RT Priority
Assess respiratory status, oxygenation, volume status, and the underlying cause of acid loss or bicarbonate excess.
Patient 5
A 56-year-old patient has had persistent vomiting for two days after a bowel obstruction. They report weakness and dizziness. The nurse notes poor oral intake and signs of dehydration.
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Learning Objectives
By the end of this case, learners should be able to interpret the ABG in a complete, clinically useful format.
1. Classify pH
Determine whether the patient is acidotic, alkalotic, or normal.
Determine whether the patient is acidotic, alkalotic, or normal.
2. Identify the primary pattern
Decide whether the disorder is respiratory, metabolic, or absent.
Decide whether the disorder is respiratory, metabolic, or absent.
3. Assess compensation
Determine whether compensation is absent, partial, complete, or not applicable.
Determine whether compensation is absent, partial, complete, or not applicable.
4. Assess oxygenation
Use PaO₂ to classify oxygenation status.
Use PaO₂ to classify oxygenation status.
Clinical Picture
A 56-year-old patient has had persistent vomiting for two days after a bowel obstruction. They report weakness and dizziness. The nurse notes poor oral intake and signs of dehydration.
RR
12/min
12/min
SpO₂
95% RA
95% RA
HR
108/min
108/min
BP
102/64
102/64
Appearance
Weak, dry mucous membranes
Weak, dry mucous membranes
Breath Sounds
Clear bilaterally
Clear bilaterally
ABG Results
pH7.49
PaCO₂48
HCO₃36
PaO₂84
Step 1
Determine the pH Status
The pH is 7.49. How should it be classified?
Step 2
Identify the Primary Pattern
Compare the pH with PaCO₂ and HCO₃. Which component explains the pH pattern?
pH 7.49 | PaCO₂ 48 | HCO₃ 36
Step 3
Evaluate Compensation
Is the opposite system trying to compensate for the primary disorder?
pH 7.49 | PaCO₂ 48 | HCO₃ 36 | PaO₂ 84
Step 4
Assess Oxygenation
The PaO₂ is 84 mm Hg. How would you classify oxygenation?
Reference: Severe 0–39 · Moderate 40–59 · Mild 60–79 · Normal 80–100 mm Hg
Step 5
Build the Complete Interpretation
Select the best final ABG interpretation.
Clinical Connection
Final interpretation: Partially Compensated Metabolic Alkalosis with Normal Oxygenation
The primary problem is increased bicarbonate or loss of acid.
Respiratory compensation may retain CO₂, but compensation is limited by oxygenation needs.
Vomiting is a classic cause because gastric acid is lost.
Possible Clinical Causes
Prolonged vomiting
Nasogastric suction
Diuretic use
Excess bicarbonate administration
Volume depletion
Board Pearl
In metabolic alkalosis, the lungs compensate by retaining CO₂, but the body cannot hypoventilate indefinitely without risking hypoxemia.