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Difficulty: MediumNephrology

Emergency medical personnel transport a 19-year-old female university student to the emergency department after her roommate found her profoundly lethargic. The roommate reports that the patient had been complaining of severe nausea, subjective fever, and a persistent ringing in her ears for the past 24 hours. Vital signs on arrival include a temperature of 38.3°C (100.9°F), pulse of 118/min, respiratory rate of 30/min, and blood pressure of 105/65 mmHg. Physical examination reveals deep, rapid respirations and dry mucous membranes. A basic metabolic panel and room air arterial blood gas are obtained:

ParameterValue
Serum sodium141141 mEq/L
Serum potassium4.14.1 mEq/L
Serum chloride104104 mEq/L
Serum bicarbonate1212 mEq/L
Arterial pH7.437.43
PaCO2PaCO_21818 mmHg
PaO2PaO_2102102 mmHg

Based on these laboratory results, which of the following best characterizes this patient's acid-base status?

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

Answer

The patient's laboratory values reveal a primary high anion gap metabolic acidosis and a concurrent primary respiratory alkalosis.
The correct answer identifies that this patient has two distinct primary acid-base disorders. The patient's presentation of tinnitus, tachypnea, nausea, and fever is the classic triad of salicylate (aspirin) toxicity. Salicylates directly stimulate the medullary respiratory center, causing a primary respiratory alkalosis (manifested by the inappropriately low PaCO2PaCO_2 of 1818 mmHg). Simultaneously, salicylates uncouple oxidative phosphorylation, leading to the accumulation of organic acids (lactic acid, ketoacids) and establishing a primary high anion gap metabolic acidosis (anion gap of 2525 mEq/L). The opposing effects of these two primary disorders result in a near-normal, slightly alkalemic pH of 7.437.43.

Step-by-Step Solution

1
Examine the arterial pH to determine the overall acid-base status.
The pH is 7.437.43, which leans toward alkalemia but is within the normal range (7.35−7.457.35 - 7.45). This strongly suggests a mixed disorder given the significantly abnormal bicarbonate and PaCO2PaCO_2.
Normal pH in the setting of abnormal HCO3−HCO_3^- and PaCO2PaCO_2 almost always indicates a mixed acid-base disorder.
2
Calculate the serum anion gap (Na+−[Cl−+HCO3−]Na^+ - [Cl^- + HCO_3^-]).
Anion Gap = 141−(104+12)=141−116=25141 - (104 + 12) = 141 - 116 = 25 mEq/L.
An elevated anion gap (normal is 8−128-12 mEq/L) diagnoses a primary high anion gap metabolic acidosis (HAGMA), regardless of the serum pH.
3
Apply Winter's formula (ExpectedPaCO2=1.5×[HCO3−]+8±2Expected PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2) to assess respiratory compensation.
ExpectedPaCO2=1.5×12+8±2=18+8±2=26±2Expected PaCO_2 = 1.5 \times 12 + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2 mmHg. The expected range is 24−2824-28 mmHg.
Winter's formula determines if the lungs are appropriately compensating for the metabolic acidosis.
4
Compare actual PaCO2PaCO_2 to expected PaCO2PaCO_2.
The actual PaCO2PaCO_2 (1818 mmHg) is significantly lower than the expected range (24−2824-28 mmHg).
When actual PaCO2PaCO_2 is less than the expected compensatory PaCO2PaCO_2, it indicates a concurrent primary respiratory alkalosis.
5
Calculate the delta-delta gap (ΔAG/ΔHCO3−\Delta AG / \Delta HCO_3^-) to check for a third disorder.
ΔAG=25−12=13\Delta AG = 25 - 12 = 13. ΔHCO3−=24−12=12\Delta HCO_3^- = 24 - 12 = 12. Ratio is 13/12≈1.113/12 \approx 1.1.
A ratio between 1 and 2 indicates an uncomplicated HAGMA without an underlying NAGMA or metabolic alkalosis.

Key Concept

Mixed acid-base disorders are diagnosed through a stepwise approach: determining primary disturbance, calculating the anion gap, assessing appropriate respiratory compensation with Winter's formula, and checking the delta-delta ratio.
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Topics

Nephrology

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