abg vs vbg

[hippaA]

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Is it always necessary to get an arterial blood gas to evaluate oxygenation and ventilation? Many patients in the icu have central lines but no arterial line.
If you draw a vbg and the pH is ok, the pco2 isn't high, and the pulse ox reading is ok, what is the utility of an arterial sample?

 

[Hernandez]

No, it isn't necessary.

SaO2 is more important than PaO2 anyways.

 

[Bostonredsox]

agree with Hernandez. As you mentioned, most of the MICU pts have CVCs and thus its easy to draw a VBG (which I do frequently to lok at ScvO2). From my own observation however, the most common reason for a MICU pt to have a CVC is for vasopressor support. There are many other reasons and many patients who are not on pressors who get CVC's. However if you think about it, everyone of those patients who got a CVC for pressor use should have an A-line. Atleast in SurvSepCa the Aline for IABP monitoring and vasopressor titration is a level 1a recommendation. Thus all those patients should get you ABGs without actually needing to be stuck repetitively. Just food for thought. I have noticed, atleast at my shop, even amongst my co residents, everyone is quick to throw in that IJ and start up their levophed but they are more than content to titrate it off a crappy cuff in a big fattys flabby upper arm. More A-lines I say. And it makes those abgs alot easier on the overworked RT staff. That said, if you dont have a true indication for an Aline but your pt has a CVC, you can use VBGs quite reliably to titrate/wean the vent. Assuming you are getting good pulse ox readings for accurate SpO2s.

 

[VentdependenT]

What is a "normal" vbg?
What is the most useful value in a vbg?

depends on what you are looking for concerning abg vs vbg. I do not use vbg's to assess ventilation or oxygenation.

 

[Hernandez]

What is a "normal" vbg?
What is the most useful value in a vbg?

depends on what you are looking for concerning abg vs vbg. I do not use vbg's to assess ventilation or oxygenation.

Why not? Granted it takes some understanding of the clinical condition and exercise physiology and making a few assumptions but it can be used and I use it all the time. In patients.

for acid base physiology there is good ER data for use of VBG in DKA and they have decent data for AECOPD.

A very rough rule of thumb I use is to correlate to ABG is pH will be 0.05 lower and PaCO2 will be 5 higher. These don't necessarily hold true in febrile, hypothermic, pts with significant metabolic activity,or sever sepsis.

 

[VentdependenT]

Like I stated, depends on what you are using it for.

I am aware of what a "normal" VBG should look like but I think there is too much wiggle room to use it routinely for anything else other than reassurance of oxygenation/ventllation. I only use them for SV02 which I find highly useful.

Ive nevet said "ok, get me a VBG for that awful looking DKAer." Learn me something. For DKA I use gap and bicarb, I dont really care about Ph as much unless its less than 7.0

 

[Hernandez]

Technically you have to have pH <7.3 to dx DKA it also surprises me how often they have complicated acid base disorders. Granted I've seen lots of well looking DKA With pH <7.0. And most of them correct in a few hours.

 

[glorfindel]

Ive nevet said "ok, get me a VBG for that awful looking DKAer." Learn me something. For DKA I use gap and bicarb, I dont really care about Ph as much unless its less than 7.0

In the ER I also use gap and bicarb (and B-hydroxybutyrate and RR and clinical exam). I get VBGs, but only because the inpatient doc wants one. It doesn't change my ER management, but I suppose it might be more useful for the IP doc.

Especially if the reason for the DKA was anything other than forgetting or refusing to take insulin, e.g. pancreatitis, mi, toxic ingestion, etc.

 

[Bostonredsox]

Like I stated, depends on what you are using it for.

I am aware of what a "normal" VBG should look like but I think there is too much wiggle room to use it routinely for anything else other than reassurance of oxygenation/ventllation. I only use them for SV02 which I find highly useful.

Ive nevet said "ok, get me a VBG for that awful looking DKAer." Learn me something. For DKA I use gap and bicarb, I dont really care about Ph as much unless its less than 7.0

In addition to technical diagnosis as hernandez eluded too, you can treat and close the gap with just the q1h fingersticks and q2h BMPs but there is an indication to give bicarb and it is based on the pH so it is useful to have it, whether it is via ABG or corrected VBG. And yes I am aware of http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3224469/ showing possibly no benefit from bicarb but several guidelines that are recent in nature still have it in the algorithim if pH is less than 7.15 if there are arrythmias, hypotension or other worsening clinical markers, particularly if they require the vent

I think VBG is plenty fine for AECOPD though I am not as familar with the data as Hern. I am only looking at the pH and CO2 and if the ph corrected for VBG difference is a happy 7.3 with a CO2 of 55 that end stage COPDr is prolly back to baseline, I do not need there PaO2.

I agree with Vent though, I am not comfortable titrating and assessing oxygenation on the vent with VBG unless I have a very reliable spO2 and a normalizing pH on VBG. but that is just from lack of experieince using it in that situation. It may well be ok in more experienced hands.

 

[sluggs]

Pure opinion: ABGs are overdrawn and overused. Often (not always) you can use O2 sat, minute ventilation, and physical exam to tell what is going on (yes, there are exceptions to this, of course, but frequent ABGs are over-rated). That having been said, a very smart pulmonologist once said to me: "when you need and abg, DO NOT substitute a vgb-- I don't even know how to read one, or what they mean, except for ScvO2".

 

[Hernandez]

That having been said, a very smart pulmonologist once said to me: "when you need and abg, DO NOT substitute a vgb-- I don't even know how to read one, or what they mean, except for ScvO2".

That's not a very smart sounding thing to me. Perhaps it's because I actually read lots of cardio-pulm stress and have a more than basic understanding of exercise physiology that I know the answers but the regional variations and arterial v venous differences in blood gas isn't mythical or hard to use to utilize. That's like saying the same thing about ETCO2 just excuse there aren't standards for everyone it's a worthless test.

As you said in your first post, 99 times out of a 100 all I need is a SaO2 and MV but VBGs can have lots of usedul info and i will even use them for acute decompensations that aren't oxygenation issues, I frequently use VBGs to assess ventilation and for the w/u of respiratory distress.