Metabolic acidosis and metabolic alkalosis are two important acid-base disorders seen in arterial blood gas, or ABG, interpretation. Both are linked to changes in bicarbonate, also called HCO3. The kidneys and metabolic processes play the main role in these disorders, while the lungs try to compensate by changing the breathing pattern.
In metabolic acidosis, the body has too much acid or too little bicarbonate. HCO3 goes down, and pH goes down. The blood becomes acidic. Common causes include diabetic ketoacidosis, sepsis, renal failure, severe diarrhea, alcoholism, and malnutrition. The lungs try to compensate by breathing faster and deeper to remove carbon dioxide.
In metabolic alkalosis, the body has too little acid or too much bicarbonate. HCO3 goes up, and pH goes up. The blood becomes alkaline. Common causes include vomiting, excessive gastric suctioning, loop diuretics, excess sodium bicarbonate use, and hyperaldosteronism. The lungs try to compensate by slowing breathing to retain carbon dioxide.
These conditions matter because severe acid-base imbalance affects the brain, heart, muscles, breathing, electrolytes, and organ function. Metabolic acidosis is defined by a primary fall in bicarbonate, while metabolic alkalosis is defined by a primary rise in bicarbonate.
What Is an Arterial Blood Gas?
An arterial blood gas test measures blood pH, carbon dioxide, bicarbonate, oxygen, and oxygen saturation. It helps assess acid-base balance, ventilation, and oxygenation.
The main ABG values are:
| ABG Value | Normal Range | Main Meaning |
|---|---|---|
| pH | 7.35 to 7.45 | Shows acid-base status |
| PaCO2 | 35 to 45 mmHg | Respiratory value controlled by lungs |
| HCO3 | 22 to 26 mEq/L | Metabolic value controlled mainly by kidneys |
| PaO2 | 80 to 100 mmHg | Oxygen level in arterial blood |
For metabolic disorders, focus first on pH and HCO3. PaCO2 helps you judge respiratory compensation.
What Is Metabolic Acidosis?
Metabolic acidosis occurs when acid builds up in the body or bicarbonate is lost. The result is a low HCO3 and low pH.
ABG Pattern of Metabolic Acidosis
| Value | Expected Finding |
|---|---|
| pH | Low, below 7.35 |
| HCO3 | Low, below 22 mEq/L |
| PaCO2 | Low if the lungs are compensating |
| PaO2 | Depends on the patient’s oxygenation status |
Simple pattern:
HCO3 goes down, pH goes down.
This means the blood is acidic because the metabolic buffer, bicarbonate, is too low.
What Causes Metabolic Acidosis?
Metabolic acidosis has three major mechanisms:
- Increased acid production
- Decreased acid excretion
- Excess bicarbonate loss
MSD Manual explains that metabolic acidosis can occur from acid accumulation, reduced acid excretion, or gastrointestinal or kidney bicarbonate loss.
Increased Acid Production
The body produces excess acid in conditions such as:
- Diabetic ketoacidosis
- Sepsis
- Lactic acidosis
- Alcohol-related ketoacidosis
- Starvation or malnutrition
In diabetic ketoacidosis, the body breaks down fat for energy because insulin is too low or ineffective. This produces ketones, which are acids.
Decreased Acid Excretion
The kidneys normally remove excess acid through urine. When kidney function fails, acids remain in the blood.
Common causes include:
- Acute kidney injury
- Chronic kidney disease
- Renal failure
- Renal tubular acidosis
Acidosis can develop in kidney injury because hydrogen ions are not excreted well.
Excess Bicarbonate Loss
Bicarbonate can be lost through the gastrointestinal tract.
Common causes include:
- Severe diarrhea
- Intestinal fistula
- Pancreatic drainage
- Certain renal tubular disorders
Severe or chronic diarrhea can cause metabolic acidosis because large amounts of bicarbonate are lost in stool.
Symptoms of Metabolic Acidosis
Symptoms depend on the cause and severity.
Common symptoms include:
- Deep, rapid breathing
- Kussmaul respirations
- Low blood pressure
- Confusion
- Weakness
- Fatigue
- Nausea or vomiting
- Headache
- Hypoxia
- Irregular heart rhythm
Kussmaul Respirations
Kussmaul respirations are deep, rapid breaths. They are a classic sign of metabolic acidosis, especially in diabetic ketoacidosis.
The lungs try to lower acid levels by blowing off CO2. Since CO2 acts like an acid, removing more CO2 helps raise pH toward normal. NCBI describes Kussmaul respirations as deep, labored breathing that often indicates compensation for metabolic acidosis.
