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Difficulty: HardNephrology

A 68-year-old man is brought to the emergency department for worsening shortness of breath, increased sputum production, and confusion over the past 24 hours. He has very severe chronic obstructive pulmonary disease (COPD) and uses home oxygen therapy. The patient's wife reports that for the past 3 days, he has also had multiple episodes of large-volume, watery diarrhea.

Vital signs are temperature 37.6°C (99.7°F), blood pressure 110/70 mm Hg, pulse 115/min, and respirations 26/min. Physical examination shows a barrel chest, diffuse bilateral wheezing, and a prolonged expiratory phase. The mucous membranes are dry.

Laboratory results from today are as follows:

Serum chemistries:
TestResult
Sodium135 mEq/L
Potassium3.5 mEq/L
Chloride102 mEq/L
Bicarbonate23 mEq/L
Blood urea nitrogen28 mg/dL
Creatinine1.2 mg/dL
Arterial blood gas:
TestResult
pH7.17
PaCO266 mm Hg
PaO250 mm Hg
HCO3-23 mEq/L

A review of the patient's electronic medical record reveals an arterial blood gas obtained during a routine clinic visit 2 months ago when he was feeling well:

Baseline Arterial blood gas:
TestResult
pH7.36
PaCO256 mm Hg
PaO262 mm Hg
HCO3-31 mEq/L

Which of the following combinations of acid-base disorders best describes this patient's current condition?

  1. Acute-on-chronic respiratory acidosis and normal anion gap metabolic acidosisAnswer
  2. B
    Acute-on-chronic respiratory acidosis and high anion gap metabolic acidosis
  3. C
    Acute respiratory acidosis and normal anion gap metabolic acidosis
  4. D
    Chronic respiratory acidosis and normal anion gap metabolic acidosis
  5. E
    Uncompensated acute respiratory acidosis

Answer

Acute-on-chronic respiratory acidosis and normal anion gap metabolic acidosis
The correct answer is acute-on-chronic respiratory acidosis and normal anion gap metabolic acidosis. The patient's baseline ABG shows a compensated chronic respiratory acidosis (PaCO2 56 mm Hg56 \text{ mm Hg}, HCO3- 31 mEq/L31 \text{ mEq/L}). Today, his PaCO2 has acutely increased to 66 mm Hg66 \text{ mm Hg} (an acute increase of 10 mm Hg10 \text{ mm Hg}). For an acute rise in PaCO2, the HCO3- should increase by 1 mEq/L1 \text{ mEq/L} per 10 mm Hg10 \text{ mm Hg} rise in PaCO2. Therefore, his expected HCO3- is 32 mEq/L32 \text{ mEq/L} (31+131 + 1). His actual HCO3- is 23 mEq/L23 \text{ mEq/L}, which is significantly lower than expected, indicating a concurrent metabolic acidosis. The anion gap is calculated as 135−(102+23)=10 mEq/L135 - (102 + 23) = 10 \text{ mEq/L}, which is normal. This normal anion gap metabolic acidosis is clinically supported by his 3-day history of severe diarrhea, which causes profound gastrointestinal bicarbonate loss.

Step-by-Step Solution

1
Analyze the baseline arterial blood gas.
The baseline ABG shows a PaCO2 of 56 mm Hg and HCO3- of 31 mEq/L, consistent with a compensated chronic respiratory acidosis.
Establishing the patient's baseline is necessary to correctly interpret the current acute changes in patients with severe COPD.
2
Evaluate the current PaCO2 and calculate the expected compensation.
The PaCO2 has acutely increased by 10 mm Hg (from 56 to 66 mm Hg). The expected acute increase in HCO3- is 1 mEq/L, making the new expected HCO3- 32 mEq/L (31 + 1).
In acute respiratory acidosis, the HCO3- increases by 1 mEq/L for every 10 mm Hg increase in PaCO2.
3
Compare actual HCO3- to expected HCO3-.
The actual HCO3- is 23 mEq/L, which is significantly lower than the expected 32 mEq/L. This indicates a superimposed metabolic acidosis.
A lower-than-expected bicarbonate level in the setting of respiratory acidosis confirms a concurrent metabolic acidosis.
4
Calculate the anion gap.
Anion Gap = Na+ - (Cl- + HCO3-) = 135 - (102 + 23) = 10 mEq/L. This is within the normal range.
The anion gap differentiates between a normal anion gap and a high anion gap metabolic acidosis. The normal gap points to GI bicarbonate loss from diarrhea.

Key Concept

Patients with chronic respiratory acidosis who develop an acute exacerbation require step-wise acid-base analysis utilizing their baseline ABG values to accurately calculate expected compensation and identify superimposed metabolic disorders.
Estimated Time:2m 0s
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Topics

Nephrology

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