A patient with chronic bronchitis has the following ABG values on a routine check-up: pH: 7.36, PaCO₂: 55 mmHg, HCO₃⁻: 34 mEq/L. Which interpretation best describes the patient's acid-base status?

Appeared in: HPSSC - 2015

Explanation

The pH of 7.36 is within the normal range (7.35-7.45), indicating that the body has fully compensated for the primary imbalance.
The elevated PaCO₂ of 55 mmHg (normal: 35-45 mmHg) is the primary respiratory acidosis, consistent with chronic bronchitis where CO₂ is retained.
The elevated HCO₃⁻ of 34 mEq/L (normal: 22-26 mEq/L) shows metabolic compensation, where the kidneys retain bicarbonate to buffer the excess acid.
Since the pH is normal but on the acidic side of 7.40, and both PaCO₂ and HCO₃⁻ are abnormal, the condition is identified as fully compensated respiratory acidosis.

Option A: Although the pH is in the normal range, the PaCO₂ and HCO₃⁻ values are significantly abnormal. A truly normal ABG would have all three parameters within their respective normal ranges.
Option B: In uncompensated respiratory acidosis, the pH would be low (acidic, below 7.35) and the HCO₃⁻ would be normal, indicating an acute issue with no time for renal compensation. Here, the pH is normal and HCO₃⁻ is high.
Option C: This diagnosis is incorrect because the primary problem is respiratory acidosis (high PaCO₂), not metabolic alkalosis. In a primary metabolic alkalosis, the pH would be high (above 7.45) and the primary abnormality would be an elevated HCO₃⁻.

Clinical Relevance

Nursing practice connection: Knowing Arterial Blood Gas (ABG) interpretation, specifically fully compensated respiratory acidosis helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Nurses must be proficient in ABG interpretation to identify life-threatening conditions and monitor the effectiveness of treatments for patients with respiratory and metabolic disorders.
In patients with chronic lung diseases like COPD or bronchitis, these 'compensated' values often represent their stable baseline. A sudden drop in pH below 7.35, even with similar PaCO₂/HCO₃⁻, would signal acute decompensation requiring immediate intervention.
What if? If this same patient was found with a pH of 7.25, PaCO₂ of 55 mmHg, and HCO₃⁻ of 34 mEq/L, the diagnosis would change to 'Partially Compensated Respiratory Acidosis' or 'Acute-on-Chronic Respiratory Failure'. This indicates a new acute problem (like pneumonia) has overwhelmed their chronic compensation, representing a medical emergency.

How to Approach the Question

First, recognize this is an analytical question requiring interpretation of lab data (ABGs).
Step 1: Evaluate the pH. Is it normal (7.35-7.45), acidic (<7.35), or alkalotic (>7.45)? Here, 7.36 is normal, indicating full compensation. Note that it leans towards the acidic side of 7.40.
Step 2: Evaluate the PaCO₂ (the respiratory component). Is it high (acidic) or low (alkalotic)? Here, 55 mmHg is high, indicating respiratory acidosis.
Step 3: Evaluate the HCO₃⁻ (the metabolic component). Is it low (acidic) or high (alkalotic)? Here, 34 mEq/L is high, indicating metabolic alkalosis.
Step 4: Synthesize the findings. The primary problem is respiratory acidosis (high PaCO₂), which aligns with the pH's acidic tendency. The body has responded with metabolic alkalosis (high HCO₃⁻). Since the pH is back to normal, it is fully compensated.

Concept Tested & Keywords

Concept Tested: Arterial Blood Gas (ABG) interpretation, specifically fully compensated respiratory acidosis.
Stem keywords: chronic bronchitis, ABG values, pH: 7.36, PaCO₂: 55 mmHg, HCO₃⁻: 34 mEq/L
Lead-in keywords: best describes
Clinical cues: The history of chronic bronchitis is a key cue, as it predisposes the patient to long-term carbon dioxide retention, allowing time for renal compensation.

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E6 Nursing Fundamentals Taylor p. 785-787

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

E6 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 2 — Subpart A (pp 1-239 of 478) p. 82-84

Practise the full HPSSC - 2015

Attempt every question from this paper in a timed mock, then review the full solution for each one.