1. ARTERIAL BLOOD GAS ANALYSIS
Andy Jackson

2. Why do we need blood gas analysis?
Pulse oximetry is an extremely useful bedside tool for monitoring capillary saturation of haemaglobin by oxygen (SpO2).
However, there are limits to this usefulness:

3. Anaemia
Reduction in red blood cells means that less oxygen can be transported.
Oximetry may therefore create a false sense of security in the face of undetected anaemia.

4. Carbon monoxide
Carboxyhaemaglobin increases infra-red light absorption, giving falsely high SpO2 readings.
Inhaled CO affects readings for up to 4 hours after a cigarette.
Dobson, F. (1993) shedding light on pulse oximetry. Nursing Standard 7 (46): 4-11

5. Hypercapnia
Pulse oximetry does not measure carbon dioxide levels or pH.

6. Poor peripheral perfusion
Pulse oximetry relies on perfusion and pulse - poor perfusion, for what ever reason will cause a poor signal.

7. Nail varnish Absorbs light, so can cause significant under-reading.
(Wahr, J.A. and Tremper, K.K. (1996) Oxygen measurement and monitoring techniques, in c. Prys-Roberts and B.R. Brown Jr. (Eds) International Practice of Anaesthesia, Oxford: Butterworth Heinemann:2/159/1-19)

8. Discomfort
Prolonged wearing of a pulse oximeter probe can cause significant discomfort.
(Woodrow P. (2000) Intensive Care Nursing: A framework for practice Routledge

9. Therefore, a more accurate assessment of the patient’s respiratory / metabolic status is required.
ABG analysis remains the gold standard.

10. What does arterial blood gas analysis tell us?
Oxygenation (PaO2),
Ability to eliminate carbon dioxide (PCO2),
pH / acid-base balance.

11. Oxygenation Oxygen delivery is dependant upon 3 factors:
cardiac output Hb
SaO2

12. Oxygenation 97% of oxygen is transported by Hb.
Hb composed of haem, a pigment, which contains 4 iron atoms and
globin which is a protein.
O2 + Hb combine to form oxyhaemoglobin.

13. Oxygenation
3% of oxygen is dissolved in plasma - this is represented on an ABG sample as the PO2.
PO2 determines how much O2 combines with haemoglobin.

14. Oxygenation
When PO2 is high, Hb binds with a large amount of oxygen and may become fully / near fully saturated.
When PO2 is low, Hb is only partially saturated.
Pulmonary capillaries, PO2 is high and ++O2 combines with Hb.
Tissue capillaries, PO2 low, O2 released from Hb to diffuse into cells.

15. Oxygenation
This relationship is illustrated by the oxygen-haemoglobin dissociation curve.

16. Oxygen dissociation curve

17. Oxygen dissociation curve
At the top “flat” part of the curve, increasing PO2 results in negligible changes in SaO2.
On the “steep” part of the curve, small changes in PO2 result in large changes in SaO2.

18. Oxygen dissociation curve
Curve can move to left or right.
Move to right decreases affinity of Hb for O2.
Move to left increases affinity of Hb for O2.

19. Oxygen dissociation curve shifts: causative factors
RIGHT
pyrexia
acidosis
high 2,3 DPG
LEFT
hypothermia
alkalosis
Low 2,3,DPG

20. Carbon dioxide transport
Carbon dioxide is transported in 3 ways:
dissolved in plasma (7%),
combined with Hb as carbaminohaemoglobin (23%),
as bicarbonate in plasma (70%).

21. Carbon dioxide transport as bicarbonate.
This can be illustrated by the following reversible equation, which occurs within red blood cells:
PCO2 + H2O H2CO3
(carbonic anhydrase)
H2CO H+ + HCO3

22. Carbon dioxide transport
H+ combine mainly with Hb.
HCO3 leaves RBC and enters plasma.
Cl - diffuse from plasma into RBC (chloride shift) and combines with K+ to form KCl.
HCO3 combines with Na to form NaHCO3.
Process is reversed in pulmonary capillaries.

23. Carbon dioxide transport
Increase in PCO2 causes increase in HCO3 and H+ ions.
This has important effects on respiratory stimulation and acid -base balance.

24. pH / acid - base balance
pH is a measure of amount of free hydrogen ions.
Increased free hydrogen ions = acidosis.
Decreased free hydrogen ions = alkalosis.
pH of blood needs to be maintained within a fairly narrow range in order for enzyme reactions to occur.

25. pH / acid - base balance Normal range for pH is 7.35 - 7.45
pH < 7.35 = acidosis
pH > 7.45 = alkalosis
pH maintained by 3 main mechanisms:
The buffer systems,
the respiratory system,
and the kidneys.

26. pH / acid - base balance
When an abnormality in pH occurs, these systems compensate for the imbalance.
Compensation mechanisms will only (try to) restore pH to normal - overcompensation will not occur acidosis will not become alkalosis).

27. Buffer system
Main buffering system is the carbonic acid / bicarbonate mechanism.
carbonic acid = weak acid
bicarbonate = weak base
Normal body physiological processes tend to produce more acid - consequently, the body maintains more bicarbonate ions compared to carbonic acid.

28. Other buffering mechanisms are:Hb
phosphates
proteins

29. Respiratory system
Function of respiratory system is to maintain normal levels of CO2 and O2.
CO2 is the most important chemical stimulus for regulating respiration.
Combines with water to form carbonic acid which dissociates into bicarbonate and free hydrogen.
Hypoxia also important respiratory stimulus, but only below PO2 of 8.8kpa.

