Blood is a dynamic fluid within which there are various reactions taking place all the time. As part of these reactions, both acid and alkali products will result. These acids and alkali have to be tightly balanced otherwise cell death may occur. The body has a number of mechanisms to counteract the changes, the most significant of which are the lungs and kidneys.
What is blood pH?
pH is a scale used to express the acidity or alkalinity of a substance. A pH of 7 is neutral, meaning that the acids and alkali balance each other. When there is more acid, the value is lower; when there is more alkali, the value is higher. Resting blood pH is around 7.4 and is kept within a narrow range (7.35 to 7.45). This tight maintenance is achieved rapidly and by use of buffering chemical compounds - for example, those found in the blood. When the pH is disturbed there is usually a rapid response trying to bring the pH back to normal by changing the breathing rate. There will also be a slower response from a change in the acid/alkali content of the urine by the kidneys. Measurement of arterial blood gases (ABGs) involves a sample of blood being taken from an artery (most commonly from the pulsation at the wrist). The ABG is then analysed on a special machine.
When are arterial blood gases taken?
ABGs are measured in many circumstances and include the following:
- Respiratory failure - for example, in acute asthma, in pneumonia and in chronic obstructive pulmonary disease (COPD).
- Any severe illness which may lead to a build-up of acid - for example:
- Organ failure, such as cardiac failure, liver failure and acute and chronic kidney injury.
- Diabetic ketoacidosis.
- Multiorgan failure.
- Patients being artificially ventilated.
- Severely unwell patients from any cause - the ABG provides information regarding outcome.
How are arterial blood gases taken?
- Arterial blood is obtained using a needle and syringe inserted into an artery - usually the radial artery at the wrist.
- A tourniquet is not required.
- Sometimes local anaesthetic will be used.
- A 1 ml sample is usually enough.
- Alternatives to the radial artery include the femoral artery in the groin and the brachial artery at the elbow. These are usually used in emergency settings.
- If multiple samples are required then a small plastic tube can be placed into the radial artery at the wrist.
- After the sample is taken, firm pressure is applied to the site for a minimum of two minutes (longer if the patient is on any blood-thinning medication).
What information does arterial blood gas give?
- Blood pH (normal range 7.35 to 7.45) - high indicates alkalosis; low indicates acidosis.
- Blood carbon dioxide level (PaCO2 level; normal range 4.7 to 6.5 KPa) - this may indicate a breathing problem.
- Bicarbonate level (represents levels of alkali; normal range 22-26 mEq/L).
- Blood oxygen levels (PO2 level; normal range 10.5 to 13.5 KPa) - if the PO2 level is low, it indicates an abnormally low concentration of blood oxygen (hypoxaemia).
What are the most common arterial blood gas abnormalities?
The most common ABG abnormalities detected relate to an excess of acid. This can either be as a result of the lungs not working properly (called respiratory acidosis) or from a build-up of acid from metabolic causes, usually as a result of tissues being starved of oxygen (called a metabolic acidosis). Other abnormalities and causes are listed below. Often there may be multiple abnormal processes at work and complex abnormalities may be seen.
Causes of metabolic acidosis
- Build-up of lactic acid: seen in shock, infection, hypoxia.
- Acute and chronic kidney injury.
- Build-up of ketones: seen in diabetes mellitus, alcohol poisoning.
- Other medications and or toxins: salicylates, metformin, ethylene glycol, methanol, cyanide.
- Severe diarrhoea.
Causes of metabolic alkalosis
- Low potassium (hypokalaemia) - for example, as can occur with diuretics.
- Excessive amounts of alkali-containing medication, such as for acid indigestion.
Causes of respiratory acidosis
- Depression of the central respiratory centre by stroke.
- Inability to ventilate adequately due to neurological or muscular disease - for example, myasthenia gravis, Guillain-Barré syndrome, muscular dystrophy.
- Acute asthma or acute exacerbation of chronic obstructive pulmonary disease (COPD).
- For example, COPD, obesity hypoventilation syndrome (Pickwickian syndrome).
Causes of respiratory alkalosis
- Hyperventilation - for example, anxiety, stroke, meningitis, altitude, pregnancy
How are abnormal arterial blood gases treated?
This depends on the underlying cause. For example, if septicaemia/sepsis is the cause then the patient will need intravenous antibiotics and intravenous fluids. If organ failure is the cause then organ support may be required.
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