The document presents a comprehensive guide on arterial blood gas (ABG) interpretation by Dr. Aman Jain, covering the preanalytical phase, ABG procedure, and detailed steps for evaluating acid-base balance. It outlines key definitions, compensation mechanisms, and provides a systematic approach to assess various acid-base disorders using specific equations. Additionally, the document includes numerous clinical case scenarios illustrating the principles of ABG interpretation.

1. Arterial Blood Gas
Interpretation
Presentor - Dr. Aman Jain
Moderator - Dr. Shambhavi
No of slides - 42
1

2. What is the preanalytical phase?
• It is the phase which includes all activities that occur prior to the sample’s insertion
into the analytical instrument.
• Sub phase of the preanalytical process in blood gas testing includes:
 Ordering,
 Identification,
 Collection,
 Transportation,
 Separation/preparation
• It has been recognized for years that the phase of testing most prone to errors is the
preanalytical phase.
• Pre-preanalytical Error includes :
• Inappropriate test ordered, order entry error, patient/specimen
misidentification, sample contamination (from indwelling catheter),
sample collection mishap (hemolysis, clotting, insufficient volume),
inappropriate collection container, improper
storage/handling/transport.
2

3. 3

4. ABG – Procedure and Precautions
 Site- (Ideally) RadialArtery
BrachialArtery
FemoralArtery
 Ideally - Pre-heparinised ABG syringes
- Syringe should be FLUSHED with 0.5ml
of 1:1000 Heparin solution and emptied.
DO NOT LEAVE EXCESSIVE HEPARIN IN THE
SYRINGE
HEPARIN DILUTIONAL
EFFECT
HCO3
PCO2
Only small 0.5ml Heparin for flushing and discard it
Syringes must have > 50% blood. Use only 2ml or less
syringe.
4

5. Interpretation ofABG
 OXYGENATION
 ACID BASE
5

6. Determination of PaO2
2
PaO is dependant upon Age, FiO ,P
2 atm
As Age the expected PaO2
• PaO2 = 109 - 0.4 (Age)
As FiO2 the expected PaO2
• Alveolar Gas Equation:
• PAO2= (PB-P h2o) x FiO2- pCO2/R
O
X
Y
G
E
N
A
T
I
O
N
A 2 B
P O = partial pressure of oxygen in alveolargas, P = barometric pressure
(760mmHg), Ph2o = water vapor pressure (47 mm Hg), FiO2 = fraction of
inspired oxygen, PCO2 = partial pressure of CO2 in the ABG, R = respiratory
quotient (0.8)
6

7. Assessment of ACID BASE Balance
Definitions and Terminology
ACIDOSIS – presence of a process which tends to
pH by virtue of gain of H +
-
or loss of HCO3
ALKALOSIS – presence of a process which tends
3
to pH by virtue of loss of H+ or gain of HCO -
If these changes, change pH, suffix ‘emia’ is added
 ACIDEMIA – reduction in arterial pH (pH<7.35)
 ALKALEMIA – increase in arterial pH (pH>7.45)
7

8. Simple Acid Base Disorder/ Primary Acid Base
disorder – a single primary process of acidosis or
alkalosis due to an initial change in PCO2 and HCO3.
Compensation - The normal response of
the respiratory system or kidneys to change in pH
induced by a primary acid-base disorder
 The Compensatory responses to a primary Acid Base
disturbance are never enough to correct the change in
pH , they only act to reduce the severity.
Mixed Acid Base Disorder – Presence of more than
one acid base disorder simultaneously .
8

9. STEP WISEAPPROACH
to
Interpretation Of
ABG reports
Six steps logical approach originally proposed by Narins and
Emmett (1980) and modified by Morganroth in 1991
9

10. Normal Values
ANALYTE Normal Value Units
pH 7.35 - 7.45
PCO2 35 - 45 mm Hg
PO2 72 – 104 mm Hg`
[HCO3] 22 – 30 meq/L
SaO2 95-100 %
Anion Gap 12 + 4 meq/L
∆HCO3 +2 to -2 meq/L
10

11. STEP0
STEP1
STEP2
• Is this ABGAuthentic?
• ACIDEMIA or ALKALEMIA?
• RESPIRA
TORY or METABOLIC?
STEP3 • Is COMPENSATION adequate?
STEP4 • If METABOLIC ACIDOSIS – ANION GAP?
STEP5
• If High gap MetabolicAcidosis– GAP GAP?
11

12. Is this ABG authentic ?
pH = - log [H+]
Henderson-Hasselbalch equation
pH = 6.1 + log HCO3
-
0.03 x PCO2
The [HCO3-] mentioned on the ABG is actually calculated
using this equation from measured values of PCO2 ndpH
• [H+] neq/l = 24 X (PCO2 / HCO3)
• pH = -log [ H+]
pHexpected = pHmeasured = ABG is authentic
12

