A 54-year-old female is brought to the emergency department by her husband due to progress… — USMLE Step 2 CK

A 54-year-old female is brought to the emergency department by her husband due to progressive lethargy and confusion over the past $24$ hours. Her husband reports she has had severe, large-volume, watery diarrhea for the last $5$ days. She has not urinated since yesterday. Vital signs include a temperature of $37.2^\circ$ C ( $99.0^\circ$ F), blood pressure of $82/50$ mm Hg, heart rate of $132$ /min, and respiratory rate of $26$ /min. Physical examination reveals delayed capillary refill, dry mucous membranes, and cold extremities.

Laboratory studies show:

Test	Value
Sodium	$138$ mEq/L
Potassium	$3.1$ mEq/L
Chloride	$114$ mEq/L
Bicarbonate	$6$ mEq/L
Blood urea nitrogen	$60$ mg/dL
Creatinine	$3.2$ mg/dL
Lactate	$7.5$ mmol/L

Arterial blood gas on room air:

Test	Value
pH	$7.17$
PaCO2	$17$ mm Hg
PaO2	$95$ mm Hg

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

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

Answer

High anion gap metabolic acidosis and concurrent normal anion gap metabolic acidosis with appropriate respiratory compensation

This patient has a severe metabolic acidosis (

pH

7.17

HCO_3^-

6

mEq/L). The calculated anion gap is

18

mEq/L (

138 - [114 + 6]

), confirming a high anion gap metabolic acidosis (HAGMA) driven by lactic acidosis from hypovolemic shock. However, calculating the delta gap reveals an additional disorder. The anion gap increased by

6

mEq/L (from a normal of

12

), but the bicarbonate plummeted by

18

mEq/L (from a normal of

24

). Because the delta ratio is

< 0.4

(

6/18 = 0.33

), there is excessive loss of bicarbonate unassociated with unmeasured anions. This is driven by her severe diarrhea, causing a concurrent normal anion gap metabolic acidosis (NAGMA). Finally, Winter's formula [

(1.5 \times 6) + 8 = 17

] exactly matches her measured

PaCO_2

17

mm Hg, confirming appropriate respiratory compensation without a superimposed respiratory disorder.

Step-by-Step Solution

Identify the primary disturbance based on pH and bicarbonate.

The pH is acidemic (

7.17

) and the bicarbonate is low (

6

mEq/L), indicating a primary metabolic acidosis.

pH and primary buffer changes dictate the foundational acid-base disorder.

Calculate the serum anion gap.

AG = Na^+ - (Cl^- + HCO_3^-) = 138 - (114 + 6) = 18

mEq/L. Since

18 > 12

, a High Anion Gap Metabolic Acidosis (HAGMA) is present.

All metabolic acidoses must be evaluated for the presence of unmeasured anions (e.g., lactate in hypovolemic shock).

Assess respiratory compensation using Winter's formula.

Expected

PaCO_2 = (1.5 \times 6) + 8 \pm 2 = 17 \pm 2

mm Hg. The actual

PaCO_2

17

mm Hg, which is appropriate.

Determines if there is a superimposed primary respiratory disorder.

Calculate the delta ratio (

\Delta AG / \Delta HCO_3^-

) to uncover additional metabolic disorders.

\Delta AG = 18 - 12 = 6

\Delta HCO_3^- = 24 - 6 = 18

. Delta ratio =

6 / 18 = 0.33

A delta ratio

< 0.4

means the drop in bicarbonate is much larger than the rise in unmeasured anions, indicating a concurrent Normal Anion Gap Metabolic Acidosis (NAGMA) from gastrointestinal bicarbonate loss.

Key Concept

Mixed metabolic acid-base disorders (HAGMA and NAGMA) are identified using the delta gap / delta ratio, while respiratory compensation is verified using Winter's formula.

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