A 45-year-old man is brought to the emergency department by his partner due to profound co… — USMLE Step 3

A 45-year-old man is brought to the emergency department by his partner due to profound confusion and recurrent vomiting over the past 24 hours. The partner reports the patient has a history of severe alcohol use disorder and was seen drinking an unknown fluid from an unmarked container in the garage yesterday. On review of systems, the patient complains that his vision is "blurry like looking through a snowstorm." Vital signs show temperature 36.8°C (98.2°F), blood pressure 110/70 mm Hg, heart rate 115/min, and respiratory rate 18/min. Physical examination reveals optic disc hyperemia.

Laboratory studies are obtained and reveal the following:

Laboratory Test	Result	Reference Range
Sodium ( $Na^+$ )	140 mEq/L	136 - 145 mEq/L
Potassium ( $K^+$ )	3.5 mEq/L	3.5 - 5.0 mEq/L
Chloride ( $Cl^-$ )	85 mEq/L	98 - 106 mEq/L
Bicarbonate ( $HCO_3^-$ )	10 mEq/L	22 - 28 mEq/L
Blood urea nitrogen (BUN)	14 mg/dL	8 - 20 mg/dL
Creatinine	1.2 mg/dL	0.7 - 1.3 mg/dL
Glucose	90 mg/dL	70 - 99 mg/dL
Serum osmolality (measured)	335 mOsm/kg	275 - 295 mOsm/kg

Arterial blood gas (room air):

Test	Result	Reference Range
pH	7.15	7.35 - 7.45
$PaCO_2$	30 mm Hg	35 - 45 mm Hg
$PaO_2$	70 mm Hg	75 - 105 mm Hg

Based on the provided laboratory data, which of the following represents the most accurate and complete interpretation of this patient's acid-base status?

High anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosisAnswer
B
High anion gap metabolic acidosis and primary respiratory alkalosis
C
High anion gap metabolic acidosis and respiratory acidosis
D
High anion gap metabolic acidosis, normal anion gap metabolic acidosis, and respiratory acidosis
E
High anion gap metabolic acidosis and metabolic alkalosis with adequate respiratory compensation

Answer

The correct interpretation is a triple acid-base disorder: high anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosis.

The patient has a triple acid-base disorder. First, the anion gap is 45 mEq/L, confirming a high anion gap metabolic acidosis (HAGMA). Second, using Winter's formula (

1.5 \times 10 + 8 = 23 \pm 2

), the expected

PaCO_2

is 21-25 mm Hg; the actual

PaCO_2

is 30 mm Hg, indicating concurrent respiratory acidosis due to inadequate ventilation. Finally, the delta ratio is

\Delta AG

(45-12) /

\Delta HCO_3^-

(24-10) =

33 / 14 = 2.36

. A ratio > 2.0 indicates a concurrent metabolic alkalosis, which is consistent with the patient's history of recurrent vomiting.

Step-by-Step Solution

Identify the primary disturbance and calculate the Anion Gap (AG).

The pH is 7.15 (acidemia) and

HCO_3^-

is 10 mEq/L, indicating a primary metabolic acidosis.

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (85 + 10) = 45

An AG greater than 12 mEq/L indicates a high anion gap metabolic acidosis (HAGMA), which in this patient is due to methanol toxicity (indicated by visual symptoms and high osmolar gap of 45 mOsm/kg).

Determine if respiratory compensation is adequate using Winter's formula.

Expected

PaCO_2 = (1.5 \times 10) + 8 \pm 2 = 23 \pm 2

(range 21-25 mm Hg). The patient's actual

PaCO_2

is 30 mm Hg.

Because the actual

PaCO_2

(30) is higher than the expected range (21-25), the patient has a concurrent respiratory acidosis (relative hypoventilation, likely from CNS depression).

Check for additional hidden metabolic disorders by calculating the Delta Ratio (

\Delta AG / \Delta HCO_3^-

\Delta AG = 45 - 12 = 33

\Delta HCO_3^- = 24 - 10 = 14

. Delta Ratio =

33 / 14 \approx 2.36

A Delta Ratio greater than 2.0 indicates a concurrent metabolic alkalosis. Despite the severe acidemia, the bicarbonate is higher than expected due to concurrent loss of

HCl

from severe vomiting.

Key Concept

Complex acid-base interpretation requires a systematic three-step approach: calculate the anion gap, apply Winter's formula to assess respiratory compensation, and calculate the delta ratio to identify hidden mixed metabolic disorders.

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