A pulmonology consultation is requested for a hospitalized patient who was admitted two da… — USMLE Step 2 CK

A pulmonology consultation is requested for a hospitalized patient who was admitted two days ago for an acute exacerbation of chronic obstructive pulmonary disease. The patient has a history of type 2 diabetes mellitus and has developed severe nausea and intractable emesis over the last 24 hours after starting a new medication regimen. Current vital signs show a temperature of $37.8^\circ C$ ( $100^\circ F$ ), blood pressure of $110/68$ mm Hg, pulse of $115$ /min, and respirations of $28$ /min. Physical examination reveals distant breath sounds with prolonged expiration and dry mucous membranes. A comprehensive metabolic panel and arterial blood gas analysis are obtained on room air.

Laboratory Test	Result	Reference Range
Sodium	140 mEq/L	136-145 mEq/L
Potassium	3.2 mEq/L	3.5-5.0 mEq/L
Chloride	84 mEq/L	98-106 mEq/L
Bicarbonate	18 mEq/L	22-28 mEq/L
Blood urea nitrogen	45 mg/dL	8-20 mg/dL
Creatinine	2.1 mg/dL	0.7-1.3 mg/dL
Glucose	480 mg/dL	70-110 mg/dL
Arterial pH	7.26	7.35-7.45
Arterial $P_{a}CO_2$	44 mm Hg	35-45 mm Hg
Arterial $P_{a}O_2$	60 mm Hg	75-105 mm Hg

Based on the laboratory data provided, which of the following accurately describes the primary and concurrent acid-base processes occurring in this patient?

High anion gap metabolic acidosis, respiratory acidosis, and metabolic alkalosisCevap
B
High anion gap metabolic acidosis and respiratory acidosis only
C
High anion gap metabolic acidosis and metabolic alkalosis only
D
High anion gap metabolic acidosis with appropriate respiratory compensation
E
Normal anion gap metabolic acidosis, respiratory acidosis, and metabolic alkalosis

Cevap

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

The correct answer accurately identifies a triple acid-base disorder through a four-step calculation. First, the acidemia (

pH = 7.26

) and low bicarbonate (

18

mEq/L) indicate a primary metabolic acidosis. Second, the anion gap (

140 - (84 + 18) = 38

) is severely elevated, confirming a high anion gap metabolic acidosis. Third, Winter's formula (

1.5 \times 18 + 8 = 35

) demonstrates that the patient's measured

P_{a}CO_2

(

44

mm Hg) is significantly higher than the expected compensatory range (

33-37

mm Hg), revealing a concurrent respiratory acidosis. Finally, the delta-delta calculation (Corrected

[HCO_3^-] = \text{Measured } [HCO_3^-] + (\text{Measured AG} - 12) = 18 + (38 - 12) = 44

) shows a corrected bicarbonate much higher than the normal

24

mEq/L, confirming a superimposed metabolic alkalosis.

Adım Adım Çözüm

Determine the primary disorder from arterial pH and serum bicarbonate.

Primary metabolic acidosis.

The arterial pH is 7.26 (acidemia), which corresponds with the low bicarbonate level of 18 mEq/L.

Calculate the serum anion gap to classify the metabolic acidosis.

Anion gap is 38 mEq/L, indicating High Anion Gap Metabolic Acidosis (HAGMA).

Anion Gap =

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

= 140 - (84 + 18) = 38. A normal gap is

12 \pm 4

. The elevated gap suggests accumulation of unmeasured acids (likely diabetic ketoacidosis given the glucose of 480 mg/dL).

Apply Winter's formula to determine if respiratory compensation is appropriate.

Expected

P_{a}CO_2

35 \pm 2

mm Hg; actual is 44 mm Hg, indicating a concurrent respiratory acidosis.

Winter's formula: Expected

P_{a}CO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5 \times 18 + 8 \pm 2 = 35 \pm 2

. Because the actual

P_{a}CO_2

(44) is greater than the expected range (33-37), there is a primary respiratory acidosis (due to COPD exacerbation).

Calculate the delta gap and corrected bicarbonate to check for a third disorder.

Corrected bicarbonate is 44 mEq/L, indicating a concurrent metabolic alkalosis.

\Delta AG

= Measured AG (38) - Normal AG (12) = 26. Corrected

[HCO_3^-] = \Delta AG +

Measured

[HCO_3^-] = 26 + 18 = 44

. Since 44 is significantly greater than the normal baseline of 24, a concurrent metabolic alkalosis is present (due to emesis).

Anahtar Kavram

Diagnosis of mixed acid-base disorders requires a sequential, stepwise calculation approach utilizing the anion gap, Winter's formula, and the delta gap.

Soru

Cevap

Adım Adım Çözüm

Anahtar Kavram

Sık Yapılan Hatalar

Konular

Bu konudan daha fazla soru