Measurement of arterial blood gas shows pH: 7.2, PaCO2 : 70 mm Hg, HCO3: 27 mmol/L and PaO2: 59 mm Hg. Interpret this:

Appeared in: Combined AIIMS - 2020 (Jodhpur, Bhopal, Patna, Raipur)

Explanation

The primary finding is a low pH (7.2), which defines the state as acidosis.
The cause of the acidosis is a high PaCO₂ (70 mm Hg), which indicates respiratory system dysfunction (hypoventilation) leading to carbon dioxide retention.
This combination of a low pH and a high PaCO₂ is the classic sign of respiratory acidosis.
The bicarbonate (HCO₃⁻) level is slightly elevated (27 mmol/L), showing that the kidneys have started to compensate by retaining base to counteract the acid, but the pH is not yet corrected.

Option B: Respiratory alkalosis is incorrect because it is characterized by a high pH (above 7.45) and a low PaCO₂ (below 35 mm Hg), which is the opposite of the patient's values.
Option C: Metabolic acidosis is incorrect because the primary cause is not metabolic. In metabolic acidosis, the HCO₃⁻ would be low (below 22 mmol/L), and while the pH is low, the PaCO₂ would be the compensating factor, not the cause.
Option D: Metabolic alkalosis is incorrect. It is defined by a high pH (above 7.45) and a high HCO₃⁻ (above 26 mmol/L). The patient's pH is low (acidotic).

Clinical Relevance

Nursing practice connection: Knowing Arterial Blood Gas (ABG) Interpretation helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Respiratory acidosis is a life-threatening condition caused by inadequate ventilation and subsequent CO₂ retention. Common causes include COPD exacerbation, pneumonia, drug overdose (opioids, sedatives), and neuromuscular diseases.
A nurse's priority is to assess and support the patient's respiratory function. Interventions may include encouraging deep breathing and coughing, administering oxygen as prescribed, positioning the patient to facilitate breathing (e.g., high-Fowler's), and preparing for potential non-invasive or mechanical ventilation.
The accompanying hypoxemia (PaO₂ 59 mm Hg) is a critical concern that requires immediate attention to prevent tissue damage.

How to Approach the Question

First, look at the pH. A value below 7.35 is acidosis. A value above 7.45 is alkalosis. Here, 7.2 means acidosis.
Second, determine if the cause is respiratory or metabolic. Use the ROME mnemonic (Respiratory Opposite, Metabolic Equal).
Check the PaCO₂ (respiratory parameter). It is 70, which is high (acidic). Since the pH is low and PaCO₂ is high, they are moving in opposite directions. This points to a respiratory cause.
Check the HCO₃⁻ (metabolic parameter). It is 27, which is slightly high (alkaline). This does not match the acidosis direction, so it is not the primary cause.
Finally, assess for compensation. The HCO₃⁻ is slightly high, indicating the metabolic system is trying to compensate for the respiratory problem. Because the pH is still abnormal, the compensation is partial.

Concept Tested & Keywords

Concept Tested: Arterial Blood Gas (ABG) Interpretation
Stem keywords: Arterial blood gas, pH: 7.2, PaCO2 : 70 mm Hg, HCO3: 27 mmol/L, PaO2: 59 mm Hg
Lead-in keywords: Interpret this
Clinical cues: pH below 7.35 indicates acidosis.
Clinical cues: PaCO2 above 45 mm Hg indicates CO2 retention.

QDF4PoOmA56E1rdhS2J6EE

E6 Nursing Fundamentals Taylor p. 785-787

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

Practise the full Combined AIIMS - 2020 (Jodhpur, Bhopal, Patna, Raipur)

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