ABG Results, Explain this to me....

Cowboyardee · Apr 10, 2016

I could hazard a guess, but my explanation would be pretty mundane.

I don't know why your blood gas wouldn't include an sao2. As for the bicarb value missing, it's missing because bicarbonate is not measured directly by ABGs but rather calculated using pH and CO2 values. Without a specific CO2 value, bicarb could not be calculated. I don't know how saO2 is calculated/measured specifically, but there may be a similar problem.

As for the CO2 and pH... I'm wondering if you saw the patient shortly before and/or during the ABG draw. Sometimes when I orient new ICU nurses, I use shallow water blackout as a regular-world example to illustrate the effects of RR on O2 and CO2, and how respiratory drives work - useful for RNs who don't yet understand how to titrate ventilator settings based on ABGs. But another takeaway from the topic is that CO2 levels can respond very quickly to hyperventilation.

Shallow water blackout - Wikipedia, the free encyclopedia

Though his RR was 10-12, it seems possible to me that an acute episode of hyperventilation before and/or during the draw could have resulted in the very low CO2 value you're seeing. Pain from repeated arterial sticks could have even caused or exacerbated an acute episode of tachypnea. His breathing may have been slower before or afterwards when you checked on him. Also bear in mind that hyperventilation is not only related to the respiratory rate but also to the depth. And since episodes of hyperventilation aren't uncommon after CVAs, that strikes me as the most likely scenario.

Wile E Coyote, ASN, RN · Apr 11, 2016

To add an aside to the the above: Serum CO2 values (venous CMP in your case) account for more sources of CO2 than that calculated in the ABG. Serum CO2 value included ALL forms of CO2...dissolved and as carbonic acid. Thus, the serum CO2 can be higher than from an ABG drawn and ran at the same time. As already noted, this pt could have had an irregular resp pattern related to his CVA or just hyperventilated from stimulation during/immed. prior to draw.

jadelpn, LPN, EMT-B · Apr 11, 2016

Love this question, and the answers.

I learn something all the time. Thank you.

I am throwing it out there--Is there a difference between what you would see on an arterial blood gas and a venous stick?

Also 6 lpm of O2 can be excessive on the drive to breathe. Of course dependent on situation, but something to consider. Perhaps type of stroke dependent on deficient.

Further, are the results skewed or could be due to the patient not being off of their O2 for whatever the policy is prior to the ABG stick?

Just thinking out loud--I do not know any of this to be necessarily correct, just some musings.....

Great question, great answers.

TheCommuter, BSN, RN · Apr 11, 2016

Your thread's been moved to our Critical Care forum for more responses.

mchesney · Apr 11, 2016

Here they are: pH 7.6, HCO3 16, CO2
Interesting blood gases. I'm not sure what a "CMP" is but I'm guessing this was a serum bicarbonate level? It is for sure a respiratory alkalosis and my guess would be a neurogenic alteration in respirations from CVA causing hypocapnea. It may have been going on a while because the kidneys have had time to start adjusting the bicarbonate levels to try and normalize the pH...this usually takes days. One might think there was possibly a metabolic acidosis (bicarb 16) and they compensated by blowing off CO2 but the body never overshoots the correction. So if the gas had been 7.30, pCO2

Laurie52 · Apr 11, 2016

I would redraw and have it run on a different machine. It doesn't make any sense that a pco2 would be so low with a respiratory rate of 10 to 12 and the fact that it didn't give a saturation is suspicious.

CarpeDiem'15 · Apr 11, 2016

Based off the ABGs, I would definitely say the patient was showing respiratory alkalosis. The patient must have had an episode of hyperventilation at some point prior to the blood draw. I need more info about the patient though to reason through why he/she was hyperventilating. The location of the CVA within the brain is a big determinant of how it affects respirations, so it would be nice to have some more info about the actual CVA (bilateral? mid-brain? low pons? medulla? upper pontie tegmentum?). Based off the fact that this patient has a near-normal paO2, I'm suspecting central neurogenic hyperventilation. The low RR is probably from the low paCO2 reflexively decreasing respirations via chemoreceptor inhibitory input into the patient's respiratory center. What were the patient's respiratory patterns like? Were they regular? If the patient had periods of apnea followed by clustered breathing (hyperventilatory pattern), that could also explain the low paCO2 and the mildly hypoxemic paO2....

offlabel · Apr 11, 2016

A PaCO2 of less than 16 mmHg is incompatible with consciousness. Artifact. All of the other indices not reading or "out of range" indicate sample or analysis error. Get another gas on another machine.

cocoa_puff · Apr 12, 2016

I'm not sure what a "CMP" is but I'm guessing this was a serum bicarbonate level?

CMP = Complete Metabolic Panel

RT2RN15, MSN · Apr 21, 2016

Patients who are neurogenic breathing breathing often exhibit "funky" breathing patterns. Often times they have huge tidal volumes (>1000 cc's)with low respiratory rates. Can be related to a brain-stem issue. Any CO2 that is outside analytical measurement will be unreportable and all abg analazyers are a little different [the one's I used the cut off was at 19]. A CO2 that is unreportable will yield a bicarb that is incalculable. If there is a PaO2 reported there is a high probability the SaO2 should have been reported by the machine. Either way - a PaO2 in the 70s on 6L via NC is not any thing to make you flustered.

If I was the RT I would have re-ran the sample to verify there wasn't an error with the machine (it can happen) but I wouldn't believe it to be a bad sample if it came back with the same results the second time. Neuro patients. . .what a wonderful thing.

pmath_RRT · Dec 5, 2017

Redraw. That is an false reading. Possibly air got into the sample or the ABG machine isn't calibrated right. Always "look at your patient". Do those results reflect how the pt looks?