Basic

2. It is the measuring of the common vital
gases dissolved in blood usually arterial to aid
in evaluation of different clinical conditions.
The usually actually measured values are
only pH, PaCO2 and PaO2 whereas the co-
oximeter measures SaO2, carboxyhemoglobin,
methemoglobin and hemoglobin content

3. pH 7.35-7.45
PaCO2 35-45 mm Hg
PaO2 70-100 mm Hg (PaO2 = 104.2 - (0.27 x age)
SaO2 93-98%
HCO3
- 22-26 mEq/L
%MetHb <2.0%
%COHb <3.0%
Base excess -2.0 to 2.0 mEq/L
CaO2 16-22 ml O2/dl

4. Equation Physiologic Process
1) PaCO2 equation Alveolar ventilation
2) Alveolar gas equation Oxygenation
3) Oxygen content equation Acid-base balance
4) Henderson-Hasselbalch equation
These 4 equations, crucial to understanding
and interpreting arterial blood gas data.
12/3/2023 4
The Key to Blood Gas Interpretation:
4 Equations, 3 Physiologic Processes

5. VCO2 x 0.863 VCO2 = CO2 production
PaCO2 = ------------------ VA = VE – VD
VA VE = minute (total) ventilation
VD = dead space ventilation
0.863 converts units to mm Hg
12/3/2023 5
PaCO2 equation: PaCO2 reflects ratio of
metabolic CO2 production to alveolar
ventilation

6. PAO2 = PIO2 - 1.2 (PaCO2)
where
PIO2 = FIO2 (PB - 47).
12/3/2023 6
Alveolar Gas Equation
PAO2 = PIO2 - 1.2 (PaCO2)
PAO2 = PIO2 - 1.2 (PaCO2)
PAO2 = PIO2 - 1.2 (PaCO2)
where PIO2 = FIO2 (PB – 47 mm Hg)
• If FIO2 and PB are constant, then as PaCO2
increases both PAO2 and PaO2 will decrease
(hypercapnia causes hypoxemia).
• If PB decreases (e.g., with altitude), and PaCO2
and FIO2 are constant, both PAO2 and PaO2 will
decrease (mountain climbing causes hypoxemia)

7. Neither the PaO2 nor the SaO2 tells how much
oxygen is in the blood.
“How much” is provided by the oxygen
content, CaO2 is calculated as:
CaO2 (ml O2/dl). =
quantity O2 bound to hemoglobin
+
quantity O2 dissolved in plasma
CaO2 = (Hb x 1.34 x SaO2) + (0.003 x PaO2)
(0 .003 is solubility coefficient of oxygen in plasma)
7
SaO2 and oxygen content

10. It is commonly called the “A-a gradient,” though it
does not actually result from an O2 pressure gradient in the
lungs. Instead, it results from gravity-related blood flow
changes within the lungs (normal ventilation-perfusion
imbalance).
Normal P(A-a)O2 ranges from 5 to 25 mmHg
breathing room air (it increases with age). A higher than
normal P(A-a)O2 means the lungs are not transferring
oxygen properly from alveoli into the pulmonary
capillaries. Except for right to left cardiac shunts, an
elevated P(A-a)O2 signifies some sort of problem within
the lungs.
(A – a)DO2 = 2.5 + (0.25 X Age in years)
10
P(A-a)O2

11. Inspired O2 – PaO2 Relationship
FIO2 (%) Predicted Min
PaO2 (mm Hg)
30 150
40 200
50 250
80 400
100 500
If PaO2 < FIO2 x 5, pt probably hypoxemic at RA

12. 12/3/2023 12
Oxygen dissociation curve:
SaO2 vs. PaO2

15. Primary acid-base disorder:
One of the four acid-base disturbances that is
manifested by an initial change in HCO3
- or
PaCO2.
Compensation:
The change in HCO3
- or PaCO2 that results
from the primary event. Compensatory changes
are not classified by the terms used for the four
primary acid-base disturbances.
In simple, uncomplicated MAc the patient
will never develop alkalemia.
12/3/2023
Acid base terminology

