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Difficulty: HardNephrology

Following an emergency exploratory laparotomy for ischemic colitis, a 6868-year-old patient remains mechanically ventilated in the surgical intensive care unit. Over the past 4848 hours, continuous nasogastric suctioning has yielded 2.52.5 liters of fluid. The patient is currently receiving continuous intravenous infusions of fentanyl and propofol. Current vital signs include a temperature of 38.9∘C38.9^\circ C (102.0∘F102.0^\circ F), pulse of 118118/min, and blood pressure of 88/5488/54 mm Hg. The ventilator is set to synchronized intermittent mandatory ventilation mode, and the patient is triggering very few spontaneous breaths.

Laboratory evaluation demonstrates:

ParameterValue
Arterial pHpH7.237.23
Arterial pCO2pCO_25050 mm Hg
Arterial pO2pO_27272 mm Hg
Serum Na+Na^+140140 mEq/L
Serum K+K^+3.13.1 mEq/L
Serum Cl−Cl^-8888 mEq/L
Serum HCO3−HCO_3^-2020 mEq/L
Blood Urea Nitrogen4242 mg/dL
Serum Creatinine2.52.5 mg/dL
Serum Lactate5.85.8 mmol/L

What is the complete characterization of this patient's acid-base status?

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

Answer

The patient has a triple acid-base disorder consisting of a high anion gap metabolic acidosis, a metabolic alkalosis, and a respiratory acidosis.
The correct answer identifies all three components of the patient's complex acid-base disturbance. The patient has acidemia (pH<7.35pH < 7.35). Step 1: The anion gap is 3232 (140−(88+20)140 - (88 + 20)), indicating a high anion gap metabolic acidosis. Step 2: The delta gap is 2020 (32−1232 - 12). The expected bicarbonate is 44 mEq/L (24−2024 - 20), but the measured bicarbonate is 2020 mEq/L. This gap indicates a concurrent metabolic alkalosis. Step 3: Winter's formula predicts an expected pCO2pCO_2 of 38±238 \pm 2 mm Hg (1.5×20+81.5 \times 20 + 8). The actual pCO2pCO_2 is 5050 mm Hg, which is higher than expected, confirming an additional respiratory acidosis.

Step-by-Step Solution

1
Calculate the anion gap to identify the primary metabolic disturbance.
AG=Na+−(Cl−+HCO3−)=140−(88+20)=32AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (88 + 20) = 32 mEq/L.
An elevated anion gap (>12>12 mEq/L) indicates a primary high anion gap metabolic acidosis (HAGMA), which in this hypotensive patient is secondary to lactic acidosis and uremia.
2
Calculate the delta-delta gap to evaluate for concurrent metabolic disorders.
ΔAG=32−12=20\Delta AG = 32 - 12 = 20 mEq/L. Expected HCO3−=24−20=4HCO_3^- = 24 - 20 = 4 mEq/L. The actual HCO3−HCO_3^- is 2020 mEq/L.
Because the actual HCO3−HCO_3^- (2020 mEq/L) is significantly higher than the expected HCO3−HCO_3^- (44 mEq/L), there must be a concurrent process adding bicarbonate. This indicates a superimposed metabolic alkalosis, driven by the continuous nasogastric suctioning (loss of H+H^+ and Cl−Cl^-).
3
Use Winter's formula to determine if respiratory compensation is appropriate.
Expected pCO2=(1.5×20)+8±2=38±2pCO_2 = (1.5 \times 20) + 8 \pm 2 = 38 \pm 2 mm Hg. The actual pCO2pCO_2 is 5050 mm Hg.
Because the actual pCO2pCO_2 (5050 mm Hg) is greater than the expected range (36−4036-40 mm Hg), the patient is hypoventilating relative to their metabolic state. This indicates a concurrent respiratory acidosis, secondary to heavy sedation and the set ventilator mode.

Key Concept

A triple acid-base disorder can be correctly diagnosed by sequentially evaluating the arterial pH, calculating the anion gap, utilizing the delta-delta calculation, and applying Winter's formula to assess respiratory compensation.
Estimated Time:2m 30s
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Nephrology

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