metabolic alkalosis, the expected compensatory response is ____.
hypoventilation to raise PaCO2
How do the kidneys compensate for respiratory acidosis?
By increasing bicarbonate reabsorption and generation, raising plasma HCO3-.
How do the kidneys compensate for respiratory alkalosis?
By decreasing bicarbonate reabsorption, lowering plasma HCO3-.
Which organ provides fast, minutes-scale compensation for a metabolic acid-base disorder?
The lungs, by changing the respiratory rate.
Which organ provides slow, days-scale compensation for a respiratory acid-base disorder?
The kidneys, by adjusting bicarbonate handling.
What formula predicts the expected PaCO2 in a simple metabolic acidosis?
Winter's formula: PaCO2 = 1.5 x HCO3- + 8 (plus or minus 2).
In metabolic acidosis, if the measured PaCO2 is higher than Winter's formula predicts, what does this indicate?
A superimposed (concurrent) respiratory acidosis.
In metabolic acidosis, if the measured PaCO2 is lower than Winter's formula predicts, what does this indicate?
A superimposed (concurrent) respiratory alkalosis.
In metabolic alkalosis, about how much does PaCO2 rise for every 1 mEq/L rise in HCO3-?
About 0.7 mmHg.
In acute respiratory acidosis, about how much does plasma HCO3- rise for every 10 mmHg rise in PaCO2?
About 1 mEq/L.

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