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Blood Gas Interpretation
Written by Tim Sheppard MBBS BSc. Created 24/8/10; last updated 17/11/10

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How do you interpret a blood gas result?

Having put an arterial blood gas sample into the machine, you'll get a print out with all of the results. If you know what each thing is, that's great - but what do the results mean? If the pH is high, why is it high? If the carbon dioxide is low, is it important?

I usually ask myself 5 questions:

What are the oxygen levels like? This is the most important question, but I easily get distracted by it when I try to work out what else is going on. Someone will die from lack of oxygen before they die from acidosis (although the two may be linked). If the pO2 is too low, then give oxygen! There are some instances where people are sensitive to oxygen, and giving too much can suppress their breathing - but such instances are rare, so low oxygen levels need treating immediately with extra oxygen! Sensitivity to oxygen can be considered afterwards.

A low oxygen suggests that someone is in respiratory failure. They will either fall into Type 1 respiratory failure, if their pCO2 is normal, or Type 2 respiratory failure if their pCO2 is raised. The cause of the oxygen problem can then be considered.

Is the pH normal? The normal pH is 7.40, so when I look at the sample, I ask myself whether or not the pH is between 7.35 and 7.45. If the pH is less than 7.35, it means that the patient has acidosis. If the pH is more than 7.45, the patient has alkalosis. The next step is to work out what is causing any change in the pH.

Does the pCO2 fit with the pH? Because of the carbonic acid equilibrium, a raised pCO2 should be associated with a low pH - that is, more carbon dioxide means more acid (because of the reaction of carbon dioxide with water to produce more hydrogen ions); and, of course, a high pH should be found with a low CO2. So you have to ask yourself - does the carbon dioxide fit with the pH? If the CO2 is high, is the pH low? If the CO2 is low, is the pH high? If the pCO2 fits with the pH, it means that any abnormality with the pH is due to the CO2 - and therefore due to the lungs (i.e. respiratory acidosis or alkalosis). If the pH is abnormal but the CO2 is doing the opposite to what you'd expect, it means that the lungs are trying to compensate for the abnormal pH.

Does the bicarbonate fit with the pH? This is a similar question, again depending on the carbonic acid equilibrium. A high bicarbonate should be found with a high pH (i.e. alkaline blood), whereas a low bicarbonate is found with a low pH (i.e. acidic). Bicarbonate levels depend on the kidneys and the metabolic processes going on in the body, so if the bicarbonate does fit with the pH, it means that any abnormality with the pH is due to the bicarbonate - and therefore is considered a metabolic acid-base problem (i.e. metabolic acidosis or alkalosis). If the pH is abnormal but the bicarbonate is doing the opposite of what you'd expect, it means that the kidneys are trying to compensate for the abnormal pH.

Abnormal bicarbonate or base excess values which do not fit with the pH can be hard to understand on their own. If the machine has the ability to provide sodium, potassium and chloride levels as well as bicarbonate, it is possible to calculate the anion gap, and if you know how to interpret it, you can explain what has caused these abnormal values. If it is possible, the anion gap is always worth calculating.

Any other abnormalities? I also have a look through the other values that are given, to see if anything else is abnormal. In normal cases, these will be fine, but it is worth noting if anything is abnormal. Of particular interest to me is the lactate, which is a marker of anaerobic respiration (i.e. a lack of sufficient oxygen). If the lactate is raised, it means the body is probably finding that it has metabolic demands which are greater than the oxygen which is being delivered - so even normal oxygen levels in the blood may not be enough. If it is particularly raised (>5) the person is probably pretty sick; if it is very raised (>10) then you need to understand that the body is undergoing a severe stress, and probably needs help!


What is acidosis?

Acidosis is when the pH of blood is below the normal range. This means any value less than 7.35. Of course, a value of 7.34 is much less concerning that 6.8. A very low pH (e.g. 5) is likely to reflect an error with the machine. Remember that because pH is a logarithm of hydrogen ion concentration, even a small change in pH reflects an enormous change in the concentration of hydrogen ions. Also, because enzyme function depends on pH, even small changes have a drastic effect on how the body works. It may sound like pH 7.0 is quite close to 7.35, but in fact it suggests someone has severe acidosis.

