1. Arterial Blood Gas
Analysis …..1
Dr Deopujari
Pediatrician
Nagpur

2. The Goal :
To provide simple and bedside approach to ABG
report
Not to:
To teach physiology .
To teach theories on acid-base regulation
To look for alternative approaches to
interpretation
In details

3. A Systematic and pointed
………approach
Use of pH for Hydrogen
Ion Activity ……..
The credit (or Blame) for
introducing the term pH,
the negative log of
hydrogen ion (H+)
concentration, goes to
S. P. L. Sørensen
(1868-1939), who
apparently was tired of
writing seven zeros in a
paper on enzyme
activity and wanted a
simpler
designation…..?.

4. H ION CONC.
N.MOLS / L. pH
20 7.70
30 7.52
40 7.40
50 7.30
60 7.22
H ION
OH ION
0
14
pH stand for "power of hydrogen"
H+ = 80 - last two digits of pH

5. The Anatomy
of a Blood Gas Report
----- XXXX Diagnostics ------
Blood Gas Report
248 05:36 Jul 22 2000
Pt ID 2570 / 00
Measured 37.0
o
C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Corrected 38.6
o
C
pH 7.439
pCO2 47.6 mm Hg
pO2 123.5 mm Hg
Calculated Data
HCO3 act 31.1 mmol / L
HCO3 std 30.5 mmol / L
BE 6.6 mmol / L
O2 CT 14.7 mL / dl
O2 Sat 98.3 %
ct CO2 32.4 mmol / L
pO2 (A - a) 32.2 mm Hg
pO2 (a / A) 0.79
Entered Data
Temp 38.6 oC
ct Hb 10.5 g/dl
FiO2 30.0 %
Measured Values the most
important
Temperature Correction:
Is there any value to it?
Calculated Data:
Which are the useful ones?
Entered Data:
As important

6. Bicarbonate:----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Corrected 38.6
o
C
Calculated Data
HCO3 act 31.1 mmol / L
HCO3 std 30.5 mmol / L
BE 6.6 mmol / L
O2 CT 14.7 mL / dl
O2 Sat 98.3 %
t CO2 32.4 mmol / L
pO2 (A - a) 32.2 mm Hg
pO2 (a / A) 0.79
Entered Data
Temp 38.6 oC
ct Hb 10.5 g/dl
FiO2 30.0 %
Henderson - Hasselbach equation:
pH = pK + Log HCO3
Dissolved CO2

7. ----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Corrected 38.6
o
C
Calculated Data
HCO3 act 31.1 mmol / L
HCO3 std 30.5 mmol / L
BE 6.6 mmol / L
O2 CT 14.7 mL / dl
O2 Sat 98.3 %
t CO2 32.4 mmol / L
pO2 (A - a) 32.2 mm Hg
pO2 (a / A) 0.79
Entered Data
Temp 38.6 oC
ct Hb 10.5 g/dl
FiO2 30.0 %
Standard Bicarbonate:
Plasma HCO3 after equilibration
to a PCO2 of 40 mm Hg
: reflects non-respiratory acid base change
: does not quantify the extent of the buffer
base abnormality
: does not consider actual buffering
capacity of blood
Base Excess:
D base to normalise HCO3 (to 24)
with PCO2 at 40 mm Hg
(Sigaard-Andersen)
: reflects metabolic part of acid base D
: no info. over that derived from pH,
pCO2 and HCO3
: Misinterpreted in chronic or mixed
disorders

8. ----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Corrected 38.6
o
C
Calculated Data
HCO3 act 31.1 mmol / L
HCO3 std 30.5 mmol / L
BE 6.6 mmol / L
O2 CT 14.7 mL / dl
O2 Sat 98.3 %
t CO2 32.4 mmol / L
pO2 (A - a) 32.2 mm Hg
pO2 (a / A) 0.79
Entered Data
Temp 38.6 oC
ct Hb 10.5 g/dl
FiO2 30.0 %
Oxygenation
Parameters:
O2 Content of blood:
Hb x O2 Sat + Dissolved O2
Oxygen Saturation:
( remember this is calculated )
Alveolar / arterial gradient:
Arterial / alveolar ratio:

