CM Acid Base

what is required for ABG analysis and how do you obtain it?

arterial blood

radial, brachial, femoral, or dorsalis pedis artery

if you are doing a radial/dorsalis pedis artery puncture, what test should you do to make sure there is collateral flow?

alen’s test

for ABG, what happens if there are air bubbles that are 1-2% of the blood volume?

falsely high PaO2

falsely low PaCO2

? normal PAO2 of blood → ? normal PaO2 of air

100

160

→ oxygen diffuses out of air bubble → plasma

→ false elevated PaO2

? normal PaCO2 of blood → ? PaCO2 of air

40

0.3

→ CO2 diffuses out of plasma and into air bubble

→ false low PaCO2

how to handle ABG specimen

cap specimen to prevent gas exchange in and out of syringe

put specimen on ice and analyze within 15 mins (to reduce O2 consumption by WBCs)

acidosis

process that increases [H+] by increasing pCO2 or reducing [HCO3-]

alkalosis

process that reduces [H+] by reducing pCO2 or by increasing [HCO3-]

normal body pH*

7.35-7.45

acidemia definition

low pH

→ acidosis is primary (high pCO2 or low HCO3-)

alkalemia definition

high pH

→ alkalosis is the primary (high pCO2 or low HCO3-)

normal pCO2*

35-45

PCO2 is regulated by _

ventilation

→ respiratory _

respiratory acidosis = _ pCO2

high (low pH)

respiratory alkalosis = _ pCO2

low (high pH)

normal HCO3*

22-26

HCO3 is regulated by _

kidneys

→ metabolic _

metabolic acidosis = _ hCO3

low (low pH)

metabolic alkalosis = _ HCO3

high (high pH)

normal pO2 = ?*

80-100

example of a volatile acid

carbon dioxide

example of a nonvolatile acid

organic/inorganic acid

UC (uncompensated) respiratory acidosis

ph: low

CO2: high

HCO3: normal

UC respiratory alkalosis

ph: high

co2: low

hco3: normal

UC metabolic acidosis

ph: low

co2: normal

hco3: low

UC metabolic alkalosis

ph: high

co2: normal

hco3: high

PC (partially compensated) respiratory acidosis

ph: low

co2: high

hco3: high

PC respiratory alkalosis

ph: high

co2: low

hco3: low

PC metabolic acidosis

ph: low

co2: low

hco3: low

PC metabolic alkalosis

ph: high

co2: high

hco3: high

C (compensated) respiratory acidosis

ph: low normal

co2: high

hco3: high

C respiratory alkalosis

ph: high normal

co2: low

hco3: low

C metabolic alkalosis

ph: low normal

co2: low

hco3: low

C metabolic alkalosis

ph: high normal

co2: high

hco3: high

what enzyme speeds up the CO2 bicarbonate buffer system?

carbonic anhydrase (make + dissociate carbonic acid)

there is less bicarbonate in your _ than your what blood

veins

arterial blood

(if you want accurate serum bicarb, grab from arterial blood)

base deficit/excess definition

the amt of base (bicarb)/acid needed to titrate serum pH back to normal

respiratory factors contribution is taken out of equation

normal -2 + 2 meq/L off of 24 (22-26)

if base deficit is -4, my base bicarb is 20 (→ low pH), so add 4 meg/L of bicarb (base) plasma = 24

if base excess is +4, base = 28, so give H+ (give acid) → 24

physiologic compensatory mechanism utilized what equation?

Henderson Hasselbach equation

pH = 6.1 + log([HCO3-] / 0.03 pCO2)

stepwise way to interpreting ABG*

1. look at pH. <7.35 = acidEMIA, >7.45 is alkaEMIA

2. look at pCO2 and HCO3-. both rise OR dec (to compensate for the pH, abnormal)→ alkalOSIS vs. acidOSIS

3. the initial (pathologic) change is what correlates with the abnormal pH (there is initial change and a compensatory change)

anion gap definition*

quantity of cations seemingly not balanced by anions

Unmeasured Anions - Unmeasured Cations  = ([Na+] + [K+]) - ([Cl-] + [HCO3-])

when should you suspect a mixed acid base disorder

whenever the pCO2 ahd HCO3- change in opposite directions (AKA whenever they’re not both INC or DEC)

T/F: compensatory responses NEVER return pH to “normal”

T

in a _ derangement, the direction of the compensatory response is?

simple

always the same as the primary pathologic change

major unmeasured cations*

Ca

Mg

Gamma globulins

major unmeasured anions*

Albumin*

Sulfate

Phosphate

Lactate

Citrate

Other organic anions

PCO2 levels

HIGH = respiratory ACIDOSIS

LOW = respiratory ALKALOSIS

HCO3 levels

HIGH: metabolic ACIDOSIS

LOW: metabolic ALKALOSIS

most common metabolic derangement

metabolic acidosis

normal anion gap*

8-16 meq/L (12 +/- 4)

when acid is added, H+ INC and HCO3- DEC

when gap is large = new acid (from med) or excess

anion gap will be falsely low due to?

low albumin

sodium and potassium are more than?

chloride and bicarb

anion gap equation

(([Na+] + [K+]) - ([Cl-] + [HCO3-])

how to correct an anion gap?*

reduction in “normal” anion gap (calculated gap + 2.5(normal albumin - measured albumin)

OR an upward adjustment in the pts correct anion gap

example: ((([Na+] + [K+]) - ([Cl-] + [HCO3-])) = anion gap

N = 144, K = 3, Cl = 110, HCO3 = 23, Albumin =2

apparent anion gap (OG): (144+3) - (110+23) = 14

pt’s corrected anion gap: 14 + 2.5(4-2) = 19

(1)compare corrected to standard, OR (2) OG to corrected normal - each 1 g/dL drop in albumin below normal (4), the expected anion gap drops by 2.5. so pt’s albumin = 2 → (2)(2.5) = 5, so standard normal = 12, pt’s normal is 12-5 = 7. Standard Range = 8 to 16 $\rightarrow$ This patient's personalized range = $8-5$ to $16-5 =$ 3 to 11.

1. 19 vs 8 to 16 (12±4) = 19 > 16 = HIGH

2. 14 vs 3 to 11(7±4) = 14 > 11 = HIGH

remember 8 to 16 is normal anion gap!

corrected anion gap is the REAL anion gap. anion gap (OG) is incorrect when albumin is low, it’s like their “normal”

what does “unmeasured” mean?

values NOT reported on a basic metabolic panel

AKA we need further tests

what causes the normal anion gap?

negatively charged plasma proteins