A 64-year-old man is evaluated in the intensive care unit for abnormal laboratory findings… — USMLE Step 2 CK

A 64-year-old man is evaluated in the intensive care unit for abnormal laboratory findings. He was admitted 2 days ago for severe community-acquired pneumonia complicated by septic shock. He was intubated yesterday for worsening hypoxia and is currently mechanically ventilated. The patient is sedated and receiving a continuous intravenous infusion of norepinephrine. Vital signs show a temperature of $38.4^\circ\text{C}$ ( $101.1^\circ\text{F}$ ), blood pressure of $92/58 \text{ mm Hg}$ , heart rate of $118/\text{min}$ , and oxygen saturation of $95\%$ on the current ventilator settings.

Laboratory studies:
Serum sodium: $140 \text{ mEq/L}$
Serum potassium: $4.1 \text{ mEq/L}$
Serum chloride: $100 \text{ mEq/L}$
Serum bicarbonate: $16 \text{ mEq/L}$
Blood urea nitrogen: $32 \text{ mg/dL}$
Serum creatinine: $1.9 \text{ mg/dL}$
Serum lactate: $5.2 \text{ mmol/L}$ (Normal: $0.5-1.5 \text{ mmol/L}$ )

Arterial blood gas (on mechanical ventilation):
pH: $7.52$
$\text{PaCO}_2$ : $20 \text{ mm Hg}$
$\text{PaO}_2$ : $78 \text{ mm Hg}$

Which of the following is the most accurate interpretation of this patient's current acid-base status?

Mixed respiratory alkalosis and high anion gap metabolic acidosisAnswer
B
Compensated high anion gap metabolic acidosis
C
Primary respiratory alkalosis with appropriate metabolic compensation
D
Mixed respiratory alkalosis and normal anion gap metabolic acidosis
E
Mixed respiratory acidosis and high anion gap metabolic acidosis

Answer

Mixed respiratory alkalosis and high anion gap metabolic acidosis

To accurately determine this patient's acid-base status, one must apply a stepwise mathematical approach. First, observe the pH: at

7.52

, the patient has alkalemia. Next, find the primary driver of this alkalemia: the

\text{PaCO}_2

is markedly low at

20 \text{ mm Hg}

, which directly causes alkalemia. This identifies a primary respiratory alkalosis, which in this ICU patient is iatrogenic due to hyperventilation by the mechanical ventilator. Crucially, acid-base analysis does not stop at the pH and

\text{PaCO}_2

; the anion gap (AG) must be calculated for every patient. The formula is

AG = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)

. Here,

AG = 140 - (100 + 16) = 24 \text{ mEq/L}

. A normal AG is

8-12 \text{ mEq/L}

. An AG greater than

12 \text{ mEq/L}

definitively proves the presence of an independent, primary high anion gap metabolic acidosis (HAGMA). In this patient, the HAGMA is caused by lactic acidosis secondary to septic shock. Finally, a delta-delta ratio calculation (

12 / 8 = 1.5

) confirms there is no third simultaneous disorder (like a concurrent normal anion gap acidosis or metabolic alkalosis). Therefore, the patient has two primary, concurrent acid-base disturbances.

Step-by-Step Solution

Analyze the pH to determine the predominant acid-base disorder.

The pH is

7.52

(alkalemia).

Identifying whether the patient is overall acidemic or alkalemic points to the primary disorder.

Examine the

\text{PaCO}_2

and

\text{HCO}_3^-

to find the driver of the alkalemia.

The

\text{PaCO}_2

20 \text{ mm Hg}

(low).

A low

\text{PaCO}_2

causes alkalemia. Therefore, a primary respiratory alkalosis is present, likely due to excessive minute ventilation on the mechanical ventilator.

Calculate the serum anion gap (AG).

AG = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (100 + 16) = 24 \text{ mEq/L}

An AG must always be calculated. A normal AG is

8-12 \text{ mEq/L}

. An AG of

24 \text{ mEq/L}

proves the presence of a primary high anion gap metabolic acidosis (HAGMA), driven here by elevated lactate from septic shock.

Evaluate the Delta-Delta ratio to check for a third disorder.

\Delta AG = 24 - 12 = 12

\Delta \text{HCO}_3^- = 24 - 16 = 8

. Ratio =

12 / 8 = 1.5

A ratio between

1

and

2

indicates an isolated HAGMA with no additional normal anion gap metabolic acidosis or metabolic alkalosis. Thus, the final diagnosis is a mixed dual disorder.

Key Concept

In mixed acid-base disorders, an alkalemic pH driven by a low

\text{PaCO}_2

defines a primary respiratory alkalosis, while an elevated anion gap definitively establishes the concurrent presence of a primary high anion gap metabolic acidosis.

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