A 52-year-old woman is brought to the emergency department by her husband due to altered m… — USMLE Step 3

A 52-year-old woman is brought to the emergency department by her husband due to altered mental status and confusion. She has a history of type 2 diabetes mellitus and severe chronic osteoarthritis of the knees, for which she takes multiple over-the-counter analgesics. For the past three days, the patient has experienced severe nausea, tinnitus, and intractable nonbilious vomiting.

Vital signs show a temperature of 38.2°C (100.8°F), pulse of 118/min, blood pressure of 105/65 mm Hg, and respiratory rate of 26/min. Physical examination reveals dry mucous membranes, diffuse mild abdominal tenderness, and tachypnea with clear lungs bilaterally.

Laboratory results are as follows:

Test	Result	Reference Range
Sodium	142 mEq/L	136-145 mEq/L
Potassium	3.1 mEq/L	3.5-5.0 mEq/L
Chloride	92 mEq/L	98-106 mEq/L
Bicarbonate	14 mEq/L	22-28 mEq/L
Blood urea nitrogen	45 mg/dL	7-18 mg/dL
Creatinine	1.8 mg/dL	0.6-1.2 mg/dL
Glucose	110 mg/dL	70-110 mg/dL
Albumin	4.0 g/dL	3.5-5.0 g/dL

Arterial blood gas (room air):

Test	Result	Reference Range
pH	7.49	7.35-7.45
PaCO2	19 mm Hg	35-45 mm Hg
PaO2	95 mm Hg	75-105 mm Hg

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

High anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosisAnswer
B
High anion gap metabolic acidosis and primary respiratory alkalosis
C
High anion gap metabolic acidosis and primary metabolic alkalosis
D
High anion gap metabolic acidosis with appropriate respiratory compensation
E
Primary respiratory alkalosis with appropriate metabolic compensation

Answer

High anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis

The patient's laboratory values reveal three independent primary acid-base disturbances, known as a triple acid-base disorder. First, the anion gap is significantly elevated at 36 mEq/L (Normal = 12), diagnostic of a high anion gap metabolic acidosis (HAGMA). Second, the delta ratio is evaluated to detect hidden metabolic derangements: the

\Delta AG

is 24 (36 - 12) and the

\Delta HCO_3^-

is 10 (24 - 14). The ratio of

24 / 10

is 2.4. A delta ratio > 2.0 confirms a concurrent metabolic alkalosis, as the bicarbonate has not dropped proportionately to the accumulated acid (in this vignette, generated by severe vomiting). Finally, Winter's formula dictates that expected

PaCO_2 = 1.5(14) + 8 = 29

mm Hg. Because the actual

PaCO_2

(19 mm Hg) is lower than the calculated expected range, an independent primary respiratory alkalosis is concurrently present. This specific triple disorder is a hallmark of combined salicylate toxicity (causing HAGMA and direct medullary respiratory alkalosis) and prolonged vomiting (causing metabolic alkalosis).

Step-by-Step Solution

Calculate the serum anion gap (AG) to identify the presence of a metabolic acidosis.

AG = Na^+ - (Cl^- + HCO_3^-) = 142 - (92 + 14) = 36

mEq/L (Normal = 12).

An elevated anion gap reveals a primary high anion gap metabolic acidosis (HAGMA), regardless of the serum pH.

Calculate the delta ratio to identify any concurrent metabolic derangements.

\Delta AG = 36 - 12 = 24

\Delta HCO_3^- = 24 - 14 = 10

. Delta ratio =

24 / 10 = 2.4

A delta ratio (

\Delta AG / \Delta HCO_3^-

) greater than 2.0 indicates that the drop in bicarbonate is much less than the rise in unmeasured anions, proving the presence of a concurrent metabolic alkalosis.

Apply Winter's formula to determine if respiratory compensation is appropriate.

Expected

PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5(14) + 8 = 29 \pm 2

mm Hg (range 27-31).

The patient's actual

PaCO_2

(19 mm Hg) is significantly lower than the expected range (27-31 mm Hg), indicating an independent primary respiratory alkalosis.

Key Concept

A triple acid-base disorder (high anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis) requires systematic interpretation using the anion gap, delta ratio, and Winter's formula. It classically occurs in mixed clinical scenarios, such as salicylate toxicity complicated by severe vomiting.

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