The two main organs which control blood pH are the lungs and the kidneys. If the cause of acidosis is the lungs, it is described as a respiratory acidosis, and will be associated with a rise in pCO2. If the cause is a metabolic process going on in the body, such as severe sepsis, or is a problem with the kidneys, it is described as a metabolic acidosis, and will be associated with a drop in bicarbonate.


What is alkalosis?

Alkalosis is when the pH of blood is above the normal range. This means any value higher than 7.45. Again, like with acidosis, the further from 7.45 it is, the more concerned we are about the pH, and even a small change in pH reflects an enormous change in the concentration of hydrogen ions.

The two main organs which control blood pH are the lungs and the kidneys. If the cause of alkalosis is breathing, it is described as a respiratory alkalosis, and will be associated with a fall in pCO2. If the cause is loss of hydrogen ions or gain in bicarbonate, it is referred to as a metabolic alkalosis.


What is respiratory acidosis?

Respiratory acidosis is when the blood pH is less than 7.35 secondary to a respiratory cause; that is, when the acidosis is caused by a rise in the pCO2 above 6kPa.

Since pCO2 is a sign of ventilation, a rise in pCO2 suggests a problem with ventilation. In other words, if you find that someone has a respiratory acidosis, it means they are not getting enough air through their lungs. That either means that they're not breathing quickly or fully enough, or it means that there is something impairing gas exchange. A very common reason for this is something like chronic airways disease from smoking, but you should also consider nerve problems (e.g. myasthenia gravis) or suppression from sedatives and other drugs.


What is respiratory alkalosis?

Respiratory alkalosis is when the blood pH is more than 7.45 secondary to a respiratory cause; that is, when the alkalosis is caused by a fall in the pCO2 below 4.5kPa.

Again, because pCO2 is a sign of ventilation, a fall in pCO2 most commonly suggests that someone is breathing too quickly. This may be caused by a panic attack, for example. If you breathe very quickly and start to feel dizzy, it is because of the respiratory alkalosis that you have generated.


What is metabolic acidosis?

Metabolic acidosis is when the blood pH is less than 7.35, but not because of the lungs. If the body has a load of extra acid, or if the kidney's aren't doing a good enough job of keeping the pH normal, the pH will be reduced.

Renal acidosis occurs when the breakdown products of protein metabolism build up alongside urea and creatinine; instead of the kidneys getting rid of all these products, their levels rise. Since some of these break down products are acids, this leads to acidosis. The bicarbonate levels are low because the bicarbonate bonds with the floating hydrogen ions to form carbonic acid, in an effort to keep the pH down.

Other common causes of metabolic acidosis include lactic acidosis and ketoacidosis. Lactic acidosis occurs when oxygen delivery to the tissues is not enough - either because oxygen levels are low, or because it's not getting to the tissues (e.g. when someone is bleeding a lot). The absense of oxygen means that oxidative phosphorylation cannot occur, and the metabolic processes take a different route to produce a lot of lactic acid.

When there are low levels of insulin, the body tries to get its energy from fat. This leads to the production of ketone bodies, which are acidic. Normally this isn't a problem, but it happens in excess when there is no glucose (e.g. in starvation), or when the body falsely thinks there is no glucose (e.g. in diabetes mellitus). When it happens so much that the pH of blood falls, it is called ketoacidosis.


What is metabolic alkalosis?

Metabolic alkalosis is when the blood pH is more than 7.45, but not because of the lungs; it is found with a high bicarbonate level. It is a complicated concept, and is not as common as the other three types of acid-base disturbance. However, the cause can be calculated by looking at chloride levels.

Low chloride levels suggest that there has been a loss of chloride along with the hydrogen ions. This is commonly due to vomiting (when hydrochloric acid from the stomach - hydrogen and chloride - is vomited out of the body), but can also be through leakage in the kidney (e.g. from high aldosterone levels). A further potential cause of low chloride and high pH is when there is a loss of water from the blood - e.g. when diuretics are used. Because water is lost but bicarbonate is still left behind, the bicarbonate levels appear to be high, and it mops up the hydrogen leading to a high pH.

