1. Blood Gas analysis
Diane Standring

2. Aims To identify when blood gas sampling is indicated
To understand blood gas interpretation
To make use of a systematic approach

3. When is a blood gas indicated?

4. When is a blood gas indicated?
When a patient’s clinical condition warrants it…….
When knowing the result will influence care given
To aid diagnosis
To assess treatment response
To monitor for existing worsening problems or the development of new ones

5. Is an arterial sample always necessary?

6. Is an arterial sample always necessary?
No, but it gives a lot of information
For assessing metabolic acidosis a well taken venous sample is sufficient
In many instances an “arterialised” capillary sample will suffice
For a lot of information and with a very sick patient an arterial sample is mandatory

7. What information do we get from a blood gas?

8. What information do we get from a blood gas?
Saturation
Base excess
HCO3-
PaO2
PaCO2 pH

9. Normal ABG values Pa 0 2 10 – 13 kPa (or 85-100 mmHg)
pH 7.35 to 7.45 (H+ 35 – 45 mmols)
P a CO – 6 kPa (or mmHg)
Pa – 13 kPa (or mmHg)
HCO 3 – (SBC) 22 – 26 mmols/L
Base Excess - 2 to + 2

10. Interpretation of arterial blood gases
Saturation
Base excess
HCO3-
PaO2
PaCO2 pH
Oxygenation
Ventilation
Acid base status
abgs can give us information about oxygenation, ventilation and acid base status.

11. Interpretation of arterial blood gases
Saturation
Base excess
HCO3-
PaO2
PaCO2 pH
Oxygenation
Ventilation
Acid base status
abgs can give us information about oxygenation, ventilation and acid base status.

12. Interpretation of arterial blood gases
Saturation
Base excess
HCO3-
PaO2
PaCO2 pH
Oxygenation
Ventilation
Acid base status
abgs can give us information about oxygenation, ventilation and acid base status.

13. Interpretation of arterial blood gases
Saturation
Base excess
HCO3-
PaO2
PaCO2 pH
Oxygenation
Ventilation
Acid base status
abgs can give us information about oxygenation, ventilation and acid base status.

14. What…
Partial pressure of arterial oxygen (PaO2) – This indicates the ability of the lungs to move oxygen into the bloodstream
Partial pressure of arterial carbon dioxide (PaCO2) – This indicates the ability of the lungs to retrieve carbon dioxide out of the bloodstream
pH – The pH is the measurement of hydrogen ions (H+) found in the bloodstream which indicates the acidity or alkalinity of the blood
Bicarbonate (HCO3-) – This is the most important buffer found in the bloodstream, it assists in returning the body from an acid state back to a normal range.

15. Clinical features of Hypoxaemia
Often non-specific (chronic versus acute)
Altered mental state
Dyspnoea, cyanosis, tachypnoea, arrhythmias, coma
Hyperventilation when PaO2 <5.3kPa (saturation <72%)
Loss of consciousness ~ 4.3 kPa
Death approximately 2.7 kPa

16. Haemoglobin (1)
Haemoglobin is a conjugate protein molecule, containing iron. The molecule contains 4 polypeptide subunits, two α and two β. This structure of haemoglobin determines its ability to bind oxygen.
In its deoxygenated state, haemoglobin has a low affinity for oxygen (also know as reduced Haemoglobin). The binding of one oxygen molecule to haemoglobin causes a conformational change in its protein structure; this then allows easier access to the other oxygen binding sites, thus increasing haemoglobin’s affinity for further binding of oxygen. Haemoglobin is capable of binding up to four molecule of oxygen.
Hb + 4O2 → Hb (02)4

17. Buffering of hydrogen ions Transport of CO2 as carbamino compounds.
Haemoglobin (2)
The main function of haemoglobin is to transport oxygen in the blood at the alveolar capillary membrane and to transport it to the tissues. However, haemoglobin has other functions;
Buffering of hydrogen ions
Transport of CO2 as carbamino compounds.
Haemoglobin binding
The amount of oxygen carried by haemoglobin in the blood is dependent on how many of the 4 binding sites are occupied. Therefore, haemoglobin can be said to be saturated or partially saturated.
Saturated – all 4 binding sites are occupied by oxygen.
Partially saturated – some O2 is bound, but not all sites are occupied.

