4 SIMPLE ACID-BASE DISORDERS

• Metabolic acid-base disorder presents as an imbalance between bicarbonate, the primary extracellular buffer, and fixed, aka, non-volatile, acids.
• Respiratory acid-base disorder presents as an imbalance between bicarbonate and carbon dioxide, the volatile acid.
• Acid-base Disorders are distinguishable by their arterial blood profiles, compensatory respiratory and renal mechanisms, and common causes.

ACIDEMIA

pH < 7.35

Metabolic acidosis

• Bicarbonate < 20 mEq/L
• Caused by either a gain in hydrogen ions and/or a loss of bicarbonate, which lowers pH.
• Consequently, the arterial blood profile shows reduced bicarbonate concentration and elevated hydrogen ion concentration.
• In response to a drop in pH, the respiratory systemcompensates with hyperventilation:
  • Excess carbon dioxide is “blown off,” which lowers the partial pressure of carbon dioxide in the blood; the arterial blood profile reflects this.
• Though slower to respond, renal bicarbonate conservation and acid excretion produce a more effective and longer-lasting pH elevation.

Anion Gap

Causes of metabolic acidosis into at least two categories: those associated with a normal anion gap (AG), and those associated with an elevated anion gap.

• In a routine blood test, only some cations and anions are measured; the anions that are not measured constitute the “anion gap.”
  • Because the total concentration of anions must be equal to the total concentration of cations, we know that the anion gap, the unmeasured anions, must be equal to:
    The measured cations (usually sodium) minus the measured anions (usually bicarbonate and chloride).
  • A normal average anion gap value is 12 mEq/L (typical range = 8 – 16 mEq/L).
• Thus, an increased anion gap indicates that bicarbonate, a measured anion, has been lost and replaced by the unmeasured ions
• A normal anion gap indicates that chloride is the ion that replaced the lost bicarbonate, which makes sense given that it is the other measured anion in the anion gap equation.
• Common causes of a normal anion gap include:
  • Diarrhea
  • Renal tubular acidosis
  • Renal failure
  • Hyperchloremia
  • Addison disease
  • Acetazolamide
  • Spironolactone
  • Saline infusion
• Common causes of an increased anion gap include MUDPILES:
  • Methanol intoxication (methanol is also called “wood alcohol,” commonly found in antifreeze and industrial settings)
  • Uremia
  • Diabetic ketoacidosis
  • Paraldehyde (which is sometimes used to treat alcoholism and certain convulsive and mental disorders)
  • Iron overdose
  • Lactic acid
  • Ethylene glycol poisoning (ethylene glycol is a compound commonly found in antifreeze)
  • Salicylate ingestion (key ingredient in aspirin, poisoning is common in children)

Respiratory acidosis

• Pco2 > 44 mmHg
• Caused by a gain in carbon dioxide and bicarbonate.
• The degree of change in pH depends on the duration of the disorder:
  • The pH is more reduced in acute acidosis than in chronic because, in chronic acidosis, sufficient time has elapsed for renal mechanisms to have some effect.
• There is no respiratory compensation when the respiratory system is itself the source of the imbalance.
  • To raise pH, the nephrons conserve bicarbonate and excrete hydrogen ions; notice that this is similar to the renal response to metabolic acidosis.

Caused by:

• Hypoventilation
  • Inhibition of the medullary respiratory center, which can be induced by sedatives or brainstem lesions
  • Neuromuscular defects that inhibit the anatomical structures responsible for ventilation
  • Gas exchange defects, such as COPD.

ALKALEMIA

pH > 7.45

Respiratory alkalosis

• Pco2 < 36 mmHg
• pH is increased in proportion to the duration of the disorder.
• There is no respiratory compensation for respiratory-induced acid-base disturbances.
• Nephrons excrete excess bicarbonate and reduce titratable acid and ammonium ion secretion to conserve hydrogen ions; this is similar to the renal response to metabolic alkalosis.

Caused by:

• Hyperventilation, which “blows off” too much carbon dioxide and lowers its arterial partial pressure.
  • Stimulation of the medullary respiratory center, hypoxemia (low blood oxygen), and physical or mental distress.

Metabolic alkalosis

• Bicarbonate > 28 mEq/L
• Caused by a loss of hydrogen ions and/or a gain in bicarbonate, which raises pH.
• Consequently, the arterial blood profile shows elevated bicarbonate concentration, and decreased hydrogen concentration.
• In response to elevated pH, hypoventilation retains carbon dioxide, which increases its arterial partial pressure; this is reflected in the arterial blood profile.
• The nephrons increase bicarbonate excretion, and, by reducing secretion of titratable acids and ammonium, conserve hydrogen ions; these actions lower pH.

Most commonly caused by

• Vomiting (HCL is lost from the body)
• Loop and thiazide diuretics, which increase bicarbonate excretion in the urine.
  • Vomiting and diuretics cause extracellular fluid volume contraction, which, as we’ve learned elsewhere, triggers hormonal responses that increase bicarbonate reabsorption and maintain the metabolic alkalosis.
• Hyperaldosteronism (excessive aldosterone secretion) causes over-excretion of hydrogen ions.