a new perspective on metabolic acidosis
TRANSCRIPT
A New Perspective on Metabolic acidosis
Taipei Veterans General Hospital, Hsin-Chu branch
Director of Nephrology
Steve Chen
H+
Analysis of Acid-Base Disorders
NORMAL ACID-BASE BALANCE
23-27 mEq/L 22-26 mEq/LStandard HCO3
42-50 mmHg 35-45 mmHgPaCO2
42-48 nEq/L35-45 nEq/LH+
7.32-7.387.35-7.45pH
VenousArterialParameter
Basic Regulation of Acid-Base Balance
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3
The lungs help control acid-base balance by blowing off or retaining CO2. The kidneys help regulate acid-base balance by
excreting or retaining HCO3
Na+
NHE-3
H+ HCO3-
Na+
NBC
H2O CO2+
CA-2
Na+
K+
Na/K ATPase
PCT: Re-absorption of HCO3-
H+HCO3-
H+ATPase
H+
H2O
OH- CO2
HCO3-
+Cl-
CA
CCD: H+ secretion
AE-1
CCD: HCO3- secretion generation
Pendrin
Types of Acids in the body
Volatile acids:– Can leave solution and enter the atmosphere.– H2C03 (Carbonic acid).– Pco2 is most important factor in pH of body
tissues.
• Pco2 is a measurement of tension or partial pressure of carbon dioxide in the blood.
Types of Acids in the body
Fixed Acids:– Acids that do not leave solution.– Sulfuric and phosphoric acids.
( H2SO4& H3PO4)– Catabolism of amino acids, nucleic acids, and
phospholipids.
Types of Acids in the body
Organic Acids:– Byproducts of aerobic metabolism, during
anaerobic metabolism and during starvation, diabetes.
– Lactic acid , Ketones
Types of Acids in the body
Toxic Acids:– Hippuratic acid
Immediate response (Hb)
1~2 X all chemical ECF buffers
1 x
3x
Chemical buffer system
– Bicarbonate/carbonic acid • major plasma buffer
– Phosphate: H2PO4- / HPO42- • major urine buffer
– Ammonium: NH3 / NH4+ • also used to buffer the urine
– Proteins: important in ICF– Hb: is the main buffer against CO2 change
~ 25%
~75%
Bicarbonate Buffer System
Carbonic acid (H2CO3)– Weak acid
Bicarbonate ion (HCO3-)
– Weak base CO2 + H20 H2CO3 H+ + HCO3
-
Works along with lung and kidney– These systems remove CO2 or HCO3
-
• Bicarbonate/Carbonic acid = 20:1 normally • Alterations in the ratio changes pH
irrespective of absolute concentrations
Phosphate Buffer System
• Dihydrogen phosphate ion (H2PO4-)
– Weak acid• Monohydrogen phosphate ion (HPO4
2-)– Weak base
• H2PO4- H+ + HPO4
2-
• More important in buffering kidney filtrate than in tissue• The amount of phosphate filtered is limited and relatively
fixed, and only a fraction of the secreted H+ can be buffered by HPO4
2-
Degree of phosphate buffering if 50 mmol/L of phosphate excreted
Segment pH HPO4 H2PO4 Amount buffered by HPO4
Filtrate 7.4 40 10 0
Proximal tubule
6.8 25 25 15
Final urine 4.8 0.5 49.5 39.5
Titratable acid excretion
15 ~40
Ammonia Buffer System
• NH4+– Weak acid
• NH3– Weak base
• NH4+ H+ + NH3-
• Ammonia is produced in the proximal tubule from the amino acid glutamine, and this reaction is enhanced by an acid load and by hypokalemia
• Under basal conditions, ~50% of the ammonia that is produced is excreted in urine and 50% is added to the systemic circulation via renal veins
NH4+ excretion
Arterial pH and urine pH on NH4 excretion
Diet acid load on NH4 excretion
Renal Control of Acid-Base Balance
• Acidosis →↑ urinary HCO3- re-absorption ↑ new HCO3- production
HCO3 reabsorption ↑ HCO3 generation↑
AE-1
H+
K+
TYPES OF ACID-BASE DISTURBANCES
Depression of the central nervous system, as evidenced by disorientation followed by coma
Excitability of the nervous system; muscles may go into a state of tetany and
convulsions
Regulatory mechanisms of metabolic acidosis in the bone microenvironment
Acid-sensing ion channels (ASIC), Transient receptor potential vanilloid channels (TRP), G-protein-associated receptors such as OGR1, Receptor activator of the nuclear factor κB ligand (RANKL) V-ATPase ion pump, an enzyme that promotes acidification of the bone surface where the resorption process will take place
In persons with chronic uremic acidosis, bone salts contribute to buffering, and the serum HCO3
- level usually remains > 12 mEq/L.
