Metabolic Acidosis Explained
Metabolic acidosis is an acid-base disorder caused by a primary decrease in bicarbonate. For respiratory therapy students, the key pattern is a low pH with a low HCO₃. The lungs may compensate by increasing ventilation to lower PaCO₂.
Core Metabolic Acidosis Pattern
Metabolic acidosis occurs when bicarbonate is low enough to push the pH below normal. Since HCO₃ is the metabolic component, the disorder is metabolic when pH and HCO₃ move in the same direction.
Low, below 7.35
Low, below 22 mEq/L
May be low if respiratory compensation is present
pH and HCO₃ move in the same direction
Normal Values to Know
| ABG Value | Normal Range | Role |
|---|---|---|
| pH | 7.35–7.45 | Determines acidotic, alkalotic, or normal status. |
| PaCO₂ | 35–45 mmHg | Respiratory component. |
| HCO₃ | 22–26 mEq/L | Metabolic component. |
| PaO₂ | 80–100 mmHg | Oxygenation status. |
How Compensation Works
In metabolic acidosis, the respiratory system attempts to compensate by increasing ventilation. This blows off CO₂, which helps move the pH back toward normal.
HCO₃ is low, pH is low, and PaCO₂ remains normal.
HCO₃ is low, pH is still low, and PaCO₂ is also low.
HCO₃ and PaCO₂ are abnormal, but pH has returned to normal.
If PaCO₂ moves the wrong way or does not fit the picture, consider a mixed disorder.
Common Causes of Metabolic Acidosis
Metabolic acidosis often occurs when the body gains acid, loses bicarbonate, or cannot clear acid effectively.
| Cause | Why It Causes Acidosis | RT Clinical Connection |
|---|---|---|
| Diabetic ketoacidosis | Ketone production increases acid load. | May cause deep, rapid Kussmaul respirations. |
| Lactic acidosis | Poor perfusion or hypoxia increases lactate. | Look for shock, sepsis, hypoxemia, or severe work of breathing. |
| Renal failure | Kidneys cannot excrete acid effectively. | May coexist with fluid overload or pulmonary edema. |
| Diarrhea | Bicarbonate is lost through the GI tract. | Assess dehydration, perfusion, and respiratory compensation. |
| Toxin ingestion | Some toxins create acid metabolites. | Requires urgent team escalation and monitoring. |
Example ABG
PaCO₂ 30 mmHg
HCO₃ 14 mEq/L
PaO₂ 92 mmHg
Step 1: pH is low, so the patient is acidotic.
Step 2: HCO₃ is low and matches the low pH, so the primary disorder is metabolic acidosis.
Step 3: PaCO₂ is low, showing respiratory compensation. Since pH is still abnormal, this is partial compensation.
Step 4: PaO₂ is normal.
Partially compensated metabolic acidosis with normal oxygenation.
Common Student Mistakes
PaCO₂ may be abnormal because it is compensating.
The low HCO₃ is the primary metabolic clue.
Always assess PaO₂ separately after acid-base status.
If both PaCO₂ and HCO₃ are pushing pH in the same direction, think mixed disorder.
Try Free Interactive ABG Cases
Practice metabolic acidosis and other ABG patterns through guided clinical case studies with step-by-step feedback.