13. Reference table for pH v/s [H+]
[H+] neq/l = 24 X (PCO2 /HCO3)
H+ ion pH
100 7.00
79 7.10
63 7.20
50 7.30
45 7.35
40 7.40
35 7.45
32 7.50
25 7.60
13

14.  Look at pH
<7.35 - acidemia
>7.45 – alkalemia
 RULE – An acid base abnormality is present even if
either the pH or PCO2 are Normal.
ACIDEMIA ORALKALEMIA?
STEP 1
14

15. IS PRIMARY DISTURBANCE RESPIRATORY OR
METABOLIC?
pH PCO2 or pH PCO2 METABOLIC
pH PCO2 or pH PCO2 RESPIRA
TORY
RULE- If either the pH or PCO2 is Normal, there is a
mixed metabolic and respiratory acid base disorder.
RESPIRATORY or METABOLIC?
STEP2
15

16. 16

17. 17
STEP 3 - COMPENSATION

18. ADEQUATE COMPENSATION?
IS THE COMPENSATORY RESPONSE ADEQUATE OR NOT?
METABOLIC DISORDER
PCO2measured ≠ PCO2expected
PCO2expected
MIXED DISORDER
RESPIRATORYDISORDER pHexpected acute-chronic
pHm ≠ pHe range MIXED DISORDER
18

19. STEP0
STEP1
STEP2
• Is this ABGAuthentic?
• ACIDEMIA or ALKALEMIA?
• RESPIRA
TORY or METABOLIC?
STEP3
STEP4
• If Respiratory – ACUTE or CHRONIC?
• Is COMPENSATION adequate?
STEP4
• If METABOLIC ACIDOSIS – ANION GAP?
STEP6
• If High gap MetabolicAcidosis–
GAP GAP?
19

20. Electrochemical Balance in Blood
80%
70%
60%
50%
40%
30%
20%
10%
0%
100%
90%
CATIONS ANIONS
Sulfate
Phosphate
Mg- OA
K - Proteins
Ca-HCO3
Na- Cl
UA
UC
Na
Cl
HCO3
20

21. Anion Gap
AG based on principle of electroneutrality:
 Total Serum Cations = Total Serum Anions
 M cations + U cations = M anions + U anions
 Na + (K + Ca + Mg) = HCO3 + Cl + (PO4 + SO4
+ Protein + Or
 Na + UC
 But in Blood there is a
= HCO3 + Cl + UA
relative abundance of Anions,
ganicAcids)
hence
Anions > Cations
 Na – (HCO3 + Cl) = UA – UC
 Na – (HCO3 + Cl) = Anion Gap
21

22. METABOLIC ACIDOSIS-
ANION GAP?
STEP4
IN METABOLIC ACIDOSIS WHAT IS THE ANION GAP?
ANION GAP(AG) = Na – (HCO3 + Cl)
Normal Value = 12 + 4 ( 8- 16 Meq/l)
Adjusted Anion Gap = Observed AG +2.5(4.5- S.Albumin)
50% in S.Albumin 75% in Anion Gap !!!
High Anion Gap MetabolicAcidosis
MetabolicAcidosis
Normal Anion GapAcidosis 22

23. High Anion Gap Metabolic
Acidosis
M
U
D
P
I
E
METHANOL
UREMIA - ARF/CRF
DIABETIC KETOACIDOSIS & other KETOSIS
PARALDEHYDE, PROPYLENE GLYCOL
ISONIAZIDE, IRON
L
LACTICACIDOSIS
ETHANOL, ETHYLENE GLYCOL
S
SALICYLATE
23

24. CO EXISTANT METABOLIC
DISORDER – “Gap Gap”?
STEP5
C/O HGAG METABOLIC ACIDOSIS,ANOTHER DISORDER?
 ∆ Anion Gap = Measured AG – NormalAG
Measured AG – 12
∆ HCO3 = Normal HCO3 – MeasuredHCO3
24 – Measured HCO3
Ideally, ∆Anion Gap = ∆HCO3
For each 1 meq/L increase in AG, HCO3 will fall by 1 meq/L
∆AG/ HCO3
- = 1  Pure High AG MetAcidosis
AG/ HCO3
- > 1  Assoc Metabolic Alkalosis
AG/ HCO3
- < 1  Assoc N AG MetAcidosis
24

25. Clinical
CASE
SCENARIOS
25

26. CASE-1
pH-7.28,PaCO2-24, HCO3-12,Na-128, K-3.8,
Step 1: Acidemia
Step 2: Metabolic
Step 3: Appropriate compensation- 26
PCO2= 1.5 (HCO3)+8(winters formula)
Step 4: Calculate AG= Na – (Cl+ HCO3)
128- (98+ 12) = 18
Elevated Anion Gap
If S alb= 2.0
c AG = 18 + 2.5(4-2) = 23
26