9. Rt. Shift
Lt.Shift

10. Alveolar-arterial Difference
Inspired O2 = 21 %
piO2 = (760-45) x . 21 = 150 mmHg
O2
CO2
palvO2 = piO2 – pCO2 / RQ
= 150 – 40 / 0.8
= 150 – 50 = 100 mm Hg
partO2 = 90 mmHg
palvO2 – partO2 = 10 mmHg

11. Alveolar- arterial Difference
O2
CO2
Oxygenation Failure
piO2 = 150
pCO2 = 40
palvO2= 150 – 40/.8
=150-50
=100
pO2 = 45
D = 100 - 45 = 55
Ventilation Failure
piO2 = 150
pCO2 = 80
palvO2= 150-80/.8
=150-100
= 50
pO2 = 45
D = 50 - 45 = 5
PAO2 (partial pres. of O2. in the alveolus.)
= 150 - ( PaCO2 / .8 )
760 – 45 = 715 : 21 % of 715 = 150

12. ----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Corrected 38.6
o
C
Calculated Data
HCO3 act 31.1 mmol / L
O2 CT 14.7 mL / dl
O2 Sat 98.3 %
t CO2 32.4 mmol / L
pO2 (A - a) 32.2 mm Hg
pO2 (a / A) 0.79
Entered Data
Temp 38.6 oC
ct Hb 10.5 g/dl
FiO2 30.0 %
Oxygenation:
Limitations of parameters:
O2 Content of blood:
Useful in oxygen transport calculations
Derived from calculated saturation
Oxygen Saturation:
Ideally measured by co-oximetry
Calculated values may be error-prone
Alveolar / arterial gradient:
Reflects O2 exchange with fixed FiO2
Impractical
Differentiates hypoventilation as cause
Arterial / alveolar ratio:
Proposed to be less variable
Same limitations as A-a gradient
20 × 5 = 100

13. ----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Calculated Data
HCO3 act 31.1 mmol / L
O2 Sat 98.3 %
pO2 (A - a) 32.2 mm Hg
Entered Data
FiO2 30.0 %
The Blood Gas Report:
pH 7.40 + 0.05
PCO2 40 + 5 mm Hg
PO2 80 - 100 mm Hg
HCO3 24 + 4 mmol/L
O2 Sat >95
Always mention and see FIO2
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Corrected 38.6
o
C
pH 7.439
pCO2 47.6 mm Hg
pO2 123.5 mm Hg
Calculated Data
HCO3 act 31.1 mmol / L
HCO3 std 30.5 mmol / L
BE 6.6 mmol / L
O2 CT 14.7 mL / dl
O2 Sat 98.3 %
t CO2 32.4 mmol / L
pO2 (A - a) 32.2 mm Hg
pO2 (a / A) 0.79
Entered Data
Temp 38.6 oC
ct Hb 10.5 g/dl
FiO2 30.0 %
The essentials

14. A ) low PAO2 ( Low Alveolar Pressure )
1) low barometric pressure,
2) low fraction of inspired oxygen (FiO2)
3) Hypercarbia – elevated (PaCO2).
B ) Wide A / a gradient ( Normal Alveolar
pressure )
1) Shunt ( cardiac or non cardiac )
2) Diffusion abnormality
Low PaO2 can be the result of

15. Technical Errors
Glass vs. plastic syringe:
Changes in pO2 are not clinically important
No effect on pH or pCO2
Heparin (1000 u / ml):
Need <0.1 ml / ml of blood
pH of heparin is 7.0; pCO2 trends down
Avoided by heparin flushing & drawing 2-4 cc
blood
Delay in measurement:
Rate of changes in pH, pCO2 and pO2 can be
reduced to 1/10 by cooling in ice slush(4
o
C)
No major drifts up to 1 hour

16. The
Steps for
Successful
Blood Gas
Analysis

17. Step 1
Look at the pH
Is the patient acidemic pH < 7.35
or alkalemic pH > 7.45
Step 2
Who is responsible for this change ( culprit )?
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
The culprit
BICARB pH
CO2 pH
BICARB pH
CO2 pH

18. Step 3
If there is a primary respiratory disturbance, is it acute?
(Acute)change in pH = 0.08 for 10 mm change in PCO2
(Chronic)change in pH = 0.03 for 10 mm change in PCO2