18. Oxygen dissociation curve

19. The oxygen dissociation curve shift
2,3-diphosphoglycerate (2,3-DPG) is a substance made in the red blood cells. It controls the movement of oxygen from red blood cells to body tissues.

20. Oxygen dissociation curve shift
Shift to the left (increased affinity)
Reduced temperature
Alkalinity
Reduced C02
Reduced 2,3 DPG
Shift to the right (increased dissociation/decreased affinity)
Increased temperature
Increased acidity
2,3 DPG

21. What does pO2 measure?

22. What does pO2 measure?
The partial pressure of oxygen dissolved in plasma!
BUT…. there is a positive association between oxyhaemaglobin and dissolved oxygen

23. Oxygen Transport in a fit person The Alveolar-arterial oxygen gradient (aADO2)
Air
100% Oxygen
Alveolar Pa O2
14 kPa
88 kPa
Arterial Pa O2
13.3 kPa
84 kPa
Oxygen sats
99%
100%
Oxygen content in mls per 100 mls of arterial blood
Bound
19.7 mls
20.1 mls
Dissolved (unbound)
0.3 mls
1.9 mls
Conversion factor: 1 KPa = 7.5 mm Hg

24. Oxygen cascade Dry atmospheric gas: 21 kPa
Humidified tracheal gas: 19.8 kPa
Alveolar gas: 14 kPa
Arterial blood: 13.3 kPa
Capillary blood: 6-7 kPa
The partial pressure of oxygen falls from dry atmospheric pressure value of of 21 kpa through a series of steps until it reaches only 1-5 kpa in the mitochondria where it used to make energy. As it is humdified in the trachea water vapour makes up more the gas mix and po2 falls. CO2 in the alveolus further reduced po2. There Is only a small gradient between the alveolus and arterial blood which we will discuss again later. As bllod moves through the circulation po2 continues to fall until it reaches 5.3 kpa in venous blood. The po2 in mitochondria is very low due to long path from the cappilary to the interior of the cell. Drops in po2 in the bllod have deleterious knock on effects for mitochondrial po2
Mitochondria: 1-5 kPa
Venous blood: 5.3 kPa
Conversion factor: 1 kPa = 7.5 mm Hg

25. Acid-Base Balance
The pH is a measurement of the acidity or alkalinity of the blood. It is inversely proportional to the number of hydrogen ions (H+) in the blood. The more H+ present, the lower the pH will be.
normal
<7.35 = acidaemia
>7.45 = alkalaemia
Blood pH below 6.8 or above 7.8 will interfere with cellular functioning, and if uncorrected, will lead to death.

26.  C0 2 + H20  H2 C03  H+ + H C03- Compensation Respiratory
Metabolic including kidney

27. Respiratory Buffer Response
CO2 is carried in the blood to the lungs, where excess CO2 combines with water (H2O) to form carbonic acid (H2CO3).
The blood pH will change according to the level of carbonic acid present. This triggers the lungs to either increase or decrease the rate and depth of ventilation until the appropriate amount of CO2 has been re-established.

28. Renal Buffer Response
To maintain the pH of the blood within its normal range, the kidneys excrete or retain bicarbonate (HCO3).
As the blood pH decreases, the kidneys will compensate by retaining HCO3-
As the pH rises, the kidneys excrete HCO3- through the urine.

