A 24-year-old man with type 1 diabetes mellitus is brought to the emergency department by … — USMLE Step 2 CK

A 24-year-old man with type 1 diabetes mellitus is brought to the emergency department by his roommate after a 2-day history of abdominal pain, nausea, vomiting, and profound weakness. The patient admits to missing several doses of his basal insulin due to a recent gastrointestinal illness. He appears lethargic and is using accessory muscles to breathe. Vital signs show a temperature of 37.6°C (99.7°F), blood pressure of 98/62 mm Hg, pulse of 124/min, and respiratory rate of 28/min with shallow breaths. Physical examination reveals dry mucous membranes and diffuse abdominal tenderness without guarding.

Laboratory evaluation shows:

Parameter	Result	Reference Range
Sodium ( $Na^+$ )	135 mEq/L	136-145 mEq/L
Potassium ( $K^+$ )	5.1 mEq/L	3.5-5.0 mEq/L
Chloride ( $Cl^-$ )	97 mEq/L	98-106 mEq/L
Bicarbonate ( $HCO_3^-$ )	12 mEq/L	22-28 mEq/L
Blood urea nitrogen	38 mg/dL	8-20 mg/dL
Serum creatinine	1.6 mg/dL	0.7-1.3 mg/dL
Glucose	495 mg/dL	70-110 mg/dL

Arterial blood gas analysis on room air:

Parameter	Result	Reference Range
pH	7.18	7.35-7.45
$pCO_2$	38 mm Hg	35-45 mm Hg
$pO_2$	82 mm Hg	75-105 mm Hg

Based on these findings, which of the following best describes this patient's acid-base status?

A
High anion gap metabolic acidosis and a superimposed respiratory alkalosis
B
High anion gap metabolic acidosis with appropriate respiratory compensation
High anion gap metabolic acidosis and a superimposed respiratory acidosisAnswer
D
Normal anion gap metabolic acidosis and a superimposed respiratory acidosis
E
Normal anion gap metabolic acidosis with appropriate respiratory compensation

Answer

High anion gap metabolic acidosis and a superimposed respiratory acidosis

This patient's presentation is consistent with diabetic ketoacidosis (DKA). Acid-base analysis begins with noting the acidemia (pH = 7.18) and low bicarbonate (12 mEq/L), identifying a primary metabolic acidosis. Next, calculating the anion gap (

135 - (97 + 12) = 26

mEq/L) reveals it is a high anion gap metabolic acidosis. To determine if respiratory compensation is adequate, Winter's formula is applied: Expected

pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2

. For this patient, the expected

pCO_2

26 \pm 2

mm Hg (range 24-28 mm Hg). Because the patient's measured

pCO_2

(38 mm Hg) is significantly higher than the expected range, there is a concurrent respiratory acidosis. This is a critical clinical finding: despite having Kussmaul respirations initially, the patient's lethargy and shallow breathing indicate respiratory muscle fatigue and impending respiratory failure.

Step-by-Step Solution

Determine the primary acid-base disturbance based on pH and bicarbonate.

The pH is 7.18 (acidemia) and the bicarbonate is 12 mEq/L (low). This establishes a primary metabolic acidosis.

Acidemia combined with a low serum bicarbonate concentration is the diagnostic hallmark of metabolic acidosis.

Calculate the serum anion gap to categorize the metabolic acidosis.

AG = Na^+ - (Cl^- + HCO_3^-) = 135 - (97 + 12) = 26

mEq/L.

An elevated anion gap (>12 mEq/L) confirms the presence of a high anion gap metabolic acidosis, driven in this case by diabetic ketoacidosis.

Calculate the expected respiratory compensation using Winter's formula.

Expected

pCO_2 = (1.5 \times 12) + 8 = 26 \pm 2

(range 24-28 mm Hg).

Winter's formula determines the expected physiological reduction in pCO2 (via hyperventilation) in response to a primary metabolic acidosis.

Compare the measured pCO2 to the expected pCO2 range to check for mixed disorders.

The measured

pCO_2

is 38 mm Hg, which is significantly higher than the expected maximum of 28 mm Hg.

A pCO2 higher than the expected compensatory range indicates a superimposed (concurrent) respiratory acidosis, suggesting the patient is fatiguing.

Key Concept

Accurate interpretation of acid-base disorders requires calculating the anion gap and applying Winter's formula to identify potentially life-threatening mixed disorders.

Estimated Time:1m 30s

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