Examkin

Question

Difficulty: HardNephrology

A 54-year-old man with a history of severe chronic obstructive pulmonary disease and major depressive disorder is brought to the emergency department by paramedics after being found unarousable in his garage. An open, empty container of antifreeze was found nearby. Vital signs are temperature 36.7°C (98.1°F), blood pressure 112/68 mmHg, pulse 110/min, and respirations 16/min. Pulse oximetry is 89% on room air. Physical examination reveals an obtunded patient with a barrel-shaped chest, diminished breath sounds, and diffuse bilateral wheezing.

Laboratory testing yields the following results:

Laboratory TestValue
Sodium142 mEq/L
Potassium4.8 mEq/L
Chloride106 mEq/L
Bicarbonate12 mEq/L
Arterial pH7.21
Arterial pCO2pCO_236 mmHg
Arterial pO2pO_262 mmHg

Which of the following best describes this patient's current acid-base status?

  1. A
    Primary metabolic acidosis with appropriate respiratory compensation
  2. B
    Primary respiratory acidosis with appropriate metabolic compensation
  3. Mixed metabolic acidosis and respiratory acidosisAnswer
  4. D
    Mixed metabolic acidosis and respiratory alkalosis
  5. E
    Mixed metabolic acidosis and metabolic alkalosis

Answer

Mixed metabolic acidosis and respiratory acidosis
The correct answer accurately identifies two distinct acid-base disorders in this patient. First, the low pH (7.21) and low bicarbonate (12 mEq/L) with an elevated anion gap of 24 mEq/L confirm a high anion gap metabolic acidosis, consistent with toxic alcohol ingestion. Second, applying Winter's formula dictates that his expected pCO2pCO_2 should be between 24 and 28 mmHg. Because his actual pCO2pCO_2 is 36 mmHg, he is retaining more CO2CO_2 than he should, indicating an independent, concurrent respiratory acidosis caused by his baseline severe COPD.

Step-by-Step Solution

1
Determine the primary acid-base disorder by looking at the pH, pCO2pCO_2, and HCO3−HCO_3^-.
The pH is 7.21 (acidemia). The HCO3−HCO_3^- is 12 mEq/L (low). This establishes a primary metabolic acidosis.
Acidemia driven by a low bicarbonate indicates a primary metabolic acidosis, which in this clinical context is likely due to ethylene glycol toxicity.
2
Calculate the anion gap to further characterize the metabolic acidosis.
AG=Na+−(Cl−+HCO3−)=142−(106+12)=142−118=24AG = Na^+ - (Cl^- + HCO_3^-) = 142 - (106 + 12) = 142 - 118 = 24 mEq/L.
An elevated anion gap (normal 8-12 mEq/L) confirms a high anion gap metabolic acidosis (HAGMA).
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=(1.5×12)+8±2=18+8±2=26±2pCO_2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2 mmHg (Range: 24 to 28 mmHg).
Winter's formula determines the appropriate ventilatory response to a metabolic acidosis.
4
Compare the actual pCO2pCO_2 to the expected pCO2pCO_2 to identify any secondary respiratory disorders.
The actual measured pCO2pCO_2 is 36 mmHg. Since 36 mmHg is greater than the upper limit of the expected range (28 mmHg), the patient is hypoventilating relative to their acidemic state.
A pCO2pCO_2 higher than the expected compensatory range indicates a concurrent primary respiratory acidosis, likely due to this patient's severe underlying COPD limiting his ability to appropriately blow off CO2CO_2.

Key Concept

In patients with metabolic acidosis, Winter's formula (1.5×[HCO3−]+8±21.5 \times [HCO_3^-] + 8 \pm 2) must be used to calculate expected pCO2pCO_2. If the measured pCO2pCO_2 is significantly higher than expected, a concurrent respiratory acidosis is present.
Rate this question

Topics

Nephrology

More questions from this topic