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Difficulty: MediumNephrology

Match each primary acid-base disorder to its corresponding expected physiological compensatory response.

  • Primary metabolic acidosisExpected PaCO2=(1.5×[HCO3−])+8±2\text{PaCO}_2 = (1.5 \times [\text{HCO}_3^-]) + 8 \pm 2
  • Primary metabolic alkalosisPaCO2\text{PaCO}_2 increases by 0.7 mmHg0.7 \text{ mmHg} for every 1 mEq/L1 \text{ mEq/L} increase in HCO3−\text{HCO}_3^-
  • Acute primary respiratory acidosisHCO3−\text{HCO}_3^- increases by 1 mEq/L1 \text{ mEq/L} for every 10 mmHg10 \text{ mmHg} increase in PaCO2\text{PaCO}_2
  • Chronic primary respiratory acidosisHCO3−\text{HCO}_3^- increases by 3 to 4 mEq/L3 \text{ to } 4 \text{ mEq/L} for every 10 mmHg10 \text{ mmHg} increase in PaCO2\text{PaCO}_2

Answer

Metabolic acidosis matches Winters' formula; metabolic alkalosis matches a 0.7 mmHg0.7 \text{ mmHg} PaCO2\text{PaCO}_2 increase per 1 mEq/L1 \text{ mEq/L} HCO3−\text{HCO}_3^- increase; acute respiratory acidosis matches a 1 mEq/L1 \text{ mEq/L} HCO3−\text{HCO}_3^- increase per 10 mmHg10 \text{ mmHg} PaCO2\text{PaCO}_2 increase; and chronic respiratory acidosis matches a 3 to 4 mEq/L3 \text{ to } 4 \text{ mEq/L} HCO3−\text{HCO}_3^- increase per 10 mmHg10 \text{ mmHg} PaCO2\text{PaCO}_2 increase.
Each primary acid-base disorder triggers a predictable and calculable compensatory response. Metabolic acidosis is compensated by hyperventilation, calculated via Winters' formula. Metabolic alkalosis is compensated by hypoventilation, where PaCO2\text{PaCO}_2 rises 0.7 mmHg0.7 \text{ mmHg} per 1 mEq/L1 \text{ mEq/L} rise in HCO3−\text{HCO}_3^-. Acute respiratory acidosis is buffered acutely by cells, raising HCO3−\text{HCO}_3^- by 1 mEq/L1 \text{ mEq/L} per 10 mmHg10 \text{ mmHg} PaCO2\text{PaCO}_2 rise. Chronic respiratory acidosis allows for full renal compensation, raising HCO3−\text{HCO}_3^- by 3 to 4 mEq/L3 \text{ to } 4 \text{ mEq/L} per 10 mmHg10 \text{ mmHg} PaCO2\text{PaCO}_2 rise.

Step-by-Step Solution

1
Identify the primary disturbance and the physiological organ system responsible for compensation.
Metabolic disorders are compensated by the respiratory system (altering PaCO2\text{PaCO}_2), while respiratory disorders are compensated by the renal system (altering HCO3−\text{HCO}_3^-).
Understanding the fundamental physiological mechanism is the first step in applying the correct numerical formula.
2
Correlate metabolic disorders with their specific respiratory compensation rules.
Metabolic acidosis requires evaluation using Winters' formula. Metabolic alkalosis is evaluated using the 0.7 rule.
These established formulas are required to determine if a superimposed secondary acid-base disorder is present.
3
Correlate respiratory disorders with their specific acute versus chronic renal compensation rules.
Acute respiratory acidosis causes a minor HCO3−\text{HCO}_3^- shift (1 for 10), while chronic respiratory acidosis allows for robust renal HCO3−\text{HCO}_3^- retention (3 to 4 for 10).
Renal metabolic compensation takes several days to reach maximal efficacy, creating distinctly different expected values for acute versus chronic respiratory conditions.

Key Concept

Acid-base compensation formulas and expected physiological responses.
Estimated Time:1m 0s
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Topics

Nephrology

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