Acid-base disorders: Difference between revisions

 
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==Background==
==Background==
*Determiners of acid-base status are:
Determiners of acid-base status are:
**CO2
*'''CO2'''
**Weak acids (primarily albumin)
*'''Weak acids (primarily albumin)'''
***If albumin goes up more acidotic (since albumin is an acid)
**If albumin goes up more acidotic (since albumin is an acid)
**Strong ion difference (SID)
*'''Strong ions'''
***Primarily Na-Cl
**Primarily Na-Cl
****Normal difference is ~38 (140-102)
**Normal difference is ~38 (140-102)
*****If difference shrinks (i.e. more Cl) more acidotic
**If difference shrinks (i.e. more Cl) more acidotic
******Principle of electrical neutrality requires more H+ to offset the additional Cl
**Principle of electrical neutrality requires more H+ to offset the additional Cl
*****If difference increases (i.e. more Na) more alkaloatic
**If difference increases (i.e. more Na) more alkalotic
******Principle of electrical neutrality requires more bicarb to offset the additional Na
**Principle of electrical neutrality requires more bicarb to offset the additional Na
*Strong ion gap (SIG)
===Strong ion gap (SIG)===
**Equivalent to anion gap
*Equivalent to anion gap
**Strong ions include Na, Cl, lactate, ketoacid, toxic alcohols
*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


==How to approach an acid/base problem==
===Base Deficit (BD)===
*Get labs  (as coincident as possible)
*Eliminates the respiratory component of acidosis so only left with the metabolic component
**VBG/ABG
*Is equivalent to the amount of base (or acid) you would have to add to get to pH 7.4
**Lactate
*Base excess of -6 = base deficit of 6
**Albumin
*Normal = -2 to +2
**Acetone
*If base deficit is normal but patient is acidotic must all be from CO2
**Chemistry
*If base deficit is abnormal must explain by SID, weak acids, or unmeasured strong ions
*Look at pH
*If no BD is available 24.2 – serum bicarb can be used as okay substitute
**If pH >7.45 pt's primary problem is alkalosis
 
**If pH <7.35 pt's primary problem is acidosis
==Differential Diagnosis==
**Rmb that the body never over-corrects any acid-base disorder!
{{Acid-base disorders DDX}}
*Look at blood gas CO2
 
**If >45 then respiratory acidosis
==Evaluation==
**If <35 respiratory acidosis
Diagnosis is based on clinical history as well as labs:
*Calculate the strong ion difference (SID)
*[[VBG]]/[[ABG]]
**SID = Na - Cl
*[[Lactate]]
***Low SID if <38
*Albumin
****Strong ion acidosis = hyperchloremic acidosis = non-gap acidosis
*Acetone
****Causes include:
*Chemistry
*****Fluid administration
*Serum Osmolarity
******Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
 
*****Renal Tubular Acidosis
==Stuart Step Wise Approach==
******Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
*''Based on a stepwise approach taught about by Dr. Weingart based on the Stewart's Strong Ion Difference<ref>http://emcrit.org/wp-content/uploads/acid_base_sheet_2-2011.pdf</ref><ref>Stewart Acid base http://www.acid-base.com/strongion.php</ref>
*******Type I: Urine pH <5.55
===Determine pH===
*******Type II: Urine pH >5.55
*If pH >7.45 then patient's primary problem is [[alkalosis]]
*******Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
*If pH <7.35 the patient's primary problem is [[acidosis]]
*****Diarrhea
*The body never over-corrects any acid-base disorder!
***High SID if >38
===Evaluate blood gas===
****This is metabolic alkalosis  
*If pCO2 >45 then [[respiratory acidosis]]
****Causes include:
*If pCO2 <35 [[respiratory alkalosis]]
*****Nasogastric suction
 