Ulcerative colitisHigh intestinal fistula Prolonged intestinal aspiration
H
Unmeasured anions: 10~16 meq/L
Anion Gap
– This is a calculated estimation of the undetermined or unmeasured anions in the blood Anion gap(AG) = (Na) - (HCO3+Cl)
– Normal anion gap ~ 10-16 meq / L
Unmeasured cations = K/Ca/Mg
Unmeasured anions ↑:↑Pi ; ↑Albumin
Unmeasured cations ↓ : ↓K ; ↓Ca ; ↓Mg
AG ↑
Anion Gap
– This is a calculated estimation of the undetermined or unmeasured anions in the blood Anion gap(AG) = (Na) - (HCO3+Cl)
– Normal anion gap ~ 10-16 meq / L– Albumin(↓1G/dl) = AG (↓2.3-2.5 meq/L) – If K included(↑), normal AG drops 4 meq/L(↓)
AG metabolic acidosis• Ketoacidosis: DKA/SKA/AKA
(Beta-hydroxybutyrate, acetoacetate)
• Lactic acidosis• Salicylate poisoning• Ethelene glycol intoxication (glycolate, oxalate)
• Methanol poisoning: Formaldehyde ( Formate); Formic acid
• Renal failure (Sulfate, phosphate, urate, and hippurate)
• Massive rhabdomyolysis (release of H + and organic anions from damaged muscle)
Non AG metabolic acidosis: ↑Cl/↓HCO3
• Acid load / Total parenteral nutrition (TPN)
• Chronic renal failure• Carbonic anhydrase inhibitors: acetazolamide
• Renal tubular acidosis(RTA)• Ureterosigmoidostomy/Intestinal fisula or
drainage• Expansion• Diarrhea
Plasma osmolar gap (POG)Posm = [2 X Na+]+ [glucose in mg/dL] /18+
[BUN in mg/dL]/2.8POG = the difference between the measured
value and the calculated one: no more than 10-15 mOsm/kg
↑ POG: Mannitol, radioactive contrast agents High-AG acidosis: Methanol, ethylene glycol, and acetone …
Urine anion gap (UAG) = Na + K – Cl: ~ NH4+ (near zero in normal)
Cl-
Na+ K+
HCO3-~ 0 meq/L
NH4 +~ 0 meq/L
Urine anion gap (UAG): negative in metabolic acidosis
Cl-
Na+ K+
NH3 + H+ = NH4 +
Acid load
Positive UAG in non AG metabolic acidosis RTA
Cl-
Na+ K+
NH3 + H+ = NH4 +
HCO3-
Simple or mixed ?Conditions Primary event Secondary response
Metabolic acidosis(30 minutes onset, 12-24H completion)
HCO3 ↓ 1 meq/L pCO2 ↓ 1.2 mmHg
Metabolic alkalosis (30 minutes onset, 12-24H completion)
HCO3 ↑ 1 meq/L pCO2 ↑ 0.7 mmHg
Respiratory acidosis Acute Chronic > 3-5days
pCO2 ↑ 10 mmHg HCO3 ↑ 1 meq/L ↑ 3.5-4 meq/L
Respiratory alkalosis Acute Chronic >3-5 days
pCO2 ↓ 10 mmHg HCO3 ↓ 2 meq/L ↓ 4-5 meq/L
General Principles of TreatmentExogenous alkali may not be required if the
acidemia is not severe (arterial pH >7.20), the patient is asymptomatic, and the underlying process, such as diarrhea, can be controlled
Bicarbonate therapy is generally not given unless the arterial pH is < 7.00 in Ketoacidosis or < 7.10 in Lactic acidosis
Potential Acids in the body
Organic Acids Potential bicarbonate – Byproducts of aerobic metabolism, during
anaerobic metabolism and during starvation, diabetes.
