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Difficulty: MediumNephrology

A 68-year-old man with a history of severe chronic obstructive pulmonary disease and hypertension is brought to the emergency department by his daughter due to worsening confusion and decreased urine output over the past 48 hours. He recently had a gastrointestinal illness with severe nausea and poor oral intake, leading to profound dehydration. His current medications include lisinopril and tiotropium. Temperature is 37.1°C (98.8°F), blood pressure is 88/52 mm Hg, pulse is 112/min, and respirations are 26/min. Physical examination shows dry mucous membranes, diminished breath sounds bilaterally with a prolonged expiratory phase, and delayed capillary refill.

Laboratory studies reveal the following:
Sodium: 136 mEq/L136\text{ mEq/L}
Potassium: 5.1 mEq/L5.1\text{ mEq/L}
Chloride: 95 mEq/L95\text{ mEq/L}
Bicarbonate: 12 mEq/L12\text{ mEq/L}
Blood urea nitrogen: 72 mg/dL72\text{ mg/dL}
Creatinine: 4.1 mg/dL4.1\text{ mg/dL} (baseline 1.1 mg/dL1.1\text{ mg/dL})

Arterial blood gas (on room air):
pH: 7.187.18
pCO2: 34 mm Hg34\text{ mm Hg}
pO2: 62 mm Hg62\text{ mm Hg}

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

  1. High anion gap metabolic acidosis and concurrent respiratory acidosisAnswer
  2. B
    High anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    High anion gap metabolic acidosis and concurrent respiratory alkalosis
  4. D
    Normal anion gap metabolic acidosis and concurrent respiratory acidosis
  5. E
    Primary respiratory acidosis with appropriate metabolic compensation

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis
The correct diagnosis is a high anion gap metabolic acidosis with a concurrent respiratory acidosis. The patient's primary disorder is a metabolic acidosis (pH 7.187.18, HCO3−HCO_3^- 12 mEq/L12\text{ mEq/L}). The anion gap is elevated at 29 mEq/L29\text{ mEq/L} (136−[95+12]136 - [95 + 12]), confirming a high anion gap metabolic acidosis (likely uremic/lactic secondary to severe hypoperfusion and AKI). To assess the respiratory response, Winter's formula must be applied: Expected pCO2=(1.5×12)+8±2=26±2 mm HgExpected\ pCO_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2\text{ mm Hg}. The patient's measured pCO2pCO_2 is 34 mm Hg34\text{ mm Hg}. Because 34 mm Hg34\text{ mm Hg} is significantly greater than the expected maximum of 28 mm Hg28\text{ mm Hg}, he has a superimposed respiratory acidosis. His history of severe COPD limits his ability to mount an appropriate hyperventilatory response.

Step-by-Step Solution

1
Analyze the basic pH and primary disturbance.
The pH is 7.187.18 (acidemia) and the HCO3−HCO_3^- is markedly low at 12 mEq/L12\text{ mEq/L}, indicating a primary metabolic acidosis.
Establishing the primary acid-base disorder is the necessary first step before assessing for mixed disorders.
2
Calculate the Anion Gap (AG).
AG=Na+−(Cl−+HCO3−)=136−(95+12)=136−107=29 mEq/LAG = Na^+ - (Cl^- + HCO_3^-) = 136 - (95 + 12) = 136 - 107 = 29\text{ mEq/L}.
An elevated AG (normal 8−12 mEq/L8-12\text{ mEq/L}) confirms a high anion gap metabolic acidosis (HAGMA), in this case likely due to severe acute kidney injury (uremia) from hypoperfusion.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=(1.5×HCO3−)+8±2=(1.5×12)+8±2=18+8±2=26±2 mm HgExpected\ pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2\text{ mm Hg} (Range: 24−28 mm Hg24-28\text{ mm Hg}).
This formula determines if the patient's respiratory system is adequately blowing off CO2CO_2 in response to the metabolic acidemia.
4
Compare expected pCO2pCO_2 to measured pCO2pCO_2.
The measured pCO2pCO_2 is 34 mm Hg34\text{ mm Hg}, which is greater than the expected range of 24−28 mm Hg24-28\text{ mm Hg}.
Because the actual pCO2pCO_2 is higher than expected, the patient has a superimposed (concurrent) respiratory acidosis, likely because his severe COPD prevents adequate compensatory hyperventilation.

Key Concept

Diagnosis of mixed acid-base disorders requires calculating the anion gap and using Winter's formula to determine if respiratory compensation is appropriate.
Estimated Time:1m 30s
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Topics

Nephrology

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