Interpretation of an arterial blood gas is an essential skill required by all doctors and most healthcare professionals.
In simplistic terms, an arterial blood gas (ABG) tells us about three main things:
To enable us to interpret oxygenation, respiratory function and acid-base balance, an ABG analyser gives us key bits of information. This includes the pH, partial pressure of oxygen, partial pressure of carbon dioxide and calculated bicarbonate.
Using these parameters, an ABG can give us information about what is going on inside the body and the likely cause of dysfunction.
In modern medicine, an ABG can give us a wealth of additional information, including:
Within the body, pH is tightly regulated by chemical buffers, the respiratory system and renal system.
Understanding how the body tightly controls pH through the respiratory and renal systems is important for ABG interpretation.
For more information, see our notes on control of pH.
As a brief summary:
When interpreting an ABG, it is important to use the same stepwise approach.
The normal partial pressure of oxygen (PaO2) is approximately 10.6–13.3 kPa.
Step 1 involves checking whether the patient has hypoxaemia (low blood oxygen levels).
Firstly, check if the patient is receiving supplemental oxygen. This is represented as a percentage (e.g. 24%, 60%). Room air is represented as 21%, which is the partial pressure of oxygen at atmospheric pressure.
The PaO2 is always lower than alveoli oxygen. Therefore, it is estimated that the Pa02 should be 10 less than the inspired oxygen (e.g. 11 kPa breathing at 21% room air).
The normal arterial pH is 7.35-7.45.
Step 2 involves checking whether the patient is ‘acidotic’ or ‘alkalotic’
The respiratory component of the ABG refers to the partial pressure of carbon dioxide (normal range 4.7-6.0 kPa).
Step 3 involves checking the PaCO2, which is a reflection of the respiratory contribution to acid-base regulation or ‘pH control’. The PaCO2 needs to be check in context of the pH.
The metabolic component of the ABG refers to the bicarbonate level (normal range 22-26 mmol/L).
Step 4 involves checking the bicarbonate level, which is a reflection of the renal (or ‘metabolic) contribution to pH control. Needs to be assessed in context of the pH and PaCO2.
Base excess/deficit is often used interchangeably with bicarbonate as a marker of the metabolic contribution to pH control.
Compensation refers to the appropriate attempt of the respiratory or renal systems to restore pH.
Both respiratory compensation and metabolic compensation can occur depending on the acid-base abnormality. Respiratory compensation, through alteration of carbon dioxide, is a quick mechanism. Metabolic (i.e. renal) compensation through alteration of bicarbonate is a delayed mechanism.
Final interpretation should state the contribution of respiratory and metabolic systems and presence of compensation.
Respiratory or metabolic component
With mixed results, it is useful to determine which is the predominant abnormality (respiratory or metabolic).
Mrs Smith’s arterial blood gas on 35% inspired oxygen
Mrs Smith has hypoxaemia (inappropriately low PaO2 for inspired oxygen) with a metabolic acidosis (low pH, low bicarbonate) and evidence of partial respiratory compensation (low PaCO2 in response to acidosis).
Modern blood gas analysers give a wealth of additional information.
Make sure you look at the additional information provided to you on a blood gas analysis. There could be multiple abnormalities including electrolyte disturbances, acute kidney injury, raised blood glucose or significant anaemia.
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