ABG is an abbreviation for arterial blood gas. ABGs are performed by taking blood from an artery instead of from a vein. ABGs are performed to give an accurate measurement of ph, oxygen and carbon dioxide levels within the blood. They are used to check the function of the lungs and how well they are able to move oxygen and remove carbon dioxide.
To begin to understand ABGs and be able to interpret them, you need to know the normal ranges:
- pH – 7.35 – 7.45
- paO² – 10 – 14 kPa
- paCO² (carbon dioxide) – 4.5 – 6.0 kPa
- HCO3 (metabolic) – 22 – 26 mmol/L
- Base excess – -2 to +2 mmol/L
- O² sats – 94-100%
paO² – partial pressure of oxygen
This should be >10kPa (kilopascal) on room air in a healthy patient.
If patient is <10kPa on room air – patient is hypoxaemic (lack of oxygen within the blood)
If patient is <8kPa on room air – patient is severely hypoxaemic and in respiratory failure, CO² would be looked at to determine type 1 or type 2.
Ph level – acidity/alkalinity of the blood
Acidotic – <7.35
Alkalotic – >7.45
The smallest change in the ph level can have detrimental effects on the human body. The causes of a change in ph level can be respiratory or metabolic. Changes in ph are caused by a CO2 imbalance (respiratory) or a HCO3 imbalance (metabolic). Co2 and HCO3- act as buffers to keep the ph level within its normal range.
paCO² – partial pressure of carbon dioxide
paCO² is the respiratory element of an ABG.
The chemistry behind respiratory acidosis/alkalosis follows:
CO² binds with H2O and forms carbonic acid (H2CO3). This is acidic, causing the ph level to decrease below 7.35. If the patient is retaining CO², the blood will become more acidic. If the patients’ respiratory rate is raised, CO² will be ‘blown off’ meaning there is less CO² in the system than usual therefore the blood becomes more alkalotic.
HCO3- is produced by the kidneys and ‘collects’ acids (H+ ions). It is a byproduct of the body’s metabolism.
When HCO3- is raised, ph level is increased due to a decrease in H+ ions (alkalosis).
When HCO3- is low, ph level is decreased due to an increase in H+ ions (acidosis).
Base excess is another marker of metabolic acidosis/alkalosis. It is the amount of acid that must be added to each litre of fully oxygenated blood to return the ph level to 7.40 and to gain a paCO² level of 5.3kPa.
If the base excess is high ( > +2mmol/L) = this indicates higher than normal HCO3- within the blood.
If the base excess is low (< -2mmol/L) = this indicates lower than normal HCO3- within the blood.
Respiratory acidosis/alkalosis can be metaolically compensated. This is done by increasing or decreasing the levels of HCO3-.
Metabolic acidosis/alkalosis can be compensated by the respiratory system. This is done by retaining or ‘blowing off’ carbon dioxide.
You can read more about acidosis and alkalosis here.