2. The Blood Gas
Report: normals…
pH 7.40 + 0.05
PaCO2 40 + 5 mm
Hg
PaO2 80 - 100 mm Hg
HCO3 24 + 4 mmol/L
O2 Sat >95
Always mention and see
The essentials
HCO3
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6. Step 2
Who is responsible for this change in pH ( culprit )
CO2 will change pH in opposite direction
Bicarb. will change pH in same direction
Acidemia: With HCO3 < 20 mmol/L = metabolic
With PCO2 >45 mm hg = respiratory
Alkalemia: With HCO3 >28 mmol/L = metabolic
With PCO2 <35 mm Hg = respiratory
Step 1
Look at the pH
Is the patient acidemic pH < 7.35
or alkalemic pH > 7.45
7. Step 3
If there is a primary respiratory disturbance, is it
acute ?
.08 change in pH ( Acute )
.03 change in pH ( Chronic )
10 mm
Change
PaCO2
=
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8. Step 4
If the disturbance is metabolic is the
respiratory
compensation appropriate?
For metabolic acidosis:
Expected PaCO2 = (1.5 x [HCO3]) + 8 ) + 2
or simply…
expected PaCO2 = last two digits of pH
For metabolic alkalosis:
Expected PaCO2 = 6 mm for 10 mEq. rise in
Bicarb.
Suspect if .............
actual PaCO2 is more than expected :
additional …respiratory acidosis
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9. Step 4 cont.
If there is metabolic acidosis, is there a wide anion gap ?
Na - (Cl
-
+ HCO3
-
) = Anion Gap usually <12
If >12, Anion Gap Acidosis : M ethanol
U remia
D iabetic Ketoacidosis
P araldehyde
I nfection (lactic acid)
E thylene Glycol
S alicylate
Common causes
1) Lactic acidosis
2) Metabolic disorders
3) Renal failure
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