Board Pearl
If the pH is still abnormal, compensation is partial. If pH has returned to normal but PaCO₂ and HCO₃ are both abnormal, compensation is complete.
Respiratory Acidosis Explained
Respiratory acidosis occurs when ventilation is inadequate and carbon dioxide builds up in the blood. For respiratory therapy students, the key ABG pattern is a low pH with an elevated PaCO₂.
What Is Respiratory Acidosis?
Respiratory acidosis is an acid-base disorder caused by inadequate alveolar ventilation. When a patient cannot remove carbon dioxide effectively, PaCO₂ rises. Because carbon dioxide acts as an acid in the blood, the pH falls.
Primary issue:
CO₂ retention from hypoventilation.
CO₂ retention from hypoventilation.
ABG clue:
pH moves down while PaCO₂ moves up.
pH moves down while PaCO₂ moves up.
Clinical concern:
Worsening ventilation, fatigue, or altered mental status.
Worsening ventilation, fatigue, or altered mental status.
RT focus:
Assess ventilation, airway, oxygenation, and need for support.
Assess ventilation, airway, oxygenation, and need for support.
Respiratory Acidosis ABG Pattern
The classic pattern is low pH with high PaCO₂. The bicarbonate level tells you whether compensation is present.
pH7.28
PaCO₂60
HCO₃26
PaO₂55
| Value | Interpretation | Why It Matters |
|---|---|---|
| pH 7.28 | Acidotic | The blood is below the normal pH range. |
| PaCO₂ 60 | Elevated | CO₂ retention is causing the acidosis. |
| HCO₃ 26 | Normal / high end of normal | No clear metabolic compensation in this example. |
| PaO₂ 55 | Moderate hypoxemia | Oxygenation also needs to be assessed. |
Common Causes of Respiratory Acidosis
Anything that decreases effective ventilation can lead to respiratory acidosis.
| Cause | How It Causes CO₂ Retention |
|---|---|
| COPD exacerbation | Airflow obstruction and air trapping reduce effective ventilation. |
| Opioid or sedative effect | Respiratory drive decreases, causing hypoventilation. |
| Neuromuscular weakness | Respiratory muscles cannot maintain adequate ventilation. |
| Severe asthma fatigue | Airway obstruction and fatigue can cause rising PaCO₂. |
| Obesity hypoventilation | Chronic hypoventilation can cause CO₂ retention. |
| Airway obstruction | Ventilation is impaired when air cannot move effectively. |
Clinical Picture
A respiratory acidosis patient may show signs of hypoventilation, increased work of breathing, or respiratory failure risk. The symptoms depend on how acute and severe the CO₂ retention is.
Ventilation clues:
Bradypnea, shallow breathing, fatigue, weak cough, or low minute ventilation.
Bradypnea, shallow breathing, fatigue, weak cough, or low minute ventilation.
Neurologic clues:
Headache, confusion, drowsiness, somnolence, or altered mental status.
Headache, confusion, drowsiness, somnolence, or altered mental status.
Oxygenation clues:
Low SpO₂, low PaO₂, cyanosis, or increased oxygen requirement.
Low SpO₂, low PaO₂, cyanosis, or increased oxygen requirement.
Compensation in Respiratory Acidosis
The kidneys compensate by retaining bicarbonate, but this takes time. Acute respiratory acidosis may have little or no bicarbonate change. Chronic respiratory acidosis often shows an elevated HCO₃.
| Pattern | ABG Clue | Interpretation |
|---|---|---|
| No compensation | pH low, PaCO₂ high, HCO₃ normal | Uncompensated respiratory acidosis |
| Partial compensation | pH low, PaCO₂ high, HCO₃ high | Partially compensated respiratory acidosis |
| Complete compensation | pH normal, PaCO₂ high, HCO₃ high | Fully compensated respiratory acidosis |
Do Not Forget Oxygenation
Respiratory acidosis describes ventilation and acid-base status. It does not automatically describe oxygenation. Always evaluate PaO₂ separately.
| PaO₂ | Oxygenation Category |
|---|---|
| 80–100 mmHg | Normal oxygenation |
| 60–79 mmHg | Mild hypoxemia |
| 40–59 mmHg | Moderate hypoxemia |
| Less than 40 mmHg | Severe hypoxemia |
Respiratory Therapy Priorities
The RT priority is to identify why ventilation is failing and support the patient while the underlying cause is addressed.
Assess ventilation
Evaluate respiratory rate, depth, pattern, PaCO₂ trend, ETCO₂ if available, and mental status.
Evaluate respiratory rate, depth, pattern, PaCO₂ trend, ETCO₂ if available, and mental status.
Support the airway
Assess airway patency, secretion burden, cough strength, and need for suctioning or airway support.
Assess airway patency, secretion burden, cough strength, and need for suctioning or airway support.
Improve oxygenation
Apply or adjust oxygen therapy while remembering oxygen does not directly remove CO₂.
Apply or adjust oxygen therapy while remembering oxygen does not directly remove CO₂.
Escalate when needed
Worsening acidosis, rising PaCO₂, fatigue, or altered mental status may require ventilatory support.
Worsening acidosis, rising PaCO₂, fatigue, or altered mental status may require ventilatory support.
Common Student Mistakes
Calling it metabolic because HCO₃ changed
In respiratory acidosis, HCO₃ may rise as compensation. The primary disorder is still respiratory if PaCO₂ matches the pH problem.
In respiratory acidosis, HCO₃ may rise as compensation. The primary disorder is still respiratory if PaCO₂ matches the pH problem.
Ignoring oxygenation
A patient can have respiratory acidosis and also be mildly, moderately, or severely hypoxemic.
A patient can have respiratory acidosis and also be mildly, moderately, or severely hypoxemic.
Assuming oxygen fixes CO₂ retention
Oxygen improves oxygenation, but ventilation removes CO₂.
Oxygen improves oxygenation, but ventilation removes CO₂.
Missing fatigue signs
Drowsiness, quiet breathing, and rising PaCO₂ can be more concerning than obvious distress alone.
Drowsiness, quiet breathing, and rising PaCO₂ can be more concerning than obvious distress alone.