Question

A 50-year-old man presents with altered sensorium and deep labored breathing. ABG shows: pH \(7.20\), \(pCO_2 = 35\) mmHg, \(HCO_3^- = 16\) mEq/L, \(Na^+ = 130\) mEq/L, \(Cl^- = 84\) mEq/L, \(PaO_2 = 80\) mmHg. What is the most likely acid-base abnormality?

• A. Acute metabolic acidosis with AG 30
• B. Chronic metabolic acidosis with AG 30
• C. Acute respiratory acidosis
• D. Chronic respiratory acidosis

Hint:

Anion gap formula: \[ AG = Na^+ - (Cl^- + HCO_3^-) \] Low pH with low bicarbonate = metabolic acidosis.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This is an acid-base problem requiring the interpretation of Arterial Blood Gas (ABG) and electrolyte values. The clinical signs (altered sensorium, Kussmaul breathing) provide context for the metabolic state.
Step 2: Key Formula or Approach:

Analyze pH: pH 7.20 (< 7.35) = Acidosis.

Analyze Primary Cause: Low HCO$_3^-$ (16 mEq/L) matches the acidic pH = Metabolic Acidosis.

Calculate Anion Gap (AG): \[ \text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) \]

Calculate AG for this patient: \[ \text{AG} = 130 - (84 + 16) = 130 - 100 = 30 \text{ mEq/L} \]

Step 3: Detailed Explanation:

Anion Gap Interpretation: A normal AG is approximately 12 $\pm$ 2 mEq/L. A gap of 30 is significantly elevated, indicating a High Anion Gap Metabolic Acidosis (HAGMA). Common causes include DKA, Lactic acidosis, Uremia, or Toxic ingestions (MUDPILES).

Compensation Check: For a primary metabolic acidosis, Winters' formula predicts the expected pCO$_2$: \[ \text{Expected pCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 \]
\[ \text{Expected pCO}_2 = (1.5 \times 16) + 8 = 24 + 8 = 32 \pm 2 \]
The measured pCO$_2$ of 35 is close to the expected 32, representing an acute attempt at respiratory compensation (the "deep labored breathing").

Acute vs. Chronic: Metabolic acidosis is rarely classified as "chronic" in a trauma/ER setting; the presentation of altered sensorium suggests an acute metabolic crisis.

Step 4: Final Answer:
The values indicate an acute High Anion Gap Metabolic Acidosis with an Anion Gap calculated as 30.