*****Diuretics
===Calculate Strong Ion Difference (SID)===
*****Hyperaldosteronism
''SID = Na - Cl''
*****Volume depletion
*'''Low SID is <38 and indicates a strong ion acidosis = hyperchloremic acidosis = non-gap acidosis and causes include'''
*Look at the lactate
**[[IVF|Fluid administration]]
**If >2 then pt has hyperlactatemia
***Any fluid that has SID of <24 can cause acidosis (e.g. [[NS]], 1/2NS, D5W)
**If >4 and pt has infection start Early Goal Directed Therapy ([[Sepsis]])
**[[Renal tubular acidosis]]
**If pt not infected consider other diagnoses: [[Lactic Acidosis (Lactate)]]
***Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
**#Type I: Urine pH <5.55
**#Type II: Urine pH >5.55
**#Type IV: [[hyperkalemia|Hyperkalemic]]; from aldosterone deficiency, diabetes
**[[Diarrhea]]
*'''High SID is >38 and indicates a metabolic alkalosis and causes include:'''
**Nasogastric suction
**[[Diuretics]]
**Hyperaldosteronism
**[[hypovolemia|Volume depletion]]
 
===Evaluate the Lactate===
*If >2 then the patient has hyperlactatemia
*If >4 and the patient has an infection they should be considered  [[Sepsis|Severe Sepsis]]
*Always consider the differential for a [[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
*If SIG >2 this is a SIG metabolic acidosis = anion gap acidosis and the causes include:
****Causes:
**Uremia
*****Uremia
**[[DKA]]
*****[[DKA]]
**[[Alcohol ketoacidosis|AKA]]
*****[[Alcohol ketoacidosis|AKA]]
**[[ASA]]
*****[[ASA]]
**[[Ethylene Glycol Toxicity|Ethylene Glycol]], methanol, propylene glycol
*****[[Ethylene Glycol Toxicity|Ethylene Glycol]], methanol, propylene glycol
**[[Iron Toxicity]]
*****[[Iron Toxicity]]
**[[INH toxicity]]
*****INH
**Paraldehyde
*****Paraldehyde
**[[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)
*If SIG is negative (very rare) the differential includes:
***If SIG negative (very rare):
**[[Hypercalcemia]]
****[[Hypercalcemia]]
**[[Hypermagnesemia]]
****[[Hypermagnesemia]]
**[[Hyperkalemia]]
****[[Hyperkalemia]]
**Immunoglobulins
****Immunoglobulins
**Bromide
****Bromide
**Nitrates
****Nitrates
**[[Lithium]]
****[[Lithium]]
===Calculate the osmolar gap===
*Think about compensations
*Indicated if have elevated SIG without explanation
**If primary is respiratory calculate the expected metabolic compensation
*Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
***Expected ΔBE (or expected decrease of SID) = 0.4 x (Chronic change in CO2)
*Positive if osm gap >10 and differential includes:
**If primary is metabolic acidosis calculate the expected respiratory compensation:
**[[Toxic alcohols]] (if Osm gap >50)
***Expected ↓CO2 = Base Deficit
**[[Methanol]]
**If primary is metabolic alkalosis calculate the expected respiratory compensation:
**[[Ethylene glycol]]
***Expected ↑ CO2 = 0.6 x Base Excess
**[[Mannitol]]
**Winter's Formula useful for figuring out PaCO2 in COPD pt:
**[[isopropanol Toxicity|Isopropanol]] (isopropyl alcohol)
***0.8 decrease in pH for every 10 mmHg increase in PaCO2 acutely
**Propylene glycol
*Calculate the osmolar gap
**[[Lithium]]
**Indicated if have elevated SIG without explanation
 
***Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
==Traditional step-wise approach==
****Positive if osm gap >10 (if Osm gap >50 almost certainly toxic alcohol induced)
===Determine pH===
*****Causes:
*If pH < 7.35, then acidemia
******[[Methanol]]
*If pH > 7.45, then alkalemia
******[[Ethylene glycol]]
*If pH within normal range, then acid base disorder not likely present.
******Mannitol
*pH may be normal in the presence of a mixed acid base disorder,  particularly if other parameters of the ABG are abnormal.
******Isopropanol (isopropyl alcohol)
===Determine the Primary Diagnosis===
******Propylene glycol
*Acidemia
******[[Lithium]]
**↓HCO3 -[[Metabolic acidosis]]
**↑PaCO2-[[Respiratory acidosis]]
*Alkalemia
**↑HCO3-[[Metabolic alkalosis]]
**↓PaCO2 - [[Respiratory alkalosis]]
===Calculate the [[Anion gap]]===
Anion gap = [Na+]– [HCO3-] – [Cl-]
===Calculate the delta gap===
*∆gap = anion gap - 12
*This is to determine a coexistent metabolic alkalosis or non-gap acidosis


