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Blood Gas Sampling
Written by Tim Sheppard MBBS BSc. Created 9/8/10; last updated 12/11/10

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What is an arterial blood gas (ABG)?

An arterial blood gas, or ABG, is a way of looking at the concentrations of certain chemicals in the blood - particularly the concentration of certain gases dissolved in the blood.

The concept of a blood gas sample is that it informs the person taking it about how well a patient is - so, as with all tests, the most important thing to do is to treat the patient. However, the idea is that it gives an indication of things which can't (currently) be found out without invasive testing (i.e. putting a needle into someone). The things which are tested on a blood gas (e.g. pH, pO2 etc.) are essential chemicals in the blood, and any change can make a person very ill; similarly, a very ill patient will often have an abnormal result. This makes the test very useful at indicating how unwell a patient is, or how well they are compensating for ill health.

Because blood acidity is controlled heavily by the carbonic acid equilibrium, both the lungs and the kidneys are involved in affecting the values on a blood gas sample. This means that any disease affecting the lungs or the kidneys may lead to abnormal results on a blood gas sample. The test is therefore a particularly good way of looking at diseases which affect these organs. Although many people find it hard to understand, understanding this equilibrium makes an ABG very simple to interpret.

What is blood pH?

A normal concentration of hydrogen ions in human blood should lead to a pH of 7.35 to 7.45. Blood pH is an important concept because there are lots of enzymes in blood, and these enzymes work best between these values; a change in pH affects how well the enzymes work. It's really important for the body to keep the blood between certain pH values, and if it isn't managing this, it is a sign that someone is not coping very well with a particular disease. There is more information about blood pH here.

If the blood pH is too low (i.e. there are too many hydrogen ions), the blood is too much like an acid. This is called acidosis. If the blood has too few hydrogen ions, it is more like a base or alkali - it is called alkalosis. Understanding what has caused this requires interpretation of the blood gas.

What is pO2?

The partial pressure of oxygen (pO2) is a measure of how much oxygen there is dissolved in the blood. Importantly, this is how much oxygen there is floating around freely in the blood, not the oxygen which is bound to something else (e.g. haemoglobin). A normal value of pO2 is around 10-13kPa (75-100mmHg).

The oxygen that is freely dissolved in the blood is the oxygen which is available to the tissues. When the blood goes around the body, oxygen diffuses out of the blood down its concentration gradient; the higher the concentration in the blood, the more that can escape the blood and the faster it will leave. When oxygen dissolves out of the blood, it will be replaced by any oxygen stuck to haemoglobin - so haemoglobin acts like a back-up store of oxygen for when the blood is 'running out'. The relationship between haemoglobin and partial pressure of oxygen is dependent on the oxygen-haemoglobin dissociation curve, and is affected by other chemicals dissolved in the blood.

A low pO2 is often a sign that there is something wrong with the lungs; either not enough oxygen is getting through the lungs (e.g. because the lungs are damaged, as in chronic obstructive pulmonary disease), or the blood isn't able to pick up the oxygen because of a clot which has got in the way (i.e. a pulmonary embolism). Oxygen levels may be high because someone is inhaling extra oxygen through an oxygen mask.

What is oxygen saturation?

Oxygen saturation is how much oxygen is stuck to haemoglobin - or, more accurately, the proportion of haemoglobin that is filled with oxygen. In other words, oxygen saturation is a measure of how saturated haemoglobin is with oxygen. The more oxygen floating around, the easier it is for haemoglobin to be full up with it; so, the higher the pO2, the higher the oxygen saturations will be.

What we're usually interested in is the pO2 - the amount of oxygen free in the blood, and accessible to the tissues. However, we can't tell what that is without taking a blood test. Oxygen saturations are useful because they are easy to interpret (they are given as a percentage, and the closer to 100 you are, the closer to normal you are), but also because you can find out what the oxygen saturation is by using a probe on the finger - you don't need a blood test.

The reason they're not quite so useful is that the relationship between partial pressure of oxygen and oxygen saturation is not linear (that is, it doesn't go up in a straight line). The relationship is affected by what we call the oxygen-haemoglobin dissociation curve (as shown in the picture on the left).

What is pCO2?

The partial pressure of carbon dioxide (pCO2) is a measure of how much carbon dioxide there is dissolved in the blood. This, like pO2, is how much carbon dioxide is floating around freely, not the oxygen which is bound to something else such as haemoglobin. The normal value of pCO2 is between 4.7-6kPa (35-45mmHg).

Because carbon dioxide is so very, very important in the carbonic acid equilibrium, measuring carbon dioxide is a very important tool. Using this equilibrium, the higher the concentration of carbon dioxide, the more carbonic acid that will be produced; in other words, high concentrations of carbon dioxide will lead to more acid. Measuring CO2 is also extremely useful in understanding how well the lungs are working.

