ABG question.

Specialties MICU

Published

Specializes in SICU, Peds CVICU.

The other day my patient's ABG came back (roughly)

ph 7.5

pco2 25

Bicarb 18

Base Excess -4

So I say he's in partially compensated Resp Alk, right? I go home, the cardiac surgeon comes in and barks at the nurse following me that the patient's acidotic because his base excess is negative.

I know base excess = the buffering ions in the patient's blood, and at my facility +/- 2 is "norm"... but isn't acid/alk. based on the pH? Is the cardiac surgeon *gasp* wrong? or is there something more to interpreting ABGs that nursing school didn't tell me?

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Specializes in Critical Care.

Seems to me the negative base excess is because he's compensating for the alkalosis. Negative excess = producing acid, which is nothing short of "well, duh!" given the alkalosis.

Not sure what the surgeon was complaining about when the pH is 7.5.

/just a student, so my statement is speculative.

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Specializes in Trauma ICU, MICU/SICU.

I'm no expert on gases and am always learning. Base deficit/excess being my weakest area?

What I do know is that base excess/deficit is calculated based on the values obtained in the ABG. It is not an actual measurement. Right everyone??? I'm pretty sure I'm right about this....

Basically, cardiac surgeon is wrong. The pt. is clearly (but not terribly) alkalotic.His base excess is negative because of the increased Bicarb (-ion). I do know that respiratory distress often starts out as respiratory alkalosis, then when pt. poops out they quickly become acidotic. Was your pt. is respiratory distress? Why was patient hyperventilated?

He is almost fully compensated. It's really not a bad gas in the scheme of things... What was he upset about? What did he want done that wasn't being done? Was the patient on a vent? If so, that is a good time to consult your respiratory therapist regarding vent changes. They're really good with gases and can often fix the problem at the pulmonary level rather than the body correcting it metabolically.

Perhaps this is what super surgeon was upset about? :confused: Too bad you weren't there... Or perhaps it was good that you weren't there. :icon_roll

Take care!

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Specializes in CCU/CVU/ICU.
The other day my patient's ABG came back (roughly)

ph 7.5

pco2 25

Bicarb 18

Base Excess -4

So I say he's in partially compensated Resp Alk, right? I go home, the cardiac surgeon comes in and barks at the nurse following me that the patient's acidotic because his base excess is negative.

I know base excess = the buffering ions in the patient's blood, and at my facility +/- 2 is "norm"... but isn't acid/alk. based on the pH? Is the cardiac surgeon *gasp* wrong? or is there something more to interpreting ABGs that nursing school didn't tell me?

Actually, these gasses represent a compensated metabolic acidosis. (or...if you want to split hairs maybe an 'over-compensated met acidosis' as the Ph is now 7.5)

Yes, you're right that per text-books acid-base balance is based on Ph...BUT, if this patient's underlying acidosis were corrected, he would likely stop hyperventilating and his Ph would normalize...

Am i confusing you?

Here's an example... Lets say you have a tubed/vented/sedated patient with a metabolic acidosis (for whatever reason). ph 7.30, CO2:40,Bicarb 19, BE -4. You could then increase the rate of the vent enough and drop the CO2 to a point where the Ph would look normal (by Ph)...or (similar to your patient) perhaps even make him Alkylotic (by Ph) if you ventilated him too much/too fast and blew off too much CO2. HOWEVER...you did nothing to really fix his metablic acidosis...he's still 'acidotic' on a cellular level...And you would be in error calling the resultant ABGs a compensated respiratory Alkylosis...it's in fact a compensated (over-compensated?) metabolic acidosis.

IF your patient was awake and not ventilated, he was simply breathing way faster than he needed to...which he couldnt sustain (a Ph of 7.5) for very long without crashing.

I think.

I obviously dont know your patient. But with the ABgs/details you gave, it's my best guess.

Sorry if i confused you...it's late!

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Specializes in Critical Care Nursing.

continuing on from previous email primary metabolic alkalosis are rare in general ICU.

Many are due to loss of hydrogen ions OR drug problems. However as I am not familiar with post cardiac surgery I am wondering if those experienced in this area may see a metabolic alkalosis in this patient cohort?

useful ABG education site

http://orlandohealth.com/pdf%20folder/Inter%20of%20Arterial%20Blood%20Gas.pdf

http://www.acid-base.com/

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The pt is in a respiratory alkalosis. pH > 7.45 with a low pCO2.

