A 62-year-old man is brought to the emergency department by his wife due to worsening leth… — USMLE Step 2 CK

A 62-year-old man is brought to the emergency department by his wife due to worsening lethargy and confusion over the past 24 hours. The patient has a history of alcohol-associated cirrhosis, portal hypertension, and ascites. His wife reports that he has had 6 to 8 large-volume, watery bowel movements daily for the past four days after taking a new over-the-counter laxative. He has also missed his last several doses of lactulose. Vital signs are temperature 36.8 C (98.2 F), blood pressure 95/60 mm Hg, pulse 102/min, and respirations 12/min. Physical examination reveals a somnolent patient who arouses to a loud voice but is oriented only to person. Scleral icterus is present. The abdomen is markedly distended, tense, and dull to percussion with a positive fluid wave. Asterixis is elicited on neurologic examination. Lung auscultation reveals decreased breath sounds at the bases bilaterally, without crackles or wheezes.

Laboratory results:
Sodium: 136 mEq/L
Potassium: 3.2 mEq/L
Chloride: 113 mEq/L
Bicarbonate: 15 mEq/L
Blood urea nitrogen: 35 mg/dL
Creatinine: 1.4 mg/dL

Arterial blood gas (room air):
pH: 7.14
$pCO_2$ : 45 mm Hg
$pO_2$ : 68 mm Hg

Which of the following best describes this patient's acid-base status?

Normal anion gap metabolic acidosis and concurrent respiratory acidosisCevap
B
Normal anion gap metabolic acidosis with appropriate respiratory compensation
C
High anion gap metabolic acidosis and concurrent respiratory acidosis
D
Primary respiratory acidosis with appropriate metabolic compensation
E
Normal anion gap metabolic acidosis and concurrent respiratory alkalosis

Cevap

Normal anion gap metabolic acidosis and concurrent respiratory acidosis

This patient has a normal anion gap (non-anion gap) metabolic acidosis and a concurrent respiratory acidosis. The primary metabolic acidosis is indicated by severe acidemia (pH 7.14) and low bicarbonate (15 mEq/L), driven by severe gastrointestinal bicarbonate loss from laxative-induced diarrhea. The calculated anion gap is 8 mEq/L (

136 - [113 + 15]

), which is within the normal range of 8 to 12 mEq/L. To evaluate for a mixed disorder, Winter's formula (

Expected\ pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2

) is used. For this patient, the expected

pCO_2

30.5 \pm 2

mm Hg. The patient's actual measured

pCO_2

is 45 mm Hg. Because the measured

pCO_2

is substantially greater than the expected range, a concurrent respiratory acidosis is present. Clinically, this hypoventilation is explained by tense ascites mechanically restricting diaphragmatic excursion and hepatic encephalopathy depressing his central respiratory drive.

Adım Adım Çözüm

Evaluate the primary acid-base disturbance based on pH,

pCO_2

, and

HCO_3^-

The pH is 7.14 (acidemia),

HCO_3^-

is 15 mEq/L (low), and

pCO_2

is 45 mm Hg (high).

Identifying the direction of pH change and correlating it with

HCO_3^-

and

pCO_2

determines the primary disorder. Since both parameters are altered in acidotic directions, this is a mixed disorder.

Calculate the anion gap (AG).

AG = Na^+ - (Cl^- + HCO_3^-) = 136 - (113 + 15) = 8 \text{ mEq/L}

Determining the AG categorizes the metabolic acidosis into high or normal (non-anion gap), narrowing the differential diagnosis. A normal AG is typically 8-12 mEq/L.

Apply Winter's formula to assess respiratory compensation.

Expected\ pCO_2 = 1.5 \times 15 + 8 \pm 2 = 30.5 \pm 2 \text{ mm Hg}

. The measured

pCO_2

is 45 mm Hg, which is significantly higher than expected.

Winter's formula determines if the respiratory response to metabolic acidosis is adequate. A measured

pCO_2

higher than the expected range indicates a concurrent respiratory acidosis.

Anahtar Kavram

Acid-base disorders with mixed primary etiologies require systematic calculation of the anion gap and expected respiratory compensation using Winter's formula.

Tahmini Süre:1m 30s

Soru

Cevap

Adım Adım Çözüm

Anahtar Kavram

Sık Yapılan Hatalar

Konular

Bu konudan daha fazla soru