1. Arterial Blood Gas Analysis
Jane Roe
Practice Educator
Wednesday 17th January 2018

2. ABG
An arterial blood gas (ABG) analysis provides information about the patient’s respiratory & metabolic function, including acid-base status.
Can gain other useful information such as electrolyte levels, Hb, lactate.

3. Indications Respiratory compromise Post cardiopulmonary arrest
Metabolic conditions e.g. DKA
Sudden/unexplained deterioration
Evaluation of interventions
Changes/titration in invasive/non-invasive ventilation settings
Major trauma
Prior to major surgery

4. Parameters
pH : overall acid-base balance of the blood sample. It is affected by both respiratory and metabolic function.
PaO2 :- 10 – 13.3kPa measurement of partial pressure of O2 dissolved in the blood sample.
PaCO2 :- 4.5 – 6.0kPa measurement of partial pressure of dissolved CO2 in the blood. In order to be carried to the lungs to be exhaled CO2 is transported in the plasma solution as carbonic acid.

5. Parameters Continued.....
Bicarbonate (HCO3-) :- 22 – 26mmol/l Most important buffer in the body. Buffers in the body act as chemical sponges which absorb excess alkali or acid.
Base Excess (BE) :- -2 mmol to +2 mmol It is the quantity of acid or base required to restore the blood to a pH of 7.4 A negative value indicates excess of acid & a positive value indicates an excess of base.
SaO2 :- 92 – 99% Arterial O2 saturation is the % of O2 that has combined with the Hb molecule.

6. Acid-base definitions
Acid – Substance capable of providing Hydrogen ions.
Base – (Alkali) substance capable of accepting Hydrogen ions.
pH - Potential hydrogen. A Measure of how acid or alkaline (base) a substance is.
Acid-base is controlled through respiratory and metabolic functions (Renal, chemical buffers).

7. Systematic Analysis of ABG Results
Assess the oxygenation (PaO2).
Is the patient hypoxic?
What supplementary O2 are they receiving?
What is their Hb?
What is their SaO2?
Determine the pH level
Is there an acidosis (pH <7.35) or alkalosis (pH >7.45) present?

8. Systematic Analysis of ABG Results
Continued....
3) Determine the respiratory component (PaCO2)
*PaCO2 > 6kPa; Acid *PaCO2 < 4.5; Alkaline
4) Determine the metabolic component (HCO3-)
*HCO3- > 26mmol/l; Alkaline *HCO3- < 22mmol/l; Acid
5) Determine for each of these parameters whether the values are acid, normal or alkaline.

9. Systematic Analysis of ABG Results Continued....
6) Combine the findings from steps 2,3,4 & 5 and determine what the primary disturbance is.
7) Determine whether there is any compensation for abnormal pH.
Complete?
Partial?
Uncompensated?

10. Acid Normal Alkaline pH <7.35 7.35 - 7.45 >7.45 PaCO2 >6kPa
0 Acidic Alkaline 14
Acid
Normal
Alkaline pH
<7.35
>7.45
PaCO2
>6kPa
kPa
<4.5kPa
HCO3-
<22 mmol/l
mmol/l
>26mmol/l

11. Classification Of Imbalance -1
Respiratory Acidosis
pH and PaCO2 are in the acid column and HCO3- is normal.
Caused by inadequate ventilation leading to retention of CO2 ie COPD, pneumonia, pulmonary oedema, mechanical chest injury, over sedation, neurological disorder.
Acid
Normal
Alkaline pH
7.24
PaCO2
8.0kPa
HCO3-
24mmol/l

12. Classification Of Imbalance -2
Metabolic Acidosis
pH and HCO3- are in the acid column and PaCO2 is normal.
Caused by excess acid production ie lactate or loss of HCO3-.
Causes include diarrhoea, cardiac arrest, DKA, renal failure etc.
Acid
Normal
Alkaline pH
7.20
PaCO2
4.7kPa
HCO3-
16mmol/l

13. Classification Of Imbalance -3
Respiratory Alkalosis
pH and PaCO2 are in the alkaline column and HCO3- is normal.
Caused by excessive ventilation leading to over excretion of CO2 ie hysteria, excessive mechanical ventilation.
Acid
Normal
Alkaline pH
7.50
PaCO2
2.5
HCO3-
22mmol/l

14. Classification Of Imbalance -4
Metabolic Alkalosis
pH and HCO3- are in the alkaline column and PaCO2 is normal.
Caused by loss of acid or ingestion of bases ie severe vomiting, overdose of antacids, diuretics etc
Acid
Normal
Alkaline pH
7.67
PaCO2
4.2kPa
HCO3-
38mmol/l

15. Classification of Imbalance - 5
Respiratory Acidosis with Metabolic Compensation
pH is normal, PaCO2 is acid, HCO3- is alkaline.
Compensation for acidosis is achieved by the other system.
Here, the kidneys are compensating for respiratory
derangement.
Acid
Normal
Alkaline pH
7.37
PaCO2
7.9kPa
HCO3- 32

16. Classification of Imbalance - 6
Metabolic Acidosis with Respiratory Compensation
pH is normal, HCO3- is in the acid column and PaCO2 is in the alkaline column. The patient will have a high respiratory rate to achieve this PaCO2 and compensate for metabolic acidosis.
Acid
Normal
Alkaline pH
7.35
PaCO2
2.7
HCO3-
12mmol/l

17. Classification of Imbalance - 7
Metabolic Alkalosis with Partial Compensation
pH is alkaline, HCO3- is in the alkaline column and PaCO2 is in the acid column. The parameter not associated with the pH is abnormal and on the opposite side of the scale indicating partial compensation.
Acid
Normal
Alkaline pH
7.48
PaCO2
6.6
HCO3- 29

18. Arterial Blood Gas Examples -1
Acid
Normal
Alkaline
PaO2 11kPa
pH 7.30
PaCO2 6.9kPa
HCO3- 24mmol/l

19. Arterial Blood Gas Examples -2
Acid
Normal
Alkaline
PaO kPa
pH 7.49
PaCO2 4.6kPa
HCO3- 30mmol/l

20. Arterial Blood Gas Examples -3
Acid
Normal
Alkaline
PaO kPa
pH 7.32
PaCO2 5.2kPa
HCO3- 18mmol/l

21. Arterial Blood Gas Examples -4
Acid
Normal
Alkaline
PaO2 14kPa
pH 7.5
PaCO2 3.3kPa
HCO3- 26mmol/l

22. Arterial Blood Gas Examples -5
Acid
Normal
Alkaline
PaO kPa
pH 7.5
PaCO2 3.3kPa
HCO3- 26mmol/l

23. Arterial Blood Gas Example -6
Acid
Normal
Alkaline
PaO kPa
pH 7.39
PaCO2 5.3kPa
HCO mmol/l

24. Arterial Blood Gas Example -7
Acid
Normal
Alkaline
PaO kPa
pH 7.38
PaCO kPa
HCO mmol/l

25. Arterial Blood Gas Example - 8
Acid
Normal
Alkaline
PaO2 8.3
pH 7.37
PaCO2 3.9kPa
HCO mmol/l

26. Arterial Blood Gas Example -9
Acid
Normal
Alkaline
PaO2 8.3
pH 7.26
PaCO2 8.3kPa
HCO3- 30mmol/l

27. Classification of Imbalance
Double Trouble…
pH is acid, PaCO2 is acid, HCO3- is acid…
This patient is very sick….
Acid
Normal
Alkaline pH
7.13
PaCO2
9.9
HCO3- 10

28. Any Questions ?