29. Henderson Hasselbach formula
[HCO3-]
pH = 6.1+ log
[pCO2x 0.23]
Homeostasis (control) of pH therefore controlled by altering ratio of bicarbonate and carbon dioxide

30. Acid-base balance / Homeostasis
Maintaining a tight range of pH is important for survival
Enzyme function
Membrane integrity
Concentration of H+ ions is low but crucial
If pH < 6.8 or > 7.7 ……..death
Acid production is a by-product of metabolism so acid – base balance is achieved by internal chemistry (buffering) and excretion of acid

31. Acid-base + H20 CO2 HCO3- H+ H2CO3
The concentration of hydrogen ions is kept very constant due to highly efficient buffering systems. The most important buffer system is the bicarbonate system. Bicarbonate and hydrogen ions are is in equibilrium with carbonic acid which is in turn in equilibrium with carbon dioxide and water. Other buffer systems include phosphate, protein and haemoglobin.

32. + + ALVEOLAR VENTILATION H20 CO2 H2CO3 HCO3- H+
Metabolic alterations in the equilibrium of the bicarbonate buffer system result in changes in alveolar ventialtion in an attempt to maintain the hydrogen ion conc. In a metabolic acidosis hydrogen ions accumulate and the equilibrium moves to the left; CO2 is blown off by increased ventilation to compensate for this. In a metabolic alkalosis the hydrogen ion concentration falls and the equilibrium shifts to the right. Co2 is retained to counteract this shift. These changes take place soon after the change in pH occurs
Normal PaCO2 = 5.3 kPa (40 mm Hg)

33. + + ALVEOLAR VENTILATION Normal HCO3- = 22-26 mmol/l H20 CO2 H2CO3
Respiratory alterations in the equilibrium will affect renal handling of bicarbonate in an attempt to maintain hydrogen ion concentration. In respiratory acidosis hypoventilation causes accumulation of co2 and the equilibrium will shift to the RIGHT. Bicarbonate must be retained by the kidney to buffer the resulting increase in hydrogen ions. In a respiratory alkalosis CO2 is blown off, the equilibrium moves to the LEFT and hydrogen ion concentration falls. Bicarbonate must be lost in order to balance the system.
RENAL HCO3- HANDLING

34. Blood gas analysis (4 or) 5 step approach: Assess oxygenation
Determine pH
Examine respiratory component (CO2)
Examine metabolic component (HCO3-)
Consider base excess

35. Base excess / deficit
Amount or acid or base that needs to be added to 1 litre of blood to return pH to normal, assuming standard conditions (temp 37oC, PaCO2 = 5.3 kPa, pressure 1 atm)
A measure of the metabolic component of a disturbance
Normal: -2 to +2 mmol/l
BE is POSITIVE in metabolic alkalosis (or compensation for a respiratory acidosis)
BE is NEGATIVE in metabolic acidosis (or compensation for a respiratory alkalosis)
Useful to follow the TREND to assess treatment.
Amount or acid or base that needs to be added to 1 litre of blood to return pH to normal, assuming standard conditions (temp 37oC, PaCO2 = 5.3 kPa
A measure of the metabolic component of a disturbance. A base excess occurs in a metabolic alkalosis (in other words there is too much base an acid must be added) A base deficit exists in a metabolic acidosis (in other words there too much acid and base must be addedd). A base deficit is the same as a negative base excess. Trends of this parameter are useful in assessing treatment: fro example a patient with shock and poor perfusion will have a lactic acidosis and a negative be. Fluid resuscitation restores perfusion and be will improve towards normal values.

36. 1. Assess oxygenation
Is the patient hypoxic?
Is there a significant alveolar-arterial gradient?(10)
2. Determine status of the pH or H+ concentration pH > 7.45– alkalaemia < 7.35– academia
3. Determine respiratory component PaCO2 > 6.0 kPa– respiratory acidosis < 4.7 kPa– respiratory alkalosis
4. Determine metabolic component HCO3- < 22 mmol l-1 – metabolic acidosis > 26 mmol l-1 – metabolic alkalosis

37. Combine the information from 2, 3 and 4 and determine which the primary disturbance is and whether there is any metabolic or respiratory compensation.
Remembering that respiratory changes occur quickly ie minutes and metabolic problems occur over hours/days.
Primary problem.
Compensatory change.

