This presentation deals with the basics of arterial blood gas interpretation. It describes the importance of each reading in the ABG report.

1. ARTERIAL BLOOD GAS ANALYSIS IN
RESPIRATORY DISORDERS
Dr.R.Selvakumar
Professor of Anaesthesiology
K.A.P.Viswanatham Govt Medical College,
Trichy-Tamilnadu-India

2. A knowledge about basics and
patient’s clinical history
is important in reading ABG..
just like ECG….

3. WHAT DO YOU MEAN BY
PARTIAL PRESSURE?
@ @
@
@
@
$ $
$
@ @
@
@
@ $
$
IN A MIXTURE OF GASES,THE PRESSURE EXERTED BY EACH GAS
IS THE SAME AS THAT WHICH IT WOULD EXERT IF IT ALONE
OCCUPIED THE CONTAINER
$
5 PSI 3 PSI 8 PSI

4. DALTON’S LAW OF PARTIAL PRESSURE
THE PROPORTION OF THE PRESSURE EXERTED BY A GAS
IN THE TOTAL PRESSURE IS EQUATED WITH THE VOLUME
IT OCCUPIES
@ @
@
@
@
$
$
$
PRESSURE OF @ GAS = 5 PSI
PRESSURE OF $ GAS = 3 PSI
TOTAL PRESSURE = 8 PSI
What is the % contribution of pressure of @ gas
in the total pressure?
5/8 x 100 = 62.5%
So, gas @ will occupy
62.5% of the total volume

5. Total atmospheric pressure -760 mm of Hg
Out of this 760, 78% is contributed by
Nitrogen and 21 % is contributed
by Oxygen
21/100 x 760 = 160
Partial pressure of Oxygen in the atmosphere

6. What do you mean by tension of a gas?
@ @
@
@
@
@ @
The gas which gets dissolved in
the water also exerts a pressure.
That, we call it as “TENSION”
SAMPLE PaO2 in mm of
Hg
PaCO2 in mm
of Hg
ARTERIAL 80 -100 35-45
VENOUS 37-42 42-50

7. What do you mean by FIO2?
•The % of O2 in the inspired gas mixture
•In room air , it is 21% or 0.21

9. How diffusion occurs in alveoli?

10. Gases crossing the
Alveolar-capillary
Basement membrane
depending on
Partial pressure gradient

11. Do all alveoli participate in gaseous exchange?

12. What is the normal O2 content of the blood?
(Hb in gm% X 1.34 X SaO2) + ( 0.003 X paO2) = CaO2 = 20.1 ml
Oxygen in the blood combined with Hb +
Oxygen dissolved in plasma as physical solution

13. How to increase the oxygen content
of the blood?
(Hb in gm% X 1.34 X SaO2) + ( 0.003 X paO2) = CaO2 =
20.1ml
(15 X 1.34 X 99) + ( 0.003 X 100 ) = 20.1 ml%
(4 X 1.34 X 99) + (0.003 x 100 ) = 5.6 ml%
(15 X 1.34 x 99) + (0.003 x 600 ) = 21.6 ml%
Which is better?
To give 100% O2 or increase the available Hb..?

14. What is the limitation of pulseoximetry?

16. With that background knowledge…
Why to measure the ABG..?
What can you infer from it..?

17. ABG reflects
 ALVEOLAR VENTILATION
 OXYGENATION
 ACID-BASE DISORDERS
 ELECTROLYTE STATUS

18. ALVEOLAR VENTILATION
• CO2 level in the blood depends upon
the production and excretion.
• Excretion depends upon alveolar ventilation
• Alveolar ventilation depends upon many factors
like central respiratory drive , neuronal pathways,
Anterior horn cells, respiratory muscle condition,
Respiratory mechanics, intactness of airway and
integrity of the pulmonary gas exchange system
If ventilation decreases, then CO2 level increases…..

19. OXYGENATION
ABG tells you about the status of oxygenation in the form of
paO2, SaO2, Oxygen Content, a/A ratio, (A-a)DO2
We can know the joint status of cardio-pulmonary system

20. ACID-BASE DISORDER
 We can measure the pH, HCO3 level
and Base excess/deficit
 We can also know about the adequacy
of compensatory mechanisms operating
in the body.

21. COMPENSATORY MECHANISMS
IN ACID-BASE DISORDER
DUE TO RESPIRATORY PROBLEMS
If CO2 accumulates, due to Hypoventilation
CO2 + H2O = H2CO3 = H+ + HCO3
1 molecule of H+ and 1 molecule of HCO3
is added to the circulation
But acidosis results….. How?

