determining if ABG was really a VBG

[bulldog]

I sometimes have to draw ABG and while blood flow seemed pretty quick and darkish red (not super bright red), pO2 occasionally comes back something like 70-80 for someone on room air while their O2sats is in the mid 90's. this might be understandable if they hae pre-existing lung dz, acute pna, etc. If it's a VBG, shouldn't the pCO2 be lower, like the 50's, or could VBG be higher have higher pO2's up to 70s?

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[HomerSD]

I sometimes have to draw ABG and while blood flow seemed pretty quick and darkish red (not super bright red), pO2 occasionally comes back something like 70-80 for someone on room air while their O2sats is in the mid 90's. this might be understandable if they hae pre-existing lung dz, acute pna, etc. If it's a VBG, shouldn't the pCO2 be lower, like the 50's, or could VBG be higher have higher pO2's up to 70s?

A pO2 in the 70s correlates with a O2 sat in the mid 90s, assuming adult hemoglobin and no elevated levels of 2,3 DPG--consistent with you drawing an ABG on that patient. In a normal individual on room air, the expected normal range of pO2s on a VBG would be in the 30s to 50s, giving a sat around 40-85%. A peripheral O2 sat can give you a rough estimate of the pO2 (assuming a good waveform and no weird situations like methemoglobin) and if there is a large disparity in the pO2 on the blood gas, it's suggestive of a VBG rather than an ABG.

 

[swpm]

I sometimes have to draw ABG and while blood flow seemed pretty quick and darkish red (not super bright red), pO2 occasionally comes back something like 70-80 for someone on room air while their O2sats is in the mid 90's. this might be understandable if they hae pre-existing lung dz, acute pna, etc. If it's a VBG, shouldn't the pCO2 be lower, like the 50's, or could VBG be higher have higher pO2's up to 70s?

Sometimes, it's just not possible to tell if a sample is venous or arterial. Fortunately, the ambiguity in your example probably has no clinical implications - if the PO2 on room air is 70, then the sat is going to be near 100, so who cares if it's arterial or venous? The patient's clearly oxygenating fine. OTOH, if the patient's on supplemental O2 and has an arterial PO2 of 70, then he has a big A-a gradient; that's a problem. If a venous sample has a PO2 of 70, then maybe he's normal, maybe he's not.

One thing to keep in mind is that there's a huge difference in the values you'll get from a true mixed-venous gas taken from a PA catheter, where your expected O2 sat in a normal person is around 70% regardless of FiO2 ... and an inadvertent wrist-vein stick in someone with a warm hand (ie, good flow and very little O2 extraction from metabolically not-so-active tissue) which may well be indistinguishable from an arterial sample for a person on room air.

O2 uptake in a warm limb can be quite low, resulting in very small differences in ABG vs VBG values measured in peripheral samples.

For someone on supplemental oxygen with basically normal lungs (NL diffusion, no shunt), a high (over ~100) PO2 on a peripheral stick all but guarantees that it's an arterial sample. Unfortunately the reverse isn't always true ... and I'm unaware of any simple, reliable rules of thumb to answer the ABG vs VBG question for those borderline cases. There are just too many confounding factors, ranging from regional flow to O2 uptake to mixing when dealing with peripheral samples.

Clear as mud.

 

[Faebinder]

When in doubt, draw another one from a different site. Antecubital is the second location I would go for... then femoral.

 

[DarthNeurology]

There is a good rule for ABGs, I think it is Surgical Recall, the 90-60, 80-50, 70-40 rule where the first number is the SaO2 and the second is the pO2, for example,

SaO2 (Pulse Ox) PaO2

100 . . . . . . . . . .70-75 to sometimes 200 on a vent with 100% O2
90. . . . . . . . . . . 60 "Borderline normal arterial blood gas"

88 . . . . . .. . . . . or 55 on room air: requirement for long-term oxygen or SaO2 89 to 90 or PaO2 56 to 60 and cor pulmonale, pulmonary hypertension, or persistent errythrocytosis and requirement for long-term oxygenation is met.

80 . . . . . . . . . . .50 A COPD patient may drop to this level when they ambulate but not at rest so pulse ox's are checked when patients ambulate too
70. . . . . . . . . . . 40 - "Normal venous blood gas"
60 . . . . . . . . . . .30 - I have seen in severely septic patients in an ICU setting

A pulse Ox of 90 is close to a pO2 of 60 (usually this is arterial blood) and below 93 on pulse Ox is possibly when to start supplemental O2 per some medicine residents I have trained with as a pO2 of 60 is concerning as this is where you start to go down that sigmoid oxygenation curve. I won't address COPDers who live with a lower pO2 in this range.

A pulse Ox of 70 (usually you get this number with an VBG or a mixed venous O2 Saturation) correlates with a pO2 of around 40, and this number reflects oxygen utilization as is used in critical care settings. When low is bad as body is sucking up oxygen at faster then normal rates and there is deficit, when too high can be bad as oxygen is being shunted around tissue or via mitochondrial shunting in sepsis (!)

Usually when you take an ABG correctly you don't need to pull up the blood into the ABG tube, it pulsates into the tube under strong arterial pressure. All of the VBGs drawn in error I have seen had a pO2 of around 40 (expectedly).

 

[Jeff05]

anesthesiology resident here.

there is NO way to tell based on the info you provided. unless you have a definite VBG for comparision.

remember that a sat of 90 corresponds to po2 of 60.

so a po2 of 70 could very well be a sat in the mid-90s.

in a healthy person, breathing normally the po2 should be 90-100 at 1atm.

 

[paleryder44]

Hello..I have quesiton. I was in ER with acute bronchitis..They did only a venous Blood gas. Chext X ray revealed left sided Atelectaseisis and acute bronchitis. What I am concerned over is the venous blood gas showed extremely low (even for venous) level of PO2 24 !! range 90-100 The O2 SAT was 40 (range 95+, The PCo2 wa 43 on venous (range 45 +_) and s also low at as well as I have chext pain on left side. Any sauggestion? Possible Pulm. embolism? Possible air bubble? The bronchitis is under control... (a weel opf prenisone) but still have chest pain left side NOT HEART. Had angiogram and alreasdy have 6 stents. Angio was clean. NO additional arterial clogging Ideas? Chest pain is intermittent yet always present (worse at times even laying in bed) (walking does not make it worse). Seems to act like a long "spasm" of tightness, pressure pn left side of chest/. Lung X rays..2 different show no COLLAPSE... just left sided Atelectaseis//idea?

 

[group_theory]

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