Acid-base disorders

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What determines acid/base

conservation of charge, neutrality, no matter what

independent vs dependent variables body is limitless pool of H+ and bicarb- - dependent variables (not the determinators)

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)

Na and Cl are prime determinors of acid base

Strong ion difference between Na and Cl is the major determinor of acid base

Normal difference is ~38 (140 Na, 102 Cl), which is also pH determiner - if difference shrinks: acidotic -more negative charges means have to have more positive charges so to get them get more H+ If difference expands: alkalotic by generating extra bicarb

Weak acids

conc of albumin in body has affect

if albumin goes up, more acid (since it is an acid), so more acidotic --- if pt is hypovolemic and correct it then the concentration changes

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

  1. Get labs
    1. VBG/ABG, lactate, albumin, acetone, chemistry (as coincident as possible)
  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
    1. SID = Na - Cl
      1. Low SID if <38
        1. This is a metabolic acidosis=strong ion acidosis=hyperchloremic acidosis (Low SID acidosis); causes include:
          1. Fluid Administration
            1. 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

          1. Renal Tubular Acidosis: Calculate Urine Anion Gap (Urine Na + K – Cl); if negative, not an RTA,
          2. consider other causes:

Type I‐Urine pH <5.55 Type II‐Urine pH >5.55 Type IV‐Hyperkalemic; from aldosterone deficiency, diabetes, Diarrhea

      1. HIGH SID IF >38
        1. This is metabolic alkalosis (High SID alkalosis); causes include:
          1. Nasogastric Suction
          2. Diuretics
          3. hyperaldosteronism
          4. 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%