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Difficulty: HardNephrology

Emergency medical services bring a 68-year-old male resident of a skilled nursing facility to the hospital for evaluation of profound lethargy and confusion. According to the transfer paperwork, the patient has a history of severe chronic obstructive pulmonary disease requiring continuous home oxygen, as well as hypertension and type 2 diabetes mellitus. Over the past four days, he has experienced up to 10 large-volume, watery bowel movements daily following a recent course of oral clindamycin. On arrival, his blood pressure is 98/60 mm Hg, heart rate is 114/min, and respiratory rate is 26/min. Physical examination reveals dry mucous membranes, decreased skin turgor, and diffuse bilateral wheezing.

Laboratory results are as follows:
Serum sodium: 138 mEq/L
Serum potassium: 3.2 mEq/L
Serum chloride: 115 mEq/L
Serum bicarbonate: 12 mEq/L
Blood urea nitrogen: 48 mg/dL
Serum creatinine: 1.8 mg/dL

Arterial blood gas (room air):
pH: 7.12
pCO2: 38 mm Hg
pO2: 58 mm Hg

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

  1. A
    Non-anion gap metabolic acidosis with appropriate respiratory compensation
  2. Mixed non-anion gap metabolic acidosis and respiratory acidosisAnswer
  3. C
    Mixed non-anion gap metabolic acidosis and respiratory alkalosis
  4. D
    Anion gap metabolic acidosis with appropriate respiratory compensation
  5. E
    Mixed anion gap metabolic acidosis and respiratory acidosis

Answer

Mixed non-anion gap metabolic acidosis and respiratory acidosis
The correct diagnosis requires a systematic approach to acid-base interpretation. First, the acidemia (pH 7.12) is driven by a low bicarbonate (12 mEq/L), establishing a primary metabolic acidosis. Next, the anion gap is calculated as 138−(115+12)=11138 - (115 + 12) = 11 mEq/L, indicating a non-anion gap metabolic acidosis caused by massive bicarbonate loss from diarrhea. Finally, Winter's formula (Expected pCO2=(1.5×HCO3−)+8±2Expected\ pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2) is used to evaluate respiratory compensation. The expected pCO2pCO_2 is 26±226 \pm 2 mm Hg. The patient's actual pCO2pCO_2 of 38 mm Hg is significantly higher than this target range. This disparity reveals that the patient has a concurrent respiratory acidosis. His severe COPD prevents him from mounting the necessary compensatory hyperventilation required by his severe metabolic acidosis.

Step-by-Step Solution

1
Evaluate the patient's overall acid-base status by assessing the pH.
The pH of 7.12 indicates significant acidemia.
Establishing whether the patient is acidemic or alkalemic is the first step in interpreting an arterial blood gas.
2
Identify the primary disturbance responsible for the acidemia.
The serum bicarbonate is low (12 mEq/L), indicating a primary metabolic acidosis.
In acidemia, a low bicarbonate indicates metabolic acidosis, whereas a high pCO2 indicates respiratory acidosis. Here, the profound drop in bicarbonate is the primary driver.
3
Calculate the serum anion gap to classify the metabolic acidosis.
The anion gap is calculated as: AG=Na+−(Cl−+HCO3−)AG = Na^+ - (Cl^- + HCO_3^-). Here, AG=138−(115+12)=11AG = 138 - (115 + 12) = 11 mEq/L.
An anion gap of 11 mEq/L falls within the normal range (8-12 mEq/L), confirming a non-anion gap (hyperchloremic) metabolic acidosis, which aligns with his history of severe diarrhea.
4
Determine if the respiratory compensation is appropriate by applying Winter's formula.
Expected pCO2=(1.5×HCO3−)+8±2pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. Thus, expected pCO2=(1.5×12)+8=26±2pCO_2 = (1.5 \times 12) + 8 = 26 \pm 2 mm Hg.
Winter's formula calculates the precise pCO2pCO_2 range the lungs should achieve through compensatory hyperventilation in response to a metabolic acidosis.
5
Compare the actual measured pCO2pCO_2 against the expected pCO2pCO_2 range.
The actual pCO2pCO_2 (38 mm Hg) is significantly greater than the expected range (24-28 mm Hg).
Because the actual pCO2pCO_2 is higher than the physiological target, there is an uncompensated accumulation of CO2CO_2, representing a concomitant primary respiratory acidosis due to the patient's underlying severe COPD.

Key Concept

Patients with metabolic acidosis must be evaluated with Winter's formula to assess for mixed acid-base disorders; a 'normal' absolute pCO2 value (35-45 mm Hg) often masks a severe concomitant respiratory acidosis if the expected pCO2 is much lower.
Estimated Time:1m 30s
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Nephrology

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