The body maintains a delicate balance of acids and bases to keep blood pH within the narrow range of 7.35–7.45. Even small deviations from this range can lead to significant physiological disturbances, organ dysfunction, or life-threatening complications. Among the most common acid-base disorders are metabolic acidosis and metabolic alkalosis.
- Metabolic acidosis occurs when there is too much acid (hydrogen ions) or too little bicarbonate in the body.
- Metabolic alkalosis occurs when there is too much bicarbonate or too little hydrogen ions.
Both conditions reflect kidney-related metabolic disturbances, with the lungs attempting to compensate by adjusting ventilation.
This guide will walk through the pathophysiology, causes, clinical features, diagnosis, and management of these two critical acid-base imbalances.
Understanding ABG Values in Metabolic Disorders
| Disorder | pH | HCO3 | Compensation by Lungs |
|---|---|---|---|
| Metabolic Acidosis | < 7.35 | < 22 mEq/L | Hyperventilation (blow off CO2) |
| Metabolic Alkalosis | > 7.45 | > 26 mEq/L | Hypoventilation (retain CO2) |
- HCO3 represents the metabolic component, regulated by the kidneys.
- The lungs compensate quickly by adjusting CO2 levels.
Metabolic Acidosis
Pathophysiology
In metabolic acidosis, there is either:
- Excess acid accumulation (e.g., ketoacids, lactic acid), or
- Loss of bicarbonate (base) (e.g., diarrhea, renal losses).
The body responds with Kussmaul’s breathing — deep, rapid respirations that help expel CO2 and raise pH.
- Problem: Too much H+ and too little HCO3
- Compensation: Lungs hyperventilate to blow off CO2
Causes of Metabolic Acidosis
The major causes can be remembered with MUDPILES (classic mnemonic):
- M – Methanol poisoning
- U – Uremia (renal failure)
- D – Diabetic ketoacidosis
- P – Propylene glycol
- I – Infection, Iron overdose, Isoniazid
- L – Lactic acidosis (shock, sepsis, hypoxia)
- E – Ethylene glycol (antifreeze poisoning)
- S – Salicylates (late aspirin toxicity)
From the infographic:
- Diabetic ketoacidosis (insulin deficiency → fat breakdown → ketone buildup)
- Kidney injury (impaired excretion of acids)
- Malnutrition (ketone overproduction due to fat breakdown)
- Severe diarrhea (loss of bicarbonate in intestinal secretions)
Signs and Symptoms of Metabolic Acidosis
Kussmaul’s breathing (deep, rapid >20 breaths/min)Hyperkalemia (due to shift of K+ out of cells):
- Muscle twitching, weakness, arrhythmias
Interventions for Metabolic Acidosis
Treatment depends on the underlying cause.
- Monitor intake and output (fluid status)
- Administer IV solutions (to restore bicarbonate and buffer acids)
- Seizure precautions (due to electrolyte disturbances)
- Monitor potassium levels (hyperkalemia correction if needed)
Special Situations:
Diabetic Ketoacidosis (DKA):
- Give IV insulin (stops fat breakdown and ketone formation)
- Monitor fluids and electrolytes (K+ replacement often required)
- Dialysis to remove acids and toxins
- Renal diet (high calories, moderate protein, controlled electrolytes)
Metabolic Alkalosis
Pathophysiology
In metabolic alkalosis, there is either:
- Excess bicarbonate accumulation (e.g., antacids, diuretics), or
- Loss of hydrogen ions (acid) (e.g., vomiting, gastric suction).
The body compensates by hypoventilation, retaining CO2 to lower pH.
- Problem: Too much HCO3, too little H+
- Compensation: Lungs hypoventilate to retain CO2
Causes of Metabolic Alkalosis
- Excessive use of antacids (sodium bicarbonate)
- Diuretics (loss of hydrogen ions in urine)
- Prolonged vomiting or gastric suction (loss of hydrochloric acid from the stomach)
- Hyperaldosteronism (increased hydrogen and potassium excretion)
Signs and Symptoms of Metabolic Alkalosis
- Hypoventilation (<12 breaths per minute)
- Hypokalemia (low K+): muscle weakness, arrhythmias, cramps
- Dysrhythmias
- Vomiting, tremors, tetany (due to hypocalcemia)
- EKG changes
- Confusion, seizures in severe cases
Interventions for Metabolic Alkalosis
- Monitor potassium and calcium levels
- Administer IV fluids to promote renal bicarbonate excretion
- Replace potassium (K+) if depleted
- Give antiemetics (Zofran, Phenergan) to control vomiting
- Watch for signs of respiratory distress (due to compensatory hypoventilation)
Comparing Metabolic Acidosis and Metabolic Alkalosis
| Feature | Metabolic Acidosis | Metabolic Alkalosis |
|---|---|---|
| pH | < 7.35 | > 7.45 |
| HCO3 | < 22 mEq/L | > 26 mEq/L |
| Primary Problem | Excess H+, low HCO3 | Excess HCO3, low H+ |
| Compensation | Hyperventilation (↓ CO2) | Hypoventilation (↑ CO2) |
| Common Causes | DKA, renal failure, diarrhea, lactic acidosis | Vomiting, diuretics, antacids, hyperaldosteronism |
| Potassium Effect | ↑ K+ (hyperkalemia) | ↓ K+ (hypokalemia) |
| Symptoms | Kussmaul’s breathing, arrhythmias, confusion | Tetany, muscle cramps, tremors, EKG changes |
| Treatment | Insulin (for DKA), dialysis, IV fluids | IV fluids, K+ replacement, antiemetics |
Clinical Importance
- Emergency medicine: Rapid recognition of DKA and lactic acidosis prevents mortality.
- Critical care: ABG monitoring ensures appropriate ventilator settings and fluid management.
- Nephrology: Acid-base disorders are common in renal failure, requiring dialysis.
- Surgery and anesthesia: Vomiting, suction, and diuretics make alkalosis frequent in surgical patients.
Frequently Asked Questions (FAQ)
Q1: What’s the main difference between metabolic and respiratory disorders?
A: Metabolic disorders are primarily due to kidney/base imbalance (HCO3), while respiratory disorders are due to lung/CO2 imbalance.
Q2: Why does potassium shift in metabolic acidosis and alkalosis?
A: In acidosis, hydrogen ions move into cells, pushing potassium out (↑ serum K+). In alkalosis, hydrogen ions move out of cells, pulling potassium in (↓ serum K+).
Q3: How do lungs compensate for metabolic acidosis?
A: By hyperventilation (Kussmaul’s breathing) to blow off CO2 and increase pH.
Q4: Can both metabolic acidosis and alkalosis occur together?
A: Yes, in mixed acid-base disorders, such as vomiting (alkalosis) with diarrhea (acidosis).
Q5: Why is monitoring electrolytes important in these conditions?
A: Because acid-base disorders are closely tied to potassium and calcium shifts, which can cause fatal arrhythmias.
