How to use blood gas analysis when caring for critical medical patients
Blood gases are probably the most common diagnostic test we perform in emergency or critical medical patients (perhaps only second to PCV and total solids!)
So, it’s vital that we know what these tests mean for our patients, and how they might change our treatment and nursing care.
Acid-base imbalances are very common in critical or emergency medical patients. By checking blood gases regularly we can both understand the severity of their disease, and track changes over time. This gives us really vital information about the effectiveness of our treatment.
Blood gases often seem baffling, but they can be interpreted simply by following a few steps.
By having a basic knowledge of blood gases, the key parameters to look out for and what they mean, the veterinary nurse can adjust their patient care and administer treatment rapidly. This can make a huge impact in an emergency or when caring for a critical patient.
In this post we’ll refresh our knowledge on acid-base balance and talk through the step-by-step interpretation of these results.
Want to learn even more about acid-base and blood gases? In just over a week’s time, I’m hosting a 2-hour interactive workshop on blood gas analysis. On September 1st, we’ll work together to understand:
What acid-base balance is, and how it works
Acidosis and alkalosis, common causes, and how to tell what's what
How to interpret blood gas results, focusing on acid-base
How to obtain arterial blood gas samples and why we run them
How to assess oxygenation and ventilation on arterial samples
We'll also work through lots of actual blood gas results... until you're a pro at interpreting them!
Grab your ticket here - and get access to the live session, a recording, a workbook to help you interpret your results, and a certificate for 2 hours of CPD!
What is acid base balance?
Acid-base balance refers to the body's ability to balance its pH.
pH is essentially the concentration of hydrogen ions in the body and the pH scale reads all the way from 0 to 14, with 7.5 being neutral. A lower pH represents a more acidic solution, and a higher pH represents a more basic (alkaline) solution.
The normal blood pH is kept within a tight range of 7.35-7.45 (we usually take ‘normal’ as 7.4, smack bang in the middle of this range). Below 7.35 is referred to as acidosis, and above 7.45 is alkalosis.
Acids are naturally generated by our bodies all the time as a result of different processes in the body. To prevent acidosis occurring as a result of this, the body has several buffer systems.
A buffer is a chemical system that prevents radical changes in pH, by dampening the change in hydrogen ion concentrations where there is either excess acid, or excess base. The buffer solution is usually a weak acid (which takes up negatively charged hydroxyl ions) or a weak base (which takes up positively charged hydrogen ions).
Our buffer systems in the body work incredibly quickly, adjusting pH within seconds. Lots of molecules inside the body can act as buffers, including:
Plasma proteins
Haemoglobin
Phosphate
Bicarbonate
Let’s look at an example…
Acidosis occurs and blood pH lowers
Bicarbonate, a natural buffer, binds to excess hydrogen ions (which are positively charged, and therefore acidic) to form carbonic acid.
Carbonic acid is then converted to carbon dioxide and water
The CO2 is then eliminated during respiration
As you can see, these buffer systems are very clever and work in careful balance to keep our blood pH normal.
There are many organs involved in maintaining acid-base balance. The main ones to remember are the lungs, which regulate the respiratory side of acid-base, and the kidneys, which regulate the metabolic side of acid-base.
Acidosis and Alkalosis
Acidosis and alkalosis can be caused by either metabolic or respiratory changes.
Respiratory
With respiratory acidosis/alkalosis, the change in pH is due to issues with ventilation.
Ventilation is the actual ‘act’ of breathing (so delivering air to the lungs - unlike oxygenation, which is the delivery of oxygen to the bloodstream and tissues). We assess ventilation by looking at CO2 levels on our blood gas results.
If the patient is hyperventilating, they will lose too much CO2 .. Too little CO2, and alkalosis results. If the patient hypoventilates, CO2 will accumulate which will result in respiratory acidosis.
We might see respiratory acidosis in patients with impaired respiratory function. This can occur if a patient becomes fatigued and can’t effectively deliver enough oxygen to their lungs/eliminate enough CO2. An example of this could be a patient with respiratory obstruction.
Metabolic
Metabolic acidosis/alkalosis arises through any body process not associated with abnormal respiratory function.
It’s associated with a loss of buffer, or excessive acid buildup in the body.
Let’s look at DKA for example. DKA is so closely associated with metabolic acidosis it has it in its name - diabetic ketoacidosis. Metabolic acidosis technically should be confirmed on blood gases in order to diagnose DKA.
In this disease, 2 of the 3 types of ketones produced are acidic compounds. In addition, the ketones overwhelm the body's buffer system, causing decreases in bicarbonate levels. This results in metabolic acidosis.
Metabolic alkalosis can be seen during disorders which affect electrolyte levels and cause increases in bicarbonate levels, for example, vomiting.
What are blood gases and how do we test them?
Blood gas analysis is a quick, point-of-care test performed in many ICU or emergency settings. It is usually performed on a small amount of whole blood taken into a heparinised syringe.
We can perform blood gas analysis on either arterial or venous blood, depending on the patient and their disease.
If you’re looking at acid-base balance, venous blood is absolutely fine. However, if you’re wanting to assess oxygenation and ventilation (e.g. in a respiratory patient), you want to do this on arterial blood if possible.
This is because the arterial blood carries our oxygenated blood to the body from the heart (via the aorta), and our pulmonary artery carries deoxygenated blood (containing carbon dioxide) to the lungs.
