Difference between revisions of "Acid-base disorders"

(Background)
(Evaluationhttp://emcrit.org/wp-content/uploads/acid_base_sheet_2-2011.pdf)
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**Chemistry
 
**Chemistry
  
===Evaluation<ref>http://emcrit.org/wp-content/uploads/acid_base_sheet_2-2011.pdf</ref>===
+
===Evaluation===
*Look at pH
+
*''Based on a stepwise approach taught about by Dr. Weingart<ref>http://emcrit.org/wp-content/uploads/acid_base_sheet_2-2011.pdf</ref>
**If pH >7.45 pt's primary problem is alkalosis
+
'''Look at pH'''
**If pH <7.35 pt's primary problem is acidosis
+
*If pH >7.45 pt's primary problem is alkalosis
**Rmb that the body never over-corrects any acid-base disorder!
+
*If pH <7.35 pt's primary problem is acidosis
*Look at blood gas CO2
+
*The body never over-corrects any acid-base disorder!
**If >45 then respiratory acidosis
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'''Look at blood gas CO2'''
**If <35 respiratory acidosis
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*If >45 then respiratory acidosis
*Calculate the strong ion difference (SID)
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*If <35 respiratory acidosis
**SID = Na - Cl
+
'''Calculate the strong ion difference (SID)'''
***Low SID if <38
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*SID = Na - Cl
****Strong ion acidosis = hyperchloremic acidosis = non-gap acidosis
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'''Low SID is <38 and indicates a strong ion acidosis = hyperchloremic acidosis = non-gap acidosis
****Causes include:
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*Causes include:
*****Fluid administration
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*#Fluid administration
******Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
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*#Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
*****Renal Tubular Acidosis
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*Renal Tubular Acidosis
******Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
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*#Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
*******Type I: Urine pH <5.55
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*#Type I: Urine pH <5.55
*******Type II: Urine pH >5.55
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*#Type II: Urine pH >5.55
*******Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
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*#Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
*****Diarrhea
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*Diarrhea
***High SID if >38
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'''High SID is >38 and indicates a metabolic alkalosis'''
****This is metabolic alkalosis  
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*Causes include:
****Causes include:
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**Nasogastric suction
*****Nasogastric suction
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**Diuretics
*****Diuretics
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**Hyperaldosteronism
*****Hyperaldosteronism
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**Volume depletion
*****Volume depletion
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'''Look at the lactate'''
*Look at the lactate
+
*If >2 then the patient has hyperlactatemia
**If >2 then pt has hyperlactatemia
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*If >4 and the patient has an infection they should be considered  [[Sepsis|Severe Sepsis]]
**If >4 and pt has infection start Early Goal Directed Therapy ([[Sepsis]])
+
*Always consider the differential for a [[Lactic Acidosis (Lactate)]]
**If pt not infected consider other diagnoses: [[Lactic Acidosis (Lactate)]]
 
 
*Calculate the strong ion gap (SIG) to explain the base deficit
 
*Calculate the strong ion gap (SIG) to explain the base deficit
**SIG = (Base Deficit) + (SID – 38) + 2.5 (4.2 ‐ Albumin (g/dL)) – lactate
+
*'''SIG = (Base Deficit) + (SID – 38) + 2.5 (4.2 ‐ Albumin (g/dL)) – lactate'''
***If SIG >2 this is a SIG metabolic acidosis = anion gap acidosis
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*If SIG >2 this is a SIG metabolic acidosis = anion gap acidosis and the causes include:
****Causes:
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**Uremia
*****Uremia
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**[[DKA]]
*****[[DKA]]
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**[[Alcohol ketoacidosis|AKA]]
*****[[Alcohol ketoacidosis|AKA]]
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**[[ASA]]
*****[[ASA]]
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**[[Ethylene Glycol Toxicity|Ethylene Glycol]], methanol, propylene glycol
*****[[Ethylene Glycol Toxicity|Ethylene Glycol]], methanol, propylene glycol
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**[[Iron Toxicity]]
*****[[Iron Toxicity]]
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**INH
*****INH
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**Paraldehyde
*****Paraldehyde
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**[[Lactic Acidosis]] (from short gut/blind loop - will not show on lactate assay)
*****[[Lactic Acidosis]] (from short gut/blind loop - will not show on lactate assay)
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*If SIG is negative (very rare) the differential includes:
***If SIG negative (very rare):
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**[[Hypercalcemia]]
****[[Hypercalcemia]]
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**[[Hypermagnesemia]]
****[[Hypermagnesemia]]
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**[[Hyperkalemia]]
****[[Hyperkalemia]]
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**Immunoglobulins
****Immunoglobulins
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**Bromide
****Bromide
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**Nitrates
****Nitrates
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**[[Lithium]]
****[[Lithium]]
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'''Also consider compensations'''
*Think about compensations
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*If primary is respiratory calculate the expected metabolic compensation
**If primary is respiratory calculate the expected metabolic compensation
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*Expected ΔBE (or expected decrease of SID) = 0.4 x (Chronic change in CO2)
***Expected ΔBE (or expected decrease of SID) = 0.4 x (Chronic change in CO2)
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*If primary disease is a metabolic acidosis then calculate the expected respiratory compensation:
**If primary is metabolic acidosis calculate the expected respiratory compensation:
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**Expected ↓CO2 = Base Deficit
***Expected ↓CO2 = Base Deficit
+
*If primary disease is ametabolic alkalosis then calculate the expected respiratory compensation:
**If primary is metabolic alkalosis calculate the expected respiratory compensation:
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**Expected ↑ CO2 = 0.6 x Base Excess
***Expected ↑ CO2 = 0.6 x Base Excess
+
**Winter's Formula useful for figuring out PaCO2 in COPD patients:
**Winter's Formula useful for figuring out PaCO2 in COPD pt:
 