– Ketones – Lactate – Conservative supply of HCO3-
Bicarbonate deficit Assuming that respiratory function is normal,
attainment of a pH of 7.20 usually requires raising the serum Bicarbonate to 10 ~ 12 meq/L
HCO3 deficit = HCO3 space x HCO3 deficit per liter
HCO3 space = [0.4 + (2.6 ÷ [HCO3])] x lean body weight (LBW, Kg) = 0.55~0.7 x LBW
Approximately 250 meq of alkali (usually as intravenous sodium bicarbonate) can be given over the first 4 to 8 hours
Positive UAG in non AG metabolic acidosis RTA
Cl-
Na+ K+
NH3 + H+ = NH4 +
HCO3-
Renal Tubular Acidosis:RTA-1 Any patient with non-AG metabolic acidosis and a
urine pH > 5.0 Re-absorb HCO3
- normally FE of HCO3
- < 3% Serum HCO3
- : variable; in some cases ( < 10 mEq/L )
Serum K+ level typically is low in patients with distal RTA; can be high if the distal RTA is secondary to voltage-dependent Hyper-kalemic RTA-1
H+ATPase
H+
H2O
OH- CO2
HCO3-
+Cl-
CA
CCD: H+ secretion ↓ in RTA-1
H+
K+
AE-1
The causes of RTA-1 Primary: Genetic or sporadic Drug-related: Amphotericin B, lithium, analgesics,
ifosfamide, topiramate, toluene Autoimmune disease: SLE, chronic active
hepatitis, Sjögren syndrome, RA, primary biliary cirrhosis
Other systemic diseases: Sickle cell disease, hyperparathyroidism, light chain disease, cryoglobulinemia, Wilson disease, Fabry disease
Tubulointerstitial disease - Obstructive uropathy, transplant rejection, medullary cystic kidney disease, hypercalciuria
Hypokalemia in RTA-1 Decreased net H + secretion results in more
Na + re-absorption in exchange for K The drop in serum HCO 3 - and, therefore, filtered
HCO 3 -, reduces the amount of Na + reabsorbed by the Na +/H + exchanger in the proximal tubule, leading to mild volume depletion. The associated activation of the RAA system increases K + secretion in the collecting duct. + secretion.
A possible defect in K +/H + –ATPase results in decreased H + secretion and decreased K + re-absorption.
Nephrocalcinosis and Nephrolithiasis in RTA-1
A constant release of calcium phosphate from bones to buffer the extracellular H +
↓ Re-absorption of calcium and phosphate hypercalciuria and hyperphosphaturia
Relatively alkaline urine promotes calcium phosphate precipitation
Metabolic acidosis and hypokalemia lead to hypocitraturia, a risk factor for stones
Renal Tubular Acidosis:RTA-2 Any non-AG metabolic acidosis with a serum
HCO3- > 15 mEq/L (usually) + acidic urine (pH <
5.0) the strong ability of the collecting duct to reabsorb some HCO3
FEHCO3- less than 3% when their serum HCO3
- is low. However, raising serum HCO3
- above their lower threshold and closer to normal levels results in significant HCO3
- wasting and an FEHCO3exceeding 15% HCO3
- loading test
Patients with type 2 RTA typically have hypokalemia and increased urinary K+wasting Bicarbonaturia
Na+
NHE3
H+ HCO3-
Na+
NBC
H2O CO2+
CA-2
Na+
K+
Na/K ATPase
PCT: Re-absorption of HCO3- in RTA-2
H+HCO3-
The causes of RTA-2 Primary: Genetic or sporadic Inherited systemic disease - Wilson disease,
glycogen storage disease, tyrosinemia, Lowe syndrome, cystinosis, fructose intolerance
Related to other systemic disease - Multiple myeloma, amyloidosis, hyperparathyroidism, Sjögren syndrome
Drug- and toxin-related - Carbonic anhydrase inhibitors, ifosfamide, gentamicin, valproic acid, lead, mercury, streptozotocin
Osteomalacia in RTA-2Any chronic acidemic stateProximal tubular conversion of 25(OH)-
cholecalciferol to the active 1,25(OH)2-cholecalciferol is impaired
Patients with more generalized defects in proximal tubular function (as in Fanconi syndrome) may have phosphaturia and hypophosphatemia, which also predispose to osteomalacia.