38. Respiratory Acidosis Hypoventilation Respiratory arrest
Physical cause for respiratory failure

39. Some examples

40. Interpret the following arterial blood gas values
On 2 litres of oxygen via nasal cannulae
pH 7.31
pO kPa (76mmHg)
pCO kPa (32mmHg)
HCO3 18 mEq/L
BE -4
Partially compensated metabolic acidosis
What is the probable diagnosis with this patient?
What would be the treatment?

41. Interpret the following arterial blood gas values
On AIR
pH 7.35
pO2 5.7kPa (43mmHg)
pCO kPa (47mm Hg)
HCO3 35mEq/L
BE +2
Compensated Respiratory Acidosis
What is the probable diagnosis with this patient?
What would be the treatment?

42. Interpret the following arterial blood gas values
On 15l reservoir bag mask
pH 7.30
pCO kPa (55mm Hg )
pO kPa (206 mm Hg)
HCO3 25mEq/L
BE +1
Uncompensated Respiratory Acidosis
What is the probable diagnosis with this patient?
What would be the treatment?

43. Interpret the following arterial blood gas values
On 40% Venturi-mask
pH 7.49
pCO kPa (40mm Hg)
pO2 11kPa (85mm Hg)
HCO3 29 mEq/L
BE +3
Uncompensated metabolic alkalosis
What is the probable diagnosis with this patient?
What would be the treatment?

44. Interpret the following arterial blood gas values
On 40% Venturi-mask
pH 7.24
pCO kPa (29mm Hg)
pO2 11 kPa (85mm Hg)
HCO3 24 mEq/L
BE -8
Uncompensated metabolic alkalosis
What is the probable diagnosis with this patient?
What would be the treatment?

45. Scenario 1 60 yr man - Pneumonia Antibiotics SpO2 86% RR 30
FiO (Hudson)
This patient has no significant medical history. He presented with a flu like illness for a few days and has been in hospital for two days. Initially well, he has clkinically deteriorated over the last few hours.
His Blood pressure is around 130 systolic – normal for him. He has recently been catheterised and is producing 30 mls of urine (He weighs 75 Kg).
In the light of this blood gas, what immediate treatment does he need, and what medical and nursing plan do you think he should have for his ongoing care? pH
PCO kPa
PO kPa BE 45

46. Scenario 2 55 yr man - pneumonia SpO2 80% RR 35
15 l min-1 reservoir bag pH
PCO kPa
PO kPa BE
Lactate mmol l-1
This man has been in hospital for 96 hours. Initially he was admitted for a routine minor operation, but was cancelled and from that point started to become unwell.
He has a blood pressure of 90 systolic and a heart rate of 115 (sinus). He has no previous medical history.
What is your interpretation of this blood gas?
What do you think the plan should be for his medical and nursing care? 46

47. Scenario 3 64 yr old female, History of COPD
Insidious onset of SOB and c/o tight chest
Fatigued with increased accessory musclel use
CXR - Hyperinflated with probable pulmonary hypertension
ABG’s – pH 7.25, PaCO2 8.3, PaO2 7.4, BE +3.7, HCO3 28.

48. Scenario 4 26 yr old male Hx asthma Admitted with acute exacerbation
On 60% via aquapak, SpO2 94% RR=26
Acute drop in SpO2, with Increased Work Of Breathing
CXR -bibasal opacity
ABG’s pH 7.33, PaCO2 6.4, PaO2 6, BE 1.3, HCO3 21

49. Summary Understanding arterial blood gases can be confusing
A logical and systematic approach using these steps makes interpretation
much easier
Applying the concepts of acid/base balance will help you evaluate the
effectiveness of care being provided
Now you need to go and practice, practice, practice!!!

50. What we have not covered…
Anion gap
Lactic acidosis
Detail of red cell buffering

51. Any questions….