27. CASE-2
pH-7.17,PaCO2-65,HCO3-22
Na-136,Cl-98,
Sr.Alb-1.6
Step 1: Acidemia
Step 2: Respirtory
Step 3: Evaluate compensation – no, its decreased- metabolic
acidosis
Step 4: Calculate AG= Na – (Cl+ HCO3)
136- (98+ 22) = 16
If S alb= 1.6
Step 5: c AG = 16 + 2.5(4-1.6) = 22 (elevated gap Met Acidosis
Step 6: Calculate Delta ratio : 22- 12/ 24- 22= 10/ 2= 5
DELTA RATIO > 2, IRRESPECTIVE OF ETIOLOGY- OF
INCREASED AG, IT POINTS TO METABOLIC
ALKALOSIS
48 yr, alcoholic , found unconscious
,soiled with vomitus, last seen at party
6 hrs back
27

28. 17 year old w/severe kyphoscoliosis, admitted
for pneumonia
pH 7. 42
paCO2 29
paO2 72
HCO3 17
Na 140
K 4.0
Cl 109
Step 1: Check pH: Normal (towards alkalosis)
Step 2: Check paCO2 : Alkalotic (respiratory alkalosis)
Step 3: Compensation: (11mmHg CO2 ~ 5 meq HCO3)
Step 4: Calculate Anion Gap AG=140 –(109+17)=14
Step 5: Delta gap= 14-12/24-17= 2/ 5= 0.4
Signifies underlying normal anion gap metabolic acidosis
28

29. 60 year male, smoker, COPD with respiratory
distress
pH 7.26
paCO2 60
paO2 55
HCO3 26
Na 140
K 4.0
Cl 104
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : High (Respiratory)
Step 3: Compensation (2 HCO3 for 20 mmHg CO2)
Step 4: AG=140-(104+26)= 10
Respiratory Acidosis
29

30. 45 yr old male, k/c/o COPD since 10 yrs
pH 7.34
paCO2 60
paO2 55
HCO3 32
Na 140
K 4.0
Cl 100
Step 1: Check pH: Acidemia
Step 2: Check paCO2 : Raised (respiratory )
Step 3: Evaluate compensation : Compensation (8 HCO3 for
20 mmHg CO2) - adequate
AG=140 –(100+30)=10
Respiratory Acidosis (Chronic) compensated
30

31. 75 yr woman, fever & profuse diarrhoea x 2ds,
T 38.5°C, HR 130/min, BP 78/30 mmHg
pH 7.29
paCO2 30
paO2 80
HCO3 14
Na 128
K 3.2
Cl 94
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : decreased ( Primary Metabolic)
Step 3: Check compensation
Expected CO2=1.5xHCO3+8=21+8=29 (appropriate)
Step 4: AG=128 –(94+14)=20, Delta Anion Gap=20-12=8
Step 5: Delta Ratio=▲AG/ ▲HCO3=8/10=0.8
High Anion Gap Metabolic Acidosis, Additional NAG
Metabolic Acidosis 31

32. 40 years,male, CRF, on maintenance haemodialys,
Missed his last appointment .
pH 7.25
paCO2 27
paO2 99
HCO3 14
Na 140
K 4.0
Cl 96
S Albumin 2.5 gm%
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : Decreased ( Primary metabolic)
Step 3: Evaluate compensation
Expected CO2=1.5xHCO3+8= 21+8=27 (appropriate)
Step 4: AG=140 –(96+12)=32, Adjusted AG=36
Step 5: Calculate Delta Ratio=▲AG/ ▲HCO3
Delta Anion Gap=36-12=24 ; Delta ratio=24/10= 2.4
High Anion Gap Metabolic Acidosis, Additional Metabolic Alkalosis 32

33. 55-year-old male admitted with a recurring bowel
Obstruction& intractable vomiting forthe last several hours
pH 7.50
paCO2 48
paO2 90
HCO3 38
Na 135
K 3.6
Cl 85
Step 1: Check pH: Alkalotic
Step 2: Check paCO2: increased ( Primary Metabolic)
Step 3: Evaluate compensation: 40= 0.7* (HCO3- 24) = 50
Appropriate so no additional resp disorder
Step 4: AG=135 –(85+38)=12, Delta Anion Gap=12-12= 0
Metabolic Alkalosis
33

