An unresponsive 68-year-old individual is admitted to the intensive care unit for altered … — USMLE Step 3

An unresponsive 68-year-old individual is admitted to the intensive care unit for altered mental status and fever. The patient has a known history of severe chronic obstructive pulmonary disease and alcohol use disorder. Family members report that the patient has experienced intractable vomiting multiple times daily for the past week. Upon arrival, vital signs reveal a blood pressure of 90/60 mmHg, a heart rate of 115/min, a respiratory rate of 12/min, and a temperature of 38.6°C (101.5°F). Physical examination demonstrates dry mucous membranes and poor skin turgor.

Laboratory evaluations yield the following results:

Parameter	Value	Reference Range
Sodium	$140$ mEq/L	$136-145$ mEq/L
Potassium	$3.2$ mEq/L	$3.5-5.0$ mEq/L
Chloride	$86$ mEq/L	$95-105$ mEq/L
Bicarbonate	$20$ 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
Lactate	$6.5$ mmol/L	$0.5-1.0$ mmol/L

An arterial blood gas sample obtained on room air shows:

Parameter	Value	Reference Range
pH	$7.26$	$7.35-7.45$
pCO₂	$48$ mmHg	$35-45$ mmHg
pO₂	$60$ mmHg	$75-105$ mmHg

Based on the provided laboratory data, which of the following best describes the complete combination of acid-base disturbances present in this patient?

High anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosisAnswer
B
High anion gap metabolic acidosis and respiratory acidosis
C
High anion gap metabolic acidosis, normal anion gap metabolic acidosis, and respiratory acidosis
D
High anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis
E
Normal anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosis

Answer

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

The patient has a primary acidemia (pH 7.26) with a calculated anion gap of 34 mEq/L (

140 - (86 + 20)

), identifying a high anion gap metabolic acidosis (HAGMA) driven by elevated lactate. Calculating the delta-delta ratio reveals an excess anion gap of 22 (

34 - 12

) and a bicarbonate deficit of 4 (

24 - 20

). The ratio of 5.5 (

\frac{22}{4}

) is greater than 2.0, revealing a concurrent metabolic alkalosis, likely secondary to intractable vomiting. Finally, applying Winter's formula (

1.5 \times 20 + 8 = 38 \pm 2

) yields an expected

pCO_2

of 38 mmHg. The measured

pCO_2

of 48 mmHg is significantly higher than expected, demonstrating an underlying respiratory acidosis due to hypoventilation in the context of severe COPD. This creates a triple acid-base disorder.

Step-by-Step Solution

Calculate the anion gap to evaluate the primary metabolic disturbance.

Anion Gap (AG) =

Na - (Cl + HCO_3) = 140 - (86 + 20) = 34

mEq/L. This signifies a High Anion Gap Metabolic Acidosis (HAGMA), likely secondary to lactic acidosis.

Evaluating the anion gap is the essential first step in determining the etiology of any metabolic acidosis.

Calculate the delta-delta ratio (

\frac{\Delta AG}{\Delta HCO_3}

) to check for a concurrent second metabolic process.

Excess AG =

34 - 12 = 22

. Bicarbonate deficit =

24 - 20 = 4

. Ratio =

\frac{22}{4} = 5.5

. Since the ratio is

> 2.0

, a concurrent metabolic alkalosis is present.

A high delta-delta ratio demonstrates that the bicarbonate level has not dropped as much as expected for the amount of unmeasured acid added, indicating an independent process increasing bicarbonate (e.g., severe vomiting).

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

Expected

pCO_2 = (1.5 \times 20) + 8 = 38

mmHg (range

36-40

This establishes the appropriate respiratory response for a net bicarbonate of 20 mEq/L.

Compare the measured

pCO_2

to the expected

pCO_2

The measured

pCO_2

(

48

mmHg) is significantly higher than the expected

pCO_2

(

38

mmHg).

Because the patient is retaining more

CO_2

than expected, there is a superimposed primary respiratory acidosis, consistent with hypoventilation in severe COPD.

Key Concept

The systematic interpretation of mixed acid-base disorders requires calculating the anion gap, analyzing the delta-delta ratio for hidden metabolic disturbances, and applying Winter's formula to evaluate respiratory compensation.

Estimated Time:2m 30s

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