Electrolyte Changes in Metabolic Acidosis
Metabolic acidosis often affects potassium.
In acidosis, hydrogen ions move into cells. Potassium may shift out of cells into the blood. This can cause hyperkalemia, or high potassium.
Hyperkalemia can lead to:
- EKG changes
- Muscle twitching
- Muscle weakness
- Dangerous dysrhythmias
This is why potassium monitoring is critical in metabolic acidosis, especially in DKA and renal failure.
Treatment of Metabolic Acidosis
Treatment depends on the cause. The goal is to correct the underlying problem, support breathing, monitor electrolytes, and prevent complications.
Common interventions include:
- Monitor ABG values
- Monitor electrolytes, especially potassium
- Monitor neurological status
- Monitor respiratory status
- Give IV fluids if ordered
- Treat infection or sepsis
- Give insulin for DKA as ordered
- Consider sodium bicarbonate only in selected severe cases
- Prepare for dialysis if renal failure or toxin buildup is present
- Start seizure precautions when clinically needed
Bicarbonate therapy is not automatic for every case. It is usually reserved for selected severe acidosis or specific clinical indications. The provider decides based on pH, cause, potassium, renal function, and patient stability.
Diabetic Ketoacidosis and Metabolic Acidosis
Diabetic ketoacidosis, or DKA, is a major cause of metabolic acidosis.
In DKA:
- Insulin is too low
- Cells cannot use glucose properly
- Fat breaks down for energy
- Ketones build up
- Blood becomes acidic
- HCO3 falls
- pH falls
DKA Nursing Priorities
Key care priorities include:
- Give regular insulin as ordered
- Monitor blood glucose
- Monitor ketones
- Monitor potassium closely
- Give IV fluids as ordered
- Watch for mental status changes
- Monitor ABG results
- Monitor urine output
A major nursing point is potassium. As acidosis improves and insulin is given, potassium shifts back into cells. This can cause hypokalemia, even if potassium was high at the start.
What Is Metabolic Alkalosis?
Metabolic alkalosis occurs when the body loses too much acid or gains too much bicarbonate. The result is high HCO3 and high pH.
ABG Pattern of Metabolic Alkalosis
| Value | Expected Finding |
|---|---|
| pH | High, above 7.45 |
| HCO3 | High, above 26 mEq/L |
| PaCO2 | High if the lungs are compensating |
| PaO2 | Depends on oxygenation status |
Simple pattern:
HCO3 goes up, pH goes up.
The blood becomes alkaline because bicarbonate is too high or hydrogen ions are too low.
What Causes Metabolic Alkalosis?
Metabolic alkalosis has two major mechanisms:
- Excess acid loss
- Excess bicarbonate gain or retention
Common causes include vomiting, hypovolemia, diuretic use, and hypokalemia.
Excess Loss of Acid
The stomach contains hydrochloric acid. When a patient loses gastric acid, the body becomes more alkaline.
Common causes include:
- Prolonged vomiting
- Excessive gastric suctioning
- Nasogastric tube drainage
This is why persistent vomiting is a classic cause of metabolic alkalosis.
Excess Bicarbonate
Too much bicarbonate can also raise blood pH.
Causes include:
- Excess sodium bicarbonate administration
- High intake of alkaline substances
- Overuse of some antacids
- Excess baking soda intake
- Milk-alkali syndrome
Hyperaldosteronism
Hyperaldosteronism can cause metabolic alkalosis through kidney effects. Aldosterone increases sodium retention and promotes hydrogen and potassium loss.
As hydrogen ions fall, bicarbonate rises.
Loop Diuretics
Loop diuretics can cause fluid and electrolyte changes that promote metabolic alkalosis. They can increase hydrogen and potassium loss through urine.
Common examples include:
- Furosemide
- Bumetanide
- Torsemide
Symptoms of Metabolic Alkalosis
Mild metabolic alkalosis may have few symptoms. Severe alkalosis can affect the nervous system, muscles, heart, and electrolytes.
Common symptoms include:
- Slow respiratory rate
- Weakness
- Lethargy
- Irritability
- Confusion
- Muscle cramps
- Tetany
- Tingling around mouth or fingers
- Hypokalemia
- EKG changes
- Positive Chvostek’s sign in hypocalcemia
Severe metabolic alkalosis can cause headache, lethargy, and tetany.
Hypokalemia and Hypocalcemia in Metabolic Alkalosis
Metabolic alkalosis is often linked with hypokalemia, or low potassium. Potassium may shift into cells, and the kidneys may lose potassium in urine.
