1. Arterial Blood Gas Analysis …..1
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Dr Satish Deopujari
Pediatrician
Hon. Prof. ( Pediatrics) JNMC
Chairman National
Intensive care chapter
Indian academy of pediatrics
Visit us at….

2. No click
The Goal :
To provide Bedside approach to
ABG analysis

3. Last message …….. “H = 80-last two digits of pH”
H ION CONC.
N.MOLS / L.
OH ION 14 pH
pH stand for "power of hydrogen"
H+ = 80 - last two digits of pH
H ION
Don’t click wait …..till
Last message …….. “H = 80-last two digits of pH”

4. Bicarbonate: No click Henderson - Hasselbach equation: pH = pK + Log
HCO3
Dissolved CO2
Bicarbonate:
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5. Standard Bicarbonate:
Plasma HCO3 after equilibration
to a PaCO2 of 40 mm Hg
: Reflects non-respiratory acid base change
: No quantification of the extent of the buffer base abnormality
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Base Excess:
D base to normalise HCO3 (to 24) with PaCO2 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

6. Oxygenation Indices: O2 Content of blood: (Don’t forget hemoglobin)
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Oxygenation
Indices:
O2 Content of blood:
Hb. x O2 Sat + Dissolved O2
(Don’t forget hemoglobin)
Oxygen Saturation: reported as ABG report
( Derived from oxygen dis. curve
not a measured value )
Alveolar / arterial gradient:
( Useful … to classify respiratory failure )

7. Normal arterio/venous difference
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PaO2
100 80
Rt. Shift
Oxygen delivered
to tissues
with normally placed curve 60
Delivered oxygen
with Rt. Shift curve 40
Normal 20
Shift of the curve ……changes saturation for a given PaO2

8. Alveolar-arterial Difference
Inspired O2 = 21 %
piO2 = (760-45) x = mmHg
palvO2 = piO2 – pCO2 / RQ
= 150 – 40 / 0.8
= 150 – 50 = mm Hg O2
CO2
PaO2 = 90 mmHg
palvO2 – partO2 = 10 mmHg
One click and wait

9. Alveolar- arterial Difference
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Alveolar- arterial Difference
Oxygenation Failure
WIDE GAP
piO2 = 150
pCO2 = 40
palvO2= 150 – 40/.8
=150-50
=100
PaO2 = 45
D = = 55
Ventilation Failure
NORMAL GAP
piO2 = 150
pCO2 = 80
palvO2= /.8 =
= 50
PaO2 = 45
D = = 5 O2
CO2
PAO2 (partial pres. of O2. in the alveolus.)
= ( PaCO2 / .8 )
760 – 45 = 715 : 21 % of 715 = 150

10. Expected PaO2 =
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FiO2 × 5 = PaO2
Normal situation
20 × 5 = 100

11. The essentials The Blood Gas Report: normals… pH 7.40 + 0.05
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The essentials
The Blood Gas Report: normals… pH
PaCO mm Hg
PaO mm Hg
HCO mmol/L
O2 Sat >95
Always mention and see FIO2
HCO3

12. The
No click 5
Steps for
Successful
Blood Gas
Analysis

13. Step 1 Step 2 CO2 will change pH in opposite direction
Look at the pH
Is the patient acidemic pH < 7.35
or alkalemic pH > 7.45
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

14. No click
Step 3
If there is a primary respiratory disturbance, is it acute ?
10 mm
Change
PaCO2
.08 change in pH ( Acute )
.03 change in pH ( Chronic ) =

15. 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
actual PaCO2 is less than expected : additional …respiratory alkalosis
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16. No click
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 pediatric causes
Lactic acidosis
2) Metabolic disorders
3) Renal failure

17. 5
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th step
Clinical correlation

18. Same direction HCO3 pH META. Same direction PaCO2 RESP. pH
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HCO3 pH
META.
Same direction
PaCO2
RESP. pH
Opposite direction

19. No click
Remember the format pH
PaCO2
PaO2

20. METABOLIC ACIDOSIS HYPER VENTILATION Low Alkali Three clicks pH PaCO2
BICARB CHANGES
pH in same direction
Compensation
Bicarbonate
Low
Alkali
Primary lesion
Primary lesion
METABOLIC ACIDOSIS

