Venous blood gases are commonly undertaken in AMU and the Emergency Department as rapid means of assessing multiple aspects of a patients blood biochemistry.
In recent years, these have commonly been undertaken to quickly obtain a lactate level as part of the infamous Sepsis Six, but it is important we understand how much more there is to be gleaned from these tests and what we might do with this information
Acid-Base Balance
There are multiple mechanisms which contribute to ensuring that the body's pH remains within an acceptable range for optimal functioning of cells and tissues.
Respiratory and metabolic problems can contribute to both acute and/or chronic derangement in blood chemistry
When considering acid-base balance, think about the following:
1. Is the pH normal (7.35-7.45), low (acidaemic), or high (alkalaemic)?
2. Is there a respiratory component, indicated by an abnormal carbon dioxide level (pCO2)?
3. Is there a metabolic component, indicated by an abnormal bicarbonate level (HCO3)?
4. Additionally it is possible to understand that there may also be a compensatory mechanism. For example, a patient with a metabolic acidosis may also have a low carbon dioxide level to try and correct their acidaemia. This might be seen in diabetic ketoacidosis (DKA), for example.
Electrolyte Derangement
Blood gases can often be useful for quickly understanding whether there is any significant abnormality with a patient's electrolytes. Each machine may check different electrolytes but almost all will rapidly check sodium (Na) and potassium (K) levels
Potassium
- High and low potassium levels can be associated with cardiac rhythm disturbances and should prompt the need for an ECG and consideration of cardiac monitoring
- Low potassium levels can be difficult to correct if magnesium levels are also low so it is worth checking this as well
Sodium
- Low sodium levels can occur for a number of reasons including infection, drugs and heat failure. Early identification of a low sodium can allow us to think about a likely cause and request additional tests if appropriate, such as urine sodium/osmolality and cortisol levels
- High sodium levels are most commonly seen in severe dehydration but can be seen in people with underlying endocrine conditions, such as diabetes insipidus
Glucose
All patients being admitted to hospital should have a blood sugar level checked on arrival, which allows for quick identification of high or low levels. VBGs are at least as reliable as bedside monitoring of glucose (BMGs, or BMs) and there is no need to do both.
Hypoglycaemia (BM<4), with or without symptoms, should be treated as a medical emergency. This should be treated in accordance with the local guidelines found with the 'hypo boxes'. The medical team should also be informed to allow for a plan for further management to be put in place.
Hyperglycaemia can be seen in the context of poor baseline blood sugar control, in the context of acute illness or as part of a new diagnosis of diabetes
Lactate
Elevated lactate levels can be caused by tissue hypoperfusion (Type A lactataemia) in conditions such as:
- sepsis,
- heart failure,
- hypovolaemia
- and in cardiac arrest.
Alternatively, lactate levels may be elevated in the absence of hypoperfusion (Type B lactataemia) with conditions such as:
- diabetes (and some of its treatments)
- malignancy
- alcohol
- beta-adrenergic agonists (e.g. salbutamol)
- HIV
Haemoglobin
Low haemoglobin (Hb) levels may be seen in the context of acute bleeding, or in more chronic conditions
If the haemoglobin level is low (particularly Hb<80g/L), consider sending an additional blood sample to the laboratory as a "group and save" sample as this may more readily allow for a blood transfusion to be arranged in a timely manner
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