ABGs are my nemesis

Oi vey. In nursing school ABGs and acid/base balance drove me nuts. This weekend my charge nurse rightly commented that ABGs are the calculus of nursing, and as we were discussing it my former preceptor came to join in, noting that she’d never really understood ABGs. Big surprise, right? So on this particular evening my patient was headed in a downward spiral, had become unresponsive, tachycardic, severely tachypneic, and I had the admitting doc, rapid response team, respiratory, and the charge nurse all on board to figure out what was going on and how to turn things around before he deteriorated even more.

Clinical picture: 86 y.o. male admitted for failure to thrive. Found to have stage 4 lung cancer with liver mets. Enough jaundice to make crayola proud, oliguric with very little orange-colored urine.  Full code. Vitals all stable. Until they weren’t. Saturday night his respirations went up to 38/min, 96% O2 on room air according to pulse oximetry. He only responded to repeated painful stimuli. HR fluctuating 100-120. BP within normal limits and unchanged from his baseline. For some reason labs hadn’t been drawn on him in 3 days, so our information was limited. We drew ABGs and found the following:

pH 7.56

pCO2 22.5

pO2 65.4

HCO3 19.7

Base excess -1.8

So, analyze that! (Pause… think on it before reading on)… Respiratory response team, charge nurse, respiratory therapist, all scratched their heads and said, “Wow. That’s messed up.” They ordered CMP, CBC w/ differential, blood cultures, UA, C&S, PT INR, and ammonia level. According to rapid response, he was both acidotic and alkalotic (huh?!), with respiratory Acidosis - Alkalosiscause driven by metabolic imbalance. Let’s back that up. He likely went into metabolic acidosis that we didn’t catch. To compensate, he started blowing off CO2 – explaining the high resp rate. He blew off too much CO2, putting him in an alkalotic state, but the alkolosis was respiratory, per the ABGs. However, the underlying problem (hidden by ineffective and overzealous compensation) was metabolic acidosis. Let’s just be honest and say this isn’t the kind of scenario they cover in nursing school.

Now, to make the situation more interesting, his ammonia level was normal. Keep in mind, the liver filters ammonia out of the blood. When the liver isn’t working (which his obviously isn’t, judging by his LFTs and coloring to match the yellow brick road), you anticipate ammonia levels off the charts, since the liver can’t filter it out.

Sodium 123.6

Potassium 5.12

Calcium 1.02

Lactate 5.07

This is a really sick guy with a poor prognosis (and yet the family insisted on full code – very sad).  In the end, my patient went to the ICU, did not have to be intubated. But they did find that the cancer had also metastasized to the pancreas, and he was positive for MRSA. They assume the ammonia level in his brain was much higher than the ammonia level in the blood, explaining the altered mental status, and reflecting hepatic encephalopathy. In short though, there is some pathological process going on (not unlikely to be neurological) that is a missing piece in this puzzle. What a learning experience! Never stop learning. Never stop asking questions.


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