22. The impact of H+ increase is more, since
the level of H+ in body is in nanomoles
and HCO3 is in mmoles
How the body compensates for
the addition of excess H+?
 It retains HCO3 through kidney.
 We call it as metabolic compensation

23. COMPENSATION RULE:
 HCO3 increases by 1 meq/L for every 10 mm rise of
CO2 in acute situations.
 It increases by 3.5 meq/L for every 10 mm rise in
chronic situations.
Memorize plz

24. COMPENSATION RULE:
 If CO2 is washed out due to hyperventilation,
HCO3 falls by 2 meq/L for every 10 mm fall in paCO2
in acute situations.
 In chronic states, HCO3 falls by 5 meq/L for every
10 mm fall of paCO2.
Memorize plz

25. How to read the ABG..?
PARAMETER NORMAL RANGE
paO2 80 -100 mm of Hg
paCO2 35 -45 mm of HG
HCO3 22 – 26 meq/L
Std HCO3 21 – 27 meq/L
Base Excess -2 to + 2 mmol/L
CaO2 16 – 20 ml%
a/A ratio 0.8
(A-a) DO2 10 -20 mm of Hg
FIO2 0.21
Hb 15g%
Temp 37.2 C

26. Examples….
A person choked with a coin in his throat. He is in
acute distress.. His ABG reads…
pH - 7.24
paO2 - 42
paCO2 - 72
HCO3 - 27
acidosis
Hypoxemia
FIO2 - 0.21
Acute retention
Slight rise
RESPIRATORY ACIDOSIS - ACUTE

27. Example -2
A COPD patient coming for hernia repair
pH - 7.34
paO2 - 68
paCO2- 72
HCO3 - 34
FIO2 - 0.21
BE - 12
Normal / acidosis
Mild hypoxemia
SaO2 - 95 %
Very high
High level.
High level
SaO2 is high for a low paO2
CHRONIC RESPIRATORY ACIDOSIS WITH
METABOLIC ALKALOSIS

28. pH 7.34
paO2 68
paCO2 72
HCO3 34
BE 12
SaO2 95
FIO2 0.21
• Why it is chronic?
• paO2 is 68, but SaO2 is 95%. How?
• Can it be chronic metabolic alkalosis
with compensatory respiratory acidosis?

29. RULES YOU MUST NEVER FORGET….
 pH drifts towards primary acid-base disorder
 Compensation never overshoots….
pH 7.34
paO2 68
paCO2 72
HCO3 34
BE 12
SaO2 95
FIO2 0.21

30. Comment about oxygenation in the first ABG…
pH 7.24
paO2 42
paCO2 72
HCO3 27
SaO2 55%
FIO2 0.21
(A-a)DO2 75
a/A ratio 0.38
Severe Hypoxia
Saturation very low
Normal -110 -90 = 20
So ,it is high
Normal 90/110 = 0.8
Very low.

31. A patient with head injury with
neurogenic hyperventilation
pH 7.51
paO2 80
paCO2 24
HCO3 20
SaO2 94
FIO2 0.21
BE -4
Alkalosis
Decreased
Decreased. ↑ Kidney
excretion
Acute Respiratory alkalosis with
Compensatory metabolic acidosis
Remember… Acidosis results either due to the
Increase of H+ ions or depletion of HCO3 ions

32. A patient living in high altitude region
pH 7.48
paO2 72
paCO2 24
HCO3 16
SaO2 96
FIO2 0.21
Mild alkalosis
Mild hypoxemia
Decreased level
Very low level. Kidney
excreted much HCO3
For a paO2 of 72,
saturation is high
Chronic respiratory alkalosis
with compensatory metabolic acidosis

33. Some more complicated ABGs…
pH 7.32
paO2 68
paCO2 69
HCO3 34
FIO2 0.21
pH 7.26
paO2 42
paCO2 78
HCO3 26
FIO2 0.40
COPD DEVELOPING
LOBAR
PNEUMONIA
More acidotic
Further lowered
Looks like normal
Chronic resp.acidosis
with compensatory met.alkalosis
Acute on Chronic resp.acidosis
with compensatory met.alkalosis
Increases further

34. Home work…. Just try it out
pH 7.4
paO2 98
paCO2 37
HCO3 24
FIO2 1
BE 0.5
SaO2 100
(A-a)DO2 467
Patient on ventilator
Can we wean him?
pH 7.41
paO2 147
paCO2 36
HCO3 23
BE -1
FIO2 0.21
SaO2 99%
Comment on this…

35. A patient with duodenal ulcer perforation given I.V
Morphine for pain relief….
pH 7.14
paO2 80
paCO2 50
HCO3 20
BE -5.2
FIO2 0.5
(A-a)DO2 198
SaO2 98%
Severe acidosis
Moderate hypoxia
Slightly elevated
Slightly lowered
Acute combined metabolic
and respiratory acidosis

36. ARDS patient on
ventilator
pH 7.37
paO2 105
paCO2 34
HCO3 22
BE -1.3
FIO2 1
(A-a)DO2 438
2 Hours after
the introduction of
PEEP 10 cm of H2O
pH 7.37
paO2 94
paCO2 32
HCO3 23.5
BE -0.3
FIO2 0.6
(A-a)DO2 239

37. Summary
 Always remember the Basic physiology
 Go through the clinical history and then try to read
 pH sways to the primary acid-base disorder side
 Compensatory phenomenon never overshoots

38. If you understand and remember the basic…
If you are methodical….
Reading ABG is a child’s play
Concluding……

39. dr.r.selvakumar
professor of anaesthesiology
k.a.p.viswanatham govt medical college
trichy