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What determines acid/base
==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


conservation of charge, neutrality, no matter what
===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


independent vs dependent variables
==Differential Diagnosis==
body is limitless pool of H+ and bicarb- - dependent variables (not the determinators)
{{Acid-base disorders DDX}}


independent variables: CO2, Atot (total weak acids (doesn't always dissociate: albumin), SID+ (strong ions - strong meaning fully dissociate in fluid: primary one is NaCl)
==Evaluation==
Diagnosis is based on clinical history as well as labs:
*[[VBG]]/[[ABG]]
*[[Lactate]]
*Albumin
*Acetone
*Chemistry
*Serum Osmolarity


Na and Cl are prime determinors of acid base
==Stuart Step Wise Approach==
*''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>
===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]]


Strong ion difference between Na and Cl is the major determinor of acid base
===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'''
**[[IVF|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: [[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]]


Normal difference is ~38 (140 Na, 102 Cl), which is also pH determiner
===Evaluate the Lactate===
- if difference shrinks: acidotic
*If >2 then the patient has hyperlactatemia
-more negative charges means have to have more positive charges so to get them get more H+
*If >4 and the patient has an infection they should be considered  [[Sepsis|Severe Sepsis]]
If difference expands: alkalotic by generating extra bicarb
*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 toxicity]]
**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 Toxicity|Isopropanol]] (isopropyl alcohol)
**Propylene glycol
**[[Lithium]]


Weak acids
==Traditional step-wise approach==
: conc of albumin in body has affect
===Determine pH===
if albumin goes up, more acid (since it is an acid), so more acidotic
*If pH < 7.35, then acidemia
--- if pt is hypovolemic and correct it then the concentration changes
*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


Strong ion gap = anion gap
{| {{table}}
Strong anions: lactate, ketoacid, toxic alcohols
| align="center" style="background:#f0f0f0;"|'''Delta Ratio'''
| align="center" style="background:#f0f0f0;"|'''Assessment Guideline'''
|-
|< 0.4
|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
|}


Base Deficit (excess)
===Calculate the starting bicarbonate===
-Get rid of the respiratory component, as if the CO2 were 40, so left w/ representation of purely metabolic acidosis, then give you number how much base or acid you would have to add to get to pH 7.4
*∆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


Base excess of -6 = base deficit = deficit of base = need to add base to get to neutral
==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


you can look at any abg/vbg and see if pt has metabolic alkalosis or acidosis or if not then it's just respiratory
==Management==
Nl = -2 to +2
===[[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


base excess lets you look at just the metabolic side
====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%


If their is base excess or deficit either because of too much Na, too much Cl (strong ion difference), lactate, weak acids (albumin), strong ions we can't measure (ketoacids, ASA)  
^Consider balanced solution (LR) in patients with low pH (e.g. [[DKA]])


When we see a large base deficit is to figure out how to fill it to figure out what's going on
==See Also==
*[[Electrolyte Abnormalities (Main)]]


Cations: Na
==References==
Anions: Cl, bicarb, albumin, strong ion gap (e.g. lactate), unmeasured
<references/>
 
[[Category:FEN]]
==How to approach an acid/base problem==
[[Category:Renal]]
 
[[Category:Critical Care]]
#Get labs
##VBG/ABG, lactate, albumin, acetone, chemistry (as coincident as possible)
#Look at pH
##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 = Na - Cl
###Low SID if <38
####This is a metabolic acidosis=strong ion acidosis=hyperchloremic acidosis (Low SID acidosis); causes include:
#####Fluid Administration
######Any fluid that has an SID of <24 can cause acidosis (i.e. NS, ½ NS, D5W) 2
liters of NS in <24 hours is enough to cause acidosis
#####Renal Tubular Acidosis: Calculate Urine Anion Gap (Urine Na + K – Cl); if negative, not an RTA,
#####consider other causes:
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 (High SID alkalosis); causes include:
#####Nasogastric Suction
#####Diuretics
#####hyperaldosteronism
#####volume depletion
 