19. Step 4
If the disturbance is metabolic is the respiratory
compensation appropriate?
For metabolic acidosis:
Expected PCO2 = (1.5 x [HCO3]) + 8 ) + 2
(Winter’s equation)
( Last two digits of pH )
For metabolic alkalosis:
Expected PCO2 = 6 mm for 10 mEq. rise in Bicarb.
If :
actual PCO2 more than expected : additional
respiratory acidosis
actual PCO2 less than expected : additional respiratory
alkalosis
The last two
digits

20. 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 :
Methanol
Uremia
Diabetic Ketoacidosis
Paraldehyde
Infection (lactic acid)
Ethylene Glycol
Salicylate
Common pediatric causes
1) Lactic acidosis
2) Metabolic disorders
3) Renal failure

21. th step
Clinical correlation

24. HCO3 META.pH
PCO2 pH RESP.
Same direction
Opposite direction
Same direction

25. 24 CO2
BICARBONATE
= H ION CONC.
24 40 = 960
BICARBONATE
= H ION CONC.
960
24
= H ION CONC. = 40
N.MOLS / L.
H+ N.MOLS / L. = 80 - last two digits of pH
N.MOLS / L.
N.MOLS / L.

26. Primary lesion
Primary lesion
compensation
pH
HCO3
CO2
METABOLIC ACIDOSIS
HYPER VENTILATION
BICARB CHANGES
pH in same direction

27. Primary lesion
compensation
pH
HCO3
CO2
METABOLIC ALKALOSIS
HYPO VENTILATION
BICARB CHANGES
pH in same direction

28. Primary lesion
compensation
pH
CO 2
BICARB
Respiratory acidosis
CO 2 CHANGES
pH in opposite direction

29. pHCO2+H20=H2CO3 = H + HCO3
+
HCO3
HCO3
RESP. ACIDOSIS ALKALOSIS META.
ACUTE RISE : PCO2 10 : pH .08
CHRONIC RISE : PCO2 10 : pH .03
PCO2
HIGH
H
HIGH
HCO3
+

30. Primary lesion
Primary lesion
compensation
pH
CO 2
BICARB
Respiratory alkalosis
CO 2 CHANGES
pH in opposite direction

31. CO2 + H20 = H2CO3 = H + HCO3
+
pH
HCO3
LOW H IONS
…LOW HCO3
RESP. ALK. ACID. META.
ACUTE FALL : PCO2 10 : pH .08
CHRONIC FALL: PCO2 10 : pH .03
CO2
+

32. Pco2 of 10 pH
Acute change .08
Chronic change .03

33. INTERPRETATION OF A.B.G.
FOUR STEP METHOD OF DEOSAT
1) LOOK FOR pH
2) WHO IS THE CULPRIT ?
3) IF RESPIRATORY ACUTE / CHRONIC ?
4) IF METABOLIC / COMP. / ANION GAP
CLINICAL CORRELATION

34. compensation considered
complete
when the
pH returns
to
normal
range Clinical blood gases by Malley

35. COMPENSIONLIMITS
METABLIC ACIDOSIS
CO2 = Up to 10 ?
METABOLIC ALKALOSIS
CO2 = Maximum 6O
RESPIRATORY ACIDOSIS
BICARB = Maximum 40
RESPIRATORY ALKALOSIS
BICARB = Up to 10

37. ----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.523
pCO2 30.1 mm Hg
pO2 105.3 mm Hg
Calculated Data
HCO3 act 22 mmol / L
O2 Sat 98.3 %
pO2 (A - a) 8 mm Hg D
pO2 (a / A) 0.93
Entered Data
FiO2 21.0 %
Case 1
16 year old female with
sudden onset of dyspnea.
No Cough or Chest Pain
Vitals normal but RR 56,
anxious.

38. Case 2 6 year old male with progressive respiratory distress
Muscular dystrophy .
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.301
pCO2 76.2 mm Hg
pO2 45.5 mm Hg
Calculated Data
HCO3 act 35.1 mmol / L
O2 Sat 78 %
pO2 (A - a) 9.5 mm Hg D
pO2 (a / A) 0.83
Entered Data
FiO2 21 %
D CO2 =76-40=36
Expected D pH ( Acute ) = .08 for 10
Expected ( Acute ) pH = 7.40 - 0.29=7.11
Chronic resp. acidosis
pH <7.35 :acidemia
respiratory acidemia : co2 and pH
Hypoxia
Normal A-a gradient
Due to hypoventilation

