What is the most consistent finding of respiratory alkalosis in an arterial blood gas (ABG)?

Appeared in: BTSC Staff Nurse 30July-2025

Explanation

Respiratory alkalosis is a state of increased blood pH (alkalemia) caused by a primary decrease in the partial pressure of arterial carbon dioxide (PaCO2).
The underlying cause is alveolar hyperventilation, which means breathing faster or deeper than metabolically necessary, leading to excessive elimination of CO2 from the body.
Since CO2 acts as an acid in the blood (by forming carbonic acid), losing too much CO2 makes the blood more alkaline.
Therefore, the defining ABG findings are a pH greater than 7.45 and a PaCO2 less than 35 mm Hg.

Option A: A low pH indicates acidosis, not alkalosis. This combination (low pH and low PaCO2) is characteristic of metabolic acidosis with respiratory compensation.
Option B: A high PaCO2 indicates CO2 retention (hypoventilation), which causes acidosis, not alkalosis. This combination (high pH and high PaCO2) points to metabolic alkalosis with respiratory compensation.
Option C: A low pH signifies acidosis. High PaCO2 combined with low pH are the classic findings for respiratory acidosis.

Clinical Relevance

Nursing practice connection: Knowing ABG interpretation for respiratory alkalosis helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Nurses must be able to quickly identify respiratory alkalosis from ABG results to assess for and manage the underlying cause of hyperventilation, which could be anxiety, pain, fever, or early-stage hypoxia.
Interventions are aimed at correcting the cause of hyperventilation. For example, if anxiety is the cause, the nurse can provide reassurance, and encourage slow, regular breathing. If pain is the cause, administering analgesics is the priority.
What if? If the ABG showed a low PaCO2, high pH, and also a low HCO3-, it would indicate that the respiratory alkalosis is chronic or partially compensated. The kidneys have started to excrete bicarbonate (a base) to try and bring the pH back down towards normal.

How to Approach the Question

First, analyze the term 'respiratory alkalosis'. 'Alkalosis' tells you the pH is high (greater than 7.45).
Second, 'Respiratory' tells you the primary problem is with the lungs and CO2 regulation. The PaCO2 value will be abnormal and will be the inverse of the pH change.
Recall that in respiratory alkalosis, the patient is hyperventilating, or 'blowing off' too much CO2.
Losing CO2 (an acid) makes the blood more alkaline. Therefore, the PaCO2 level will be low (less than 35 mm Hg).
Combine these two findings: a high pH and a low PaCO2.
Finally, scan the options to find the one that matches this combination.

Concept Tested & Keywords

Concept Tested: ABG interpretation for respiratory alkalosis
Stem keywords: respiratory alkalosis, arterial blood gas, ABG, consistent finding
Lead-in keywords: What is
Negative lead-in flag: false

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E6 Nursing Fundamentals Taylor p. 785-787

Practise the full BTSC Staff Nurse 30July-2025

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