Blood Gases

Procedure

Arterial Blood Gas

Arterial stab to usually radial or ulnar artery. Also femoral stab can be performed. 

Venous Blood Gas

Venous blood taking similar to venepuncture. 

Fill up particular tube and insert into blood gas machine (usually located in the emergency department, ICU or HDU). 

Indications

VBG

Indication Test Utilised Potential Action

Unwell patient, e.g. septic: Lactate, pH, Na+ and K+ Vasopressors

Exacerbation of COPD: pC02, bicarb and pH BiPAP

Congestive heart failure exacerbation pC02 BiPAP

Electrolyte derangement (severe) Quick Na, K, Mg, Ca Change IVF/ escalate to HDU/ ICU

Lactate: sepsis, ischaemia (e.g. bowel) Lactate Change IVF

ABG

• Assessment for long term oxygen therapy

• Acute hypoxia / unwell of unknown cause (A/a gradient)

  • • Requiring intubation/ as per anaesthetics or respiratory

• Appears clinically hypoxic with normal saturations

  • • Methylene overdose

    • Carbon monoxide poisoning

• Unable to get venous access

Downsides

Contraindications

Cautions

Side effects

Complications

Interpretation

Interpreting acid - base implications of ABG/ VBGs is a common exam question. Here is a simple schema to follow:

1. pH: low = acidotic, high = alkalotic,

2. pC02: high causes acidosis, low causes alkalosis

   1. 1. 1. If pH and CO2 in agreement, e.g. pH low and C02 high = respiratory acidosis

         2. If pH and CO2 in disagreement, e.g. pH low and C02 high = acidosis with respiratory compensation

3. Bicarbonate: high causes alkalosis, low causes acidosis

   1. 1. 1. If pH and Bicarbonate in agreement, e.g. pH low and bicarbonate low = metabolic acidosis

         2. If pH and Bicarbonate in disagreement, e.g. pH low and bicarbonate high= acidosis with metabolic compensation

These can be put together, for example if pH low, CO2 low and Bicarb high = respiratory and metabolic acidosis, also called a mixed acidosis. 

Further points

• In COPD, a raised bicarbonate can be a sign of chronic metabolic compensation for respiratory acidosis and a sign this patient is a C02 retainer, therefore requires sats 88-92%. 

I have used the term blood gas to contain arterial (ABG) and venous (VBG) blood gases. 

Benefits of a VBG

> quick results on core parameters

Key results: 

1. Electrolytes- Na, K+, Calcium

2. Haemoglobin

3. Lactate

4. pH, Bicarb

5. PC02

Indications

1. Monitoring electrolytes- e.g. hypo or hyperkalaemia; DKA

2. Acute bleeding: GI bleeding, retroperitoneal etc

3. Any unwell patient- I commonly tell juniors to do a VBG on any unwell patient, for the lactate

4. Respiratory distress ?T2RF- looking for CO2 retention: exacerbations of COPD or asthma, pneumonia, pulmonary embolus

5. Acid base balance- renal patients can become acidotic, severe gastro enteritis

Indications for an ABG, rather than a VBG

1. LTOT requirement

2. Increased wob with a normal sats trace ?CO ?methylglobuminaemia

3. Unwell patient

4. Unable to take blood from venous source

Distinction between ABG and VBG: PO2

A VBG produced a PC02, lactate and H+ comparable to the ABG. I note a correction factor should be used for PC02, subtract 4 from ABG to VBG. However

The PO2 can be reliably achieved from the sats probe and therefore the requirement for an ABG has diminished. Historically ICU would put in arterial lines and therefore produce many ABGs and expects ward to do so, but they have fallen out of fashion. There is some evidence the pain of ABGs cause asthmatics to delay presenting. 

Therefore the indication for an ABG is when the PO2 isn't reliable enough, i.e. CO poisoning. 

In an unwell patient, it is unlikely performing an ABG will be frowned upon as it is safe medicine. 

I have had to perform an arterial stab when there is no venous access. Generally I prefer to insert an USS cannula/ venflon in patients with difficult access. 

Do you need USS to do an ABG? 

In the UK this is unheard of but in Australia much more common. I personally don't, but if its helpful then why not. 

Page written in 2024.