21. METABOLIC ALKALOSIS HYPO VENTILATION Three clicks pH PaCO2
BICARB CHANGES
pH in same direction
Compensation
Bicarbonate
High
Alkali
Primary lesion
METABOLIC ALKALOSIS

22. Respiratory acidosis Three clicks Wait for red circle pH CO 2 CHANGES
pH in opposite direction
BICARB
compensation
PaCO 2
High
CO2
Primary lesion
Respiratory acidosis

23. HCO3 HCO3 RESP. ACIDOSIS pH ACUTE RISE : PCO2 10 : pH .08
four clicks
ALKALOSIS META.
RESP. ACIDOSIS
PCO2 pH +
CO2+H20=H2CO3 = H + HCO3
HIGH H
HCO3 +
HCO3
HCO3
ACUTE RISE : PCO2 10 : pH .08
CHRONIC RISE : PCO2 10 : pH .03

24. Respiratory alkalosis
Three clicks
Wait for red circle pH
PaCO 2 CHANGES
pH in opposite direction
BICARB
compensation
PaCO 2
Primary lesion
Low
PaCO2
Primary lesion
Respiratory alkalosis

25. RESP. ALK. pH ACID. META. SERUM HCO3 + CO2 LOW H IONS …LOW HCO3
Six clicks
RESP. ALK.
ACID. META. pH +
CO2 + H20 = H2CO3 = H HCO3
CO2
SERUM
HCO3 +
LOW H IONS …LOW HCO3
Bicarbonate

26. No click pH
PaCO2 of 10
Acute change
Chronic change .03

27. INTERPRETATION OF A.B.G. FOUR STEP METHOD OF DEOSAT LOOK FOR pH
WHO IS THE CULPRIT ?
IF RESPIRATORY ACUTE / CHRONIC ?
IF METABOLIC / COMP. / ANION GAP
CLINICAL CORRELATION
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28. compensation considered complete when the pH returns to normal range
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Clinical blood gases by Malley

29. COMPENSION LIMITS METABLIC ACIDOSIS PaCO2 = Up to 10 ?
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METABLIC ACIDOSIS
PaCO2 = Up to 10 ?
METABOLIC ALKALOSIS
PaCO2 = Maximum 6O
RESPIRATORY ACIDOSIS
BICARB = Maximum 40
RESPIRATORY ALKALOSIS
BICARB = Up to 10
COMPENSION LIMITS

30. Acute respiratory alkalosis
One click for answer
Case 1
16 year old female with
sudden onset of dyspnea.
No Cough or Chest Pain
Vitals normal but RR 56,
anxious.
Blood Gas Report
Measured 37.0o C pH
PaCO mm Hg
PaO mm Hg
Calculated Data
HCO3 act 22 mmol / L
O2 Sat %
PO2 (A - a) 8 mm Hg D
PO2 (a / A) 0.93
Entered Data
FiO %
Acute respiratory alkalosis
And why acute ?

31. Case 2 6 year old male with progressive respiratory distress
Muscular dystrophy .
Five clicks
pH <7.35 :acidemia
Blood Gas Report
Measured 37.0o C pH
PaCO mm Hg
PaO mm Hg
Calculated Data
HCO3 act mmol / L
O2 Sat 78 %
PO2 (A - a) 9.5 mm Hg D
PO2 (a / A) 0.83
Entered Data
FiO2 21 %
Res. Acidemia : High PaCO2 and low pH
D CO2 =76-40=36
Expected D pH for ( Acute ) = .08 for 10
Expected ( Acute ) pH = =7.11
Chronic resp. acidosis
Hypoxemia
Normal A-a gradient
Chronic respiratory acidosis
With hypoxia due to hypoventilation
Hypoventilation

32. Acute respiratory change
Last two digits pH
80 – PaCO2
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PaCO2 pH 70
7.10 60
7.20 50
7.30 40
7.40 30
7.50 20
7.60
Acute respiratory change

