Acid-base disorders: Difference between revisions

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What determines acid/base
==Background==
 
#Determiners of acid-base status are:
conservation of charge, neutrality, no matter what
##CO2
 
##Weak acids (primarily albumin)
independent vs dependent variables
###If albumin goes up more acidotic (since albumin is an acid)
body is limitless pool of H+ and bicarb- - dependent variables (not the determinators)
##Strong ion difference (SID)
 
###Primarily Na-Cl
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)  
####Normal difference is ~38 (140-102)
 
#####If difference shrinks (i.e. more Cl) more acidotic
Na and Cl are prime determinors of acid base
######Principle of electrical neutrality requires more H+ to offset the additional Cl
 
#####If difference increases (i.e. more Na) more alkaloatic
Strong ion difference between Na and Cl is the major determinor of acid base
######Principle of electrical neutrality requires more bicarb to offset the additional Na
 
#Strong ion gap (SIG)
Normal difference is ~38 (140 Na, 102 Cl), which is also pH determiner
##Equivalent to anion gap
- if difference shrinks: acidotic
##Strong ions include Na, Cl, lactate, ketoacid, toxic alcohols
-more negative charges means have to have more positive charges so to get them get more H+
#Base Deficit
If difference expands: alkalotic by generating extra bicarb
##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
Weak acids
##Base excess of -6 = base deficit of 6
: conc of albumin in body has affect
##Normal = -2 to +2
if albumin goes up, more acid (since it is an acid), so more acidotic
##If base deficit is normal but pt is acidotic must all be from CO2
--- if pt is hypovolemic and correct it then the concentration changes
##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
Strong ion gap = anion gap
Strong anions: lactate, ketoacid, toxic alcohols
 
Base Deficit (excess)
-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
 
Base excess of -6 = base deficit = deficit of base = need to add base to get to neutral
 
you can look at any abg/vbg and see if pt has metabolic alkalosis or acidosis or if not then it's just respiratory
Nl = -2 to +2
 
base excess lets you look at just the metabolic side
 
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)
 
When we see a large base deficit is to figure out how to fill it to figure out what's going on
 
Cations: Na
Anions: Cl, bicarb, albumin, strong ion gap (e.g. lactate), unmeasured


==How to approach an acid/base problem==
==How to approach an acid/base problem==
 
#Get labs (as coincident as possible)
#Get labs  
##VBG/ABG
##VBG/ABG, lactate, albumin, acetone, chemistry (as coincident as possible)
##Lactate
##Albumin
##Acetone
##Chemistry
#Look at pH
#Look at pH
##If pH >7.45 pt's primary problem is alkalosis
##If pH >7.45 pt's primary problem is alkalosis
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##If >45 then respiratory acidosis
##If >45 then respiratory acidosis
##If <35 respiratory acidosis
##If <35 respiratory acidosis
#Calculate the strong ion difference
#Calculate the strong ion difference (SID)
##SID = Na - Cl
##SID = Na - Cl
###Low SID if <38
###Low SID if <38
####This is a metabolic acidosis=strong ion acidosis=hyperchloremic acidosis (Low SID acidosis); causes include:
####Metabolic acidosis = strong ion acidosis = hyperchloremic acidosis
#####Fluid Administration
####Causes include:
######Any fluid that has an SID of <24 can cause acidosis (i.e. NS, ½ NS, D5W) 2
#####Fluid administration
liters of NS in <24 hours is enough to cause acidosis
######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 an RTA,
#####Renal Tubular Acidosis
#####consider other causes:
######Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
Type I‐Urine pH <5.55
#######Type I: Urine pH <5.55
Type II‐Urine pH >5.55
#######Type II: Urine pH >5.55
Type IV‐Hyperkalemic; from aldosterone deficiency, diabetes,
#######Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
Diarrhea
#####Diarrhea
 
###High SID if >38
###HIGH SID IF >38
####This is metabolic alkalosis  
####This is metabolic alkalosis (High SID alkalosis); causes include:
####Causes include:
#####Nasogastric Suction
#####Nasogastric suction
#####Diuretics
#####Diuretics
#####hyperaldosteronism
#####Hyperaldosteronism
#####volume depletion
#####Volume depletion
 
#Look at the lactate
STEP V‐LOOK AT THE LACTATE
##If >2 then pt has hyperlactatemia
If >2 then the patient has hyperlactatemia
##If >4 and pt has infection start Early Goal Directed Therapy ([[Sepsis]]
If >4 and the patient has an infection, start EGDT
##If pt not infected consider other diagnoses: [[Lactic Acidosis (Lactate)]]
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
#Calculate the strong ion gap (SIG) to explain the base deficit
-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).
##SIG = [[(Base Deficit) + (SID – 38)]] + [2.5 (4.2 ‐ Albumin (g/dL))] – lactate
Most of the toxins under SIG acidoses will also cause elevated lactate.
###If SIG >2 this is a SIG metabolic acidosis = anion gap acidosis
Rare causes: pyroglutamic acidemia (from taking tylenol in combination with severe sepsis, renal fx, or hepatic fx); Shoshin beri beri (from severe thiamine deficiency).
####Causes:
 
#####Uremia
STEP VI‐CALCULATE THE STRONG ION GAP (SIG) (to explain the base deficit/excess)
#####DKA
SIG = [[(Base Deficit) + (SID – 38)]]  
#####AKA
 
#####ASA
+ [2.5 (4.2 ‐ Albumin (g/dL))] – Lactate
#####Ethylene glycol, methanol, propylene glycol
 