34. 47yearoldpostop,presentingwith fever,hypotensionandvomiting
pH 7.15
paCO2 40
paO2 55
HCO3 15
Na 140
K 4.0
Cl 92
Step 1: Check pH: Acidotic
Step 2: Check paCO2 Normal, Check HCO3- : Acidotic (Metabolic Acidosis)
Step 3: Evaluate compensation
Expected CO2 =1.5xHCO3+8=30.8, so additional respiratory acidosis
Step 4: AG=140 –(92+15)=33, Delta Anion Gap=33-12= 21
Step 5: Delta Ratio=▲AG/ ▲HCO3=21/9= 2.3
High AG Metabolic Acidosis with respiratory acidosis Additional
metabolic alkalosis (Triple metabolic disorder)
34

35. 12 yearold diabetic presents with Kussmaul breathing
pH 7.05
paCO2 15
paO2 68
HCO3 5
Na 140
K 4.0
Cl 98
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : Decreased (Metabolic Acidosis)
Step 3: Evaluate compensation:
Expected CO2 =1.5xHCO3+8=15, so no additional respiratory disorder
Step 4: AG=140 –(98+12)=30, Delta Anion Gap=30-12= 18
Step 5 : Delta Ratio=▲AG/ ▲HCO3=18/19=1
High AG Metabolic Acidosis,
35

36. Pt of severe pneumonia with pulmonary oedema
pH 7.30
paCO2 38
HCO3 18
Na 140
K 4
Cl 102
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : N : Check HCO3- : Acidotic
Step 3: Evaluate compenstaion
Metabolic Acidosis
Expected CO2 =1.5x(HCO3)+8 =35 (slight resp acidosis)
Step 4: AG=140 –(102+18)=20, Delta Anion Gap=20-12= 8
Step 5: Delta Ratio=▲AG/ ▲HCO3=8/6=1.2
High AG Metabolic Acidosis
36

37. 48yr male,Alcoholic, found unconscious, after a party
pH 7.17
paCO2 65
HCO3 22
Na 136
K 3.4
Cl 98 S Alb=1.6 gm%
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : Acidotic (Respiratory Acidosis)
Step 3: Evaluate Compensation
Resp acidosis: Compensation ?? NO
AG=136-(98+22)=16, Adjusted AG=22, Delta Anion Gap=22-12=10
Delta Ratio=▲AG/ ▲HCO3=10/2=5
High Anion Gap Metabolic acidosis with Metabolic Alkalosis
Expected CO2 =1.5x(HCO3)+8 =42
Respiratory Acidosis High AG Metab acidosis , with M Alkalosis
37

38. Mixed disorders with normal pH
38

39. 55 yrmale,alcoholic,disoriented, c/o recurrent vomiting,
Hematemesis
pH 7.45
paCO2 34
HCO3 23
Na 135
K 4.0
Cl 85
Step 1: Check pH: Normal
Step 2: Check paCO2 : slightly alkalotic
Step 3: evaluate compensation ---- Bicarb……almost normal
ABG Normal ???
Expected CO2 =1.5x(HCO3)+8 =42
Step 4: AG=135-(85+23)=27, Delta Anion Gap=27-12=15
Step 5: Delta Ratio=▲AG/ ▲HCO3=15/1=15
High Anion Gap Metabolic acidosis with Metabolic Alkalosis
High AG Metab acidosis with M Alkalosis with Respiratory
Alkalosis 39

40. 61yr, morbidly obese, COPD, CHF
pH 7.41
paCO2 55
paO2 75
HCO3 34
Na 140
K 4.0
Cl 95
Step 1: Check pH: Normal
Step 2: Check paCO2 : Acidotic ??
Step 3: Evaluate compensation ?? COPD, chronic
Compensation : chronic Respiratory acidosis…6 in this case
HCO3= 34 compensation not appropriate …Metabolic Alkalosis
Step 4: AG=140-(95+34)=11, Delta Anion Gap=11-12=-1
Metabolic alkalosis with Primary resp acidosis (chronic)
40

41. 32 yr pregnant, 4days of non stop vomiting, afebrile,HR
145/min, BP78/62
pH 7.41
paCO2 42
HCO3 26
Na 146
K 3.2
Cl 92
Step 1: Check pH: Normal
Step 2: Check paCO2 : Normal, Check HCO3- : Normal
Step 3: Evaluate Compensation ABG Normal ????
Step 4: AG=146-(92+26)=28, Delta Anion Gap=28-12=16
Step 5: Delta Ratio=▲AG/ ▲HCO3=16/-2=-8 (negative delta ratio
with N pH)
41

42. References
1) Harrison’s Principles of Internal Medicine, 17th edition,
Chap 48 – Acidosis andAlkalosis
2) Paul L.Marino – The ICU Book, 3rd Edition
3) Guyton and Hall – Textbook of Medical Physiology, 11th
edition
4) Davenport – The ABC of Acid Base Chemistry, 6thedition
5) Cohen and Kassirer – Acid Base
6) Hansen JE, Clinics in Chest medicine10(2), 1989, 227-37
7) Lippincott’s-Fluid Balance, NM Metheny
8) World Wide Web
42