A 60-year-old man with a history of peptic ulcer disease is brought to the emergency depar… — USMLE Step 2 CK

A 60-year-old man with a history of peptic ulcer disease is brought to the emergency department due to 5 days of severe, intractable nausea and nonbilious vomiting. He reports being unable to tolerate any oral intake and feels profoundly weak. On physical examination, the patient appears lethargic. His mucous membranes are markedly dry, and capillary refill is prolonged. Temperature is 37.1 C (98.8 F), blood pressure is 88/55 mm Hg, pulse is 118/min, and respirations are 16/min. Laboratory studies are as follows:

Laboratory test	Result	Reference range
Serum sodium	140 mEq/L	136-145 mEq/L
Serum potassium	3.1 mEq/L	3.5-5.0 mEq/L
Serum chloride	86 mEq/L	98-106 mEq/L
Serum bicarbonate	24 mEq/L	22-28 mEq/L
Blood urea nitrogen	75 mg/dL	8-20 mg/dL
Serum creatinine	3.8 mg/dL	0.7-1.3 mg/dL

Arterial blood gas (room air)	Result	Reference range
pH	7.40	7.35-7.45
PaCO2	40 mm Hg	35-45 mm Hg
PaO2	90 mm Hg	75-105 mm Hg

Which of the following is the most accurate interpretation of this patient's acid-base status?

A
Normal acid-base status with isolated prerenal azotemia
High anion gap metabolic acidosis and metabolic alkalosisAnswer
C
Normal anion gap metabolic acidosis and metabolic alkalosis
D
Primary metabolic alkalosis with appropriate respiratory compensation
E
High anion gap metabolic acidosis with appropriate respiratory compensation

Answer

High anion gap metabolic acidosis and metabolic alkalosis

The correct answer accurately identifies a mixed acid-base disorder by utilizing the anion gap and delta-delta calculations. Even when the pH and bicarbonate are normal, an anion gap must be calculated. The anion gap is significantly elevated at

30 \text{ mEq/L}

(normal is approximately

12 \text{ mEq/L}

), indicating a high anion gap metabolic acidosis (HAGMA) due to poor perfusion and acute kidney injury. The delta gap (

\Delta AG = 30 - 12 = 18

) indicates that the bicarbonate should have fallen by

18 \text{ mEq/L}

to an expected level of

6 \text{ mEq/L}

. Because the measured bicarbonate is

24 \text{ mEq/L}

, there is a concurrent severe metabolic alkalosis perfectly balancing the acidosis. This is consistent with his severe vomiting, which causes hypochloremic metabolic alkalosis via the loss of gastric

H^+

and

Cl^-

Step-by-Step Solution

Evaluate the primary acid-base values from the arterial blood gas.

The pH is 7.40,

PaCO_2

is 40 mm Hg, and

HCO_3^-

is 24 mEq/L. These values are all within strictly normal limits.

Establishing the baseline acid-base status is the required first step in interpretation, but normal values do not preclude mixed disorders.

Calculate the serum anion gap using the formula:

AG = Na^+ - (Cl^- + HCO_3^-)

Anion gap =

140 - (86 + 24) = 140 - 110 = 30 \text{ mEq/L}

The anion gap must always be calculated, even if pH and

HCO_3^-

are normal, to avoid missing a hidden high anion gap metabolic acidosis (HAGMA).

Calculate the expected bicarbonate level using the delta gap (

\Delta AG

) to check for a mixed disorder.

Expected

HCO_3^- = 24 - (\text{Measured AG} - \text{Normal AG}) = 24 - (30 - 12) = 6 \text{ mEq/L}

. Since measured

HCO_3^-

(24 mEq/L) is much higher than the expected

HCO_3^-

(6 mEq/L), there is a concurrent metabolic alkalosis.

A delta gap analysis reveals additional metabolic disturbances that are masked by opposing effects on the serum bicarbonate level.

Correlate the calculated mixed acid-base disorder with the patient's clinical presentation.

The HAGMA is driven by lactic acidosis (hypoperfusion) and uremia (acute kidney injury), while the metabolic alkalosis is driven by severe vomiting (loss of

H^+

and

Cl^-

Clinical correlation confirms the pathophysiologic etiology of the identified disorders.

Key Concept

In patients with severe vomiting and hypoperfusion, opposing acid-base disturbances (metabolic alkalosis and HAGMA) can result in a deceptively normal pH and bicarbonate. Calculating the anion gap and delta gap is essential to unmask the mixed disorder.

Alternative Method

The delta ratio (

\frac{\Delta AG}{\Delta HCO_3^-}

) can also be used to evaluate mixed disorders. Delta ratio =

\frac{30 - 12}{24 - 24} = \frac{18}{0}

. As the denominator approaches zero, the ratio approaches infinity. Any delta ratio

> 2.0

strongly indicates a concurrent metabolic alkalosis.

Estimated Time:1m 30s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Alternative Method

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