Acidosis & Alkalosis - Causes and Compensations
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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.
- 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:
- 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.