Introduction to arterial blood gas interpretation

1. Parameter Normal
Range
Unit Definition
pH H+
concentration
= log 1 .
[H+
]
7.35 –
7.45
Negative log of hydrogen ion activity
If [H+
] = 10-7.4
then pH = 7.4
PaO2
Partial pressure
of O2
80 – 100 mm
Hg
Partial pressure of oxygen in arterial
blood
PaCO2
Partial pressure
of CO2
35 - 45 mm
Hg
Partial pressure of CO2
in arterial blood
HCO3
-
Bicarbonate
Level
21 – 27 mEq/L Bicarbonate plasma concentration
(calculated and not measured)
SaO2
Oxygen
saturation
95 - 100 % Percent of oxygen content to maximum
oxygen carrying capacity of blood
FiO2
Fraction of
inspired oxygen
21 % Fraction of oxygen in relation to
inspired air
PaO2 ↓ with age (= 100 – age in years above 40)
FiO2 at time of ABG sampling should be included in the report.
Arterial Blood Gas AnalysisArterial Blood Gas Analysis
(ABG)(ABG)

2. Step Look at Think: Is there…
1 PaO2, PaCO2 Respiratory Failure
2 PaO2/FiO2
Acute Respiratory
Distress Syndrome
(ARDS)
3-5
pH, PaCO2,
Bicarb
Acid/Base
Disturbance
6
If severe metabolic acidosis,
Calculate IV Na Bicarb required

3. Respiratory failure is a syndrome of inadequate gas exchange due to
dysfunction of one or more essential components of the respiratory
system.
(Airways, Lungs, Respiratory Muscles, Nerve and Blood Supply)
Step 1)Step 1) Look at PaOLook at PaO22 & PaCO& PaCO22
Is there Respiratory Failure?Is there Respiratory Failure?
Type Name Definition
Mechanisms,
Examples
1 Hypoxic RF PaO2 < 60 mm Hg
Shunt, DD, V/Q
Mismatch: ARDS,
IPF, Pneumonia, PE
2 Hypercapnic RF PaCO2 > 46 mm Hg
Hypoventilation:
Central and sleep related
hypoventilation
3
Hypoxic/
Hypercapnoeic RF
PaO2 < 60 mm Hg,
PaCO2 > 46 mm Hg
Advanced or
combined
disorders

4. ARDS Severity PaO2/FiO2 Mortality
Mild < 300 27%
Moderate < 200 32%
Severe < 100 45%
Step 2)Step 2) Calculate PaOCalculate PaO22/FiO/FiO22
Is there ARDS?Is there ARDS?
Acute Respiratory Distress Syndrome is an acute condition characterized
by bilateral pulmonary infiltrates and severe hypoxaemia in absence of
evidence for cardiogenic pulmonary oedema (Non-Cardiogenic Pulmonary
Oedema).
PaO2/FiO2 (Carrico Index) is a quick and simple measure for integrity of
lung tissue and its capacity to oxygenate the blood.
Normal > 300 – 500 mmHg (at sea level)
PaO2/FiO2 can also indicate the degree of lung tissue injury in other
pulmonary disorders, eg Pneumonia

5. Examples for Calculation of PaO2/FiO2
PaO2 FiO2 PaO2/FiO2 Comment
95 0.21 452 Normal
80 0.5 160 Moderate ARDS

6. Two cases of ARDS with bilateral patchy opacities in
middle and lower lung zones.
Common Causes: severe infection, aspiration, irritant
gases, extensive trauma, multi-organ failure.

7. 7.33
7.34
7.35
7.36
7.37
7.38
7.39
7.40
7.41
7.42
7.43
7.44
7.45
7.46
7.47
7.48
7.49
7.5
7.51
7.52
7.53
7.54
7.55
7.28
7.29
7.30
7.31
7.32
7.24
7.25
7.26
7.27
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
5
51
52
53
54
55
28
29
30
31
32
24
25
26
27
24
25
26
27
20
21
22
23
16
17
18
19
14
15
32
33
28
29
30
31
pH pCO2Bicarb
Steps 3-5)Steps 3-5) Look atLook at
pH, PaOpH, PaO22, PaCO, PaCO22
Is there Acid/BaseIs there Acid/Base
Disturbance?Disturbance?

8. 7.33
7.34
7.35
7.36
7.37
7.38
7.39
7.40
7.41
7.42
7.43
7.44
7.45
7.46
7.47
7.48
7.49
7.5
7.51
7.52
7.53
7.54
7.55
7.28
7.29
7.30
7.31
7.32
7.24
7.25
7.26
7.27
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
5
51
52
53
54
55
28
29
30
31
32
24
25
26
27
24
25
26
27
20
21
22
23
16
17
18
19
14
15
32
33
28
29
30
31
pH PCO2Bicarb
Severe Diarrhoea
1ry Defect Compensation
↓Bicarb
(M Ac)
↓PCO2
(R Alk)
M Ac & Comp R Alk
M Ac & R Ac
M Ac & R Alk

9. 7.33
7.34
7.35
7.36
7.37
7.38
7.39
7.40
7.41
7.42
7.43
7.44
7.45
7.46
7.47
7.48
7.49
7.5
7.51
7.52
7.53
7.54
7.55
7.28
7.29
7.30
7.31
7.32
7.24
7.25
7.26
7.27
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
5
51
52
53
54
55
28
29
30
31
32
24
25
26
27
24
25
26
27
20
21
22
23
16
17
18
19
14
15
32
33
28
29
30
31
pH PCO2Bicarb
Persistent Vomiting
1ry Defect Compensation
↑ Bicarb
(M Alk)
↑ PCO2
(R Ac)
M Alk & Comp R Ac
M Alk & R Ac
M Alk & R Alk