==[[IV Fluids]]==
{| {{table}}
*Normal SIG (Na-Cl) is 38
| align="center" style="background:#f0f0f0;"|'''Delta Ratio'''
**Fluid that has SIG of 38 would be basic b/c it would dilute out the albumin (weak acid)
| align="center" style="background:#f0f0f0;"|'''Assessment Guideline'''
**Fluid that has SIG identical to pt's serum bicarb is pH neutral
|-
***If SID of fluid is greater than pt's bicarb level then it is alkalotic
|< 0.4
***If SID of fluid is less than pt's bicarb level then it is acidotic
|Hyperchloremic [[non anion gap acidosis|normal anion gap acidosis]]
|-
|0.4 - 0.8
|Consider combined high AG & normal AG acidosis BUT note that the ratio is often <1 in acidosis associated with renal failure
|-
|1 to 2
|
*Usual for uncomplicated [[anion gap acidosis|high-AG acidosis]].
*[[Lactic acidosis]]: average value 1.6
*[[DKA]] more likely to have a ratio closer to 1 due to urine ketone loss (esp if patient not dehydrated)
|-
| > 2
|
Suggests a pre-existing elevated HCO3 level so consider:
*a concurrent metabolic alkalosis
*a pre-existing compensated respiratory acidosis
|}


===Examples===
===Calculate the starting bicarbonate===
*∆gap + (HCO3) = “starting bicarbonate”
*The purpose of this calculation is to assess the body’s ability to change HCO3 in response to a metabolic acid. In cases with a pure anion gap metabolic acidosis, the rise in anion gap from 12 should equal the fall in HCO3 from from 24
===Calculate compensations===
*Will allow for identification of a secondary process
 
==Determinants of compensation==
'''Metabolic acidosis:'''
*PaCO2 = 1.5 (HCO3) + 8 ± 2
*PaCO2 = last two digits of pH
*PaCO2= ↓ 1.0–1.5per ↓ 1mEq/L HCO3
'''Metabolic alkalosis'''
*PaCO2 = 0.9 (HCO3) + 9
*PaCO2= ↑ 0.5–1.0 mm per ↑ 1mEq/L HCO3
'''Respiratory acidosis and alkalosis (acute acid-base changes based on PCO2 and HCO3):'''
*∆H+=0.8 (∆PaCO2)
*For every ↑ or ↓ of PCO2 by 1 the pH changes by 0.008
*For every ↑ or ↓ of HCO3 by 1 the  pH changes by 0.015
'''Estimate of baseline PCO2 in patients with Acute Respiratory Acidosis:'''
*Estimated baseline PCO2 = 2.4 (admission measured HCO3 – 22)
'''Chronic respiratory acidosis'''<ref>Brandis K. Anesthesia MCQ. Rules for Metabolic Acid-Base Disorders. http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php</ref>
*HCO3 increases by 4 for every 10 mmHg ↑ in pCO2 above 40
*∆H+=0.4 (∆PaCO2)
*In chronic respiratory acidosis, kidneys retain HCO3, which takes a few days
'''Chronic respiratory alkalosis'''
*HCO3 decreases by 5 for every 10 mmHg decrease in pCO2 below 40
*∆H+=0.5 (∆PaCO2)
*Takes few days also
*Maximal compensation is HCO3 ~12-15 mEq/L
 
==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 patient's serum bicarb is pH neutral
***If SID of fluid is greater than patient's bicarb level then it is alkalotic
***If SID of fluid is less than patient's bicarb level then it is acidotic
 
====Examples====
*NS or 1/2NS
*NS or 1/2NS
**(SID = 0) so is acidotic so causes hyperchloremic acidosis
**(SID = 0) so is acidotic so causes hyperchloremic acidosis
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**SID is 892 (very alkalotic) is 8.4%
**SID is 892 (very alkalotic) is 8.4%


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


==See Also==
==See Also==
*[[Electrolyte Abnormalities (Main)]]
*[[Electrolyte Abnormalities (Main)]]
*[[Metabolic Acidosis]]
*[[Metabolic Alkalosis]]
*[[Respiratory Acidosis]]
*[[Respiratory Alkalosis]]
==Source==
*EMCrit Acid/Base Lecture


==References==
<references/>
[[Category:FEN]]
[[Category:FEN]]
[[Category:Renal]]
[[Category:Critical Care]]

