Acid base

Kidney contributes acid base homeostasis by

• reclamation of bicarbonate in proximal tubule

• generation of new bicarbonate in the distal tubule to replenish body buffer stores

• active secretion of hydrogen ion

Daily filtered load of bicarbonate is 4500 mEq

Proximal tubular absorption of bicarbonate is increased by

• ECF volume contraction

• increase in Pco2

• hypokalemia

Acid anion is filtered in glomerulus as sodium salt

H+ ion is excreted in distal nephron

factors increasing H+ secretion in distal nephron

• aldosterone

• elevated Pco2

Bicarbonate equivalent to H+ secretion is generated and absorbed in distal nephron

Normal pH is 7.38-7.42

pH values compatible with life 6.8-7.8

Bicarbonate is in both ICF and ECF  and participates in buffering capacity

Acid production

• Endogenous production of acid

  • combustion of glucose and fatty acids

  • carbon dioxide (volatile acid) and water

  • Handled by ventilation

• Cellular metabolism

  • non volatile acids

Blood brain barrier

• Freely permeable to co2

  • responses occur instantaneously

• lag in equilibrating with bicarbonate

  • early stages of metabolic acidosis 2-3 hour lag in respiratory response

  • hyperventilation may persist even after correction of metabolic acidosis

Anion gap

• anion gap = Na- (CL+HCO3)

• Anion gap is due unmeasured phosphates, sulphates and albumin

• the normal value for this anion gap is 10-12 mEq/l

• increase in anion gap indicates metabolic acidosis

Metabolic acidosis

• Normal anion gap

  • abnormally high bicarbonate loss

    • Kidney fails to reabsorb

      • renal tubular acidosis

    • Kidney fails to regenerate

      • Diuretics

    • extra renal loss of bicarbonate

      • Diarrhea

      • ileal drainage

    • acidifying salts have been added

      • hyperalimentation

      • ammonium chloride

• Increased anion gap

  • reduced excretion of inorganic acids

    • renal failure

      • retention of sulphates and phosphates

      • impaired net acid excretion (ATN)

      • impaired ammonia excretion (Chronic renal failure)

  • accumulation of organic acids

    • ketoacidosis

      • diabetic

      • starvation

      • alcoholic

    • lactic acidosis (impaired cellular respiration with anaerobic glycolysis)

      • shock

      • septicemia

      • profound hypoxemia

    • ingestion

      • salicilates

      • methanol

Treatment

• Renal failure: Bicarbonate is not required unless plasma bicarb falls below 16 mEq/L

• Distal tubular acidosis: 30-60 mEq of bicarbonate daily

• GI losses: Bicarb when the pH falls below 7.1

• Lactic acidosis: improve tissue perfusion

• Diabetic ketoacidosis: Insulin, bicarb only when the pH falls to 7.0-7.1

Metabolic alkalosis

• loss of hydrogen ions from the body

  • vomiting 

  • gastric suction

• net rate of renal bicarbonate generation is greater than normal

  • volume contraction

  • potassium depletion

  • increased delivery of sodium to distal tubule (Loop diuretics)

  • minerelocorticoid excess

• Rapid correction of ventilation in a patient with chronic CO2 retention

In severe alkalosis

• Cardiac arrhythmia

• Hypoventilation

Single vs. Mixed metabolic disorders

Single acid base disturbance: the change in concentration of one anion is balanced by a reciprocal change in one other anion.

Mixed acid base disturbance: the anion patterns are more complex

Respiratory acidosis

• impaired alveolar ventilation

• acute respiratory acidosis (normal bicarbonate)

  • narcotic overdose

  • respiratory muscle paralysis

  • acute airway obstruction

• Chronic respiratory acidosis (increased bicarbonate)

  • COPD

  • kyphoscoliosis

  • Obesity hypoventilation syndromes

Respiratory alkalosis

• Acute hyperventilation (Light headedness, paresthesias, circumvoral numbness, tingling of extremities and tetany.)

  • acute salicylism

  • fever

  • sepsis

  • CHF

  • PE

  • Mechanical ventilation

  • anxiety (breath with a paper bag)

• Chronic hyperventilation

  • high altitude

  • hepatic insufficiency

  • pregnancy