A 71-year-old man is brought to the emergency department by emergency medical services due… — USMLE Step 2 CK

A 71-year-old man is brought to the emergency department by emergency medical services due to severe shortness of breath and altered mental status. His past medical history is significant for severe chronic obstructive pulmonary disease (COPD), cor pulmonale, and hypertension. Home medications include inhaled tiotropium, oral furosemide, and lisinopril. His wife reports he has experienced a productive cough with green sputum and a fever for the past three days.

Vital signs include a temperature of 38.5°C (101.3°F), blood pressure of 82/50 mm Hg, heart rate of 118/min, and respiratory rate of 28/min. Oxygen saturation is 84% on room air. On physical examination, he is somnolent but arousable. Jugular venous distention is present. Lung auscultation reveals bilateral diffuse wheezes and coarse rhonchi.

Laboratory results are as follows:

Test	Result
Sodium	$140\text{ mEq/L}$
Potassium	$3.1\text{ mEq/L}$
Chloride	$86\text{ mEq/L}$
Bicarbonate	$30\text{ mEq/L}$
Blood urea nitrogen	$48\text{ mg/dL}$
Creatinine	$1.9\text{ mg/dL}$
Lactate	$6.0\text{ mmol/L}$

Arterial blood gas (room air):

Test	Result
pH	$7.26$
$PaCO_2$	$70\text{ mm Hg}$
$PaO_2$	$50\text{ mm Hg}$
$HCO_3^-$	$30\text{ mEq/L}$

Based on the laboratory results, which of the following best describes this patient's primary acid-base disturbances?

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

Answer

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

The patient's arterial blood gas shows an acidemia (pH 7.26) with an elevated

PaCO_2

(70 mm Hg), indicating a primary respiratory acidosis (due to COPD exacerbation). Next, calculating the anion gap (AG) reveals a high anion gap metabolic acidosis (HAGMA):

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (86 + 30) = 24\text{ mEq/L}

(normal 10-14). Finally, to check for a third disorder, the delta gap (

\Delta AG

) is calculated and added to the measured

HCO_3^-

to find the corrected

HCO_3^-

. The

\Delta AG = 24 - 12 = 12\text{ mEq/L}

. The corrected

HCO_3^- = 30 + 12 = 42\text{ mEq/L}

. Since the corrected

HCO_3^-

is significantly greater than the normal expected value of 24 mEq/L, an independent metabolic alkalosis is present (likely secondary to loop diuretic use). Therefore, this is a triple acid-base disorder.

Step-by-Step Solution

Evaluate the pH and PaCO2 to identify the primary acid-base disorder.

The pH is 7.26 (acidemia) and PaCO2 is 70 mm Hg (hypercapnia), indicating a primary respiratory acidosis.

Matching the direction of pH change with the PaCO2 change determines the initial primary respiratory disturbance.

Calculate the serum anion gap to evaluate for a high anion gap metabolic acidosis (HAGMA).

AG = Na - (Cl + HCO3) = 140 - (86 + 30) = 24 mEq/L. This indicates a HAGMA is present.

An elevated anion gap (> 12 mEq/L) independently signifies the presence of a HAGMA (in this case, lactic acidosis from sepsis/hypoperfusion), regardless of the overall pH.

Calculate the delta gap (ΔAG) and corrected bicarbonate to check for a hidden third disorder.

ΔAG = Calculated AG - Normal AG = 24 - 12 = 12 mEq/L. Corrected HCO3- = Measured HCO3- + ΔAG = 30 + 12 = 42 mEq/L.

The corrected bicarbonate reveals what the HCO3- would be without the HAGMA. Since 42 mEq/L > 24 mEq/L, a concurrent metabolic alkalosis is also present (likely from chronic furosemide use).

Key Concept

Stepwise interpretation of complex acid-base disorders, including the identification of a triple acid-base disorder using the anion gap and delta gap calculations.

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