Acid-Base Balance and Blood pH

• Maintaining stable blood pH (7.35-7.45, avg. 7.40) is vital for physiological processes.
• Acid-base disturbances are classified as acidosis (pH below normal) or alkalosis (pH above normal).

Determining Acid-Base Status

• Biochemistry profiles (chemistry panels) and blood gas profiles are used.
• Biochemistry profiles provide TCO2, anion gap, and electrolytes, where TCO2 approximates bicarbonate.
• Decreased TCO2 indicates metabolic acidosis, while increased TCO2 indicates metabolic alkalosis.
• Anion gap characterizes metabolic acidosis, but this method does not assess respiratory issues.
• Blood gas profiles measure pH, pCO2, pO2, electrolytes, and provide calculated values like bicarbonate and base excess.
• Blood gas profiles allow evaluation of respiratory acid-base imbalances.

Key Concepts

• Blood pH is influenced by pCO2 (ventilation changes), addition/removal of acids and strong ion movements (Na+, K+, Cl-).
• Serum proteins and phosphates can also contribute to pH.
• Buffers (bicarbonate, phosphate, ammonia, hemoglobin, albumin) resist pH changes.
• Regulatory systems include renal H+ excretion (slower) and alveolar ventilation for CO2 regulation (faster).

Interpreting Biochemistry Data

• Metabolic acidosis is characterized by decreased TCO2.
  • Titrational metabolic acidosis (increased anion gap) is due to increased unmeasured anions like ketones or lactate.
  • Secretional metabolic acidosis (normal anion gap) is associated with bicarbonate loss, often leading to increased chloride (hyperchloremic).
• Metabolic alkalosis is characterized by increased TCO2, often with decreased chloride in excess of sodium (hypochloremic).
• Mixed acid-base disturbance is suspected when data doesn't fit a single category.

Anion Gap (AG)

• Anion gap represents the difference between measured cations and anions, estimating unmeasured ions.
• Anion gap calculation: [Na+ + K+] - [Cl- + HCO3-].
• Increased anion gap suggests increased unmeasured anions.
• Decreased anion gap is uncommon and may be caused by hypoalbuminemia, hypercalcemia, or hypermagnesemia.

Law of Electroneutrality

• The law of electroneutrality is when the sum of cations equals the sum of anions.
• Ion changes affect others to maintain electrical balance, e.g., bicarbonate excretion with chloride retention.

Blood Gas Analysis

• Blood gas analysis is performed on electrochemical analyzers using heparinized whole blood (venous or arterial).
• Avoid air exposure and process quickly to prevent pH and pO2 changes.
• Measured parameters include pH, pCO2, pO2, electrolytes.
• Calculated parameters include HCO3- and base excess.
• pH below the reference interval indicates acidemia, and above indicates alkalemia.
• Low HCO3- indicates metabolic acidosis; high HCO3- indicates metabolic alkalosis.
• High pCO2 indicates respiratory acidosis (hypoventilation); low pCO2 indicates respiratory alkalosis (hyperventilation).
• Base excess < 0 indicates metabolic acidosis, and base excess > 0 indicates metabolic alkalosis.

Compensatory Mechanisms

• Compensatory mechanism are physiological responses to normalize blood pH without overcompensating.
• Respiratory compensation (fast):
  • For metabolic acidosis (low HCO3-), ventilation increases to decrease pCO2.
  • For metabolic alkalosis (high HCO3-), ventilation decreases to increase pCO2.
• Metabolic compensation (slow):
  • For respiratory acidosis (high pCO2), kidneys retain bicarbonate and excrete H+.
  • For respiratory alkalosis (low pCO2), kidneys excrete bicarbonate.
• Adequate compensation results in pH between 7.4-7.6; otherwise, suspect a mixed disturbance.

Stepwise Approach

• Evaluate pH to determine acidemia or alkalemia.
• Evaluate HCO3- for metabolic acidosis or alkalosis.
• Evaluate pCO2 for respiratory acidosis or alkalosis.
• Determine the primary disturbance and compensation; consider mixed disturbance if unclear.
• Determine if compensation is adequate.

Respiratory Acid-Base Disturbances

• Respiratory acidosis (CO2 retention, hypoventilation): pH ↓, pCO2 ↑, HCO3- ↑ (compensatory).
  • Causes: Anesthesia, pulmonary/CNS disease.
  • Compensation: Renal bicarbonate retention (slow).
• Respiratory alkalosis (CO2 release, hyperventilation): pH ↑, pCO2 ↓, HCO3- ↓ (compensatory).
  • Causes: Hypoxemia, pain, anxiety.
  • Compensation: Renal bicarbonate excretion (slow).

Metabolic Acid-Base Disturbances

• Metabolic acidosis (decreased bicarbonate): pH ↓, pCO2 ↓ (compensatory), HCO3- ↓.
  • Titrational: Increased nonvolatile acids (ketones, lactate); decreased TCO2, elevated anion gap.
  • Secretional: Base loss (diarrhea, renal tubules); decreased TCO2, normal anion gap, increased chloride.
  • Compensation: Increased ventilation (short-term), renal H+ excretion (long-term).
• Metabolic alkalosis (increased bicarbonate): pH ↑, pCO2 ↑ (compensatory), HCO3- ↑.
  • Causes: Excessive renal H+ loss (diuretics, hypokalemia), hypochloremia (vomiting).
  • Compensation: Decreased ventilation (short-term), renal H+ retention (long-term).
  • Biochemical findings: Increased TCO2, normal anion gap, decreased Cl- in excess of Na+.
• Paradoxical Aciduria: Acidic urine in metabolic alkalosis due to hypochloremia; treat with NaCl.

Mixed Acid-Base Disturbances

• Occur when two or more primary disturbances occur simultaneously.
• Suspect in cases of high anion gap/chloride changes with normal TCO2, data that doesn't fit, or compensatory responses in the wrong direction.
• Example: Diabetic ketoacidosis with vomiting.