Metabolic Acidosis-Normal Anion Gap (Non-Anion Gap Metabolic Acidosis, NAGMA)


Etiology

Type 1 Distal RTA (see Type 1 Distal Renal Tubular Acidosis)

Genetic Disease

Tubulointerstitial Renal Disease

Nephrocalcinosis Syndromes

Autoimmune Disease

Hypergammaglobulinemic States

Drugs/Toxins

Other

Type 2 Proximal RTA (see Type 2 Proximal Renal Tubular Acidosis)

Genetic Disease

  • Carbonic Anhydrase II Deficiency
  • Cystinosis
  • Galactosemia
  • Hereditary Fructose Intolerance
  • Glycogen Storage Disease Type I
  • Lowe Syndrome
  • Metachromatic Leukodystrophy
  • Methylmalonic Acidemia
  • Proximal Tubule Cell Sodium Bicarbonate Co-Transporter (NBCe1) Defect
  • Pyruvate Carboxylase Deficiency
  • Tyrosinemia
  • Wilson Disease (see Wilson Disease): produces both distal and proximal RTA

Renal Interstitial Disease

Carbonic Anhydrase-Related Conditions

  • Acetazolamide (Diamox) (see Acetazolamide)
    • Physiology: carbonic anhydrase inhibition -> bicarbonate loss in urine
  • Carbonic Anhydrase II Deficiency/Osteopetrosis
  • Dichlorphenamide (Keveyis) (see Dichlorphenamide)
    • Physiology: carbonic anhydrase inhibition -> bicarbonate loss in urine
  • Mafenide Acetate (Sulfamylon) (see Mafenide Acetate)
    • Mechanism: topical sulfonamide antibiotic (which is rapidly absorbed systemically in burn patients) with carbonic anhydrase inhibitor properties -> bicarbonate loss in urine
  • Sulfanilamide
  • Topiramate (Topamax) (see Topiramate)
    • Mechanism: carbonic anhydrase inhibitor properties -> bicarbonate loss in urine

Dysproteinemias

Drugs/Toxins

Other

Type 4 Renal Tubular Acidosis (RTA)/Hypoaldosteronism (see Type 4 Renal Tubular Acidosis and Hypoaldosteronism)

Decreased Aldosterone Synthesis

Aldosterone Resistance

Gastrointestinal (GI) Bicarbonate Loss

  • Biliary Drainage (Biliary Drain or T-Tube)
    • Mechanism: external loss of biliary fluid (biliary fluid contains 35 mmol bicarbonate per L)
  • Cholestyramine (see Cholestyramine)
    • Mechanism: cationic resin which that is administrated as a chloride salt -> formation of calcium carbonate or the bicarbonate salt of cholestyramine within the gastrointestinal lumen -> increased gastrointestinal bicarbonate loss
  • Diarrhea (see Diarrhea)
    • Mechanism: gastrointestinal loss of bicarbonate in stool
  • Enterovesical Fistula/Colovesical Fistula
    • Mechanisms: both require retrograde movement of urine into the intestine
      • Urinary Chloride is Exchanged for Bicarbonate by Chloride/Bicarbonate Transporter, Resulting in Bicarbonate Loss in the Stool
      • Urinary Urea is Metabolized by Colonic Bacterial Ureases into Ammonium Ions with Ammonium Ion Reabsorption Via Colonic Sodium-Hydrogen Antiporter (as Ammonium Takes the Place of Sodium): result in a net gain of ammonium and chloride ions and a loss of bicarbonate
  • Ileal Conduit (Neobladder) (see Ileal Conduit)
    • Epidemiology: since metabolic acidosis is a common complication of ureteroenterostomy, ileal conduits have largely replaced this procedure (however, metabolic acidosis still occurs in 10% of ileal conduit cases, especially in the presence of obstruction)
    • Mechanisms
      • Intestinal Mucosal Exchange of Urinary Chloride for Bicarbonate: results in urinary bicarbonate loss and hyperchloremia
      • Intestinal Mucosal Absorption of Urinary NH4: results in hepatic conversion to NH3 and hydrogen ion
        • Increased risk of this occurring when large loop of bowel used to make the neo-bladder, stoma is obstructed, or when sigmoid colonic section is used instead of ileal section
  • Laxative Abuse (see Laxative Abuse)
    • Mechanism: gastrointestinal loss of bicarbonate in stool
  • Oral Calcium Chloride (see Calcium Chloride)
    • Epidemiology: with oral administration only
    • Mechanism: calcium chloride administered orally is excreted as calcium bicarbonate (due to exchange of the chloride for bicarbonate) -> results in gastrointestinal bicarbonate loss and metabolic acidosis
  • Oral Magnesium Sulfate (see Magnesium Sulfate)
    • Mechanism: increases gastrointestinal bicarbonate loss
  • Pancreaticocutaneous Fistula
    • Mechanism: results in external loss of pancreatic fluid (pancreatic fluid contains 115 mmol bicarbonate per L)
  • Pancreaticopleural Fistula
    • Mechanism: loss of pancreatic fluid into pleural space (pancreatic fluid contains 115 mmol bicarbonate per L)
  • Small Intestinal Enterocutaneous Fistula
    • Mechanism: external loss of small intestinal fluid (duodenal secretions contain 10 mmol bicarbonate per L, ileal secretions contain 30 mmol bicarbonate per L)
  • Ureteroenterostomy/Ureterosigmoidostomy
    • Epidemiology: metabolic acidosis is a common complication of ureteroenterostomy
    • Mechanism: urinary diversion into the colon -> urinary ammonium is absorbed by the colonic mucosa, resulting in bicarbonate loss in the stool
  • Villous Adenoma
    • Mechanism: secretion of bicarbonate-rich fluid into the gastrointestinal lumen