Oxygen levels in the blood usually limited by how much surface area there is to cross, or how good the blood supply is to the lungs. This means that if someone is breathing more or less quickly, it is unlikely to make much difference to how much oxygen there is in the blood. However, carbon dioxide is more affected by how quickly someone is breathing. The quicker someone is breathing, the more carbon dioxide that can be removed from the blood; the slower they breathe, the more that will be left in the blood. This means the levels of carbon dioxide in the blood can be a good measure of whether or not someone is breathing as quickly as they need to in order to leave the right amount of carbon dioxide (see respiratory acidosis or alkalosis).

Since breathing more or less frequently affects the levels of carbon dioxide, it is a really good way that the body is able to affect the acid levels of the blood.

What is bicarbonate?

Bicarbonate is a substance that is on the opposite side of the carbonic acid equilibrium from carbon dioxide; it's basically carbonic acid without the hydrogen, which makes it the conjugate base of carbonic acid. Normal levels are between 22-26mEq/L.

As a base (or alkali), bicarbonate is able to reduce how acidic blood is; in fact, that is exactly how it works in the human body. If there's too much spare hydrogen floating around, adding more bicarbonate can 'soak up' the hydrogen (and get rid of it in the form of carbon dioxide and water). This is mainly done by the kidneys by reabsorbing more bicarbonate.

Higher than normal levels of bicarbonate show that the kidneys, either deliberately or because of malfunction, are absorbing too much bicarbonate - which would make the blood more alkaline. Low levels show that bicarbonate is getting used up trying to deal with blood that is too acidic.

What is the base excess (BE)?

The base excess is not a chemical dissolved in blood - it is a theoretical idea. In a particular blood sample, how much acid would I need to add to bring the pH back to normal (7.4)? This is the base excess. In other words, it tells you how much excess there is of base or alkali in the blood. In a normal person, the base excess should be between -2 and +2 mEq/L.

If the base excess is less than 0 (i.e. a negative number), then really what you're saying is there isn't an excess of base at all - on the contrary, you would need to add base in order to get the pH back to normal. In that situation the 'base excess' answers the question, 'how much base would I need to add to bring the pH back to normal?'

A raised base excess is therefore because the pH is high (too alkaline). This is usually because there's too much bicarbonate kicking around. A low base excess is either because there is not enough bicarbonate, or because there's too much acid in the blood. You can work out which one of these it is by calculating and interpreting the anion gap. If the cause is too much acid, you'd also expect a raised anion gap; if the problem is that bicarbonate has leaked out, chloride would have been swapped in, keeping the anion gap normal.

What is lactate?

Lactate, or lactic acid, is the chemical which is produced when it doesn't have enough oxygen to go around. Oxygen is needed to efficiently use up the fuel which goes into the body (particularly glucose). When you don't have enough oxygen to burn the fuel properly, a less efficient way of generating energy can be used that doesn't need oxygen - but it produces this acidic waste product: lactic acid.

Lactic acid is what causes that burning feeling when you've been running and you're not used to it. It's a build up of this lactate chemical which happens when you've needed loads of energy and can't breathe in enough oxygen in time. It makes sense, then, that you'll also get a build up of this chemical when your body needs lots of energy for other reasons. For instance, when you have a serious infection, you need loads of energy - and regardless of how hard your body is working, you might struggle to get enough oxygen to the right place quickly enough.

Lactate should normally be at least less than 2. If it's higher than this, it probably suggests that the metabolic demands are high. Most patients in hospital who have a raised lactate cannot blame going for a jog. It's usually a sign that someone is very unwell, and needs immediate attention.

What other types of blood gas sampling are there?

There are basically three main types of blood gas that may be taken. The first, arterial, has been the discussion of this article. However, trying to get blood from an artery isn't always easy - they were deliberately deeper down in the flesh in order to protect them. Many arteries are better protected by bones or other body structures. Veins and capillaries, on the other hand, are often readily accessible from the surface.

A venous blood gas works in much the same way as an arterial gas - in that most of the values will be just as true in the veins as they are in the arteries. The pH and lactate should normally be the same. It is the gases - oxygen and carbon dioxide - which will have swapped over since being in the arteries, so you need to remember that oxygen will be a lower value, and carbon dioxide will probably be higher than what is normal in arterial blood. Because bicarbonate is often calculated using the carbon dioxide concentration, this may also not be an accurate reflection of arterial blood.

If you can't access any blood vessels (which is very rare, but a common issue in very young children / babies) then you can fill a capillary tube to get a capillary blood gas. This can be difficulty because you have to prick the skin and then gather the blood one drop at a time; however, you can still get useful information to explain what is going on in the blood - and, indirectly, in the lungs and kidneys.

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