RT Priority
Assess ventilation, mental status, oxygenation, and need for ventilatory support. Worsening somnolence with hypercapnia is a red flag.
Patient 2
A 68-year-old patient with severe COPD arrives in the emergency department with increasing somnolence, shallow breathing, and worsening dyspnea. Family reports he has been “harder to wake up” today.
Step 1Step 2Step 3Step 4Step 5Step 6Step 7Step 8Step 9
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 68-year-old patient with severe COPD arrives in the emergency department with increasing somnolence, shallow breathing, and worsening dyspnea. Family reports he has been “harder to wake up” today.
RR
8/min, shallow
8/min, shallow
SpO₂
86% RA
86% RA
HR
104/min
104/min
BP
148/86
148/86
Appearance
Drowsy, labored breathing
Drowsy, labored breathing
Breath Sounds
Diminished with scattered wheezes
Diminished with scattered wheezes
ABG Results
pH7.28
PaCO₂60
HCO₃26
PaO₂55
Step 1
Determine the pH Status
The pH is 7.28. 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 7.28 | PaCO₂ 60 | HCO₃ 26
Step 3
Evaluate Compensation
Is the opposite system trying to compensate for the primary disorder?
pH 7.28 | PaCO₂ 60 | HCO₃ 26 | PaO₂ 55
Step 4
Assess Oxygenation
The PaO₂ is 55 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: Uncompensated Respiratory Acidosis with Moderate Hypoxemia
The main problem is hypoventilation with CO₂ retention.
The normal HCO₃ suggests this is acute or not yet compensated.
The patient is also moderately hypoxemic.
Possible Clinical Causes
COPD exacerbation with fatigue
Opioid or sedative effect
Neuromuscular weakness
Severe airway obstruction
Board Pearl
Respiratory acidosis means the lungs are not removing enough CO₂. Think hypoventilation first.