Dilutional Metabolic Acidosis

  • Rapid Infusion of Bicarbonate-Free (and Lactate-Free) Normal Saline (see Normal Saline)
    • Physiology
      • Dilutional Metabolic Acidosis Results Predominantly from an Expansion in the Extracellular Fluid Volume by Fluids Which are Bicarbonate-Free or Contain No Organic Acid Salts that Could Potentially Be Metabolized to Bicarbonate (Such as Lactate or Acetate)
      • Mechanisms Favoring the Development of Dilutional Metabolic Acidosis
        • Narrowing of strong ion difference between sodium and chloride
        • Volume expansion -> decreased renal bicarbonate absorption
      • Mechanisms Countering the Development of Dilutional Metabolic Acidosis
        • Movement of bicarbonate from bone and intracellular stores into the extracellular space
        • Binding of hydrogen ions by proteins (albumin, hemoglobin)
    • Clinical Significance
      • However, in a Dog Model with a 28% Expansion of the Extracellular Volume with Isotonic Saline, the Serum Bicarbonate Only Decreased 10% (Am J Physiol, 1973) [MEDLINE]
        • This Suggests that Dilutional Acidosis is Unlikely to Occur Unless Extremely Large Amounts of Bicarbonate-Free Intravenous Fluids are Administered

Other

  • Acidic Salt Infusion
    • Ammonium Chloride (see Ammonium Chloride): intravenous ammonium chloride is a systemic and urinary acidifying agent, which is converted to ammonia and hydrochloric acid through hepatic oxidation
    • Calcium Chloride (see Calcium Chloride): generates hydrogen chloride
    • Arginine Hydrochloride: generates hydrogen chloride
  • D-Lactic Acidosis with Normal Renal Function (see Lactic Acidosis)
    • Mechanism: the proximal tubule sodium/L-lactate co-transporter is stereospecific and does not transport D-lactate
      • Therefore, filtered D-lactate is rapidly excreted in the urine (assuming normal renal function)
    • Diagnosis: delta anion gap/delta bicarbonate ratio is 1 or <1 (obviously, as the delta anion gap/delta bicarbonate ratio approaches zero, this would be observed as a non-anion gap metabolic acidosis)
      • This is in contrast with L-lactic acidosis, where the delta anion gap/delta bicarbonate ratio is typically between 1.1-1.6
  • Hydrochloric Acid (HCl) Infusion (see Hydrochloric Acid): hydrogen chloride added to the extracellular space results in replacement of bicarbonate by chloride in an equimolar basis
  • Late Phase of Diabetic Ketoacidosis (DKA) (see Diabetic Ketoacidosis and Hyperosmolar Hyperglycemic State)
    • Mechanism: urinary loss of ketoanions with sodium and potassium -> this is equivalent to a loss of potential bicarbonate, since each ketoanion (if retained) would have consumed a proton and been converted to a new bicarbonate molecule
  • Methionine Intoxication (see Methionine)
  • Moderate Renal Failure
    • Acute Kidney Injury (AKI) (see Acute Kidney Injury)
    • Chronic Kidney Disease (CKD) (see Chronic Kidney Disease)
      • Mechanisms
        • Early Kidney Disease: greater dysfunction in acid excretion than acid anion excretion -> typically have non-anion gap metabolic acidosis or anion gap metabolic acidosis with delta anion gap/delta bicarbonate ratio <1
        • Later Kidney Disease: typically have anion gap metabolic acidosis with delta anion gap/delta bicarbonate ratio >1
  • Total Parenteral Nutrition (TPN) (see Total Parenteral Nutrition)
  • Post-Hypocapnic Metabolic Acidosis
    • Mechanism
      • Chronic Hypocapnia Results in Renal Loss of Bicarbonate
      • When Hypocapnia is Remedied (Often in the Setting of Placing the Patient on Mechanical Ventilation, etc), Metabolic Acidosis is Observed Until the Kidney is Able to Compensate by Reabsorbing Bicarbonate
  • Toluene Intoxication (see Toluene)
    • May Present as Anion Gap Metabolic Acidosis (see Metabolic Acidosis-Elevated Anion Gap)
      • Early in the Course
      • With Impaired Renal Function
    • May Present as Non-Anion Gap Metabolic Acidosis:
      • Late in the Course

