Arterial blood gases

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 vital for the cells of the body, and our body's systems to function normally. It 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 by the kidneys to change the composition of the urine, helping to balance the body's pH.

Arterial blood gases (ABGs) are measured in many circumstances, particularly in patients who are very unwell or who are suspected of having abnormalities in acid-base balance. Patients who are artificially ventilated to help them breathe may have repeated ABGs to monitor their acid-base balance.

The ABGs test result can help to diagnose the cause of illness, determine treatment and monitor a person's response to treatment.

1. Arterial blood is obtained using a needle and syringe inserted into an artery - usually the radial artery at the wrist. Other arteries in the body may be used if necessary. A tourniquet is not required. Sometimes local anaesthetic will be used.

2. 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).

• 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)

• 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)

ABG results reveal if a person has acid-base balance which is normal, too acidic (an acidosis), too alkaline (an alkalosis), and whether this is "respiratory" or "metabolic". In respiratory causes, the respiratory system is responsible for causing the acid-base abnormality. In metabolic causes it is the result of the metabolism (activity) of cells of the body.

The most common ABG abnormality is too much acid in the body. 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). Some common 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, a waste product of body cells: seen in shock, infection, hypoxia.

• Acute and chronic kidney injury.

• Build-up of ketones: seen in diabete mellito, alcohol poisoning.

• Certain medications and or toxins: salicylates, metformin, ethylene glycol, methanol, cyanide.

• Severe diarrhoea.

Causes of metabolic alkalosis

• Vomito.

• Low potassium (hypokalaemia).

• Excessive amounts of alkali-containing medication, such as acid indigestion medication.

• Ustioni.

Causes of respiratory acidosis

• Respiratory acidosis is caused by the inability to ventilate adequately (respiratory failure), causing a build-up of carbon dioxide in the body. Carbon dioxide is an acid. It is removed from the body by being exhaled (breathed out). If it is not adequately removed it causes respiratory acidosis. There are many reasons why this might happen. For example, this may be because of a neurological or muscular disease - miastenia grave, Sindrome di Guillain-Barré, distrofia muscolare, or a problem with the respiratory system such as an asma attack or worsening of chronic obstructive pulmonary disease (COPD).

Causes of respiratory alkalosis

• Respiratory alkalosis is caused by too much carbon dioxide being exhaled. Any condition which causes hyperventilation (breathing too fast) can cause this - for example, ansia, ictus, meningite, altitude, pregnancy.

This usually depends on the underlying cause. Management of the underlying cause will usually improve or correct the acid-base balance of the body. In other instances, chemical buffers may be used to try and correct the disturbance.