 
***pCO2 = 1.5 [HCO3] + 8 mmHg +/- 2
 
***pCO2 = 1.5 [HCO3] + 8 mmHg +/- 2
 
***0.8 decrease in pH for every 10 mmHg increase in PaCO2 acutely
 
***0.8 decrease in pH for every 10 mmHg increase in PaCO2 acutely
*Calculate the osmolar gap
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'''Calculate the osmolar gap'''
**Indicated if have elevated SIG without explanation
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*Indicated if have elevated SIG without explanation
***Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
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*Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
****Positive if osm gap >10 (if Osm gap >50 almost certainly toxic alcohol induced)
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*Positive if osm gap >10 and differential includes:
*****Causes:
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**Toxic alcohols (if Osm gap >50)
******[[Methanol]]
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**[[Methanol]]
******[[Ethylene glycol]]
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**[[Ethylene glycol]]
******Mannitol
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**Mannitol
******Isopropanol (isopropyl alcohol)
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**Isopropanol (isopropyl alcohol)
******Propylene glycol
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**Propylene glycol
******[[Lithium]]
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**[[Lithium]]
  
 
==Management==
 
==Management==

Revision as of 16:00, 12 December 2015

Background

Determiners of acid-base status are:

  • CO2
  • Weak acids (primarily albumin)
    • If albumin goes up more acidotic (since albumin is an acid)
  • Strong ion difference (SID)
    • Primarily Na-Cl
    • Normal difference is ~38 (140-102)
    • If difference shrinks (i.e. more Cl) more acidotic
    • Principle of electrical neutrality requires more H+ to offset the additional Cl
    • If difference increases (i.e. more Na) more alkaloatic
    • Principle of electrical neutrality requires more bicarb to offset the additional Na
  • Strong ion gap (SIG)
    • Equivalent to anion gap
    • Strong ions include Na, Cl, lactate, ketoacid, toxic alcohols
  • Base Deficit
    • Gets rid of respiratory component of acidosis so only left with the metabolic component
    • How much base (or acid) you would have to add to get to pH 7.4
    • Base excess of -6 = base deficit of 6
    • Normal = -2 to +2
    • If base deficit is normal but pt is acidotic must all be from CO2
    • If base deficit is abnormal must explain by SID, weak acids, or unmeasured strong ions
    • If no BD is available 24.2 – serum bicarb can be used as okay substitute

Clinical Features

Differential Diagnosis

Acid-base disorders

Diagnosis

Work-up

  • Get labs (as coincident as possible)
    • VBG/ABG
    • Lactate
    • Albumin
    • Acetone
    • Chemistry

Evaluation

  • Based on a stepwise approach taught about by Dr. Weingart[1]

Look at pH

  • If pH >7.45 pt's primary problem is alkalosis
  • If pH <7.35 pt's primary problem is acidosis
  • The body never over-corrects any acid-base disorder!

Look at blood gas CO2

  • If >45 then respiratory acidosis
  • If <35 respiratory acidosis

Calculate the strong ion difference (SID)

  • SID = Na - Cl

Low SID is <38 and indicates a strong ion acidosis = hyperchloremic acidosis = non-gap acidosis

  • Causes include:
    1. Fluid administration
    2. Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
  • Renal Tubular Acidosis
    1. Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
    2. Type I: Urine pH <5.55
    3. Type II: Urine pH >5.55
    4. Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
  • Diarrhea

High SID is >38 and indicates a metabolic alkalosis

  • Causes include:
    • Nasogastric suction
    • Diuretics
    • Hyperaldosteronism
    • Volume depletion

Look at the lactate

  • If >2 then the patient has hyperlactatemia
  • If >4 and the patient has an infection they should be considered Severe Sepsis
  • Always consider the differential for a Lactic Acidosis (Lactate)
  • Calculate the strong ion gap (SIG) to explain the base deficit
  • SIG = (Base Deficit) + (SID – 38) + 2.5 (4.2 ‐ Albumin (g/dL)) – lactate
  • If SIG >2 this is a SIG metabolic acidosis = anion gap acidosis and the causes include:
  • If SIG is negative (very rare) the differential includes:

Also consider compensations

  • If primary is respiratory calculate the expected metabolic compensation
  • Expected ΔBE (or expected decrease of SID) = 0.4 x (Chronic change in CO2)
  • If primary disease is a metabolic acidosis then calculate the expected respiratory compensation:
    • Expected ↓CO2 = Base Deficit
  • If primary disease is ametabolic alkalosis then calculate the expected respiratory compensation:
    • Expected ↑ CO2 = 0.6 x Base Excess
    • Winter's Formula useful for figuring out PaCO2 in COPD patients:
      • pCO2 = 1.5 [HCO3] + 8 mmHg +/- 2
      • 0.8 decrease in pH for every 10 mmHg increase in PaCO2 acutely

Calculate the osmolar gap

  • Indicated if have elevated SIG without explanation
  • Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
  • Positive if osm gap >10 and differential includes:

Management

IV Fluids

  • Normal SID (Na-Cl) is 38
    • Fluid that has SID of 38 would be basic b/c it would dilute out the albumin (weak acid)
    • Fluid that has SID identical to pt's serum bicarb is pH neutral
      • If SID of fluid is greater than pt's bicarb level then it is alkalotic
      • If SID of fluid is less than pt's bicarb level then it is acidotic

Examples

  • NS or 1/2NS
    • (SID = 0) so is acidotic so causes hyperchloremic acidosis
  • LR
    • SID of 24-28
  • D5W
    • SID of 0
  • NaBicarb
    • SID is 892 (very alkalotic) is 8.4%

^Consider balanced solution (LR) in pts w/ low pH (e.g. DKA)

See Also

References