Teaching the Basics: Blood Gases

Author: Elizabeth Herrieven / Editor: Nikki Abela / Codes: C3AP3, CC23, RP2, SLO6 / Published: 13/10/2020

Teaching is a huge part of emergency medicine. Whether you’re preparing for the FRCEM OSCE or some shopfloor supervision of medical students, you need to be able to explain things to others that, for you, have now become second nature. That means being able to go back to basics, dissect out the details and describe them to people who may have no idea about the topic. You need to absolutely understand the subject yourself to be able to do that. Saying “It just does, ok!” doesn’t work any better with adult learners than it does with my kids at home. Let’s see how we might tackle teaching someone about blood gases.

What’s in a blood gas result?

First up is the pH, a measure of the number of hydrogen ions (H+), or how acid or alkali the blood is. Normal is 7.35-7.45. Higher is alkalosis, lower is acidosis.

The PaO2 and PaCO2 are the partial pressure (a measure of dissolved gas) of oxygen and carbon dioxide in the blood. In an arterial sample, the PaO2 should be about 10 in a healthy person breathing air. For those on oxygen, a rough guide is about 10 less than the percentage of inspired oxygen (so someone on 40% FiO2 should have a PaO2 of about 30). A normal PaCO2 would be about 4.7-6.0, on an arterial sample.

Base Excess is a weird one. Technically, it’s the amount of acid you would need to add to the blood to get the pH back to normal. Or the amount of alkali you’d need to add, if it’s a negative number. Basically (no pun intended), it’s a surrogate measure of how acid or alkali the blood is. So a Base Excess of 6 means you’d need to add lots of acid to get back to normal, as the blood is alkalotic. A Base Excess of – 6 (that’s MINUS 6) means you’d need to take acid away, or add alkali, to get it back to normal, because the blood is acidic. To make it more tricky, a negative Base Excess is sometimes called a Base Deficit. Confused?? The bigger the positive number, the more the alkalosis. The more negative, the more the acidosis. 

Most blood gas results will have a lactate. Lactate is produced during anaerobic metabolism, so can be raised in poor perfusion or increased oxygen demand by the tissues. It can also be raised in the hypermetabolic stress state associated with adrenaline release and by various drugs, including salbutamol. It is removed by the liver, so liver disease can cause a raised lactate. Generally, think of it as a marker of badness (thanks St Emlyn’s!).

You might get a calcium level. About 40% of the calcium in the blood is bound to proteins and acts as a store. The unbound calcium, the stuff that is ready to use, is also known as the ionised calcium. This is what is measured by most blood gas analysers, rather than the total amount, measured in the usual lab serum sample. So your gas calcium result will be much lower than the lab one (but more physiologically relevant).

The other bits of the blood gas are easier to understand and more familiar. It depends on your machine and settings, but typically you’ll get a glucose, sodium, potassium, bicarbonate, haemoglobin and haematocrit (the amount of the total volume of blood which is red blood cells, usually about 40%). You might get a carboxyhaemoglobin, which is useful in carbon monoxide poisoning. You might even get a bilirubin or a chloride. 

You might be able to “correct for temperature” on your machine. What’s that all about? Blood gas analysers warm blood to 37 degrees to process it. PaO2, PaCO2 and pH all change slightly with temperature. It doesn’t really make any practical difference, though, and leaving the temperature uncorrected is fine in most cases (plus allows you to monitor changes a little easier). For hypothermic patients it might make a difference a their warmed blood gas will show a higher PaO2, higher PaCO2 and a lower pH than their cool blood will actually have. 

Types of blood gas

When I was a new, more energetic doctor, we did plenty of arterial blood gases. It was a bit embarrassing to get a venous sample. But actually, arterial blood gases are blooming painful and not often needed. The only bit of information it will give you that a venous gas won’t is an indication of oxygenation. Most of the time you don’t need that, as an oxygen saturation monitor will give you a good surrogate measure. If you are going to do an arterial blood gas in an awake patient, use local anaesthetic. 

So, venous gases are the way forward. Less painful, easier to collect and give some great information. There’s little difference in base excess, pH, bicarb and lactate compared to an arterial sample. The PaCO2 might be very slightly higher, but we can factor that into our evaluation. 

How about capillary samples? We use those a fair bit in paediatric practice, squeezing blood into a teeny tube from a heel prick or finger prick. Maybe we should use them in adults too? Particularly if our patient is difficult to bleed. Being capillary blood, it’s a mix of oxygenated arteriolar blood and deoxygenated venous blood, so the results aren’t difficult to interpret. There can be a bit of squeezing involved in collecting it, particularly if the digit is cold or poorly perfused. It’s worth giving it a good rub first! If the sample is squeezed, cell lysis will give a falsely raised potassium result. Haemostasis in the limb or digit during trying to squeeze out the sample will raise the lactate too (read more here), as will any kicking or thrashing or general exertion from the child. If your result shows an unexpectedly raised potassium and/or lactate, consider whether you need a venous sample – as with all things in medicine, look at the results in context.

What’s it all about? 

Yes, I’ve been putting this bit off. Here goes:

The body needs a pH of 7.35-7.45 to be able to function, for all the necessary enzymes and chemical reactions to work. It does a good job, usually, of keeping the pH between these levels. This equation is partly how it does it:

 

Think of this like a see-saw or a set of scales, that always wants to be level. If the H+ ions go up (so there is an acidosis, with low pH), the body tries to get rid of CO2 (by breathing faster and deeper) so the chemical reaction can run from right to left, to turn H+ eventually into CO2, to be breathed away. 

