A 62-year-old man is brought to the emergency department by emergency medical services due… — USMLE Step 2 CK

A 62-year-old man is brought to the emergency department by emergency medical services due to severe confusion and lethargy. His family reports that he has been increasingly difficult to arouse since yesterday morning. The patient has end-stage renal disease secondary to hypertensive nephrosclerosis and normally receives hemodialysis three days a week. However, he missed his last two scheduled treatments due to transportation issues. His past medical history is also significant for severe chronic obstructive pulmonary disease, for which he requires continuous home oxygen therapy.

Upon arrival, his temperature is $36.8^\circ C$ ( $98.2^\circ F$ ), blood pressure is $158/92$ mm Hg, heart rate is $105$ /min, and respiratory rate is $12$ /min and shallow. He is somnolent and oriented only to person. Pulmonary examination reveals a prolonged expiratory phase with diffuse expiratory wheezing.

Arterial blood gas and serum laboratory analysis reveal the following:

Parameter	Result	Reference Range
Arterial Blood Gas (on 2L O₂)
pH	$7.14$	$7.35 - 7.45$
PaCO₂	$42$ mm Hg	$35 - 45$ mm Hg
PaO₂	$68$ mm Hg	$75 - 105$ mm Hg
Serum Chemistry
Sodium	$136$ mEq/L	$136 - 145$ mEq/L
Potassium	$6.4$ mEq/L	$3.5 - 5.0$ mEq/L
Chloride	$98$ mEq/L	$95 - 105$ mEq/L
Bicarbonate	$14$ mEq/L	$22 - 28$ mEq/L
Blood urea nitrogen	$112$ mg/dL	$8 - 20$ mg/dL
Creatinine	$8.8$ mg/dL	$0.7 - 1.3$ mg/dL

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

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

Answer

The patient has a mixed acid-base disorder characterized by a high anion gap metabolic acidosis and a concurrent respiratory acidosis.

The correct diagnosis is a mixed disorder consisting of a high anion gap metabolic acidosis and a concurrent respiratory acidosis. The patient's missed hemodialysis sessions have led to uremia, resulting in a primary metabolic acidosis with a calculated anion gap of 24 mEq/L (

136 - [98 + 14]

). To determine if respiratory compensation is adequate, Winter's formula is applied: Expected PaCO2 =

(1.5 \times 14) + 8 = 29 \pm 2

mm Hg. The patient's actual measured PaCO2 is 42 mm Hg. Because the actual PaCO2 is substantially higher than the expected compensated value, there is a coexisting primary respiratory acidosis, caused by his severe COPD and respiratory muscle fatigue.

Step-by-Step Solution

Assess the primary acid-base disturbance by looking at the pH and bicarbonate.

The pH is acidemic (

7.14

) and the serum bicarbonate is low (

14

mEq/L), establishing a primary metabolic acidosis.

Identifying the primary direction of the pH and the primary driver (HCO3- vs PaCO2) is the first step in acid-base interpretation.

Calculate the serum anion gap (AG) to classify the metabolic acidosis.

AG = Sodium - (Chloride + Bicarbonate) = 136 - (98 + 14) = 24

mEq/L. This indicates a high anion gap metabolic acidosis (HAGMA).

An elevated anion gap (

> 12

mEq/L) reveals the presence of unmeasured anions, which in this case are uremic toxins due to missed hemodialysis.

Apply Winter's formula to calculate the expected respiratory compensation (expected PaCO2) for the metabolic acidosis.

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

. Therefore,

(1.5 \times 14) + 8 = 21 + 8 = 29 \pm 2

mm Hg (range

27-31

mm Hg).

Winter's formula determines how much the lungs should physiologically hyperventilate to compensate for the metabolic acid load.

Compare the actual measured PaCO2 to the expected PaCO2 calculated in Step 3.

The actual PaCO2 is

42

mm Hg, which is significantly greater than the expected range of

27-31

mm Hg.

Because the actual PaCO2 is higher than what is required for adequate compensation, the patient is hypoventilating relative to their metabolic state, indicating a superimposed primary respiratory acidosis (due to COPD).

Key Concept

Mixed Acid-Base Disorders: Utilizing Winter's Formula

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