16. Metabolic acidosis:
PCO2  1.2 mmHg per 1.0 meq/L  HCO3
-
Metabolic alkalosis:
PCO2  0.7 mmHg per 1.0 meq/L HCO3
-
Expected changes in PaCO2 for a 1
meq/L change in HCO3
-

17. Two general rules hold up for the respiratory
response to a metabolic alkalosis:
• A patient will increase PaCO2 above 40 but
not greater than 50-55 to compensate for a
metabolic alkalosis.
• A patient will be alkalotic (pH > 7.45) if the
PaCO2 is elevated to compensate for a
metabolic alkalosis (If the patient is acidotic,
pH < 7.35, then an additional respiratory
acidosis is present).

18. • If no other metabolic disturbance exists, then
the following calculation would result in 24:
Corrected HCO3
- =
measured HCO3
- + (anion gap - 12)
• If the corrected HCO3
- is greater than 24, a
metabolic alkalosis co-exists.
• If the corrected HCO3
- is less than 24 then a
non-gap acidosis co-exists.

19. TIP 1.
Don’t interpret any blood gas data for acid-
base diagnosis without closely examining the
serum electrolytes: Na+, K+, Cl-
and CO2.
Note that serum CO2 may be normal in the
presence of two or more acid-base disorders.
12/3/2023
Tips to diagnosing mixed
acid-base disorders

20. TIP 2 .
Single acid-base disorders do not lead to
normal blood pH. a normal pH with distinctly
abnormal HCO3
-
and PaCO2 invariably suggests
two or more primary disorders.
12/3/2023 20

21. TIP 3.
Simplified rules predict the pH and HCO3
-
for a given change in PaCO2. If the pH or HCO3
-
is higher or lower than expected for the change in
PaCO2, the patient probably has a metabolic acid-
base disorder as well.
12/3/2023 21

22. Stepwise approach to diagnosing acid-base
disorders
****************Valid or not then ****************
Step 1: Acidemic or Alkalemic?
Step 2: Is the primary disturbance respiratory or metabolic?
Step 3. For a respiratory disturbance, determine whether it
. is acute or chronic.
Step 4. For a metabolic acidosis, determine whether an
. anion gap is present.
Step 5. Determine whether other metabolic disturbances
. co-exist with an anion gap acidosis.
Step 6. Assess the normal compensation by the respiratory
. system for a metabolic disturbance.

23. Test for validity

25. pCO2 , HCO3 
pCO2 , HCO3 N
Resp + Met Alkalosis
Uncomp Resp Alkalosis
pCO2 N, HCO3  Uncomp Met Alkalosis
pCO2 , HCO3  Comp Met Alkalosis
pCO2 , HCO3  Comp Resp Alkalosis
pH 

26. pCO2 , HCO3 
pCO2 , HCO3 N
Resp + Met Acidosis
Uncomp Resp Acidosis
pCO2 N, HCO3  Uncomp Met Acidosis
pCO2 , HCO3  Comp Resp Acidosis
pCO2 , HCO3  Comp Met Acidosis
pH 

27. pCO2 , HCO3 
Comp Met Alkalosis
pCO2 N, HCO3 N N Acid Base Homeostasis
pCO2 , HCO3 
Met acidosis
+
Resp alkalosis
pH
N
or
N
Comp Met Acidosis
Comp Resp Alkalosis
Comp Resp Acidosis
Resp Acidosis
+
Met Alkalosis

28. Arterial hypocapnia can be observed in
patients with profound depression of cardiac
function and pulmonary perfusion but with
relative preservation of alveolar ventilation (
including patients undergoing CPR).
Severely reduced pul BF limits CO2
delivered to lungs for excretion  PvCO2.
Increased V/Q ratio causes removal of a
larger-than-normal amount of CO2 per unit of
blood traversing the pulmonary circulation
arterial eucapnia or frank hypocapnia.
Pseudorespiratory Alkalosis

29. To rule out pseudorespiratory
alkalosis in a patient with circulatory
failure, blood gas monitoring must
include sampling of mixed (or central)
venous blood.