Examkin

Question

Difficulty: MediumNephrology

A 5454-year-old man is admitted to the intensive care unit directly from the medical triage area due to high fever and altered mental status. His wife reports that he has been complaining of painful urination and flank discomfort for three days. His temperature is 39.2∘C39.2^\circ\text{C} (102.6∘F102.6^\circ\text{F}), blood pressure is 82/48 mm Hg82/48 \text{ mm Hg}, heart rate is 130/min130\text{/min}, and respiratory rate is 30/min30\text{/min}. Physical examination shows a lethargic man with mottling of the lower extremities and marked suprapubic tenderness.

Laboratory results:
Serum sodium: 140 mEq/L140 \text{ mEq/L}
Serum potassium: 4.5 mEq/L4.5 \text{ mEq/L}
Serum chloride: 104 mEq/L104 \text{ mEq/L}
Serum bicarbonate: 16 mEq/L16 \text{ mEq/L}
Blood urea nitrogen: 32 mg/dL32 \text{ mg/dL}
Serum creatinine: 1.6 mg/dL1.6 \text{ mg/dL}

Arterial blood gas (room air):
pH: 7.457.45
pCO 2_2: 24 mm Hg24 \text{ mm Hg}
pO 2_2: 88 mm Hg88 \text{ mm Hg}

What is the most accurate description of this patient's acid-base disorder?

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

Answer

The correct answer is a mixed high anion gap metabolic acidosis and primary respiratory alkalosis.
The patient's laboratory values reveal a mixed acid-base disorder consisting of a high anion gap metabolic acidosis (HAGMA) and a primary respiratory alkalosis. First, the elevated anion gap of 20 mEq/L20 \text{ mEq/L} (140−[104+16]140 - [104 + 16]) definitively identifies a HAGMA, which in this clinical context is likely lactic acidosis secondary to septic shock. Second, applying Winter's formula [(1.5×16)+8=32±2 mm Hg(1.5 \times 16) + 8 = 32 \pm 2 \text{ mm Hg}] establishes the expected respiratory compensation. Because the patient's actual pCO 2_2 (24 mm Hg24 \text{ mm Hg}) is significantly lower than the expected range, there is excessive hyperventilation indicating a superimposed primary respiratory alkalosis. This combination of HAGMA and respiratory alkalosis is a hallmark of early sepsis.

Step-by-Step Solution

1
Analyze the pH to determine the overall acid-base state.
The pH is 7.457.45, which indicates alkalemia.
A pH greater than 7.407.40 establishes an alkalemic state, meaning a primary alkalosis must be present as a driving disorder.
2
Calculate the serum anion gap to evaluate for a hidden metabolic acidosis.
Anion gap =140−(104+16)=20 mEq/L= 140 - (104 + 16) = 20 \text{ mEq/L}.
An anion gap greater than 12 mEq/L12 \text{ mEq/L} confirms the presence of a primary high anion gap metabolic acidosis (HAGMA), regardless of the overall pH. This is likely due to lactic acidosis from septic shock.
3
Calculate the expected respiratory compensation for the metabolic acidosis using Winter's formula.
Expected pCO 2_2 =(1.5×16)+8=32±2 mm Hg= (1.5 \times 16) + 8 = 32 \pm 2 \text{ mm Hg}.
This calculation determines if the patient's pCO 2_2 level is purely compensatory for the acidosis or if an additional primary respiratory disorder is present.
4
Compare the actual pCO 2_2 to the expected pCO 2_2.
The actual pCO 2_2 (24 mm Hg24 \text{ mm Hg}) is less than the expected range (30−34 mm Hg30-34 \text{ mm Hg}).
Because the pCO 2_2 is significantly lower than expected, there is excessive respiratory drive, indicating a superimposed primary respiratory alkalosis (common in early sepsis).

Key Concept

Acid-base analysis requires calculating the anion gap in all patients, as a high anion gap metabolic acidosis can exist concurrently with an opposing primary acid-base disorder (such as respiratory alkalosis in early sepsis), resulting in a normal or alkalemic pH.
Estimated Time:1m 30s
Rate this question

Topics

Nephrology

More questions from this topic