Metabolic Acid-Base Imbalance

Acid-Base Balance and Imbalances Due to Metabolic Reasons

Overview of Acid-Base Imbalances

  • This lecture discusses metabolic causes of acid-base imbalances, specifically focusing on:

    • Metabolic Acidosis

    • Metabolic Alkalosis

Metabolic Acidosis

  • Definition: Result of a deficiency in bicarbonate or excess of noncarbonic acids.

  • pH Level: Falls below 7.35.

Causes of Metabolic Acidosis

  1. Bicarbonate Deficiency

    • Loss of excessive intestinal secretions (e.g., diarrhea, suction, or fistula).

    • Renal loss of bicarbonate.

  2. Accumulation of Noncarbonic Acids

    • Lactic Acidosis: Accumulation during shock or cardiac arrest due to anaerobic metabolism (insufficient oxygen).

    • Ketoacidosis: Related to uncontrolled diabetes, excessive alcohol consumption, starvation, or ketogenic diets.

    • Ingestion of toxic substances such as methanol, ethylene glycol, or acetylsalicylic acid (aspirin).

  3. Decreased Renal Function

    • Inability of the kidneys to secrete hydrogen ions or conserve bicarbonate; common in the elderly.

  4. Hyperchloremic Acidosis

    • Caused by excessive chloride use (e.g., IV solutions).

    • Ammonium chloride dissociates into NH₄⁺ and Cl⁻, leading to bicarbonate deficit and increased acid via hydrochloric acid formation.

Anion Gap in Metabolic Acidosis

  • Definition: Helps identify unmeasured anions and conditions causing metabolic acidosis.

  • Normal Values: Typically between 6 to 9 milliequivalents per liter.

  • Calculation: Anion gap = (Sodium) - (Bicarbonate + Chloride).

Manifestations of Metabolic Acidosis

  • Normal bicarbonate level is around 25 milliequivalents per liter.

  • Symptoms observed in patients:

    • Headache, malaise, weakness, fatigue.

    • Anorexia, nausea, and vomiting.

    • Sympathetic nervous system effects: vasodilation causes flushed, warm skin.

    • Decreased membrane excitability leading to decreased consciousness, stupor, and coma.

    • Cardiovascular effects include decreased contractility and cardiac output, leading to dyspnea and shock.

Compensation Mechanisms

  • Acute Metabolic Acidosis: Respiratory compensation develops through Kussmaul respirations to expel CO₂.

  • Chronic Metabolic Acidosis: Body compensates over time; symptoms might not be as severe.

Treatment of Metabolic Acidosis

  • Depends on the cause:

    • Supplemental bicarbonate may not be effective for increased anion gap conditions (e.g., lactic acidosis).

    • Best approach: correct underlying problems and restore fluid/electrolyte balance.

Metabolic Alkalosis

  • Definition: Occurs when bicarbonate levels rise above 27 (or above 25 for practical purposes) with a pH exceeding 7.45.

Causes of Metabolic Alkalosis

  1. Excess of Bicarbonate

    • Ingestion of antacids or bicarbonate products (e.g., citrate during blood transfusion).

  2. Deficit of Acid

    • Loss of hydrogen ions due to vomiting, gastric suction, or diuretics causing hypokalemia.

  3. Fluid Loss

    • Conditions leading to volume depletion (e.g., excessive diuretics or vomiting).

Manifestations of Metabolic Alkalosis

  • Symptoms include:

    • Hypotension, volume depletion (postural hypotension).

    • Mental confusion, hyperactive reflexes, tingling, tetany, potential seizures.

    • Severe cases may lead to respiratory failure or coma.

Treatment of Metabolic Alkalosis

  • Compensatory Mechanism: Hypoventilation to retain CO₂.

  • Treatment aims at addressing the underlying cause:

    • Correcting chloride or potassium deficits.

    • Fluid replacement (normal or half-normal saline) if extracellular fluid loss is present.

Acid-Base Interpretation

  • Trig Acid-Base Balance Method: A simple interpretational chart for assessing acid-base status in clinical scenarios.

Steps to Interpret Acid-Base Status

  1. Identify pCO₂:

    • Determine if normal or indicative of a respiratory problem.

  2. Determine if pCO₂ is acidotic or alkalotic:

    • Values below 40 indicate respiratory alkalosis, while values above 40 indicate respiratory acidosis.

  3. Compare pH to expected values:

    • If pH aligns with pCO₂ and expected values from respiratory component, the condition is uncompensated.

    • If there’s a mismatch, a metabolic component is present leading to compensated metabolic acidosis or alkalosis, as applicable.

Key Points for Quiz and Exam Preparation

  • Evaluate both respiratory and metabolic components.

  • Recognize possible mixed acid-base disorders, where metabolic issues may exacerbate a respiratory condition.

  • Understand the shifting impacts on pH and compensatory mechanisms interacting in acid-base disturbances.