A 74-year-old woman is admitted to the intensive care unit with profound dyspnea, altered … — USMLE Step 2 CK

A 74-year-old woman is admitted to the intensive care unit with profound dyspnea, altered mental status, and hypotension. She has a history of severe chronic obstructive pulmonary disease (COPD). For the past three days, she has experienced a worsening productive cough and subjective fevers.

Upon arrival, she is lethargic and easily arousable but confused. Vital signs are temperature $38.6^\circ\text{C}$ ( $101.5^\circ\text{F}$ ), blood pressure $82/50 \text{ mmHg}$ , pulse $124/\text{min}$ , and respirations $28/\text{min}$ . Oxygen saturation is $84\%$ on her baseline home oxygen requirement of $2 \text{ L/min}$ via nasal cannula.

Physical examination reveals dry mucous membranes, delayed capillary refill, and diffuse expiratory wheezes with right lower lobe crackles.

Laboratory results:

Test	Result
Sodium	$138 \text{ mEq/L}$
Potassium	$4.6 \text{ mEq/L}$
Chloride	$98 \text{ mEq/L}$
Bicarbonate	$14 \text{ mEq/L}$
Blood urea nitrogen	$48 \text{ mg/dL}$
Creatinine	$2.4 \text{ mg/dL}$ (baseline $1.1 \text{ mg/dL}$ )
Lactic acid	$5.8 \text{ mmol/L}$

An arterial blood gas analysis on

40\%

Venturi mask reveals:

Parameter	Value
pH	$7.15$
PaCO2	$42 \text{ mmHg}$
PaO2	$64 \text{ mmHg}$
HCO3	$14 \text{ mEq/L}$

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

A
High anion gap metabolic acidosis with adequate respiratory compensation
B
Mixed high anion gap metabolic acidosis and respiratory alkalosis
Mixed high anion gap metabolic acidosis and respiratory acidosisAnswer
D
Normal anion gap metabolic acidosis with concurrent respiratory acidosis
E
Primary respiratory acidosis with appropriate metabolic compensation

Answer

Mixed high anion gap metabolic acidosis and respiratory acidosis

The patient's arterial blood gas shows acidemia (pH

7.15

) driven by a primary metabolic acidosis (HCO3

14 \text{ mEq/L}

). The first step in evaluating metabolic acidosis is calculating the anion gap (AG):

\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 138 - (98 + 14) = 26 \text{ mEq/L}

. A normal AG is typically

8-12 \text{ mEq/L}

. An AG of 26 indicates a high anion gap metabolic acidosis (HAGMA), which in this patient is due to lactic acidosis from septic shock and acute kidney injury. The next step is to determine if the respiratory compensation is adequate using Winter's formula:

\text{Expected PaCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2

. Applying the values:

1.5 \times 14 + 8 = 21 + 8 = 29 \pm 2 \text{ mmHg}

. The patient's expected PaCO2 range is

27-31 \text{ mmHg}

. The measured PaCO2 is

42 \text{ mmHg}

. Since the measured PaCO2 is significantly higher than the expected PaCO2, there is a concurrent primary respiratory acidosis. In this patient with severe COPD and pneumonia, the inability to mount an adequate hyperventilatory response signals impending respiratory failure.

Step-by-Step Solution

Determine the primary acid-base disorder from the pH, PaCO2, and HCO3.

The pH is

7.15

(acidemia), driven by a low HCO3 of

14 \text{ mEq/L}

. This indicates a primary metabolic acidosis.

Establishing the primary disturbance is the necessary first step before assessing compensation or anion gap.

Calculate the anion gap (AG).

\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 138 - (98 + 14) = 26 \text{ mEq/L}

. Since normal AG is

8-12 \text{ mEq/L}

, this is a high anion gap metabolic acidosis.

Classifying the metabolic acidosis narrows the differential diagnosis (here, lactic acidosis and acute kidney injury).

Calculate the expected respiratory compensation using Winter's formula.

\text{Expected PaCO}_2 = (1.5 \times 14) + 8 = 29 \pm 2 \text{ mmHg}

(range

27-31 \text{ mmHg}

Metabolic acidosis stimulates respiratory drive. Winter's formula predicts the appropriate PaCO2 drop. A mismatch indicates a secondary respiratory disorder.

Compare the actual PaCO2 to the expected PaCO2.

The actual PaCO2 (

42 \text{ mmHg}

) is significantly higher than the expected range (

27-31 \text{ mmHg}

Because the actual PaCO2 is higher than expected, the patient is failing to ventilate adequately, revealing a superimposed respiratory acidosis.

Key Concept

Interpretation of mixed acid-base disorders in the critical care setting using the anion gap and Winter's formula.

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