Renal Tubular Acidosis:RTA-4 Any patient with a mild non-AG metabolic
acidosis : Diminished ammoniagenesis CKD stages 2-3 in most patients; Diabetes mellitus (in
approximately 50% of patients) Serum HCO3
- > 15 mEq/L (usually), and the urine pH is < 5.0
A TTKG less than 5 in the presence of hyperkalemia indicates aldosterone deficiency or resistance
Hyperkalemia also reduces proximal tubular NH4
+ production and decreases NH4+absorption by the
thick ascending limb: ↓ the ability of the kidneys to excrete an acid load
The causes of RTA-4 Hyporeninemic hypoaldosteronism (diabetes mellitus/mild
renal impairment, chronic interstitial nephritis, nonsteroidal anti-inflammatory drugs, beta-blockers)
Hypoaldosteronism (high renin) - Primary adrenal defect (isolated: congenital hypoaldosteronism; generalized: Addison disease, adrenalectomy, AIDS), inhibition of aldosterone secretion (heparin, ACE inhibitors, AT1 receptor blockers)
Aldosterone resistance (drugs) - Diuretics (amiloride, triamterene, spironolactone), calcineurin inhibitors (cyclosporine, tacrolimus), antibiotics (trimethoprim, pentamidine)
Aldosterone resistance (genetic) - Pseudohypoaldosteronism (PHA) types I and II
L-Lactic acidosis Daily L-lactate production in a healthy person is
substantial (approximately 20 mEq/kg/d), and this is usually metabolized to pyruvate in the liver, the kidneys, and, to a lesser degree, in the heart.
Serum lactate > 5 mEq/L Type A lactic acidosis occurs in hypoxic states,
while type B occurs without associated tissue hypoxia
D-lactic acidosis is a form of lactic acidosis that occurs from overproduction of D-lactate by intestinal bacteria. It is observed in association with intestinal bacterial overgrowth syndromes
L-Lactic acidosis Definition of acute lactic acidosis: blood lactate level ≥ 5
mEq/L, blood pH ≤ 7.35, and serum bicarbonate concentration ≤ 20 mEq/L
Sustained hyperlactatemia in sepsis or low-flow states carries mortality ≥ 60%
Sodium bicarbonate does not improve cardiac function or reduce mortality
In individuals predisposed to develop intracellular acidification with bicarbonate, other buffers (such as THAM [tris-hydroxymethyl aminomethane] or buffers containing disodium carbonate) should be considered
Hyperventilation to reduce carbon dioxide accumulation and infusion of calcium to stabilize calcium concentration improve myocardial function
Lactate-guided therapy with the goal of normalizing blood lactate levels (to <2 mEq/L) has shown some benefit
Renal failure CKD (GFR 20 ~ 50 mL/min): normal AG metabolic acidosis
Ammoniagenesis ↓ NH3 reabsorption and recycling↓ ; medullary interstitial NH3 concentration ↓ Serum HCO3
- > 12 mEq/L GFR < 20: high AG metabolic acidosis
Accumulation of sulfates, urates and phosphates Serum HCO3
- > 12 mEq/L, but significant loss of bone calcium with resulting osteopenia and osteomalacia
Methanol poisoningMethanol is metabolized by alcohol
dehydrogenase to formaldehyde and then to formic acid
High AG: formic acid, lactic acid, and ketoacid
Formaldehyde: optic nerve and CNS toxicity
Retinal edema, CNS depression, and unexplained metabolic acidosis with high anion and osmolar gaps
Ethylene glycol poisoning Ethylene glycol is converted by alcohol
dehydrogenase first to glycoaldehyde and then to glycolic and glyoxylic acids. Glyoxylic acid then is degraded to several compounds, including oxalic acid, which is toxic, and glycine, which is relatively innocuous
High AG: accumulation of these acids + mild lactic acidosis
CNS symptoms ( slurred speech, confusion, stupor or coma) , myocardial depression, and renal failure with flank pain
Oxalate crystals in the urine; elevated osmolar gap
Toluene Toxicity –Renal Renal tubular acidosis (RTA)HypokalemiaHypophosphatemiaHyperchloremiaAzotemiaSterile pyuriaHematuriaProteinuria
Toluene Toxicity -CNS Acute intoxication from inhalation is characterized
by rapid onset of CNS symptoms: euphoria, hallucinations, delusions, tinnitus, dizziness, confusion, headache, vertigo, seizures, ataxia, stupor, and coma.
