ABG Interpretation | Including Case Studies

ABG (Arterial Blood Gas) Interpretation | Including Case Studies

This is simpler than it seems. Let’s start with the basics.

The pH scale ranges from 1–14, where 1 is the strongest acid, 14 is the strongest alkali and 7 is neutral (‘normal’ in terms of acid-base balance).

Normal arterial blood pH = 7.35-7.45

I.e. the body normally controls the pH of blood within a tight range. One must remember that pH is a logarithmic scale and so a change from 8 to 7 is a ten-fold increase in H+ concentration.

• pH < 7.35 – blood is acidic
• pH > 7.45 – blood is alkaline.

For acidic and alkaline states, there are 2 groups of causes (respiratory and metabolic).

In a patient with acidosis (pH <7.35)

• pCO2 > 6 kPa – respiratory acidosis (usually due to cardiorespiratory failure, e.g. exacerbation of COPD or acute heart failure)
• HCO3 < 22 mmol/L – metabolic acidosis (usually due to acute kidney injury (AKI), diabetic ketoacidosis (DKA), lactic acidosis or salicylate overdose)
• Base excess – more negative than -2 mmol/L – indicates a metabolic acidosis (negative base excess).

In a patient with alkalosis (pH > 7.45)

• pCO2 < 4.5 kPa – respiratory alkalosis (less common)
• HCO3 > 26 mmol/L – metabolic alkalosis (less common)
• Base excess – more positive than +2 mmol/L – indicates a metabolic alkalosis (positive base excess).

Lactate > 2.0 mmol/L is hyperlactataemia – and requires action today; levels above 4.0 mmol/L are severe and need action now.

It is a bit more complicated than this, as the following cases represent.

Case Studies

Case Study 1. Arterial BGs (ABGs) in a stable COPD patient at home

• pH: 7.36 (7.35-7.45)
• pO2: 8.0 (10–13). Patients with chronic lung disease can be stable at this level of hypoxia
• pCO2: 7.6 (4.5–6.0). Patients with chronic lung disease can be stable at this level of hypercapnia
• HCO3: 31 (22-26)
• BE: +5 (-2 to +2).

Interpretation

• This is a stable compensated respiratory acidosis (mild).
  • This does not represent acute pathology
  • Rather it reflects a compensation for a chronic respiratory acidosis secondary to chronic obstructive pulmonary disease (COPD)
  • Note this is an acidosis, not an acidaemia (pH normal, but only due to compensatory mechanisms: the high bicarbonate).

Case Study 2: Venous BGs (VBGs) in an unwell 7 year old child

You are called to see a 7 year old child with diabetes who has diarrhoea and vomiting.

Her mum reports she is drowsy and her sugars have been high. You notice that she is sighing when breathing. You take a venous blood gas.

Interpretation

• This is a metabolic acidosis with respiratory compensation (DKA)
  • This child is most likely in DKA and has a metabolic acidosis. She is sighing as she attempts to breathe off the CO2 to compensate for the acidosis
  • She will need aggressive management
  • DKA is a life threatening complication of type I diabetes.