A 21-year-old collegiate athlete is brought to the emergency department due to acute confu… — USMLE Step 2 CK

A 21-year-old collegiate athlete is brought to the emergency department due to acute confusion, tinnitus, and severe nausea for the past 24 hours. He reports repeatedly taking "handfuls of pain pills" over the last three days to manage a severe ankle sprain. The patient has also had multiple episodes of forceful, non-bilious emesis today. His temperature is 38.0°C (100.4°F), heart rate is 115/min, blood pressure is 112/68 mm Hg, and respiratory rate is 32/min.

Laboratory Test	Result	Reference Range
Sodium ( $Na^+$ )	140 mEq/L	136-145 mEq/L
Potassium ( $K^+$ )	3.4 mEq/L	3.5-5.0 mEq/L
Chloride ( $Cl^-$ )	92 mEq/L	98-106 mEq/L
Bicarbonate ( $HCO_3^-$ )	16 mEq/L	22-28 mEq/L

Arterial Blood Gas (Room Air)	Result	Reference Range
pH	7.50	7.35-7.45
$PaCO_2$	21 mm Hg	35-45 mm Hg
$PaO_2$	98 mm Hg	75-105 mm Hg

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

Primary respiratory alkalosis, primary elevated anion gap metabolic acidosis, and primary metabolic alkalosisCevap
B
Primary elevated anion gap metabolic acidosis with appropriate respiratory compensation
C
Primary respiratory alkalosis with appropriate metabolic compensation
D
Primary respiratory alkalosis and primary elevated anion gap metabolic acidosis only
E
Primary elevated anion gap metabolic acidosis and primary metabolic alkalosis only

Cevap

Primary respiratory alkalosis, primary elevated anion gap metabolic acidosis, and primary metabolic alkalosis

This patient's presentation and lab values indicate a classic triple acid-base disorder. The pH of 7.50 and

PaCO_2

of 21 mm Hg establish a primary respiratory alkalosis (due to central hyperventilation from salicylate toxicity). The anion gap of 32 establishes a primary high anion gap metabolic acidosis. Finally, the delta-delta calculation (

\Delta AG = 20

; Expected

HCO_3^- = 24 - 20 = 4

mEq/L) reveals that the actual

HCO_3^-

of 16 mEq/L is inappropriately high, proving the existence of a concurrent primary metabolic alkalosis caused by gastric acid loss from severe vomiting.

Adım Adım Çözüm

Analyze the pH to determine the predominant acid-base status.

The pH is 7.50, which indicates alkalemia.

Identifying the primary direction of the pH shift helps isolate at least one of the primary disorders. Alkalemia means a primary alkalosis (either respiratory or metabolic) must be present.

Evaluate the

PaCO_2

in the context of the alkalemia.

The

PaCO_2

is 21 mm Hg (normal 40 mm Hg).

A low

PaCO_2

in the setting of alkalemia confirms the presence of a primary respiratory alkalosis. This is classically seen in salicylate toxicity due to direct stimulation of the medullary respiratory center.

Calculate the Anion Gap (AG) to assess for hidden metabolic acidosis.

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (92 + 16) = 32

An AG greater than 12 confirms the presence of a primary elevated anion gap metabolic acidosis (caused by uncoupling of oxidative phosphorylation and accumulation of lactic/salicylic acid).

Apply Winter's formula to determine expected respiratory compensation for the metabolic acidosis.

Expected\ PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2 = (1.5 \times 16) + 8 \pm 2 = 32 \pm 2

mm Hg.

Since the actual

PaCO_2

(21 mm Hg) is significantly lower than the expected range (30-34 mm Hg), it confirms the primary respiratory alkalosis is an independent process, not just compensation.

Calculate the Delta Gap (

\Delta AG

) and expected

HCO_3^-

to check for a third disorder.

\Delta AG = Actual\ AG - Normal\ AG = 32 - 12 = 20

Expected\ HCO_3^- = 24 - 20 = 4

mEq/L.

If this were a pure high AG metabolic acidosis, the

HCO_3^-

should drop to 4 mEq/L. Because the actual

HCO_3^-

(16 mEq/L) is much higher than 4 mEq/L, there must be a concurrent process adding bicarbonate to the blood: a primary metabolic alkalosis due to severe vomiting.

Anahtar Kavram

Diagnosis of mixed triple acid-base disorders using the anion gap, Winter's formula, and the delta-delta ratio.

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