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ABG - Arterial blood gas - interpreting results

Edit this Article | Original article by Tom Leach | Last updated on 28/6/2014 | View Revisions
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 Normal Values

pH
7.35-7.45
CO2
4.7-6.0 kPa
O2
9.3-13.3 kPa
HCO3-
22-28 mmol/L
SaO2(Oxygen saturation)
92-98%
PA – means pressure in the alveoli
Pa – means pressure in the artery
  • If you keep the saturation above 88-90% and the patient is not at risk of dying from hypoxia.
  • This correlates to an oxygen level of about 7kPa.
To convert kPa to mmHg, multiply by 7.5
 

The oxygen dissociation curve

 
 
Note how:
  • The saturation declines rapidly at some points, and barely moves at other points
  • Various factors can shift the curve to the right or the left
 
You have to know the patient history to be able to accurately discuss blood gas results
  • Renal disease
  • Diabetes
  • Drugs; diuretics, aspirin(+are they on oxygen?!)
    • You can tell this by adding the PO2 and PCO2 – if the sum of these is >19 then they have to be on inspired oxygen. If the level is lower than this, they are likely to be breathing room air. Rule of 19.
  • Symptoms and onset (lung disease?)
 

Basic interpretation Rules

  1. Look at the pH - is it acidosis or alkalosis?
  2. Look at the CO2 – is it normal or abnormal? Then look is this change in keeping with the pH? If it is, then it is likely to be uncompensated respiratory acidosis/alkalosis. If not, then it could be a metabolic disturbance compensated for by the respiratory system, or a respiratory disturbance compensated for metabolically.
  3. Look at the HCO3- - is it normal or abnormal? Is the change in keeping with the pH? If it is, then it is likely to be uncompensated metabolic acidosis/alkalosis.
    1. Note the changes in bicarb and base excess take at least a couple of days to occur after the initial causatory event.
  4. If the changes aren’t in keeping with the levels, then it is likely to be some sort of compensation! More on how to tell this later on.
 

Advanced Interpretation

The A-a gradient – need to know of it, but not necessarily know about it. If it is increased, it hints at lung disease. It is a ratio of alveolar and blood O2.
Low PaO2 is caused by VQ mismatch.
The breathing centre – in the medulla – sensitive to H+ levels – when H+ levels rise as a result of high CO2, will force you to breathe- but people with prolonged high CO2 become sensitised to this, and they require on hypoxia to stimulate their breathing. The hypoxia stimulates the carotid bodies to stimulate breathing in these cases.
 

Signs of CO2 retention

  • Confusion – as a result of peripheral vasodilation
  • Asterixis (renal failure, type 2 resp failure, liver failure)
  • Warm extremeties
  • Bounding pulse
  • Morning headache – cO2 particularly high at these times.
 
To determine acid/base balance – do a blood test and check for HCO3-  – then you can do an ABG.
 

The Anion Gap

This is used to help diagnose acid base disorders. It is usually used in suspected cases of metabolic acidosis.
Anions are negatively charged ions.
Anions are hard to measure accurately. The anion gap is the difference between the number of measured anions, and the number of unmeasured anions.
Negatively charged proteins make up most of the unmeasured anions in a normal individual, and the measured anions are things like HCO3-  and sulphates.
In metabolic acidosis, you produce a large number of both measured (e.g. HCO3-  ) anions, and unmeasured anions (the proteins). BUT, the HCO3-  will bind to H+ (which is also produced in excess) and be turned into CO2 which is blown off by the lungs. So although you have produced more HCO3- , the amount of HCO3-  is low on an ABG sample because of all the H+ binding to it. So the proportion of protein anions compared to measured anions INCREASES and so in metabolic acidosis the anions gap increases.
 
To calculate the anion gap, you work out the difference between plasma cations and measureable plasma anions. The actual sum is:
Anion gap = [Na+] - [Cl-] - [HCO3-]
Some calculations also include potassium:
AG = [Na+] + [K+] - [Cl-] - [HCO3-]
 
Because of the high number of variables, the anion gap can actually be quite inaccurate.
 

Respiratory alkalosis

  • PaCO2 lowers, so pH rises
  • Hyperventilation / anxiety
  • Hypoxia (due to acute illness or altitude)
 

Respiratory acidosis

  • COPD
  • Depressed respiratory drive
  • Hypoventilation
  • As long as sats are >90% you don’t need to give O2! You could even kill them by doing this. Don’t worry about PaO2.
 

Example 1

Patient breathing room air
  • PaO2     6.6 – very low
  • PaCO2   6.5 – high
  • pH 7.14
  • HCO3     23
This is a primary respiratory acidosis without compensation – because pH is low (acidosis) and CO2 is high (respiratory) and HCO3 is normal – so there is not metabolic compensation.
 
This iis type 2 respi failure. If you gave them O2, then checked the blood gasses, their O2 would be normal, but CO2 would still be high – and still be acidotic – so you would still be able to tell they were in resp failure.
Try to use the lowest amount of O2 possible because you don’t want to increase his CO2! You would put him on a mask.
 
His acidosis is acute because it is not compensated.
 

Nasal specs or mask?

Specs give you a much more variable amount of O2 intake – if you have low breathing rate that is very quick the % of oxygen will be reduced. If you have a low breathing rate, the amount of O2 breathed in will be much greater because the specs have been gradually filling up the nose in between breaths.
Venturi mask – the masks have vents – different coloured masks have different size vents. As oxygen travels through the mask it pulls air in through the holes to give a set concentration of oxygen.
 
 

Example 2

  • PaO2                     7.8 (low)
  • PaCO2                  8.0 (high)
  • pH                          7.35 (normal)
  • HCO3                    31 (high)
High CO2 is likely to mean resp acidosis – but the increased bicarb and the normal pH means he is compensated.
This is likely to be chronic resp failure
Tell tale sign is CO2 – if this is abnormal then something is wrong! The CO2 shows if they are ventilating properly.
 
 

Example 3

  • FlO2                  .21 (this means 21% oxygen) which means he is breathing room air
  • PaO2                8.0 low
  • PaCO2             5.0 (normal) – therefore he is ventilating adequately
  • pH                     7.51 High
  • HCO3               30
 
CO2 is normal – therefore not likely to be hyperventilation
This i metabolic alkylosis – with a possible other cause of the hypoxia.
%COHb – this is related to smoking – smoking reduces your oxygen carrying capability. This patients abnormal results were caused by a diuretic.
Aspirin – overdose will cause an acidosis.
 

Example 4

Patient is on 3L oxygen
2L – 24%
4L – 28%
 
  • PaO2                9.5 (low)
  • PaCO2             2.8 (low) – this means he is likely to be breathing very fast! – but he is still noly maintaining PaO2 of 9.5
  • pH                     7.40 (normal)
  • HCO3               12 -very low
  • O2 sats            95%
 
Low bicarbonate hints at acidosis (metabolic acidosis) – low CO2 hints at alkylosis (respiratory alkylosis)