33. 8-year-old male asthmatic with resp. distress
Six clicks
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.
Blood Gas Report
Measured 37.0o C pH
PaCO mm Hg
PaO mm Hg
Calculated Data
HCO3 act 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
PaCO2 >45; respiratory acidemia
D CO2 = = 9
Expected D pH ( Acute ) = 9/10 x 0.08 = 0.072
Expected pH ( Acute ) = = 7.328
Acute resp. acidosis
Hypoxia
30 × 5 = 150
WITH INCREASE IN CO2 BICARB MUST RISE ?
Bicarbonate is low………
Metabolic acidosis + respiratory acidosis
piO2 = 715x.3= / palvO2 = /.8=153 Wide A / a gradient

34. Case 4 8 year old diabetic with respi
Case 4 8 year old diabetic with respi. distress fatigue and loss of appetite.
Three clicks
Blood Gas Report
Measured 37.0o C
pH 7.23
PaCO mm Hg
PaO mm Hg
Calculated Data
HCO3 act 14 mmol / L
O2 Sat %
PO2 (A - a) mm Hg D
PO2 (a / A)
Entered Data
FiO %
pH <7.35 ; acidemia
Last two digits of pH
Correspond with co2
HCO3 <22; metabolic acidemia
If Na = 130,
Cl = 90
Anion Gap = ( )
= 130 – 104 = 26

35. Case 5 : 10 year old child with encephalitis
Four clicks
Blood Gas Report
Measured 37.0o C
pH 7.46
PaCO mm Hg
PaO mm Hg
Calculated Data
HCO3 act mmol / L
O2 Sat %
PO2 (A - a) mm Hg D
PO2 (a / A)
Entered Data
FiO %
pH almost within normal range
Mild alkalosis
PaCO2 is low , respiratory
low by around 10
( Acute ) by .08
(Chronic ) by .03
Bicarb looks low ?
Is it expected ?
BICARBINATURIA

36. Case 6………….
One click
pH PCO2 l5mmHg HCO3 8mmol/L PaO2 90 mmHg
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.
For metabolic acidosis: FULL COMPENSATION
Expected PaCO2 = (1.5 x [HCO3]) + 8 ) + 2
(Winter’s equation)
PCO 2 ……SHOULD BE 20

37. Adolescent boy with appendicitis , posted for surgery , he is a known
Case 7……….
Adolescent boy with appendicitis , posted for surgery , he is a known
case of SLE.
His pre-op ABG shows
: Room air pH pCO2 l5mmHg paO mmHg
HCO3 8mmol/L
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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.
What is the probable cause for the above findings ? Are they OK
as far as oxygenation is concerned ?

38. No click
Patient was hypo volumic , received Normal Saline bolus...
Corrected acidosis
He was operated ….but post-op became drowsy
His ABG……..
FiO2….30%
pH 7.38
PaCO2 38
PaO2 60
1) Why hypoxemia ?
2) Were the lungs bad to begin with ? ( Pre OP PaO mmHg )
3) Micro atelectesis during surgery ? Anesthetist goofed up the case
4) Pure and simple hypoventilation …..Sedation ?

39. Oxygenation status good …..?
One click
Why hypoxemia ?
Lungs were bad to begin with ?
Micro atelectesis during surgery
Pure and simple hypoventilation ? sedation
PRE OP ….ABG on room air
pH PaCO2 l5mmHg PaO2 90 mmHg
HCO3 8mmol/L
Oxygenation status good …..?
Pre OP .....A/a gradient
palvO2 = PiO2 – PaCO2 / RQ
= 150 – 15 / 0.8
= 150 – 18 = mm Hg
132 – 90= WIDE A / a gradient

40. Apparently the lungs looked good with PaO2 of 90…….
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Apparently the lungs looked good with PaO2 of 90…….
But have a good look at the ABG again
With wash out of CO 2 ……….
The expected PaO2 should have been more than 90 .
This coupled with correction of acidosis
( normalizing PaCO2 )
Lowered the PaO2 …post operatively.
Conclusion ……..
Lungs were not normal to begin with ( SLE )……..

41. Learning point Correlate PaO2 with FiO2
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Learning point
Correlate PaO2 with FiO2
But please also correlate with PaCO2

42. pH 7.583 PCO2 19.8 HCO3 18.7 What is the Diagnosis Respiratory
Case 8,,,,,,,,,,,,,,,,,,
What is the
Diagnosis
Click for answer pH
PCO
HCO
Respiratory
Alkalosis
Is it acute ?

43. pH
Bicarb.
CO2
THANKS