Low potassium can cause:
- Weakness
- Fatigue
- Muscle cramps
- Constipation
- EKG changes
- Dysrhythmias
Alkalosis can also reduce ionized calcium by increasing calcium binding to albumin. This can cause symptoms of hypocalcemia, such as paresthesia, muscle cramps, tetany, and positive Chvostek’s sign.
Treatment of Metabolic Alkalosis
Treatment focuses on correcting fluid loss, electrolyte imbalance, and the cause of acid loss or bicarbonate excess.
Common interventions include:
- Stop vomiting with antiemetics as ordered
- Replace fluids with IV fluids as ordered
- Monitor intake and output
- Monitor potassium and chloride
- Replace potassium if ordered
- Review diuretic use
- Stop or reduce loop diuretics if ordered
- Monitor EKG changes
- Start seizure precautions if clinically needed
- Use acetazolamide if ordered in selected cases
Acetazolamide, also called Diamox, can increase bicarbonate loss through urine. It is not used for every patient. It depends on volume status, kidney function, electrolytes, and provider judgment.
Lung Compensation in Metabolic Disorders
The lungs compensate for metabolic problems by changing CO2 levels.
Compensation in Metabolic Acidosis
In metabolic acidosis, the lungs try to remove CO2 by breathing faster and deeper.
This causes:
- PaCO2 to go down
- pH to rise toward normal
- Respiratory rate to increase
This is why deep, rapid breathing is seen in severe metabolic acidosis.
Compensation in Metabolic Alkalosis
In metabolic alkalosis, the lungs try to retain CO2 by slowing breathing.
This causes:
- PaCO2 to go up
- pH to move down toward normal
- Respiratory rate to decrease
This compensation is limited because the body still needs oxygen. The lungs cannot slow breathing too much without risking hypoxia.
Metabolic Acidosis vs Metabolic Alkalosis
| Feature | Metabolic Acidosis | Metabolic Alkalosis |
|---|---|---|
| Main problem | Too much acid or too little bicarbonate | Too little acid or too much bicarbonate |
| pH | Low, below 7.35 | High, above 7.45 |
| HCO3 | Low, below 22 mEq/L | High, above 26 mEq/L |
| Hydrogen ions | Increased | Decreased |
| Lung compensation | Fast, deep breathing | Slower breathing |
| PaCO2 compensation | Goes down | Goes up |
| Potassium issue | Often hyperkalemia | Often hypokalemia |
| Common causes | DKA, renal failure, diarrhea, sepsis | Vomiting, suctioning, diuretics, bicarbonate excess |
| Major breathing sign | Kussmaul respirations | Slow respirations |
| Key treatment focus | Treat acid cause and monitor potassium | Replace fluids/electrolytes and stop acid loss |
ABG Interpretation: How to Identify Metabolic Disorders
Use this simple step-by-step method.
Step 1: Check pH
- pH below 7.35 = acidosis
- pH above 7.45 = alkalosis
- pH 7.35 to 7.45 = normal, but check compensation
Step 2: Check HCO3
- HCO3 below 22 = metabolic acidosis
- HCO3 above 26 = metabolic alkalosis
Step 3: Check PaCO2
PaCO2 tells you if the lungs are compensating.
- Low PaCO2 with metabolic acidosis means the lungs are blowing off CO2
- High PaCO2 with metabolic alkalosis means the lungs are retaining CO2
Step 4: Check Compensation
| Compensation Type | pH | HCO3 | PaCO2 |
|---|---|---|---|
| Uncompensated metabolic acidosis | Low | Low | Normal |
| Partially compensated metabolic acidosis | Low | Low | Low |
| Fully compensated metabolic acidosis | Normal but acid-leaning | Low | Low |
| Uncompensated metabolic alkalosis | High | High | Normal |
| Partially compensated metabolic alkalosis | High | High | High |
| Fully compensated metabolic alkalosis | Normal but base-leaning | High | High |
Clinical ABG Examples
Example 1: Metabolic Acidosis
| Value | Result |
|---|---|
| pH | 7.25 |
| PaCO2 | 30 mmHg |
| HCO3 | 14 mEq/L |
Interpretation:
- pH is low, so the patient is acidotic.
- HCO3 is low, so the cause is metabolic.
- PaCO2 is low, so the lungs are compensating.
- Result: partially compensated metabolic acidosis.
Possible causes include DKA, renal failure, lactic acidosis, severe diarrhea, or sepsis.
Example 2: Metabolic Alkalosis
| Value | Result |
|---|---|
| pH | 7.50 |
| PaCO2 | 48 mmHg |
| HCO3 | 34 mEq/L |
Interpretation:
- pH is high, so the patient is alkalotic.
