Understanding Metabolic Acid-Base Disorders: A Focus on Metabolic Acidosis

What is Metabolic Acidosis?

Types of Metabolic Acidosis

Metabolic acidosis can be classified into two main types based on the underlying mechanisms: high anion gap metabolic acidosis and normal anion gap metabolic acidosis.

1. High Anion Gap Metabolic Acidosis: This occurs when there is an accumulation of unmeasured anions in the blood. Common causes include:
   • Lactic Acidosis: Resulting from tissue hypoxia, sepsis, or strenuous exercise.
   • Ketoacidosis: Often associated with uncontrolled diabetes, prolonged fasting, or excessive alcohol consumption.
   • Renal Failure: The kidneys' inability to excrete acid leads to its accumulation.
2. Normal Anion Gap Metabolic Acidosis: This type occurs when bicarbonate is lost, but there is no accumulation of unmeasured anions. Common causes include:
   • Diarrhoea: Loss of bicarbonate through the gastrointestinal tract.
   • Renal Tubular Acidosis: A condition where the renal tubules fail to adequately excrete hydrogen ions.

Causes of Metabolic Acidosis

The causes of metabolic acidosis can be grouped into three main categories:

1. Increased Acid Production:
   • Lactic Acidosis: This condition arises from the accumulation of lactic acid due to conditions such as sepsis, shock, or strenuous exercise. Lactic acid is produced when the body undergoes anaerobic metabolism, often when oxygen delivery to tissues is insufficient.
   • Ketoacidosis: Commonly seen in uncontrolled diabetes, this occurs when the body breaks down fats for energy, leading to an increase in ketone bodies. These ketones can accumulate and cause acidosis.
2. Loss of Bicarbonate:
   • Diarrhoea: Excessive loss of bicarbonate through diarrhoea can lead to a decrease in serum bicarbonate levels, resulting in acidosis. This is particularly common in conditions such as inflammatory bowel disease.
   • Renal Tubular Acidosis: In this condition, the kidneys fail to excrete acids effectively, leading to the collection of hydrogen ions and a simultaneous decrease in bicarbonate levels.
3. Decreased Acid Excretion:
   • Chronic Kidney Disease (CKD): Compromised renal function in CKD impacts the ability of the kidneys to excrete acids causing metabolic acidosis. This is more commonly seen in patients with advanced stages of kidney disorders.

Clinical Features

The clinical manifestations of metabolic acidosis can vary depending on the underlying cause and severity of the condition. Common symptoms include:

• Fatigue and Weakness: Patients often feel unusually tired and weak due to the effects of acidosis on muscle function.
• Headache: Increased acid levels can lead to cerebral vasodilation, causing headaches.
• Confusion or Altered Mental Status: Severe acidosis may impact neurological function, leading to confusion or altered consciousness.
• Rapid, Shallow Breathing: This is known as Kussmaul respiration and occurs as the body attempts to compensate for acidosis by increasing carbon dioxide elimination through hyperventilation.
• Nausea and Vomiting: Gastrointestinal symptoms can arise due to the body’s response to acidosis.

In severe cases, metabolic acidosis is known to cause more serious conditions like shock, coma, and even death.

Diagnosis

It is diagnosed through clinical evaluation and laboratory tests. Main components of the diagnostic process include:

1. Arterial Blood Gas (ABG) Analysis: This test measures blood pH, bicarbonate levels, and partial pressure of carbon dioxide (pCO2). In metabolic acidosis, the pH is usually below 7.35, and bicarbonate levels are decreased, indicating an acidotic state.
2. Serum Electrolytes: Checking serum electrolytes, especially potassium, is important, as metabolic acidosis can lead to hyperkalemia (elevated potassium levels). 
3. Anion Gap Calculation: The anion gap helps determine the cause of metabolic acidosis. It is calculated using the formula:

Anion Gap=[Na+]−([Cl−]+[HCO3−])Anion Gap=[Na+]−([Cl−]+[HCO3−​])

A normal anion gap suggests a loss of bicarbonate, often seen in conditions like diarrhoea, while an increased anion gap tells of the presence of unmeasured anions, commonly associated with lactic acidosis or ketoacidosis.

Management

The management of metabolic acidosis focuses on treating the root cause and quickly restoring the acid-base balance. Fundamental management strategies include:

Identifying and Treating the Underlying Condition:  For instance, administering insulin and fluids in cases of diabetic ketoacidosis or providing intravenous fluids and antibiotics in patients with septic shock can significantly improve patient outcomes.

Bicarbonate Therapy

In severe cases of acidosis, doctors might use sodium bicarbonate to reduce the extra acid in the body. This approach is generally reserved for serious situations because it doesn't help everyone. During this treatment, close monitoring of bicarbonate levels in the blood is important to ensure safety.

Supportive Care

People with metabolic acidosis need careful observation. This includes checking vital signs, giving oxygen if needed, ensuring proper hydration, and keeping track of electrolytes, especially potassium levels, to prevent complications.

Prognosis

The outcome depends on how quickly treatment starts and what caused the acidosis in the first place. Early treatment usually leads to better recovery. For those with chronic conditions like kidney disease or diabetes, regular check-ups are essential to prevent it from happening again.

Conclusion

Education about metabolic acid-base disorders is important for both healthcare professionals and patients. Those seeking further knowledge on this topic or other essentials asked in the AMC, USMLE, OPRA, SPLE, DHA, Oman Prometric Exam (OPE) or other pharmacy licensure exams click here.