Difference between revisions of "Acid-base disorders"

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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
 
#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 if <38
 
####Strong ion acidosis = hyperchloremic acidosis = non-gap acidosis
 
####Causes include:
 
#####Fluid administration
 
######Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
 
#####Renal Tubular Acidosis
 
######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: Hyperkalemic; from aldosterone deficiency, diabetes
 
#####Diarrhea
 
###High SID if >38
 
####This is metabolic alkalosis
 
####Causes include:
 
#####Nasogastric suction
 
#####Diuretics
 
#####Hyperaldosteronism
 
#####Volume depletion
 
#Look at the lactate
 
##If >2 then pt has hyperlactatemia
 
##If >4 and pt has infection start Early Goal Directed Therapy ([[Sepsis]])
 
##If pt not infected consider other diagnoses: [[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
 
####Causes:
 
#####Uremia
 
#####[[DKA]]
 
#####[[AKA]]
 
#####[[ASA]]
 
#####[[Ethylene Glycol Toxicity|Ethylene Glycol]], methanol, propylene glycol
 
#####[[Iron Toxicity]]
 
#####INH
 
#####Paraldehyde
 
#####[[Lactic Acidosis]] (from short gut/blind loop - will not show on lactate assay)
 
###If SIG negative (very rare):
 
####[[Hypercalcemia]]
 
####[[Hypermagnesemia]]
 
####[[Hyperkalemia]]
 
####Immunoglobulins
 
####Bromide
 
####Nitrates
 
####[[Lithium]]
 
#Think about 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 is metabolic acidosis calculate the expected respiratory compensation:
 
###Expected ↓CO2 = Base Deficit
 
##If primary is metabolic alkalosis calculate the expected respiratory compensation:
 
###Expected ↑ CO2 = 0.6 x Base Excess
 
##Winter's Formula useful for figuring out PaCO2 in COPD pt:
 
###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 (if Osm gap >50 almost certainly toxic alcohol induced)
 
#####Causes:
 
######[[Methanol]]
 
######[[Ethylene glycol]]
 
######Mannitol
 
######Isopropanol (isopropyl alcohol)
 
######Propylene glycol
 
######[[Lithium]]
 
  
==[[IV Fluids]]==
+
==Differential Diagnosis==
#Normal SIG (Na-Cl) is 38
+
{{Acid-base disorders DDX}}
##Fluid that has SIG of 38 would be basic b/c it would dilute out the albumin (weak acid)  
+
 
##Fluid that has SIG identical to pt's serum bicarb is pH neutral
+
==Evaluation==
###If SID of fluid is greater than pt's bicarb level then it is alkalotic  
+
Diagnosis is based on clinical history as well as labs:
###If SID of fluid is less than pt's bicarb level then it is acidotic  
+
*VBG/ABG
#Examples
+
*Lactate
##NS or 1/2NS
+
*Albumin
###(SID = 0) so is acidotic so causes hyperchloremic acidosis
+
*Acetone
##LR
+
*Chemistry
###SID of 24-28
+
*Serum Osmolarity
##D5W
+
 
###SID of 0
+
==Stuart Step Wise Approach==
##NaBicarb
+
*''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>
###SID is 892 (very alkalotic) is 8.4%
+
===Determine pH===
#Consider balanced solution (LR) in pts w/ low pH (e.g. DKA)
+
*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===
 +
*If pCO2 >45 then respiratory acidosis
 +
*If pCO2 <35 respiratory alkalosis
 +
 
 +
===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'''
 +
**Fluid administration
 +
***Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
 +
**Renal Tubular Acidosis
 +
***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: Hyperkalemic; from aldosterone deficiency, diabetes
 +
**[[Diarrhea]]
 +
*'''High SID is >38 and indicates a metabolic alkalosis and causes include:'''
 +
**Nasogastric suction
 +
**Diuretics
 +
**Hyperaldosteronism
 +
**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
 +
*'''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:
 +
**Uremia
 +
**[[DKA]]
 +
**[[Alcohol ketoacidosis|AKA]]
 +
**[[ASA]]
 +
**[[Ethylene Glycol Toxicity|Ethylene Glycol]], methanol, propylene glycol
 +
**[[Iron Toxicity]]
 +
**INH
 +
**Paraldehyde
 +
**[[Lactic Acidosis]] (from short gut/blind loop - will not show on lactate assay)
 +
*If SIG is negative (very rare) the differential includes:
 +
**[[Hypercalcemia]]
 +
**[[Hypermagnesemia]]
 +
**[[Hyperkalemia]]
 +
**Immunoglobulins
 +
**Bromide
 +
**Nitrates
 +
**[[Lithium]]
 +
===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:
 +
**Toxic alcohols (if Osm gap >50)
 +
**[[Methanol]]
 +
**[[Ethylene glycol]]
 +
**Mannitol
 +
**Isopropanol (isopropyl alcohol)
 +
**Propylene glycol
 +
**[[Lithium]]
 +
 
 +
==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===
 +
*Acidemia
 +
**↓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
 +
 
 +
{| {{table}}
 +
| align="center" style="background:#f0f0f0;"|'''Delta Ratio'''
 +
| align="center" style="background:#f0f0f0;"|'''Assessment Guideline'''
 +
|-
 +
|< 0.4
 +
|Hyperchloraemic 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'''<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
 +
**(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==
 
==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 11:56, 24 September 2016

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:

  • VBG/ABG
  • Lactate
  • Albumin
  • Acetone
  • Chemistry
  • Serum Osmolarity

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

  • If pCO2 >45 then respiratory acidosis
  • If pCO2 <35 respiratory alkalosis

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
    • Fluid administration
      • Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
    • Renal Tubular Acidosis
      • Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
      1. Type I: Urine pH <5.55
      2. Type II: Urine pH >5.55
      3. Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
    • Diarrhea
  • High SID is >38 and indicates a metabolic alkalosis and causes include:
    • Nasogastric suction
    • Diuretics
    • Hyperaldosteronism
    • Volume depletion

Evaluate 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:

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:

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

  • Acidemia
    • ↓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
Delta Ratio Assessment Guideline
< 0.4 Hyperchloraemic 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