If chloride levels are high or normal, it suggests something else is going on. It could simply be that bicarbonate is not leaving through the kidneys properly. Sometimes people take things by mouth (e.g. antacids) which are designed to reduce acid levels, but they have taken too much of them so it affects the blood as well. The third cause is when potassium levels are low in the blood; because potassium is so important, it leaks out of the cells into the blood to keep levels in the right range. Unfortunately this leads to hydrogen swapping places with potassium, and disappearing into the cells - leading to a raised blood pH.


What are respiratory and metabolic compensation?

Because the blood has a number of different ways of keeping the blood at the same pH, when one method stops working, the others kick in to try and compensate. Since the kidneys and the lungs are two of the main ways of controlling the pH, they are the two things which most commonly adjust to try to keep the pH stable.

Imagine the lungs aren't doing their job properly. There is a build up of CO2, so the pH of the blood falls. In order to oppose this change in the pH, the kidneys decide to keep hold of some of the bicarbonate that they would normally be leaking out. If the bicarbonate stays, it mops up some of the hydrogen ions that are floating around. This means that you will get a rise in CO2 (which you would expect in respiratory acidosis) but a rise in bicarbonate (which you would expect in metabolic alkalosis). The kidneys are compensating for the pH change.

Just like with equilibria, there is a shift to oppose the change. So when you're interpreting the blood gas result, the best idea is to work out what is causing the pH change (e.g. CO2), and then work out if the other thing (e.g. bicarbonate) is trying to shift to oppose the change.

Metabolic compensation is when the bicarbonate has moved in the opposite direction to the CO2 in order to compensate for a respiratory change. If you have respiratory acidosis, metabolic compensation is when the bicarbonate is high (to try to mop up the hydrogen). If you have respiratory alkalosis, metabolic compensation is when the bicarbonate is low.

Respiratory compensation, then, means that the CO2 has moved in the opposite direction to the bicarbonate in order to try to compensate (or make up for) the metabolic pH change. If you have metabolic acidosis, respiratory compensation makes the CO2 low (i.e. you're breathing more quickly). If you have metabolic alkalosis, respiratory compensation makes the CO2 high (i.e. you're breathing more slowly). This is one of the reasons why a change in the respiratory rate is a good marker of illness.

This could make it difficult to work out which way round it is - if the CO2 is high and the bicarbonate is high, which came first? Is it respiratory acidosis with metabolic compensation, or metabolic alkalosis with respiratory compensation? The way to work it out is to look at the pH. You never over-compensate. So any change in the pH will be due to the original problem, not the compensation. In the example just mentioned, if the pH is high then it's metabolic alkalosis with respiratory compensation. If the pH is low, then its respiratory acidosis with metabolic compensation. If the pH is normal, then you have to work out what the problem is by looking at the patient.

One important thing to remember is that the kidneys take a while to kick in - so if there is metabolic compensation, it means the lungs have been having problems for a while (for hours or days, not just for a few minutes). The lungs, on the other hand, tend to respond quite quickly; if the kidneys stop working properly, the lungs start working to correct the problem straight away.


What is mixed acidosis or alkalosis?

What happens if both the carbon dioxide and the bicarbonate can explain the pH - what happens when it looks like it's caused by the lungs and by metabolism? Then it is called a mixed acidosis or alkalosis. It is both respiratory and metabolic in nature - both have gone wrong at the same time.

If the carbon dioxide value was high and the bicarbonate was low, the pH would be low and it would be called mixed acidosis. This would happen when both the lungs and metabolic processes are causing acidosis, such as when someone has a cardiac arrest (e.g. stops breathing and stops getting the blood around the body) or when someone with COPD (i.e. chronic respiratory acidosis) develops a severe infection (i.e. acute metabolic acidosis).

If the carbon dioxide value was low and the bicarbonate was high, the pH would be high and it would be called mixed alkalosis. This could happen if someone on diuretic therapy (which can cause metabolic alkalosis) develops a pulmonary embolism (and starts hyperventilating).


Further Reading