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Difficulty: HardNephrology

During an evening shift, a critical care team evaluates a patient who was transferred from an outside hospital following a suicide attempt. The patient was initially found stuporous by a roommate next to an empty bottle of an unknown over-the-counter medication. Prior to transfer, the patient experienced several episodes of large-volume, nonbilious emesis. Current vital signs include a temperature of 38.6∘C38.6^\circ\text{C} (101.5∘F101.5^\circ\text{F}), blood pressure of 110/68110/68 mmHg, heart rate of 122/min122/\text{min}, and respiratory rate of 34/min34/\text{min}.

Laboratory results are obtained:

Arterial blood gas (room air)
pH: 7.327.32
pCO 2_2: 1616 mmHg
pO 2_2: 9898 mmHg

Serum chemistry
Na +^+: 142142 mEq/L
K +^+: 3.43.4 mEq/L
Cl −^-: 9292 mEq/L
HCO 3−_3^-: 1212 mEq/L
BUN: 2222 mg/dL
Creatinine: 1.31.3 mg/dL

Based on the laboratory data provided, which of the following combinations best describes this patient's primary acid-base disturbances?

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

Answer

The correct combination of disturbances is high anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis.
This patient's clinical presentation (tinnitus, fever, tachypnea, vomiting after an overdose) is highly suspicious for salicylate toxicity, which classically causes a mixed acid-base disorder. The laboratory values demonstrate a triple acid-base disturbance. First, the acidemic pH (7.327.32) with low HCO 3−_3^- (1212 mEq/L) establishes a primary metabolic acidosis. Second, the anion gap is elevated at 3838 (142−[92+12]142 - [92 + 12]), defining a high anion gap metabolic acidosis. Third, Winter's formula (1.5×12+8=26±21.5 \times 12 + 8 = 26 \pm 2) predicts a compensatory pCO 2_2 of 24−2824-28 mmHg; the patient's actual pCO 2_2 of 1616 mmHg is lower than expected, revealing a concurrent respiratory alkalosis. Finally, the delta ratio (Δ\Delta AG / Δ\Delta HCO 3−_3^-) is calculated as (38−12)/(24−12)=26/12=2.16(38 - 12) / (24 - 12) = 26 / 12 = 2.16. A delta ratio greater than 2.02.0 signifies that a concurrent metabolic alkalosis is present, consistent with the patient's history of significant vomiting.

Step-by-Step Solution

1
Determine the primary driving disorder using pH and serum bicarbonate.
Primary metabolic acidosis.
The pH is acidemic (7.327.32) and the bicarbonate is low (1212 mEq/L).
2
Calculate the serum anion gap to determine the etiology of the metabolic acidosis.
Anion gap = 142−(92+12)=38142 - (92 + 12) = 38 mEq/L. This indicates a High Anion Gap Metabolic Acidosis (HAGMA).
Differentiates between anion gap and non-anion gap metabolic acidosis.
3
Apply Winter's formula to evaluate the expected respiratory compensation.
Expected pCO 2=1.5×12+8±2=26±2_2 = 1.5 \times 12 + 8 \pm 2 = 26 \pm 2 mmHg. The actual pCO 2_2 is 1616 mmHg. Because 16<2416 < 24, there is a concurrent primary Respiratory Alkalosis.
Determines if the patient's hyperventilation is a normal physiological response or an independent primary disorder (common in salicylate toxicity).
4
Calculate the delta-delta ratio (Δ\Delta Ratio) to identify hidden concurrent metabolic disturbances.
Δ\Delta AG = 38−12=2638 - 12 = 26. Δ\Delta HCO 3−_3^- = 24−12=1224 - 12 = 12. Δ\Delta Ratio = 26/12=2.1626 / 12 = 2.16. A ratio >2.0> 2.0 indicates a concurrent Metabolic Alkalosis.
A ratio >2.0> 2.0 means the increase in unmeasured anions is much larger than the drop in bicarbonate, implying that a process is simultaneously adding bicarbonate to the blood (in this case, from severe vomiting).

Key Concept

The systematic approach to complex acid-base disorders requires calculating the anion gap, applying Winter's formula to check for primary respiratory derangements, and calculating the delta ratio to uncover hidden metabolic disturbances.
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Topics

Nephrology

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