Arterial blood pH: acidosis, alkalosis and how the body compensates

The body produces acid constantly, yet blood pH has to stay within tight limits for enzymes, the heart and the brain to work. Lab Tests Online UK explains that pH is a measure of the level of hydrogen ion in blood: it decreases, becoming more acidic, with increased amounts of carbon dioxide and other acids, and it rises, becoming more alkaline, with decreased carbon dioxide or increased amounts of bases such as bicarbonate.

A pH below the usual range is called acidaemia (the process behind it is acidosis); one above is alkalaemia (alkalosis). Your hospital report prints the reference interval it uses, and it is the interval printed there, not a number from elsewhere, that your clinical team applies.

Blood gas tests are requested when someone has difficulty breathing, shortness of breath or rapid breathing, to monitor oxygen therapy, during operations, and for people with head or neck trauma, prolonged anaesthesia, ventilators, or known respiratory, metabolic or kidney disease with distress. In COPD the test may assess whether long-term oxygen therapy is needed. Lab Tests Online UK adds that testing the cord blood of newborns who show breathing difficulty can reveal respiratory problems and acidity.

Most blood gas analysis is done in hospital because the sample must be analysed straight away on specialised equipment; most GP surgeries do not have it.

Arterial blood is almost always used because gas and pH levels differ between arteries and veins. It is usually taken from the radial artery at the wrist, sometimes after an Allen test checks that the hand has adequate circulation, and occasionally from the brachial artery in the elbow or the femoral artery in the groin. Collection is usually more painful than a normal blood test, with moderate discomfort, and the site must be pressed firmly for at least 5 minutes, up to 10 to 15 minutes if you take blood thinners or aspirin.

No preparation is typically needed. If you are on oxygen, it may be turned off for 20 to 30 minutes for a room-air test, or the amount of oxygen you are receiving is recorded.

Acid-base problems are mainly due to breathing (the respiratory component) or kidney and metabolic function (the metabolic component), and because the systems are interrelated, one compensates for the other to bring the pH back toward balance. Lab Tests Online UK divides imbalances into four categories.

Abnormal results can mean the body is not getting enough oxygen, is not getting rid of enough carbon dioxide, or has a kidney problem. Left untreated, these imbalances could eventually be life threatening. The blood gas does not name the cause, but it points toward a respiratory or metabolic problem, and severity depends on whether the cause is sudden or long-term and whether the body can compensate.

A normal pH can hide two opposing disorders that cancel each other out, so doctors look at carbon dioxide, bicarbonate and sometimes lactate, glucose, ketones and the anion gap as well.

Treatment is directed at the cause: oxygen, assisted breathing, fluids, insulin for ketoacidosis, treating infection or stopping a harmful drug. Doctors repeat the gas to see the response. Pulse oximetry is a useful non-invasive way to follow oxygen, but it does not detect an acid-base imbalance. If you have sudden severe breathlessness, confusion or drowsiness, call 999.