- HCO3 is high, so the cause is metabolic.
- PaCO2 is high, so the lungs are compensating.
- Result: partially compensated metabolic alkalosis.
Possible causes include vomiting, gastric suctioning, loop diuretics, or excess bicarbonate.
Nursing Assessment Priorities
A good nursing assessment looks beyond the ABG sheet. Always assess the patient.
Check:
- Respiratory rate and depth
- Oxygen saturation
- Blood pressure
- Heart rhythm
- Mental status
- Muscle strength
- Seizure risk
- Intake and output
- Vomiting or diarrhea
- Medication history
- Diuretic use
- Blood glucose
- Kidney function
- Potassium, chloride, calcium, and bicarbonate levels
Emergency Warning Signs
Escalate care fast if the patient has:
- Severe confusion
- Loss of consciousness
- Severe dyspnea
- Kussmaul respirations
- Seizure activity
- Severe hypotension
- Dysrhythmias
- Very high or very low potassium
- Chest pain
- Signs of shock
- Severe dehydration
- Rapidly worsening ABG values
Severe acid-base imbalance can become life-threatening. Treat the patient, not only the lab value.
Common Student Mistakes
Avoid these common ABG errors:
- Confusing metabolic and respiratory causes
- Forgetting HCO3 is the metabolic value
- Forgetting CO2 is the respiratory value
- Assuming all low pH is respiratory acidosis
- Ignoring compensation
- Missing potassium shifts
- Ignoring vomiting and diarrhea history
- Forgetting DKA can cause severe acidosis
- Giving bicarbonate without understanding the cause
- Not reassessing after treatment
Quick Memory Tips
Use these simple memory points:
- HCO3 is base
- Low HCO3 means metabolic acidosis
- High HCO3 means metabolic alkalosis
- Low pH means acidosis
- High pH means alkalosis
- Metabolic acidosis: HCO3 down, pH down
- Metabolic alkalosis: HCO3 up, pH up
- Acidosis often shifts potassium out of cells
- Alkalosis often shifts potassium into cells
- Lungs compensate for metabolic problems
FAQs
1. What is metabolic acidosis?
Metabolic acidosis is an acid-base disorder where the body has too much acid or too little bicarbonate. On ABG, it usually shows low pH and low HCO3. Common causes include DKA, renal failure, sepsis, lactic acidosis, and severe diarrhea.
2. What is metabolic alkalosis?
Metabolic alkalosis is an acid-base disorder where the body has too little acid or too much bicarbonate. On ABG, it shows high pH and high HCO3. Common causes include vomiting, gastric suctioning, loop diuretics, and excess bicarbonate intake.
3. What ABG values show metabolic acidosis?
Metabolic acidosis usually shows pH below 7.35 and HCO3 below 22 mEq/L. PaCO2 may be low if the lungs are compensating. This low PaCO2 happens because the patient breathes faster to remove carbon dioxide.
4. What ABG values show metabolic alkalosis?
Metabolic alkalosis usually shows pH above 7.45 and HCO3 above 26 mEq/L. PaCO2 may be high if the lungs are compensating. This happens because breathing slows to retain carbon dioxide.
5. Why does diarrhea cause metabolic acidosis?
Severe diarrhea causes loss of bicarbonate through stool. Since bicarbonate is a base, losing it makes the blood more acidic. This can lead to low HCO3 and low pH.
6. Why does vomiting cause metabolic alkalosis?
Vomiting causes loss of stomach acid. When acid is lost, the body becomes more alkaline. This can raise bicarbonate levels and increase blood pH.
7. What is Kussmaul breathing?
Kussmaul breathing is deep, rapid breathing seen in severe metabolic acidosis. It is the body’s attempt to remove carbon dioxide and raise blood pH. It is commonly linked to diabetic ketoacidosis.
8. Why does metabolic acidosis cause hyperkalemia?
In acidosis, hydrogen ions move into cells. Potassium may shift out of cells into the blood. This can increase serum potassium and raise the risk of dangerous heart rhythm changes.
9. Why does metabolic alkalosis cause hypokalemia?
In alkalosis, potassium may shift into cells, and kidneys may lose more potassium in urine. This can cause hypokalemia. Low potassium can lead to weakness, cramps, EKG changes, and dysrhythmias.
10. What is the main difference between metabolic acidosis and metabolic alkalosis?
The main difference is the direction of bicarbonate and pH. In metabolic acidosis, HCO3 goes down and pH goes down. In metabolic alkalosis, HCO3 goes up and pH goes up.
