HOT TAKE - ABGs
- Dr. Zak Flaming
- Feb 22, 2021
- 2 min read
Every time I’m asked to get an arterial blood gas (ABG) for a consultant or attending, I shudder. Well, I stop and think, and then 9 times out of 10, I shudder. Since the dawn of time there have been arteries, and ever since then, healthcare providers have tried to get their draculaesque hands on some of the sweet, red mélange. Maybe it only seems like forever. Although arterial blood gas (ABG) analysis was not invented until the late 1950s, it still predates the Glasgow Coma Scale, home pregnancy tests, and dexamethasone, and should therefore be considered as old as time itself.
However, since then we have developed (read: popularized) a magnificent piece of technology that essentially antiquates the ABG: the pulse oximeter. The pulse oximeter has been widely available since the 1990s and is now at a negligible cost. With good waveform it can predict O2 saturations within +/-2% (American Thoracic Society), which can easily be converted to paO2 values. The accurate paO2 is frequently cited as one of the reasons for getting an ABG (over a VBG), so we fortunately don’t need to worry about obtaining this value anymore. The remainder of the differences can be summarized below:
This post is not to debate the accuracy of the difference between ABGs and VBGs; that is well-rooted in the evidence. This post is to question if we still need ABGs. The three main reasons I hear about why an ABG is needed are the follows 1) “to get an accurate paO2,” 2) “to calculate an A-a gradient,” and 3) “to know the actual paCO2.” Let’s explore these reasons:
1) As we discussed above, the pulse ox can accurately predict the paO2.
***Caveat: the pulse ox must have a good waveform, so this may not be useful in patients with poor peripheral perfusion or in severe acidosis***
2) Interestingly, in a study of 530 patients, the only pathology in which the ABG and A-a gradient provided diagnostic accuracy was with hyperventilation from anxiety disorder.
3) The paCO2 is primarily used in decisions on the far ends of the spectrum when it identifies whether a patient is compensating or retaining CO2 (and if this is affecting their mental status). Fortunately using a cutoff of a venous pCO2 of <45 mmHg, there is a 100% NPV of hypercarbia. And remember (referenced above), the pCO2 is fairly accurate on a VBG.
As the benefits of the ABG disappear, the downsides start to heavily outweigh them. Depending on your institution, respiratory therapists (RTs) may or may not do them, contributing to the ~$200 cost of obtaining an ABG. There is the time cost for the RT/provider. Additionally, there is the pain inflicted on the patient. There isn’t much published data on the potential damage to the radial nerve with repeated pokes, but I doubt the damage is zero. Many patients (both in evidence and in my experience) require more than 1 stick, and I’ve even had patients tell me the reason for their delayed presentation to the ED or reason for wanting to leave AMA is that they dread the ABGs.
Next time someone asks for an ABG, think about getting a VBG instead.
References:
7) https://rebelem.com/the-veinart-trial-vbg-vs-abg/
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