If there is too much CO2 (for example, with respiratory failure or apnoea), then the equation is pushed from left to right, turning the CO2 into bicarbonate and hydrogen ions. 

The bicarb level is adjusted using buffering systems involving plasma proteins and the kidneys, to try to mop up the extra hydrogen. If it can’t, there will be an acidosis. 

How to interpret a blood gas

  1. Is there an acidosis or alkalosis? i.e. what’s the pH?
  2. If there’s an acidosis or alkalosis, is it caused by a respiratory problem or a metabolic problem, or is it mixed?  A respiratory problem will have an abnormal CO2, a metabolic problem an abnormal bicarbonate and likely abnormal Base Excess and/or lactate.
  3. Is there any compensation? The body is great at compensating. So in a metabolic acidosis it will hyperventilate to blow off CO2 and try to bring the pH back to normal. In a respiratory acidosis the bicarbonate level will go up, to try to bring the pH back to normal. Respiratory compensation is quick, metabolic slower. Compensation never “over-shoots”, i.e. the pH won’t go the other side of normal unless something else is going on.
  4. Then look at EVERYTHING else – step by step, noting any abnormalities. 

Blood gas results, without compensation

 

pH

PaCO2

Bicarbonate

Respiratory Acidosis

 

N

Resp Alkalosis

 

 

N

Metabolic Acidosis

 

 

N

Met Alkalosis

 

 

N

With compensation

Primary Problem

Compensation

Respiratory Acidosis

Raised bicarbonate

Respiratory Alkalosis

Low bicarbonate

Metabolic Acidosis

Low PaCO2

Metabolic Alkalosis

Raised PaCO2

Causes

Respiratory acidosis (low pH, high PaCO2) occurs when the lungs can’t get rid of CO2, for example in asthma, bronchiolitis, apnoea, lower respiratory tract infection or COPD. 

Respiratory alkalosis (high pH, low PaCO2) occurs when too much CO2 is being blown off: hyperventilation seen in PE for example, or anxiety, or during mechanical ventilation. 

Metabolic acidosis (low pH, normal PaCO2 or low PaCO2 if compensating, low bicarbonate) happens when there are too many acidic compounds in the blood. The anion gap can help to differentiate which acidic compounds these may be, but causes include DKA, sepsis, overdose with aspirin or tricyclic antidepressants or renal failure.

Metabolic alkalosis (high pH, high bicarbonate) happens when there has been a loss of acidic compounds from the body – most often seen in excess vomiting (e.g. pyloric stenosis). 

Mixed acidosis (low pH, low bicarbonate, high PaCO2) is often seen in fitting patients – they may have a lactic acidosis due to exertion, with or without an element of sepsis, plus poor ventilation causing a high PaCO2. It can also be seen in patients with chest sepsis, or a severe asthma exacerbation (with the metabolic component due to loss of fluid, poor perfusion and salbutamol-associated lactic acidosis). 

Anion Gap

Feel free to ignore this bit until you’re confident with the rest.

The anion gap is useful for helping us find out what might be causing a metabolic acidosis. The blood has an overall neutral electric charge, but has a mix of positively charged ions (cations – sodium, potassium) and negatively charged ions (anions – chloride, bicarbonate, albumin, organic acids and other proteins). Some of these things are measured directly, some are not. Various other compounds may be present in illness, which are also anions. These are estimated in the anion gap.

Anion gap = [sodium + potassium] – [chloride + bicarbonate] = 3-11(ish) mEq/L normally

The potassium level can be quite variable and is only a small number, so often the calculation is done without it:

Anion gap = [sodium] – [chloride + potassium]

Metabolic acidosis with normal anion gap: This is most often caused by diarrhoea. Other causes include Addisonian crisis, renal tubular acidosis and pancreatic insufficiency.

Metabolic acidosis with low anion gap: This is usually because of a low amount of various negatively charged proteins, such as in hypoalbuminaemia, or a high amount of positively charged substances such as in hypercalcaemia or hypermagnesaemia.

Metabolic acidosis with high anion gap: These are more interesting, with a fancy mnemonic and everything. The commonest causes, though (if you can’t remember the very unmemorable mnemonic), are related to lactate, toxins, ketosis and renal problems.

CAT MUDPILES  
C Carbon monoxide, cyanide
A Alcoholic ketoacidosis
T Toluene
M Metformin, methanol
U Uraemia
D DKA
P Paracetamol, paraldehyde
I Iron, isoniazid, inborn errors of metabolism
L Lactic acidosis
E Ethanol, ethylene glycol
S Salicylates

References and further reading

  1. RCEMLearning Arterial Blood Gas Analysis
  2. Michael Gradmedic Acidbase
  3. LITFL Lactate and Lactic Acidosis
  4. Stemlynsblog.org Lactate Lacthate
  5. Don’t forget the bubbles Blood Lactate freshly squeezed
  6. LITFL Anion Gap
  7. Embeds.co.uk Anion Gap Metabolic Acidosis
  8. Geekymedics ABG Interpretation
  9. Geekymedics ABG Quiz

(and thanks to Charlotte Davies for sharing her PowerPoint presentation with me 😊)

4 Comments

  1. Mr. Martin Anderson says:

    Good resource

  2. pickettna says:

    Thank you, nicely written.
    Made the anion gap make sense.

  3. Samantha Jayne Oliphant says:

    valuable resource

  4. Miss Nicola Rae says:

    Great little refresher, clear and well explained.

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