arterial blood gas is a sampling of arterial blood taken from either a direct needle aspirate of an artery, or from an arterial line (“art line”)
it can provide valuable information on the acid-base status, ventilation status (via interpretation of the pCO2 level) and oxygenation.
in addition, many ABG analyser machines also provide measures of Na, K, Hb, and ionised calcium.
blood gas analysers usually do NOT detect whether samples are haemolysed and thus the K+ reading may be falsely high without warning of the error
blood gas samples are generally much more costly than normal lab tests so should only be performed when clinically indicated
arterial blood gas sampling procedure is quite painful and risks local haematoma and arterial damage - best used for patients who already have an arterial line in situ
usual clinically reasonable indications for a blood gas
suspected carbon monoxide poisoning (assuming the analyzer has CoHB read outs)
COPD exacerbation - moderate to severe with suspicion of carbon dioxide narcosis
Critically unwell patients.
Diabetic Ketoacidosis including suspected.
Hyperkalaemia including suspected - although a serum U&E is cheaper and may be more appropriate
Ischaemic bowel including suspected - although a serum lactate is cheaper and may be more appropriate
Overdose with suspected acid-base disturbance e.g. toxic alcohol, massive / delayed paracetamol toxicity.
before you do an ABG
do you really need to do one?
a venous blood gas may provide you with sufficient data to assist your decision making and with less pain and risk to the patient.
venous blood gas
free flowing venous blood can be used if following taken into account:
pCO2 higher in venous by 7-8mmHg;
pH lower by 0.3-0.4;
calculated actual bicarb. higher by 1.8mM;
is it safe to do one?
patients with coagulopathy or likely to undergo thrombolysis may result in difficult to control local bleeding
patients with inadequate collateral arterial supply eg. absent ulnar artery are at risk of arterial insufficiency if the artery is damaged.
should you place an arterial line first?
patients who are likely to need multiple ABGs may benefit from an arterial line which will also provide more accurate blood pressure recording, particularly those at risk of becoming hypotensive such as cardiogenic or septic shock.
what are these results?
pH, PCO2, Cl-, Na+, K+, can be measured directly but the following need to be calculated:
Actual Bicarbonate:
from H-H equation above (ie. from pH & pCO2);
Std. Bicarbonate:
the bicarbonate level adjusted if pCO2=40:
the pt. where CO2 titration line intersects the bicarb. scale on Siggaard-Andersen curve nomogram;
CO2 titration line:
determined by buffer levels;
Base excess:
amount of acid or base that would restore 1L of blood to normal acid-base at pCO2=40;
if pCO2>40, then there is resp. acidosis which is:
compensatory if there is a primary alkalosis;
primary event if there is a primary acidosis;
Step 3: is the degree of compensation appropriate?
if primary metabolic condition:
PaCO2 (normal = 40) should have changed to compensate & should be:
equal to last 2 digits of pH (ie. 7.XX rule)
also:
CO2 falls by 1 for each 1.3 fall in HCO3
CO2 rises by 6 for each 10 rise in HCO3
if primary respiratory condition:
HCO3 (normal = 24) should have changed to compensate & for each change of 10mmHg PaCO2 should change (to a max. of ~31-32) according to 1-2-3-4 rule:
1mM for acute resp. acidosis
2mM for acute resp. alkalosis
3.5mM for chronic resp. acidosis (eg. COAD)
4-5mM for chronic resp. alkalosis (eg. high altitude, pregnancy)
also:
change in pH per 10mmHg change in PCO2:
resp. acidosis: acute = 0.08; chronic = 0.03;
resp. alkalosis: acute = 0.08; chronic = 0.017;
Step 4: is there a mixed disorder where 2 or more primary acid-base abnormalities co-exist?
look at anion gap to exclude presence of co-existing increased anion gap metabolic acidosis (eg. lactic acidosis)
if raised anion gap consider checking osmolal gap & measuring lactic acid +/- salicylate, etc
in general, only one primary respiratory condition exists at one time EXCEPT for chronic respiratory acidosis with superimposed acute respiratory alkalosis.