Chronic CNS sequelae: neuropsychosis, cerebral and cerebellar degeneration with ataxia, seizures, choreoathetosis, optic and peripheral neuropathies, decreased cognitive ability, anosmia, optic atrophy, blindness, tinnitus, and hearing loss
Toluene Toxicity -CP Toluene has direct negative effects on
cardiac automaticity and conduction and can sensitize the myocardium to circulating catecholamines.
"Sudden sniffing death" secondary to cardiac arrhythmias has been reported.
Pulmonary effects include bronchospasm, asphyxia, acute lung injury (ALI), and aspiration pneumonitis.
Toluene Toxicity -GI GI symptoms from inhalation and ingestion:
abdominal pain, nausea, vomiting, and hematemesis.
Hepatotoxicity: ascites, jaundice, hepatomegaly, and liver failure.
A rare form of hepatitis: hepatic reticuloendothelial failure (HREF)
Hepatitis secondary to toluene toxicity, not just infectious causes, should be considered in the differential diagnosis in the younger population
Mechanisms AG↑ Normal AG
Acid production ↑ Lactic acidosisKetoacidosisMethanol intoxicationEthylene glycol Diethylene glycolPropylene glycol Aspirin Pyroglutamic acid(5 oxo proline)Toluene
Toluene ( if preserved renal function/excretion of Na and K hippurate in urine later)
Loss of HCO3 or its precursors
Diarrhea (tube drainage)Other intestinal lossesT2RTACA inhibitorsUreteral diversion(ileal loop) Post-treatment of ketoacidosis
Renal acid secretion↓ CKD-5 (GFR <20) CKD (GFR 20~50)
T1RTAT4RTA(hypoaldosteronism)
RhCG in urinary ammonium excretion
RhCG
RhCG
NH3
H-ATPaseAE1
H/K ATPase
CO2+H2OHCO3HCl
K
H
NH3
CA
C (cortical) CD
Lithium
Model of collecting duct ammonia secretion
NH4+ excretion
Glutamine synthase (GS)
HCO- transporter: NBC e1
GS: NH4 + + glutamate + ATP -> glutamine + H+ + ADP + Pi.
PDG: glutamate dehydrogenasePEPCK
RhCG in urinary ammonium excretion
RhCG
RhCG
NH3
H-ATPaseAE1
H/K ATPase
CO2+H2OHCO3HCl
K
H
NH3
NaK ATPaseK(NH4)
CA
Inner medullay
CD
Lithium
NaKCCK(NH4)
Metabolic acidosisIssues Traditional views New aspectsDefinition PHCO3↓ HCO3 content if ECFV↓
Look for new Anions by P anion gap Adjusted when P albumin (adjusted by P albumin is low) is high if ECFV is low = Na-Cl-HCO3 in plasma Detect new anions in urine (UAG=Na+K+NH4-Cl) Detect NH4(urine) UAG = Na+K-Cl Uosm gap(UOG): best indirect Urine pH indicator for NH4Compare fall in PHCO3 Expect 1:1 Calculate HCO3 content in with rise in P anion gap ECFV to estimate deficit
Examine effectiveness of Rely only on PaCO2 Use capillary PCO2 in HCO3 buffer system skeletal muscle (reflected by brachial venous PCO2)