A patient presents to the emergency department with a serum pH of 7.25, pCO2 of 40 mmHg, HCO3 of 22 mEq/L, sodium of 150 mEq/L, and chloride of 100 mEq/L. What is the primary acid-base disturbance in this patient?

1. A

   Metabolic acidosis

2. B

   Respiratory acidosis

3. C

   Metabolic alkalosis

4. D

   Respiratory alkalosis

Appeared in: NORCET 6 Prelims -2024

Explanation

Listen in Hindi

• The patient's pH of 7.25 is below the normal range of 7.35-7.45, indicating acidosis.
• The primary cause is determined to be metabolic because the bicarbonate (HCO₃⁻) level is on the low end of normal (22 mEq/L), and the respiratory parameter (pCO₂) is normal (40 mmHg).
• The diagnosis is further confirmed by calculating the anion gap: Na⁺ – (Cl⁻ + HCO₃⁻) = 150 – (100 + 22) = 28 mEq/L.
• This high anion gap (normal is 8-12 mEq/L) is a hallmark of metabolic acidosis caused by the accumulation of unmeasured acids in the blood.

Why Other Options Were Wrong

• Option B: The patient's pCO₂ is normal (40 mmHg). Respiratory acidosis is defined by a low pH and an elevated pCO₂ (greater than 45 mmHg) due to hypoventilation.
• Option C: The patient's pH is low (7.25). Metabolic alkalosis is defined by a high pH (greater than 7.45) and an elevated HCO₃⁻ (greater than 26 mEq/L).
• Option D: The patient's pH is low (7.25). Respiratory alkalosis is defined by a high pH (greater than 7.45) and a low pCO₂ (less than 35 mmHg) due to hyperventilation.

Related Visual

Clinical Relevance

• Nursing practice connection: Prioritize focused assessment, early escalation, and real-time monitoring when managing Interpretation of Arterial Blood Gas (ABG) results and anion gap calculation to identify primary acid-base disturbances in acute care settings.
• Rapid and accurate ABG interpretation is a critical skill in emergency and critical care nursing to identify life-threatening conditions and guide immediate treatment.
• Identifying a high anion gap metabolic acidosis (HAGMA) prompts the nurse and medical team to search for underlying causes, often remembered by the mnemonic MUDPILES (Methanol, Uremia, Diabetic ketoacidosis, Paraldehyde, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates).
• What if? If the patient's pCO₂ was 30 mmHg instead of 40 mmHg, it would indicate respiratory compensation for the metabolic acidosis. The body attempts to 'blow off' CO₂ (an acid) to raise the pH. The diagnosis would then be 'partially compensated metabolic acidosis'.

How to Approach the Question

• First, analyze the pH. A value below 7.35 indicates acidosis; a value above 7.45 indicates alkalosis.
• Second, determine if the cause is respiratory or metabolic. Look at the pCO₂ (the respiratory parameter) and the HCO₃⁻ (the metabolic parameter). The parameter that matches the pH's direction (acidosis or alkalosis) reveals the primary problem.
• Use the ROME mnemonic: Respiratory Opposite, Metabolic Equal. In respiratory disorders, pH and pCO₂ move in opposite directions. In metabolic disorders, pH and HCO₃⁻ move in the same direction (both down for acidosis, both up for alkalosis).
• Third, if metabolic acidosis is identified, always calculate the anion gap (Na⁺ - [Cl⁻ + HCO₃⁻]) to determine if it is a high anion gap or normal anion gap acidosis, which helps narrow down the cause.
• Finally, evaluate the other options to confirm they are inconsistent with the patient's data.

Concept Tested & Keywords

• Concept Tested: Interpretation of Arterial Blood Gas (ABG) results and anion gap calculation to identify primary acid-base disturbances.
• Stem keywords: serum pH, pCO2, HCO3, sodium, chloride, acid-base disturbance
• Lead-in keywords: What is the primary acid-base disturbance
• Clinical cues: pH: 7.25 (low)
• Clinical cues: pCO2: 40 mmHg (normal)

Question ID

QyE7SQOkhpNt48WaeMkvU7

Copy ID

Reference Book

E6 Nursing Fundamentals Potter Perry 12e Part 5 p. 56-58

E6 Physiology Guyton 4SAE Part 2B pp. 79-81, 75-77