#####Iron
This can also be thought of as the corrected base deficit, or put a minus sign in front and it is the corrected
#####INH
base excess
#####Paraldehyde
IF SIG>2, THIS IS A SIG METABOLIC ACIDOSIS
#####DLactic Acidosis (from short gut/blind loop - will not show on lactate assay)
-start looking for anion causes:
###If SIG negative (very rare):
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
####Hypercalcemia
 
####Hypermagnesemia
NEGATIVE SIG (very rare)
####Hyperkalemia
Hypercalcemia, Hypermagnesemia, Hyperkalemia, Immunoglobulins, Bromide, Nitrates, Lithium
####Immunoglobulins
Overdose
####Bromide
 
####Nitrates
STEP VII‐THINK ABOUT COMPENSATIONS
####Lithium
If primary is respiratory and you feel it is chronic, you can calculate the expected metabolic compensation
#Think about compensations
Expected Δ BE (or expected decrease of SID) = 0.4 x (Chronic Change in CO2)
##If primary is respiratory calculate the expected metabolic compensation
If the primary problem is metabolic acidosis
###Expected ΔBE (or expected decrease of SID) = 0.4 x (Chronic change in CO2)
Expected ↓ CO2=Base Deficit
##If primary is metabolic acidosis calculate the expected respiratory compensation:
If the primary problem is metabolic alkalosis
###Expected ↓CO2 = Base Deficit
Expected ↑ CO2=0.6 x Base Excess
##If primary is metabolic alkalosis calculate the expected respiratory compensation:
Old school formula may be useful for figuring out to correct PaCO2 in a COPD Patient
###Expected ↑ CO2 = 0.6 x Base Excess
0.8 decrease in pH = for every 10 mmHg increase in PaCO2 acutely
##Winter's Formula useful for figuring out PaCO2 in COPD pt:
 
###0.8 decrease in pH for every 10 mmHg increase in PaCO2 acutely
STEP VIII‐OSMOLAR GAP
#Calculate the osmolar gap
If elevated SIG without explanation, get osmolar gap
##Indicated if have elevated SIG without explanation
Osm Gap=Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
###Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
Positive if osm gap >10
####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
#####Causes:
If Osm Gap is >50, almost certainly toxic alcohol induced
######Methanol
Notes:
######Ethylene glycol
######Mannitol
######Isopropanol (isopropyl alcohol)
######Propylene glycol
######Lithium




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
Fluids

Revision as of 19:58, 3 August 2011

Background

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

How to approach an acid/base problem

  1. Get labs (as coincident as possible)
    1. VBG/ABG
    2. Lactate
    3. Albumin
    4. Acetone
    5. Chemistry
  2. Look at pH
    1. If pH >7.45 pt's primary problem is alkalosis
    2. If pH <7.35 pt's primary problem is acidosis
  3. Look at blood gas CO2
    1. If >45 then respiratory acidosis
    2. If <35 respiratory acidosis
  4. Calculate the strong ion difference (SID)
    1. SID = Na - Cl
      1. Low SID if <38
        1. Metabolic acidosis = strong ion acidosis = hyperchloremic acidosis
        2. Causes include:
          1. Fluid administration
            1. Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
          2. Renal Tubular Acidosis
            1. 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
          3. Diarrhea
      2. High SID if >38
        1. This is metabolic alkalosis
        2. Causes include:
          1. Nasogastric suction
          2. Diuretics
          3. Hyperaldosteronism
          4. Volume depletion
  5. Look at the lactate
    1. If >2 then pt has hyperlactatemia
    2. If >4 and pt has infection start Early Goal Directed Therapy (Sepsis
    3. If pt not infected consider other diagnoses: Lactic Acidosis (Lactate)
  6. Calculate the strong ion gap (SIG) to explain the base deficit
    1. SIG = (Base Deficit) + (SID – 38) + [2.5 (4.2 ‐ Albumin (g/dL))] – lactate
      1. If SIG >2 this is a SIG metabolic acidosis = anion gap acidosis
        1. Causes:
          1. Uremia
          2. DKA
          3. AKA
          4. ASA
          5. Ethylene glycol, methanol, propylene glycol
          6. Iron
          7. INH
          8. Paraldehyde
          9. DLactic Acidosis (from short gut/blind loop - will not show on lactate assay)
      2. If SIG negative (very rare):
        1. Hypercalcemia
        2. Hypermagnesemia
        3. Hyperkalemia
        4. Immunoglobulins
        5. Bromide
        6. Nitrates
        7. Lithium
  7. Think about compensations
    1. If primary is respiratory calculate the expected metabolic compensation
      1. Expected ΔBE (or expected decrease of SID) = 0.4 x (Chronic change in CO2)
    2. If primary is metabolic acidosis calculate the expected respiratory compensation:
      1. Expected ↓CO2 = Base Deficit
    3. If primary is metabolic alkalosis calculate the expected respiratory compensation:
      1. Expected ↑ CO2 = 0.6 x Base Excess
    4. Winter's Formula useful for figuring out PaCO2 in COPD pt:
      1. 0.8 decrease in pH for every 10 mmHg increase in PaCO2 acutely
  8. Calculate the osmolar gap
    1. Indicated if have elevated SIG without explanation
      1. Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
        1. Positive if osm gap >10 (if Osm gap >50 almost certainly toxic alcohol induced)
          1. Causes:
            1. Methanol
            2. Ethylene glycol
            3. Mannitol
            4. Isopropanol (isopropyl alcohol)
            5. Propylene glycol
            6. Lithium


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)

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