How to.. interpret blood gas results
Today’s post inspiration comes from Jude and Hayleigh who asked for a refresher on blood gases and the Henderson-Hasselbalch equation. Blood gases are performed commonly in medical or emergency patients and provide us with vital information very quickly. Blood gases often seem baffling, but they can be interpreted simply by following a few steps. By having a basic knowledge of blood gases, key parameters to look out for, and what they mean, the veterinary nurse can adjust their patient care and administer treatment rapidly in an emergency patient. Ready to jump in? Let’s go…
Acid/Base balance – a recap
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. Below this is referred to as acidosis, and above this, alkalosis.
Acids are naturally generated by our bodies all the time, as a result of different processes in the body. To prevent acidosis forming as a result of these, the body has several buffer systems. Buffers are substances in the body which affect hydrogen ions to maintain a neutral pH.
For example: if the blood became acidic, bicarbonate, a natural buffer, would bind to excess hydrogen ions (which are positively charged, and therefore acidic) to form carbonic acid. This is converted to carbon dioxide and water, and the CO2 is eliminated during respiration.
These syndromes are categorised as respiratory - where the change in pH arises from abnormalities of the respiratory system or metabolic - through any body process not associated with abnormal respiratory function.
Respiratory acidosis occurs due to hypoventilation, which causes an increase in blood carbon dioxide levels. Conversely, hyperventilation causes decreased CO2 levels, which leads to respiratory alkalosis.
Metabolic acidosis is associated with a loss of buffer or excessive acids in the body. For example, ketones produced during diabetic ketoacidosis overwhelm the body's buffer system, causing decreases in bicarbonate levels and 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?
Blood gas analysis is a common first-line test in many critical medical or emergency patients. It can be performed on a venous or arterial blood sample – ideally, an arterial sample is preferred but in many cases, a venous sample is fine (unless you’re assessing oxygenation and ventilation – e.g. in a respiratory patient). It requires only a small volume of whole blood, which is usually collected in a heparinised blood gas syringe.
Though the specific parameters assessed during blood gas analysis vary depending on the specific analyser, the following are generally included:
pH: This tells us whether acidaemia or alkalaemia is present. A normal blood pH is 7.35-7.45.
pO2: This is the partial pressure of oxygen in the blood. A normal pO2 in a patient breathing in 21% oxygen is 90-100mmHg on an arterial sample.
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. CO2 levels are used to assess any respiratory component to a patient’s acid/base balance, as CO2 is converted to carbonic acid, an acid.
sO2: This is the oxygen saturation; the percentage of haemoglobin saturated with oxygen. A normal reading would be >95% on an arterial sample.
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. Base excess is a calculated figure (one which is not directly measured by the analyser, but calculated from other results. It provides an estimate of the metabolic component of the patient’s acid-base balance and is reported in positive or negative numbers. E.g. a base excess of -3mmol/L would show that there is a deficit of bicarbonate in the body, which would be seen with metabolic acidosis.
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 and can be calculated using the following formula: AG = sodium – (chloride + bicarbonate). An elevated anion gap suggests the presence of metabolic acidosis. In order for this calculation to be accurate, the patient’s albumin and phosphate levels need to be normal.
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. Patients with poor perfusion (e.g. hypovolaemic patients) cannot effectively deliver oxygen to cells, so anaerobic respiration occurs, resulting in elevated lactate. This is known as a type A hyperlactataemia. Another form of hyperlactataemia occurs in patients with normal perfusion; this is known as type B hyperlactataemia and is seen where oxygen cannot be appropriately utilised for cellular respiration – e.g. in cases of hypoglycaemia.
Electrolytes: Most blood gas analysers also include sodium, potassium, chloride and ionised calcium +/- magnesium levels within their minimum database, providing important information about any electrolyte abnormalities present; this is very useful in medical patients, and emergency patients where abnormalities are common.
Glucose: Most blood gas analysers also include glucose levels in addition to blood gas values. This allows the rapid detection and correction of hypoglycaemia, which is commonly seen in emergency/medical patients.
Interpreting blood gases: the six-step method
We can use the following six steps to easily and quickly interpret our blood gas results:
1. Examine the pO2
Our blood gases as well as telling us about acid-base balance, also tell us about oxygenation level when running on an arterial sample. If you have a respiratory patient and the PaO2 is low, you likely need to intervene with supportive oxygen therapy (or adjust the oxygen therapy they are already receiving).
2. Examine the pH
Is it low or high? Is your patient acidotic or alkalotic?
3. Examine the pCO2
Is this high or low? If so, you have a respiratory component to your acid-base imbalance. This could be the primary cause of the imbalance, or secondary (eg. the body is compensating for metabolic changes).
4. Examine the bicarbonate
Is this high or low? If so, you have a metabolic component to your 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
This will change in the same direction as the pH (e.g. low bicarbonate and a low pH = metabolic acidosis).
6. Determine which is the secondary change (if present)
This will be the compensatory change, as the metabolic system adapts to work against the respiratory system to try and keep the blood pH level normal and vice versa. On your blood gas results, the compensatory parameter will change the opposite way to the pH. E.g. a low pH (acidosis), low bicarbonate (acidic) and low CO2 (alkaline) = a metabolic acidosis with compensatory respiratory change.
What is the Henderson Hasselbalch equation?
The Henderson Hasselbalch equation is an equation used to estimate the pH of a solution, from the bicarbonate and carbon dioxide levels. Simply put, it states that the pH of arterial blood is proportional to the ratio of bicarbonate to arterial carbon dioxide levels. It can be calculated using the following simplified formula:
pH = HCO3- / paCO2
(NB. the ‘a’ in paCO2 is for arterial blood)
From this formula we can make the following deductions:
1. If the HCO3- to paCO2 ratio remains normal, the pH will remain normal
2. pH will increase (resulting in alkalosis) if either the HCO3- increases, or paCO2 decreases
3. pH will decrease (resulting in acidosis) if either the HCO3- decreases, or paCO2 increases
4. If both the HCO3- and paCO2 increase or decrease but by roughly the same amount, the ratio between them stays the same, and so the pH will remain normal.
So that’s blood gases, simplified! I hope this helped demystify what can be quite a brain-busting topic! Do you run blood gases in practice? If so let me know your experiences below!
References
Aldridge P & O'Dwyer L. 2013. Practical Emergency & Critical Care for Veterinary Nurses.
Higgins, C. 2004. An Introduction to Acid-Base Balance in Health and Disease. Acute Care Testing. Available from: https://acutecaretesting.org/en/articles/an-introduction-to-acidbase-balance-in-health-and-disease
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.
Sirois, M. 2020. Laboratory Procedures for Veterinary Technicians, 7th edition.
