This post is accompanied by a podcast. Listen to and/or download it here: DitchMedics - Episode 4

Adventures in ABGs

Intro:

Welcome back to DitchMedics.com. Today’s topic is arterial blood gas interpretation. Which means today’s topic is about acid-base balance. (I could hear the collective groans from all across the country) Now you may not find this topic as exciting as how to deal with multi-system trauma or how to manage a cardiac arrest. But let me let you in on a little secret… If you understand acid-base balance, if you understand what arterial blood gas interpretation represents, you’ll be better at managing multi system trauma. You’ll better understand how to manage patients in peri-arrest conditions. And if you desire to transition to critical care transport, understanding this will be mandatory.

When we talk about the life-threatening conditions that we deal with in the prehospital environment, every one of those represents a derangement in acid-base balance. Either as a cause of the life-threat or as its consequence. Arterial blood gas interpretation is how we assess that level of imbalance. And while it’s true you may not have the equipment to perform an ABG assessment in the field, simply understanding the physiology of arterial blood gases makes you a better clinician. And that’s why this site exists. That’s why you’re here.

This is definitely a podcast where you want to reference the show notes, before or while listening. For many of you ABG interpretation is going to be a new thing. So today’s discussion is an introduction into arterial blood gases. And more importantly how we tie this into your EMS practice. Let’s take a look at ABG’s…

ABGs:

An ABG looks at partial pressures. It determines the partial pressure of CO2 and O2. It also quantifies the HCO3 and pH of the blood. It is through the measurement of these levels that the clinician can assess respiratory and metabolic function of the patient. In addition to those levels, arterial blood can be assessed for lactate, glucose, as well as a host of other metrics. An ABG is usually assessed in the hospital. Very frequently with the intent of determining ventilatory effectiveness. But knowledge of this system and how it pertains to your patients will improve your care. And you’ll impress the hell out of the doctors when you exhibit knowledge of ABGs… And that’s all we want, right? A little damn recognition! ;)

 

Let’s define the components of an ABG:

pH – simply, the acidity of the blood. Normal pH level is 7.35 – 7.45< 7.4 = acidosis. > 7.4 = alkalosis.

PaCO2 – the partial pressure of CO2 in the blood. In ABGs, CO2 is representative of respiratory function. Normal range is 35 – 45 mmHg. <35 = respiratory alkalosis. >45 = respiratory acidosis.

HCO3 – the serum bicarbonate level. HCO3 is a vital component of the pH buffering system. It is representative of metabolic function. Normal range for HCO3 is 22 – 26 mmol/L. < 22 = metabolic acidosis. >26 = metabolic alkalosis.

PaO2 – the partial pressure of O2 in the blood. Represents the adequacy of oxygenation. Normal range is 80 – 100 mmHg. <80 = hypoxia. >100 = hyperoxia. 

 

ABG interpretation system:

  1. Does the pH represent acidosis or alkalosis?
  2. Find the matching component. Look at the CO2 and the HCO3 and determine which matches the pH. This is the cause of the pH derangement. Either respiratory or metabolic.
  3. Look for compensation. Look again at the CO2 and HCO3 and determine if there is any compensation in the opposite component. For instance…if there is respiratory acidosis, do you see compensatory metabolic alkalosis (HCO3 >26)? If there is no compensatory response, this is an uncompensated ABG derangement.
  4. If there IS compensation of the opposite component, look at the pH again. Is the pH in normal range (7.35-7.45)? If the pH is in normal range, it is a fully compensated ABG derangement. If the pH remains outside of normal range, it is a partially compensated ABG.
  5. Lastly look at the PaO2. Is it outside of normal range? < 80 is hypoxia. >100 is hyperoxia. In the emergency setting, hypoxia is of greatest concern. But more and more research is indicating the prolonged periods of hyperoxia, specifically >250 mmHg, is harmful for patients.

 

ABG interpretation examples: (these examples are discussed in the podcast)

Example #1:

pH – 7.38, CO2 – 50, HCO3 – 30

Look at the pH. It represents acidosis.

Look for the matching component. CO2 matches the acidosis at 50. So, this is respiratory acidosis.

Look at the opposite component for compensation. HCO3 is 30, which represents metabolic alkalosis. So there is compensation.

Look at the pH again. Is it within normal range? The pH is 7.38, within normal range. So this is fully compensated respiratory acidosis. 

We’ll discuss where PaO2 comes into the interpretation in the next example.

 

Example #2:

pH – 7.50, CO2 – 55, HCO3 – 38, PaO2 – 85

Look at the pH. It represents alkalosis.

Look for the matching component. HCO3 matches the alkalosis at 38. This is metabolic alkalosis.

Look for compensation. The opposite component, CO2, represents acidosis at 55. So there is compensation.

Look at the pH again. It is outside of normal range at 7.50. This is partially compensated metabolic alkalosis

Lastly, look at the PaO2. At 85 is within normal range. If this patient is on room air, oxygenation is adequate. (If they’re on supplemental O2, that is another discussion for another day!)

 

Example #3:

pH – 7.27, CO2 – 60, HCO3 – 25, PaO2 – 68

pH = acidosis

The matching component is the CO2 at 60. Thus, this is respiratory acidosis.

Look at the opposite component, the HCO3. The HCO3 is within normal range in this patient, so there is no compensation. This is uncompensated respiratory acidosis.

The PaO2 is 68, outside of the normal range. This patient is hypoxemic. So…. The full interpretation isuncompensated respiratory acidosis with hypoxia.

 

Example #4:

pH – 7.25, CO2 – 58, HCO3 – 18, PaO2 – 100

pH = acidosis

Look for the matching component. CO2 matches the acidosis at 58. But wait there’s more… The HCO3 matches the acidosis that 18. Well that’s new… Since they both match the pH, this is called a mixed acidosis. The cause of the acidosis in this patient is both respiratory and metabolic in nature.

And once again we finish off by looking at the PaO2 which is within normal range. Not a factor.

 

Those are some pretty simple examples to get you started down the road of becoming proficient at ABG interpretation. This skill, like any other, takes practice. Fortunately for you there is a wealth of resources online to help you perfect your ABG interpretation skills. A simple YouTube search of the term “ABG interpretation” will present you with many options. If you want to become skilled at this… practice, practice, practice.

In the podcast we finish off by discussing some common causes of these ABG derangements. A very simple look at some frequently encountered problems. If after that discussion you would like more information about causes of ABG abnormalities, check out this link: lifeinthefastlane.com/investigations/acid-base. A great resource that delves pretty deep into the topic.

And another interesting post of ours re: DKA as a cause of metabolic acidosis can be found here. Check it out.

 

Conclusion:

Well that’s it for episode #4. I want to reiterate, while you may not find this knowledge applicable in your day to day job as a paramedic, I beg to differ. Understanding acid-base balance gives you a critical insight to the pathophysiology of serious illness or injury. This knowledge makes you a better clinician. Understanding what is going on inside of your patient’s body gives you a much greater understanding of what intervention will have positive impact on their outcome and WHY it will have that impact. That is the point. We need to start understanding the why of our job.

And who knows, the day may come sooner than later when were assessing blood gases in the field. You will be prepared before your peers are…and that makes you awesome.

Thanks for joining us again. Please, please, please leave some feedback. I’d love to hear what you thought of this post. I’d love to hear any topics you would like discussed in the future. I’d love to hear what your favorite color is. Just talk to me. We’ll be back soon with more posts and episode five of our podcast series. See you then. – Derrick

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