10. 7.33
7.34
7.35
7.36
7.37
7.38
7.39
7.40
7.41
7.42
7.43
7.44
7.45
7.46
7.47
7.48
7.49
7.5
7.51
7.52
7.53
7.54
7.55
7.28
7.29
7.30
7.31
7.32
7.24
7.25
7.26
7.27
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
5
51
52
53
54
55
28
29
30
31
32
24
25
26
27
24
25
26
27
20
21
22
23
16
17
18
19
14
15
32
33
28
29
30
31
pH PCO2Bicarb
Hypoventilation
1ry Defect Compensation
↑ PCO2
(R Ac)
↑ Bicarb
(M Alk)
R Ac & Comp M Alk
R Ac & M Alk
R Ac & M Ac

11. 7.33
7.34
7.35
7.36
7.37
7.38
7.39
7.40
7.41
7.42
7.43
7.44
7.45
7.46
7.47
7.48
7.49
7.5
7.51
7.52
7.53
7.54
7.55
7.28
7.29
7.30
7.31
7.32
7.24
7.25
7.26
7.27
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
5
51
52
53
54
55
28
29
30
31
32
24
25
26
27
24
25
26
27
20
21
22
23
16
17
18
19
14
15
32
33
28
29
30
31
pH PCO2Bicarb
Hyperventilation
1ry Defect Compensation
↓ PCO2
(R Alk)
↓ Bicarb
(M Ac)
R Alk & Comp M Ac
R Alk & M Alk
R Alk & M Ac

12. Prediction of Compensatory Response
Disorder Predicted Compensation
Metabolic Acidosis pCO2
= (1.5 X Bicarb) + 8 + 2
Metabolic Alkalosis pCO2
= (0.7 X Bicarb) + 20 + 5
Acute Respiratory Acidosis ↑ Bicarb = 0.1 ↑ pCO2
Chronic Respiratory Acidosis ↑ Bicarb = 0.4 ↑ pCO2
Acute Respiratory Alkalosis ↓ Bicarb = 0.2 ↓ pCO2
Chronic Respiratory Alkalosis ↓ Bicarb = 0.5 ↓ pCO2
↑ or ↓ Bicarb in relation to 24 mEq/L
↑ or ↓ PCO2 in relation to 40 mm Hg
Another useful tool in estimating the PCO2 in metabolic acidosis is the recognition that
pCO2 is approximately equal to the last 2 digits of the pH.

13. Na Bicarb Required =
0.5 X Wt (Kg) X desired increase in serum bicarbonate (mEq/L)
Apply only if severe metabolic acidosis (pH < 7.2)
Rather than the normal bicarb level of 24 mEq/L, target a lower level,
eg. 12 mEq/L
Step 6)Step 6) If severe metabolic acidosis,If severe metabolic acidosis,
calculate Na Bicarbonate Requiredcalculate Na Bicarbonate Required
Risks of Bicarbonate Therapy in Metabolic Acidosis:
• Paradoxical transient intracellular acidosis.
• Shift of O2- Hb dissociation curve to the left.
• Hypokalaemia.
• Hypernatraemia and Hypervolaemia

14. Paradoxical Transient Intracellular Acidosis
Whereas the arterial pH tends to ↑ rapidly after
administration of Na bicarb, intracellular pH ↑ more slowly:
• Slow passage of bicarb ion across the cell membrane.
• Rapid conversion of plasma bicarb to carbonic acid which
dissociates producing CO2 and water. CO2 diffuses into
cells more rapidly than bicarb
→ ↓ intracellular bicarb/CO2 ratio → ↓ intracellular pH.
The intracellular acidosis will persist as long as bicarb
administration exceeds CO2 elimination.
Therefore, adequate tissue perfusion and ventilation should
be secured in line with Na bicarb administration.

15. Shift of O2-Hb Dissociation Curve to the Left
(↑ Hb Affinity for Oxygen)
→ ↓ Oxygen delivery to tissues → ↑ tissue hypoxia
→ ↑ tendency for lactic acidosis

16. Hypokalaemia
Acidosis stimulates movement of K+
from intracellular to
extracellular compartment. Correction of acidosis by alkali
has the reverse effect: movement of K+
from extracellular
to intracellular compartment:
Roughly, for each 0.1 rise in pH,
serum K+
↓ by about 0.5 mEq/L
Hypernatraemia and Hypervolaemia
These are problematic in patients with cardiac
and/or renal dysfunction

17. 1 2 3 4 5 6
pH 7.34 7.5 7.6 7.3 7.43 7.14
PaO2
58 75 80 74 55 94
PaCO2
50 41 30 32 39 28
HCO3
- 30 32 24 19 25 8
FiO2
0.5 0.3 0.21 0.21 0.21 0.21
Disorder Predicted Compensation
Metabolic Acidosis pCO2
= (1.5 X Bicarb) + 8 + 2
Metabolic Alkalosis pCO2
= (0.7 X Bicarb) + 20 + 5
Acute Respiratory Acidosis ↑ Bicarb = 0.1 ↑ pCO2
Chronic Respiratory Acidosis ↑ Bicarb = 0.4 ↑ pCO2
Acute Respiratory Alkalosis ↓ Bicarb = 0.2 ↓ pCO2
Chronic Respiratory Alkalosis ↓ Bicarb = 0.5 ↓ pCO