Physiology

Biliary/Duodenal/Pancreatic Secretions are Normally Alkaline

  • Biliary/Duodenal/Pancreatic Secretions Serve to Neutralize the Acidity of Gastric Secretions
    • Jejunal Mucosal (Luminal) Sodium/Hydrogen Ion Exchanger Normally Functions in Jejunum to Reabsorb Sodium Bicarbonate, Resulting in Normal Stool Having Small Amounts of Bicarbonate
      • However, Increased Stool Volume (Diarrhea) Results in Increased Bicarbonate Excretion, Resulting in Metabolic Acidosis

Diagnosis

Serum Chemistry

  • Serum Bicarbonate: decreased
  • Serum Chloride: hyperchloremia
  • Anion Gap: normal
    • Calculation of Anion Gap = Na – (Cl + HCO3)
      • Anion Gap Reflects the Difference Between Unmeasured Cations and Anions (i.e. the Anions in the Blood Which are Not Routinely Measured)

Serum Lactate (see Serum Lactate)

  • Normal

Arterial Blood Gas (ABG) (see Arterial Blood Gas)

  • Decreased Serum Bicarbonate with Respiratory Compensation

Urine Anion Gap (see Urine Anion Gap)

  • Rationale: renal ammonia excretion is the predominant component of renal net acid excretion
    • Proximal Tubule: apical Na+/H+ exchanger, NHE-3, is a major mechanism of preferential NH4+ secretion
    • Thick Ascending Loop of Henle: apical Na+-K+-2Cl- cotransporter, NKCC2, is a major contributor to ammonia reabsorption and the basolateral Na+/H+ exchanger, NHE-4, appears to be important for basolateral NH4+ exit
    • Collecting Duct: major site for renal ammonia secretion, involving parallel H+ secretion and NH3 secretion
  • Calculation of Urine Anion Gap = (Urine Na + Urine K) – (Urine Cl)
    • Normal Urine Anion Gap: 20 to 90 mEq/L
      • On a Typical Western Diet, the Quantity of Sodium and Potassium Absorbed from the Gastrointestinal Tract Exceeds the Quantity of Absorbed Chloride: therefore, renally excreted urine Na and K is greater than the amount of renally excreted urine Cl, making the urine AG positive
    • Urine AG >-20 mEq/L (Indicating Increased Renal NH4 Ion Excretion): urine AG becoming more negative indicates gastrointestinal bicarbonate loss
      • Diarrhea Results in Gastrointestinal Loss of Na, K, and Non-Chloride Anions (Including Bicarbonate, Butyrate, Citrate, and Lactate)
        • Gastrointestinal Loss of Bicarbonate Results in Increased Renal Excretion of Hydrogen Ion (H+) in the Form of Ammonium (NH4+) Chloride: urine chloride serves as a surrogate for NH4 ion excretion
      • Hypovolemia occurs with decreased sodium delivery to distal nephron
      • Bicarbonate is replaced by chloride in serum (producing hyperchloremia)
    • Urine AG Positive (Indicating Normal-Low Renal NH4 Ion Excretion): indicates altered urinary acidification, suggestive of distal renal tubular acidosis
  • Clinical Situations Where the Urine Anion Gap is Unreliable
    • Urine Anion Gap is Not Reliable in Neonates: neonates excrete other unmeasured anions at relatively high rates
    • Diabetic Ketoacidosis: due to increased urinary excretion of unmeasured non-chloride anions, beta-hydroxybutyrate and acetoacetate -> this alters the relationship between the urine NH4 and urine anion gap
    • Toluene Intoxication: due to increased urinary excretion of unmeasured non-chloride anion, hippurate
    • Proximal RTA Treated with Alkali Therapy: due to increased urinary excretion of unmeasured non-chloride anion, bicarbonate
    • D-Lactic Acidosis: due to increased urinary excretion of unmeasured non-chloride anion, D-lactate
    • Pyroglutamic Acidosis: due to increased urinary excretion of unmeasured non-chloride anion, 5-oxoproline

Urine Osmolal Gap

  • Rationale: urine osmolal gap performs better as a surrogate of the urine NH4 concentration when urinary ammonium (NH4) ion is excreted with an anion other than chloride (such as beta-hydroxybutyrate, acetoacetate, bicarbonate, or hippurate)

Clinical Manifestations

Pharmacologic Manifestations

Enhanced Effect of Neuromuscular Junction Antagonists (see Neuromuscular Junction Antagonists)

  • Physiology
    • Acidosis Potentiates the Effect of Neuromuscular Junction Antagonists
  • Agents

Renal Manifestations

  • Non-Anion Gap Metabolic Acidosis

Treatment

Treat or Remove the Underlying Etiology

  • xxxx

Sodium Bicarbonate (see Sodium Bicarbonate)

  • xxxx

References