Latest revision as of 16:09, 15 October 2019

Background

Determiners of acid-base status are:

  • CO2
  • Weak acids (primarily albumin)
    • If albumin goes up more acidotic (since albumin is an acid)
  • Strong ions
    • 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 alkalotic
    • 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 (BD)

  • Eliminates the respiratory component of acidosis so only left with the metabolic component
  • Is equivalent to the amount of 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 patient 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

Differential Diagnosis

Acid-base disorders

Evaluation

Diagnosis is based on clinical history as well as labs:

Stuart Step Wise Approach

  • Based on a stepwise approach taught about by Dr. Weingart based on the Stewart's Strong Ion Difference[1][2]

Determine pH

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

Evaluate blood gas

Calculate Strong Ion Difference (SID)

SID = Na - Cl

  • Low SID is <38 and indicates a strong ion acidosis = hyperchloremic acidosis = non-gap acidosis and causes include
  • High SID is >38 and indicates a metabolic alkalosis and causes include:

Evaluate the Lactate

Calculate the osmolar gap

Traditional step-wise approach

Determine pH

  • If pH < 7.35, then acidemia
  • If pH > 7.45, then alkalemia
  • If pH within normal range, then acid base disorder not likely present.
  • pH may be normal in the presence of a mixed acid base disorder, particularly if other parameters of the ABG are abnormal.

Determine the Primary Diagnosis

Calculate the Anion gap

Anion gap = [Na+]– [HCO3-] – [Cl-]

Calculate the delta gap

  • ∆gap = anion gap - 12
  • This is to determine a coexistent metabolic alkalosis or non-gap acidosis
Delta Ratio Assessment Guideline
< 0.4 Hyperchloremic normal anion gap acidosis
0.4 - 0.8 Consider combined high AG & normal AG acidosis BUT note that the ratio is often <1 in acidosis associated with renal failure
1 to 2
  • Usual for uncomplicated high-AG acidosis.
  • Lactic acidosis: average value 1.6
  • DKA more likely to have a ratio closer to 1 due to urine ketone loss (esp if patient not dehydrated)
> 2

Suggests a pre-existing elevated HCO3 level so consider:

  • a concurrent metabolic alkalosis
  • a pre-existing compensated respiratory acidosis

Calculate the starting bicarbonate

  • ∆gap + (HCO3) = “starting bicarbonate”
  • The purpose of this calculation is to assess the body’s ability to change HCO3 in response to a metabolic acid. In cases with a pure anion gap metabolic acidosis, the rise in anion gap from 12 should equal the fall in HCO3 from from 24

Calculate compensations

  • Will allow for identification of a secondary process

Determinants of compensation

Metabolic acidosis:

  • PaCO2 = 1.5 (HCO3) + 8 ± 2
  • PaCO2 = last two digits of pH
  • PaCO2= ↓ 1.0–1.5per ↓ 1mEq/L HCO3

Metabolic alkalosis

  • PaCO2 = 0.9 (HCO3) + 9
  • PaCO2= ↑ 0.5–1.0 mm per ↑ 1mEq/L HCO3

Respiratory acidosis and alkalosis (acute acid-base changes based on PCO2 and HCO3):

  • ∆H+=0.8 (∆PaCO2)
  • For every ↑ or ↓ of PCO2 by 1 the pH changes by 0.008
  • For every ↑ or ↓ of HCO3 by 1 the pH changes by 0.015

Estimate of baseline PCO2 in patients with Acute Respiratory Acidosis:

  • Estimated baseline PCO2 = 2.4 (admission measured HCO3 – 22)

Chronic respiratory acidosis[3]

  • HCO3 increases by 4 for every 10 mmHg ↑ in pCO2 above 40
  • ∆H+=0.4 (∆PaCO2)
  • In chronic respiratory acidosis, kidneys retain HCO3, which takes a few days

Chronic respiratory alkalosis

  • HCO3 decreases by 5 for every 10 mmHg decrease in pCO2 below 40
  • ∆H+=0.5 (∆PaCO2)
  • Takes few days also
  • Maximal compensation is HCO3 ~12-15 mEq/L

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 patient's serum bicarb is pH neutral
      • If SID of fluid is greater than patient's bicarb level then it is alkalotic
      • If SID of fluid is less than patient'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 patients with low pH (e.g. DKA)

See Also

References