ABG

Acid Base Balance

Overview

The acid-base balance is critical for maintaining homeostasis in the human body, ensuring that the hydrogen ion concentration remains compatible with life. A variety of physiological mechanisms work together to achieve this balance, primarily through the regulation of blood pH.

Sources of Hydrogen Ions:

  • Volatile Acids: These acids can easily be converted to gas and include Carbonic acid (H2CO3). Carbonic acid is formed from the reaction of CO2 with water in the blood and is eliminated through respiration.

  • Non-Volatile Acids: These are produced from metabolism and include Sulfuric acid, Phosphoric acids, and Lactic acid. Non-volatile acids are not readily expelled from the body and must be managed mainly by the kidneys.

Defense Mechanisms

Immediate responses to H+ accumulation are crucial to preventing acidosis. The body employs buffer systems:

  • Bicarbonate Buffers: The primary buffer in the blood, bicarbonate (HCO3-) can quickly react with excess hydrogen ions.

  • Non-bicarbonate Buffers: Proteins and phosphate ions also serve as buffers, though they play a secondary role to bicarbonate.

Key Organs Involved in Acid Excretion:

  • Lungs: The lungs excrete carbon dioxide, a major determinant of blood acidity. Through ventilation, CO2 is expelled, thereby affecting H2CO3 levels and pH.

  • Kidneys: The kidneys contribute to acid-base balance by adjusting bicarbonate levels in the blood and eliminating fixed acids through urine production.

Acid Elimination

  • Volatile Acids: Primarily removed through the lungs via exhalation. Regulation of ventilation adjusts blood acidity.

  • Fixed Acids: Eliminated by the kidneys through urine; the kidneys also reabsorb bicarbonate to maintain plasma levels.

pH Control Equation

The pH of blood is influenced by:

  • HCO3- levels: Base controlled by the kidneys.

  • PCO2 levels: Acid controlled by the lungs. The balance between these entities is critical for maintaining normal physiological function.

Importance of CO2 and HCO3- Ratio

Maintaining a proper ratio of CO2 to HCO3- is essential for normal pH levels. The quick responses of the lungs can adjust CO2 levels rapidly, while the kidneys take longer to modify bicarbonate levels.

  • Lungs: Have sole responsibility for CO2 control, providing rapid adjustments to acid-base balance.

  • Kidneys: Adjust HCO3- levels over hours to days, retaining or excreting bicarbonate as necessary.

Normal Acid Base Balance

Normal Ranges:

  • pH: 7.35 - 7.45 (a precise range is crucial for cellular metabolism)

  • paCO2: 35 - 45 mm Hg

  • [HCO3-]: 22 - 26 mEq/L

  • PaO2: 80 - 100 mm Hg

Acid-Base Disorders

Definitions

  • Acidemia: pH < 7.35, indicating an excess of hydrogen ions.

  • Alkalemia: pH > 7.45, indicating a deficiency of hydrogen ions.

Examples of Abnormal Readings:

  • pH = 7.32, PCO2 = 55, HCO3- = 24 → Suggests respiratory acidosis.

  • pH = 7.33, PCO2 = 50, HCO3- = 29 → Suggests a mixed disorder.

  • pH = 7.35, PCO2 = 50, HCO3- = 32 → Mild metabolic alkalosis could be indicated.

Interpretation of Acid-Base Disorders

Uncompensated Disorders

  1. Determine the Type: Based on pH, classify as Acidosis or Alkalosis.

  2. Determine Origin: Metabolic or Respiratory based on the corresponding CO2 or HCO3 values.

  3. Use Prefixes: Uncompensated if the remaining value is within the normal range.

Compensated Disorders

  • Partially Compensated: Remaining value is not normal but is moving in the opposite direction to aid pH normalization.

  • Fully Compensated: pH is within the normal range but on the abnormal end of the scale (7.35-7.40 or 7.40-7.45).

Examples

  • Uncompensated Respiratory Acidosis:

    • pH = 7.30, PCO2 = 55 mm Hg, HCO3- = 23 mEq/L

  • Compensated Respiratory Acidosis:

    • pH = 7.36, PCO2 = 55 mm Hg, HCO3- = 35 mEq/L

Respiratory Disorders

Respiratory Acidosis

  • Indicators: ↓ pH, ↑ PCO2 suggests hypoventilation.

Causes:

  • CNS Depression (e.g., Anesthesia, Narcotics)

  • Neuromuscular diseases (e.g., Myasthenia Gravis, Guillain-Barré Syndrome)

  • Restrictive disorders (e.g., Obesity, COPD)

Compensation Mechanisms:

  • The kidneys retain bicarbonate and excrete acids to normalize pH.

Clinical Implications for Physical Therapists

Physical therapists should focus on improving conditions that lead to hypoventilation, such as:

  • Deep breathing exercises

  • Incentive spirometry to enhance lung function

Respiratory Alkalosis

  • Indicators: ↑ pH, ↓ PCO2 indicates hyperventilation.

Causes:

  • Normal lungs (e.g., Anxiety, Acute pain)

  • Abnormal lungs (e.g., Asthma, Pneumonia)

Compensation in Metabolic Disorders:

  • The kidneys will excrete more bicarbonate during respiratory alkalosis.

Examples:

  • Uncompensated: pH = 7.57, PCO2 = 23 mm Hg, HCO3- = 24 mEq/L

  • Compensated: pH = 7.44, PCO2 = 26 mm Hg, HCO3- = 17 mEq/L

Metabolic Disturbances

Metabolic Acidosis

  • Characterized by a decrease in bicarbonate (↓ HCO3-) and a decrease in pH.

Causes:

  • Increased lactic acid production, renal failure, gastrointestinal loss of bicarbonate.

Compensation for Metabolic Acidosis:

  • Hyperventilation occurs to increase blood pH by reducing CO2 levels.

Metabolic Alkalosis

  • Characterized by an increase in bicarbonate (↑ HCO3-) and an increase in pH.

Causes:

  • Loss of hydrogen ions through vomiting or use of diuretics.

Compensatory Mechanisms:

  • Hypoventilation occurs in metabolic alkalosis to raise CO2 levels, working to restore balance.

Clinical Implications for Metabolic Disturbances

Physical therapists have limited roles in direct management, but should understand the importance of medical intervention for metabolic disturbances to ensure proper patient treatment.