There's no such thing as overcompensating, the body just doesnt do it when it comes to acid/base and ABG's.

Bicarb could be low d/t renal function.

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Specializes in CCU/CVU/ICU.
The pt is in a respiratory alkalosis. pH > 7.45 with a low pCO2.

There's no such thing as overcompensating, the body just doesnt do it when it comes to acid/base and ABG's.

Bicarb could be low d/t renal function.

The term 'overcompensation' was a made up word.

The idea of 'overcompensation' that i brought up was referring(sp?) to a hyperventilated patient on a ventilator (man made, not body-made)---which happens.

Patient starts out acidotic...the vent settings (TV, RR, etc) are too high and CO2 drops lower than desired...giving you an alkylotic Ph.

AND...you dont/shouldnt treat metabolic acidosis with ventilator manipulation. It's poor practice. Makes numbers(ph) look better but does nothing to address(sp?) the underlying issue.

The OP would agree with you (and others) that if you follow the Text-books on ABG interpretation, step one is to look at Ph, step two look at CO2. The OP did this...and correctly determined (as did you) respiratory Alkylosis.

But...The origional question was why did the surgeon say the patient was Acidotic.

And yes..bicarb could be low d/t renal function...or a bazillion other reasons. But (whats funny) is that when you say this, you're speaking out of both sides of your mouth ie. 'patient's alkylotic...but could be acidotic from renal failure'.

Any human being that has (by ABGs) a bicarb of 18 and a BE of -4 wont be walking around feeling OK (with marginal kidney function as you suggest). They'd be sick as all get-out, huddled on a bed somewhere hyperventilating. This hyperventilation IS a compensatory mechanism and the patient MAY have a normal ph. (or even slightly alkylotic) but is still acidotic. ((i guess this last example is to show that by hyperventilating you are able to develope a (temporary)respiratory alkylosis in the face of a metabolic acisodis...and could in effect actually be a body-driven(rather than ventilator-driven) 'overcompensation'))...

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Specializes in CCU/CVU/ICU.
The other day my patient's ABG came back (roughly)

ph 7.5

pco2 25

Bicarb 18

Base Excess -4

So I say he's in partially compensated Resp Alk, right? I go home, the cardiac surgeon comes in and barks at the nurse following me that the patient's acidotic because his base excess is negative.

I know base excess = the buffering ions in the patient's blood, and at my facility +/- 2 is "norm"... but isn't acid/alk. based on the pH? Is the cardiac surgeon *gasp* wrong? or is there something more to interpreting ABGs that nursing school didn't tell me?

And one other thing to consider...

A pure respiratory alkylosis means a person is hyperventilating enough to throw off the body's acid-base balance. If this person has hyperventilated to this point he/she is going to feel VERY strange/sick/dizzy/whatever...(which would make it VERY hard for person to maintain)

The kidneys (body's primary 'bicarb factory') can take many days/weeks to compensate for respiratory-driven acid/base disturbances. (Which is why they cant help/compensate in acute hypercapnia's, etc...)

So...again assuming a person is hyperventilating enough to become alkylotic (and feeling all the symptoms that go with it), they would have to maintain this hyperventilation for many days/weeks before you might see any type of metabolic compensation....or a 'compensated/partially compensated respiratory alkylosis'...and would be an extremely unusual case.

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Specializes in ICU, CVICU.
I'm no expert on gases and am always learning. Base deficit/excess being my weakest area?

What I do know is that base excess/deficit is calculated based on the values obtained in the ABG. It is not an actual measurement. Right everyone??? I'm pretty sure I'm right about this....

You are right on- in fact our pulmonary docs don't even want to know that stuff when you call with the gas results. All they want is pH, pO2, pCO2 because those are the only things directly measured from the blood while everything else is a calculation. Anyhoo- I think the cardiac surgeon was just being a cardiac surgeon (if you know what I mean).:chuckle

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Specializes in Critical Care Nursing.

there is a saying where in australia when you make a clanger (mistake) , its "folks are dumb where I come from":imbar

you shouldnt answer emails when you haven't had much sleep.

humbled me

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