Match the following arterial blood gas and serum electrolyte profiles with their most like… — USMLE Step 2 CK

Match the following arterial blood gas and serum electrolyte profiles with their most likely underlying clinical etiology.

$\text{pH}: 7.32$ , $\text{PaCO}_2: 28\text{ mmHg}$ , $\text{HCO}_3^-: 14\text{ mEq/L}$ , $\text{Na}^+: 140\text{ mEq/L}$ , $\text{Cl}^-: 115\text{ mEq/L}$ Severe gastrointestinal bicarbonate loss from excessive laxative consumption
$\text{pH}: 7.46$ , $\text{PaCO}_2: 20\text{ mmHg}$ , $\text{HCO}_3^-: 14\text{ mEq/L}$ , $\text{Na}^+: 142\text{ mEq/L}$ , $\text{Cl}^-: 105\text{ mEq/L}$ Toxicity from acetylsalicylic acid causing central respiratory stimulation and cellular metabolic uncoupling
$\text{pH}: 7.26$ , $\text{PaCO}_2: 25\text{ mmHg}$ , $\text{HCO}_3^-: 11\text{ mEq/L}$ , $\text{Na}^+: 138\text{ mEq/L}$ , $\text{Cl}^-: 100\text{ mEq/L}$ Ketone body accumulation due to absolute insulin deficiency
$\text{pH}: 7.33$ , $\text{PaCO}_2: 60\text{ mmHg}$ , $\text{HCO}_3^-: 31\text{ mEq/L}$ , $\text{Na}^+: 138\text{ mEq/L}$ , $\text{Cl}^-: 97\text{ mEq/L}$ Chronic alveolar hypoventilation due to advanced obstructive airway disease
$\text{pH}: 7.50$ , $\text{PaCO}_2: 48\text{ mmHg}$ , $\text{HCO}_3^-: 36\text{ mEq/L}$ , $\text{Na}^+: 140\text{ mEq/L}$ , $\text{Cl}^-: 94\text{ mEq/L}$ Significant hydrogen chloride loss from prolonged emesis

Answer

Profile 1 corresponds to laxative consumption (normal gap acidosis), Profile 2 to salicylate toxicity (mixed disorder), Profile 3 to insulin deficiency (high gap acidosis), Profile 4 to obstructive airway disease (chronic respiratory acidosis), and Profile 5 to prolonged emesis (metabolic alkalosis).

Each profile uniquely identifies a distinct acid-base disturbance. Applying the principles of anion gap calculation and expected respiratory compensation separates the varying etiologies accurately.

Step-by-Step Solution

Calculate the anion gap for every profile using the formula

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

Profile 1:

11

, Profile 2:

23

, Profile 3:

27

, Profile 4:

10

, Profile 5:

10

Determining the anion gap is an essential first step in evaluating any acid-base disorder, revealing hidden metabolic derangements even when the pH is alkalemic.

Determine the primary acid-base disturbance by examining the pH in conjunction with the changes in PaCO2 and HCO3-.

Profiles 1 and 3 are primary metabolic acidoses. Profile 2 is a mixed primary respiratory alkalosis and primary metabolic acidosis. Profile 4 is a primary respiratory acidosis. Profile 5 is a primary metabolic alkalosis.

Identifying the primary physiological disturbance dictates which secondary compensatory formulas must be applied.

Apply Winter's formula (

1.5 \times \text{HCO}_3^- + 8 \pm 2

) to the patients with metabolic acidosis.

For Profile 3, the predicted

\text{PaCO}_2

24.5\text{ mmHg}

, matching the measured

25\text{ mmHg}

(simple compensated high gap acidosis). For Profile 2, the predicted

\text{PaCO}_2

29\text{ mmHg}

, but the measured is

20\text{ mmHg}

, confirming an additional primary respiratory alkalosis.

This mathematical step definitively distinguishes simple, appropriately compensated acid-base disorders from complex mixed disorders.

Correlate the mathematical acid-base diagnoses with the provided pathophysiological states.

Normal gap acidosis matches intestinal bicarbonate loss. High gap acidosis matches ketone production. The mixed respiratory alkalosis and high gap acidosis matches acetylsalicylic acid toxicity. Chronic compensated respiratory acidosis matches COPD. Metabolic alkalosis matches gastric acid loss.

Final confirmation requires aligning the objective laboratory values with the clinical mechanism of disease.

Key Concept

Diagnosis of complex and mixed acid-base disorders using the anion gap, Winter's formula, and compensation rules.

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