39. Case 3
8-year-old male asthmatic;
3 days of cough, dyspnea
and orthopnea not
responding to usual
bronchodilators.
O/E: Respiratory distress;
suprasternal and
intercostal retraction;
tired looking; on 4 L NC.
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7. 24
pCO2 49.1 mm Hg
pO2 66.3 mm Hg
Calculated Data
HCO3 act 18.0 mmol / L
O2 Sat 92 %
pO2 (A - a) mm Hg D
pO2 (a / A)
Entered Data
FiO2 30 %
153-66= 87
pH <7.35 ; acidemia
pCO2 >45; respiratory acidemia
piO2 = 715x.3=214.5 / palvO2 = 214-49/.8=153 Wide A / a gradient
Hypoxia
WITH INCREASE IN CO2 BICARB MUST RISE ?
Metabolic acidosis + respiratory acidosis
30 × 5 = 150
D CO2 = 49 - 40 = 9
Expected D pH ( Acute ) = 9/10 x 0.08 = 0.072
Expected pH ( Acute ) = 7.40 - 0.072 = 7.328
Acute resp. acidosis

40. Case 4 8 year old diabetic with respi. distress fatigue and loss of appetite.
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.23
pCO2 23 mm Hg
pO2 110.5 mm Hg
Calculated Data
HCO3 act 14 mmol / L
O2 Sat %
pO2 (A - a) mm Hg D
pO2 (a / A)
Entered Data
FiO2 21.0 %
pH <7.35 ; acidemia
HCO3 <22; metabolic acidemia
Last two digits of pH
Correspond with co2
If Na = 130,
Cl = 90
Anion Gap = 130 - (90 + 14)
= 130 – 104 = 26

41. ----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.46
pCO2 28.1 mm Hg
pO2 55.3 mm Hg
Calculated Data
HCO3 act 19.2 mmol / L
O2 Sat %
pO2 (A - a) mm Hg D
pO2 (a / A)
Entered Data
FiO2 24.0 %
Case 5 : 10 year old child with encephalitis
pH almost within normal range
Mild alkalosis
Co2 is low , respiratory
Co2 low by around 10
( Acute ) by .08
(Chronic ) by .03
Bicarb looks low ?
Is it expected ?
More cases

42. ABG OF THE DAY
The arterial blood gas report : Room air
pH 7.39
PCO2 l5mniHg
HCO3 8mmol/L
PaO2 90 mmHg
PCO2 24
BICARBONATE
H ION CONCENTRATION =
= 45 nmol/lit

43. 1) These findings are most consistent with….
a) Metabolic acidosis with compensatory Hypocapnia.
b) Primary metabolic acidosis with
respiratory alkalosis.
c) Acute respiratory alkalosis fully compensated.
d) Chronic respiratory alkalosis fully compensated.
pH 7.39
PCO2 l5mniHg
HCO3 8mmol/L
PaO2 90 mmHg
For metabolic acidosis: FULL COMPENSATION
Expected PCO2 = (1.5 x [HCO3]) + 8 ) + 2
(Winter’s equation)
PCO 2 ……SHOULD BE 20

44. 2) What is the oxygenation status
a) Normal oxygenation status
b) Hypoxemia
c) None of the above
palvO2 = piO2 – pCO2 / RQ
= 150 – 15 / 0.8
= 150 – 18 = 132 mm Hg
132 – 90 = 42 WIDE A / a gradient
pH 7.39
PCO2 l5mniHg
HCO3 8mmol/L
PaO2 90 mmHg

45. pCO2 pH
70 7.10
60 7.20
50 7.30
40 7.40
30 7.50
20 7.60

46.  When pH is normal and:
Bicarbonate is high ( Metabolic alkalosis + respiratory
acidosis )
Bicarbonate is low ( Metabolic acidosis + resp. alkalosis)
 Bicarbonate is normal and:
anion gap is high ( Metabolic Acidosis + Metabolic alkalosis)
 When bicarbonate is normal and:
pH is in acidic range ( Chronic resp. acidosis + resp alk.)
pH is in alkalemic range ( Metab.alk. + resp alk.)
 Anion gap is elevated and:
clinical and laboratory data suggest a diagnosis other than
metabolic acidosis
 PCO2 level and bicarbonates are shifted from normal in
opposing directions.

47. THANKS