PaO2 (arterial oxygen): measuring oxygen in the blood

Oxygen in blood exists in two forms: a large amount bound to haemoglobin and a small amount dissolved in plasma. PaO2 captures the dissolved part, expressed as a pressure. Lab Tests Online UK defines PO2 as the partial pressure of oxygen, the amount of oxygen gas dissolved in blood, and notes that a decreased PO2 may result in an elevated PCO2 and decreased pH.

The dissolved oxygen sets how fully haemoglobin loads up as blood passes the lungs, which is why PaO2 and oxygen saturation are linked, although the relationship is not a straight line.

Doctors want arterial oxygen when someone is struggling to breathe, has rapid breathing, or has a lung, heart, kidney or metabolic illness affecting gas exchange. It is used to monitor the effectiveness of oxygen therapy, during operations, for people on ventilators, with head or neck trauma, or after prolonged anaesthesia, and for newborns with breathing difficulty.

In COPD, Lab Tests Online UK says the test may be used to assess whether someone needs long-term oxygen therapy, which is why blood gases are done when the person is stable rather than only in emergencies.

A pulse oximeter clipped to a finger or earlobe measures oxygen saturation continuously and painlessly using light. Lab Tests Online UK points out its accuracy can be affected by abnormal forms of haemoglobin such as carboxyhaemoglobin in carbon monoxide poisoning, by poor circulation, and by very low haemoglobin in severe anaemia, and that oximeters do not detect acid-base imbalance. Blood gases give oxygen, carbon dioxide and pH together.

If a co-oximeter is used, the report may also list carboxyhaemoglobin and other altered haemoglobins. Haemoglobin binds carbon monoxide about 210 times more strongly than oxygen, which is why carbon monoxide poisoning is dangerous even when the oxygen pressure in blood looks reasonable.

Blood is usually taken from the radial artery at the wrist after an Allen test checks hand circulation; the brachial and femoral arteries are alternatives and a heel-prick capillary sample may be used in babies. Pressure is held for at least 5 minutes. For someone on oxygen, the oxygen may be stopped for 20 to 30 minutes for a room-air test, or the fraction of inspired oxygen or litres per minute is recorded so results can be interpreted.

A low PaO2 means the body is not getting enough oxygen. Causes include lung diseases such as pneumonia and COPD, severe asthma, fluid on the lungs, blood clots in the lungs, and over-sedation that slows breathing. The pattern with PCO2 helps: a low oxygen with a high carbon dioxide suggests under-ventilation, whereas a low oxygen with low carbon dioxide suggests a problem with gas transfer in the lung.

Whether the problem is sudden or long-term and whether the body can compensate determines severity. Treatment is directed at the cause and may include supplemental oxygen, medication, or assisted breathing.

The team treats the cause, repeats the gas, and adjusts oxygen. Carefully controlled oxygen is used for people at risk of carbon dioxide retention. For long-term lung disease, repeat gases when stable guide decisions on home oxygen: the NHS says that if COPD causes a low level of oxygen in the blood you may be advised to have oxygen at home, that it should be used for at least 15 hours a day, and that you must not smoke while using it because oxygen is highly flammable. Call 999 for severe breathlessness, blue lips or confusion, and tell clinicians if you have been exposed to smoke or carbon monoxide.