Though the specific parameters assessed during blood gas analysis vary depending on the specific analyser, the following are generally included:
pH
PO2: This is the partial pressure of oxygen in the blood. If you’re running an arterial sample, it’s referred to as PaO2 (the little ‘a’ is for arterial). A normal PaO2 in a patient breathing in 21% oxygen is 90-100mmHg.
PCO2: This is the partial pressure of carbon dioxide in the blood. It correlates well with end-tidal CO2, so a normal reading would be 35-45mmHg.
SO2: This is the oxygen saturation.
HCO3-: This is the bicarbonate level. Bicarbonate is a basic/alkaline substance which is controlled by the kidneys, in order to ‘buffer’ acid levels within the body.
BE: This is base excess. It provides an estimate of the metabolic component of the patient’s acid-base balance.
Anion gap: The anion gap is a calculated variable used to evaluate metabolic acidosis. It is the difference between positively-charged and negatively-charged ions in the blood.
Lactate: Lactate is a by-product of anaerobic cellular respiration (when cells in the body form energy in the absence of oxygen). Lactate levels, therefore, provide us with information on how oxygen is utilised or delivered in the body.
Electrolytes: Most blood gas analysers also include sodium, potassium, chloride and ionised calcium +/- magnesium levels.
Glucose: Most blood gas analysers also include glucose levels in addition to blood gas values.
How to interpret blood gases quickly and easily
We can use the following six steps to easily and quickly interpret our blood gas results:
1. Examine the PaO2
If you have a respiratory patient and you’re running an arterial blood gas, look at the PaO2. If it’s low, you likely need to intervene with supportive oxygen therapy - or adjust the oxygen therapy you’re already giving your patient!
2. Examine the pH
Next, look at the pH. Is it low or high? Is your patient acidotic or alkalotic?
3. Examine the PCO2
Now it’s time to look at the PCO2.Is this normal or abnormal? High or low?If it’s abnormal, you have a respiratory component to your acid-base imbalance (acidosis if it’s high, alkalosis if it’s low).
These changes could be the primary cause of the acid-base imbalance, or secondary (if the body is compensating for metabolic changes by adjusting CO2 levels).
4. Examine the bicarbonate and base excess/anion gap
We’re now looking for evidence of metabolic change.So what are your metabolic parameters - your bicarbonate and base excess (and the anion gap if your analyser gives you this) - telling you?
A low bicarbonate / base excess or a high anion gap is associated with metabolic acidosis, whereas a high bicarbonate and/or base excess, or a low anion gap, is associated with metabolic alkalosis.
If these parameters are abnormal, you have a metabolic component to your patient’s acid-base imbalance.
Again, this could be the primary cause of the imbalance or secondary compensation for respiratory changes.
5. Determine which is the primary change
If your patient has changes to both their respiratory and metabolic parameters, how do you know which is the primary change and which is secondary/compensatory?
This is how to figure it out:
The primary change will explain the change in the patient’s pH.So, if you have a primary metabolic acidosis, the changes to the metabolic parameters would cause the pH to drop.
If you have a primary respiratory acidosis, the changes to the CO2 would cause the pH to drop.
If you have a primary metabolic alkalosis, the changes to the metabolic parameters would cause the pH to rise.
If you have a primary respiratory alkalosis, the changes to the CO2 would cause the pH to rise.
6. Determine which is the secondary change (if present)
Any compensatory change causes the opposite effect to above - because it’s trying to neutralise the pH.
Let’s look at an example:
Venous sample
pH: 7.24
Bicarbonate: 16.4mmol/L (normal range: 20-22)
Base Excess: -6mmol/L (normal range: -3 to +3)
CO2: 26.5mmHg
It’s a venous sample, so we can’t assess PaO2 (and don’t need to!)
The pH is low = acidosis
The CO2 is low, which would cause alkalosis (as the patient would be hyperventilating, eliminating more CO2)
The bicarbonate and base excess (metabolic parameters) are low, which would cause acidosis (due to a loss of buffer)
The metabolic parameters explain the pH change, and the respiratory parameters do the opposite.These results demonstrate a primary metabolic acidosis with respiratory compensation.
So that’s an overview on blood gas analysis for the veterinary nurse! As you can see, it’s quite a complex topic, but gives us a lot of information about our patient’s acid-base balance.
And, alongside that, we also get lots of information about fluid balance, electrolyte levels, perfusion, delivery of oxygen and our patient’s ability to ventilate normally - a LOT of tests on a small volume of blood!
Want to learn even more about blood gas analysis? Don’t forget to grab your ticket for the workshop on September 1st, so we can go through these results together in even more detail! Save your spot here and DM me on instagram and let me know once you’ve got your place!
References
Aldridge P & O'Dwyer L. 2013. Practical Emergency & Critical Care for Veterinary Nurses.
Biga, LM. et al. undated. Anatomy and Physiology, chapter 26.4, acid base balance. Available from: https://open.oregonstate.education/aandp/chapter/26-4-acid-base-balance/
LaCorte, R. 2019. Veterinary Technician’s Guide to Reading Blood Gases. DVM360. Available from: https://www.dvm360.com/view/veterinary-technician-s-guide-reading-blood-gasses.
Poli, G. 2022. Blood gas analysis part 1: why everyone needs to know about it. Vet Times, August 2022.
Sirois, M. 2020. Laboratory Procedures for Veterinary Technicians, 7th edition.
