A 65-year old man had a heart attack and experiences cardiopulmonary arrest while being transported to the emergency department. Following lab values were obtained from arterial blood- Plasma pH = 7.12; Plasma PCO2 = 60 mmHg; Plasma HCO3 concentration= 19mEq/L which of the following options best describes his acid base disorder? A. Respiratory acidosis with partial renal compensation B. Metabolic acidosis with partial respiratory compensation C. Mixed acidosis: combined metabolic and respiratory acidosis D. Mixed alkalosis: combined respiratory and metabolic alkalosis
Quick Answer:
The ABG shows pH 7.12 (acidotic), PCO2 60 mmHg (elevated - respiratory acidosis), and HCO3 19 mEq/L (low - metabolic acidosis). Both components are driving the pH down simultaneously, making this a mixed acidosis (Option C).
Here is the reasoning:
Step-by-step ABG interpretation:
| Parameter | Value | Normal | Interpretation |
|---|---|---|---|
| pH | 7.12 | 7.35-7.45 | Acidosis |
| PCO2 | 60 mmHg | 35-45 mmHg | Elevated → Respiratory acidosis |
| HCO3 | 19 mEq/L | 22-26 mEq/L | Low → Metabolic acidosis |
Why not Option A (Respiratory acidosis with partial renal compensation)?
In pure respiratory acidosis with renal compensation, the kidneys retain bicarbonate to buffer the acidosis. You would expect HCO3 to be elevated or normal, not low (19 mEq/L). Here, HCO3 is actually below normal, meaning the kidneys are not compensating - they are contributing to the acidosis.
Why not Option B (Metabolic acidosis with partial respiratory compensation)?
In pure metabolic acidosis with respiratory compensation, the lungs blow off CO2 (hyperventilation), so PCO2 would be low, not high. A PCO2 of 60 mmHg is the opposite of compensation.
Why Option C is correct:
- PCO2 is high (60 mmHg) → lungs are failing to ventilate (cardiopulmonary arrest stops breathing) → respiratory acidosis
- HCO3 is low (19 mEq/L) → tissue ischemia from the cardiac arrest generates lactic acid, consuming bicarbonate → metabolic acidosis
- Both defects are independently pushing the pH down to 7.12
This is the classic ABG picture of cardiopulmonary arrest: no ventilation causes CO2 retention, and ischemic tissues produce lactic acid that depletes bicarbonate. Neither component is compensating for the other - they are both pathological.
The answer is C - Mixed acidosis: combined metabolic and respiratory acidosis.
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