30. Respiratory system
Hydrogen ions stimulate chemosensitive area in medulla.
Increased CO2 stimulates increased rate and depth of respiration until CO2 normal.
Decreased CO2 decreases rate and depth of respiration until CO2 normal.

31. Respiratory system
Respiratory response to acidosis is rapid (3 mins) and is maximised in hours.

32. The kidneys The kidneys regulate pH by: excreting hydrogen ions
producing and reabsorbing filtered bicarbonate and phosphate (chemical buffers).
Process is slow, may take hours to show effect and days to maximise.

33. ACIDOSIS Clinical effects include:
Central Nervous System depression leading to coma
myocardial depression
arrythmias
MAY BE RESPIRATORY OR METABOLIC IN ORIGIN.

34. Respiratory acidosis - causes:
Lung disease (COPD, CF, pneumonia)
Opiates
Impaired neuromuscular function (Guillan -Barre, spinal cord injury)
Severe head injury

35. Respiratory acidosis Compensation by kidneys (increase HCO3).
Treatment = NIPPV, IPPV, physiotherapy

36. Metabolic acidosis - causes:
Inadequate elimination of H+, loss of HCO3 or excessive accumulation of acids other than H2CO3.
Due to:
renal failure
diarrohea
DKA
Lactic acidosis

37. Metabolic acidosis
Compensation by the respiratory system to eliminate CO2.
Treatment = HD, CVVH insulin, fluids inotropes.

38. ALKALOSIS Clinical effects include: muscle spasm / convulsions
Hypokalaemia
MAY BE RESPIRATORY OR METABOLIC IN ORIGIN.

39. Respiratory alkalosis - causes:
Hyperventilation (anxiety, PE)
oxygen deficiency (high altitiude, PE)
CVE or severe head injury

40. Respiratory alkalosis
Compensation is by the kidneys, which remove bicarbonate and reabsorb / reduce secretion of hydrogen ions.
Oxygen, rebreath CO2.

41. Metabolic alkalosis - causes:
Excessive bicarbonate ions, due to:
loss of hydrogen ions (vomiting, Gastro Intestinal fistula)
over-administration of bicarbonate
diuretics.

42. Metabolic alkalosis
Compensation is by respiratory system - hypoventilation (not very effective).
Treat the cause (e.g. anti-emetics)
IT IS HARDER TO TREAT A METABOLIC ALKALOSIS THAN AN ACIDOSIS.

43. Interpretation of blood gases
Look at the PO2 - is it normal?
Look at pH - < 7.35, > 7.45?
Look at base excess : <-3 = acidosis, > +3 = alkalosis.

44. Base excess
Definition: the number of mols of acid required to restore 1 litre of blood to a pH of 7.4, while the PCO2 is held constant.
Can be used to calculate the dose (in mmols) of NaHCO3 to correct a metabolic acidosis:
BE x 0.2 x weight (kg) = mmols NaHCO3

45. Interpretation of blood gases
PCO2 - is it normal? (respiratory problem)
HCO3 - is it normal?(metabolic problem)

46. Scenario 1
pH = 7.48
PO2 = 12.6
PCO2 = 3.7
HCO3 = 22
BE = +5

47. Scenario 2
pH = 7.2
PO2 = 18
PCO2 = 14.6
HCO3 = 30
BE = +6

48. Scenario 3
pH = 7.1
PO2 = 8.6
PCO2 = 7.3
HCO3 = 10.5
BE = -13

49. Scenario 4
pH = 7.53
PO2 = 14
PCO2 = 5.5
HCO3 = 33
BE = +7

50. Scenario 5
pH = 7.37
PO2 = 8.8
PCO2 =7.8
HCO3 = 35
BE = +7

51. Scenario 6
pH = 7.35
PO2 = 12
PCO2 = 3.1
HCO3 = 15
BE = -9

52. Scenario 7
pH = 7.28
PO2 = 10
PCO2 = 4.5
HCO3 = 12
BE = -8

53. Further reading:
Woodrow, P. (2000) Intensive Care Nursing: A framework for practice Routledge.
T.E. Oh, A.D. Bersten and N. Soni (Eds) (2003) Oh’s Intensive Care Manual 5th Edition Butterworth Heinemann

54. Abbreviations
CO2 CO
pH – a measure of acidity or alkalinity of a solution
ABG – Arterial blood gas
Hb – Heamoglobin
SaO2 Saturation of oxygen in arterial blood

55. SpO2 – Saturation of oxygen in capillary blood
O2 - Oxygen
PO2 – Partial pressure of oxygen
H2CO3 - Carbonic Acid
HCO3 - Bicarbonate
H+ - Hydrogen
PCO2 – Partial Pressure of Carbon Dioxide
H2O – Water
CL – Choloride
RBC – Red Blood Cells
K – Potassium
KCL – Potassium Chloride

56. Na – Sodium
NaHCO3 – Sodium Bicarbonate
CNS – Central Nervous System
NiPPV – Non invasive Positive Pressure Ventilation
IPPV – Invasive Positive Pressure Ventilation
DKA – Diabetic Ketoacidosis
HD – Heamodialysis
CVVH Insulin – Insulia via continuous veno-venous heamo-filtration
PE- Pulmonary Embolism
CVE – Cerebrovascular event
BE – Base excess