STEP V‐LOOK AT THE LACTATE
If >2 then the patient has hyperlactatemia
If >4 and the patient has an infection, start EGDT
If patient not infected, consider any other shock state, seizures, dead gut, hepatic failure, malignancies or just from hyperlactetemic state such as exercise or the use of b‐agonists, or
-Toxicologic causes of elevated lactate include Cyanide, Carbon Monoxide, Metformin, Didanosine, Stavudine, Zidovudine, Linezolid, Strychnine, Emtriva, Rotenone (Fish Poison), NaAzide (Lab Workers), Apap (if Liver Fx), Phospine (rodenticide), NaMonofluoroacetate (Coyote Poison‐Give Etoh as antidote), Inh (if patient seizes), Hemlock, Depakote, Hydrogen Sulfide, Nitroprusside (If cyanide toxic), Ricin & Castor Beans, Propofol, Linezolid, Sympathomimetics (Cocaine, Methamphetamine), Jequirty peas (Abrus precatorius), Prunus Amygdalus Plants as well as Crab Tree Apple Seeds & Cassava (yucca).
Most of the toxins under SIG acidoses will also cause elevated lactate.
Rare causes: pyroglutamic acidemia (from taking tylenol in combination with severe sepsis, renal fx, or hepatic fx); Shoshin beri beri (from severe thiamine deficiency).
 
STEP VI‐CALCULATE THE STRONG ION GAP (SIG) (to explain the base deficit/excess)
SIG = [[(Base Deficit) + (SID – 38)]]  
 
+ [2.5 (4.2 ‐ Albumin (g/dL))] – Lactate
 
This can also be thought of as the corrected base deficit, or put a minus sign in front and it is the corrected
base excess
IF SIG>2, THIS IS A SIG METABOLIC ACIDOSIS
-start looking for anion causes:
Uremia, DKA, AKA, Tox‐ASA, ethylene glycol, methanol, propylene glycol (ativan, valium, dilantin infusions), iron, INH, paraldehyde, DLactic Acidosis‐from short gut/blind loop. Will not show on lactate assay
 
NEGATIVE SIG (very rare)
Hypercalcemia, Hypermagnesemia, Hyperkalemia, Immunoglobulins, Bromide, Nitrates, Lithium
Overdose
 
STEP VII‐THINK ABOUT COMPENSATIONS
If primary is respiratory and you feel it is chronic, you can calculate the expected metabolic compensation
Expected Δ BE (or expected decrease of SID) = 0.4 x (Chronic Change in CO2)
If the primary problem is metabolic acidosis
Expected ↓ CO2=Base Deficit
If the primary problem is metabolic alkalosis
Expected ↑ CO2=0.6 x Base Excess
Old school formula may be useful for figuring out to correct PaCO2 in a COPD Patient
0.8 decrease in pH = for every 10 mmHg increase in PaCO2 acutely
 
STEP VIII‐OSMOLAR GAP
If elevated SIG without explanation, get osmolar gap
Osm Gap=Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
Positive if osm gap >10
Causes: Methanol, Ethylene glycol, mannitol, isopropanol (isopropyl alcohol), propylene glycol, lithium
If Osm Gap is >50, almost certainly toxic alcohol induced
Notes:
 
 
If no BD is available, 24.2 – serum bicarb can be used as a poor man’s substitute
The more complex but correct formula for SID is (Na + K + Ionized Mg + ICal – Cl) If this formula is used, then normal
should be considered 42. In clinical practice, if the patient is not hyperkalemic, this more complex formula is not
necessary.
 
Fluids
- Normal Na Cl difference is 38
- Fluid that has 38 would be basic b/c it would dilute out the albumin and therefore would be alkalotic
- Magic number for pH neutral fluid is 24-28 or whatever the pt's bicarb is
- If SID of fluid is greater than pt's current bicarb level then fluid is alkalotic
-NS or 1/2NS (SID = 0) so is acidotic so causes hyperchloremic acidosis
LR has SID of 24-28
D5W has SID of 0
 
NaBicarb is 8.4%
 
Consider balanced solution in pt with DKA and low pH (LR)

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

  1. ↑ http://emcrit.org/wp-content/uploads/acid_base_sheet_2-2011.pdf
  2. ↑ Stewart Acid base http://www.acid-base.com/strongion.php
  3. ↑ Brandis K. Anesthesia MCQ. Rules for Metabolic Acid-Base Disorders. http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php
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