Arterial Blood Gases

We as respiratory therapists are experts in arterial blood gas procurement and interpretation. This places a burden on students of the profession as their preceptors and instructors expect a high level of understanding and performance with regards to arterial blood gases (ABGs).

An approach that may get a novice started is to work on the initial classification system. To help with this there are many tools but the tradition of using HCO3, bicarbonate, as a basis of interpretation is often problematic in my opinion. An alternative is to focus on the base excess (BE) to guide the classification. Here is a link to my website where these free tools are offered, check out the "Resources for  Clinicians" (http://respiratory-education.usefedora.com/). I have also created a Google Play application that also guides you through the classification and also provides some insight into the pathology or patient conditions that would be potentially consistent with an ABG classification.

This leads me to the next level of ABG interpretation. A practitioner should be aware of how the ABG results aid to the understanding of the patient's condition and treatment plan. Clearly this takes time to develop this level of understanding and students will gain experience by studying cases, a patient's history and clinical situations.

So as a student where does this leave us? Well using the tools and becoming an expert in the mechanics of classification and relating that to pathologies is a good start. My suggestion would be then to work on the application of this knowledge and using one of the most fundamental rules that I believe should always be applied.

Never interpret an ABG without knowledge of the patient's condition and history at the time of the ABG.

I can highlight the critical nature of this with a simple example, a patient has an ABG with a PaO2 of 85, SO2 of 98%, PaCO2 of 40, pH of 7.40, HCO3 of 24, BE of 0. Often when presented with these ABG values a student will suggest the patient has no immediate problems, then when confronted with the facts that the patient is on a non-rebreathing mask and a respiratory rate of 28 b/min the view of the ABG changes.

Another aspect of an ABG that is often overlooked is an assessment of oxygenation. Examination of the PaO2 and SO2 is only part of the equation, oxygen delivery is the other critical component so hemoglobin and cardiac output are critical and leads to my next rule of ABG assessment.

Never interpret an ABG or the oxygenation status of a patient without knowledge of the hemoglobin and cardiac status of the patient.

With these two simple rules in mind and in practice a novice practitioner of the respiratory arts is well on their way to being an expert. An important aspect of the application of these rules is to always ask for the patient information and provide the critical information when discussing ABGs.