Acid base 2 part – the answer

Not to beat a dead horse, but if I was a resident starting out, I wouldn’t know what to make of this discussion. I know blood gases can get complicated but in all likelihood, this case shouldn’t be that difficult.

Your contributors are intelligent, all good theories, but look at the spectrum of what has been postulated – acute respiratory alkalosis, chronic respiratory alkalosis, acute metabolic alkalosis, chronic metabolic alkalosis, acute metabolic acidosis, acute respiratory acidosis. In other words, every blood gas disorder but chronic respiratory and metabolic acidosis. I mean how is a person new to the field supposed to understand these theories?

My point is, and this is right in your wheelhouse, get back to basics. HAve the residents draw an ABG initially when the bicarb is 7 (my bicarb indication is a blood gas for any value less than 17 or greater than 30). Is that so hard? If we had an initial blood gas we could analyze the values and the changes fairly quickly and make them understandable to our residents.

Let me weigh in for a little bit of dialogue.

Patrick, you are correct in saying they wouldn’t alter management here. And their greatest value is in respiratory problems. But to say they are for entertainment otherwise is, I believe, incorrect.
It is the mentality that permeates testing today and the main reason ABGs are no longer drawn as frequently.
Every test does not alter management – there is no test that fulfills that criterion. To fail to draw blood gases because they don’t alter management is to ignore the fact they help you better understand the physiology of problems and that while they may not change today’s case or tomorrow’s for that matter, there will be situations where they will -and those who do not understand them will make the wrong decision.
Virtually every situation where there is a metabolic derangement – bicarb less than 16 or greater than 30- has some educational value even if it doesn’t change management. Sometimes it’s just the degree of metabolic alkalosis your diuretics have induced.

I have seen a number of respiratory arrests after surgery or opiate adminisitration in patients who had bicarbs of 35-40 and never had ABGs. No one considered the elevated bicarbs were a chronic respiratory acidosis.

Why get CBCs in everyone? They usually don’t change management. U/As virtually never change management unless there is urinary tract or kidney pathology but hey are ubiquitous in the ED.

Back to this case -even in this case the patient might have had a volatile alcohol ingestion and the management might have turned out quite differently.
It didn’t but it’s not hard to see it could have.

This is what they said about pulmonary artery catheters – and there is no doubt there were many situations where they were of no help or hurt patients But if you knew how and when to use them occasionally you saved a life. That is unlikely to happen anymore because the intrinsic knowledge and analysis that should have occurred didn’t and the technology withered. So no one knows how to use them.

There is much to be learned from ABGs in metabolic situations about diagnosis and severity of illness that will be lost because they are regarded as “entertainment”. They aren’t evidence based but I can guarantee you the physician who understands blood gas analysis will out perform the one who doesn’t more times than not. Just my opinion as an intensivist who studied blood gases for 30 years.

It’s turning into a fun discussion.
Point well taken – not everyone should have an ABG, only a fraction of people should have them compared to a u/a.
But you still sort of skirt the issue of why to obtain either test. One of the fundamental questions of medicine. Do we simply obtain tests that will change management today or do we develop indications so we know when and how to apply those tests in the future? Certainly part of the answer is the invasiveness of the test- we don’t cath all chest pain – but we do cath many negatives.

Most u/a’s don’t change management- if changing management were the reason for getting them, you wouldn’t get most of them (not that I’m opposed to it, for the same reason – it is information that adds to our collective information base), painless or not. Getting it because it is painless is low down on the reason for getting it. IT provides information on that aspect of all patients’ physiology which only occasionally becomes useful, but sometimes very useful. The fact it is painless just means we get more and have a lot more info to sift through. I won’t venture into whether this is right or wrong.

ABG’s when done properly with a 21 butterfly, with local anesthetic by a skilled practitioner, are generally not too painful (I’ll grant you there are exceptions and unskilled practitioners make them worse than they are). In a borderline case you might be able to rationalize not getting them for pain, true, but more often it is the inconvenience or ignorance of the unskilled or lazy practitioner that is the real reason. Which again leads to a cycle- people don’t like to get them so they don’t get them when they need them, leading to less information, occasional bad decisions and overall diminution of knowledge of acid base physiology. That’s one reason we speculate so much with acid-base problems today.

I’ve seen people come in short of breath, get 100% O2, noninvasive ventilation and be put on ventilators all without a single blood gas. It’s hard for me to see all those interventions and rationalize the possibility of small pain from the test not being done. I’ve seen unrecognized pCO2 of 90 because they were delayed. Let me come back once more to the patient in our example. the odds are the patient’s lytes will normalize, but supposing they don’t? Then the whole question of what’s going on remains unsettled -we still are n’t sure of what the actual problem is right now and while our best guess is probably right future events could prove us wrong. The patient might need another ABG, depending – and this might have been unnecessary with the initial ABG that was indicated. Remember we’re assuming Dr. C is telling us the end of the story, usually but not always, the right assumption.

And remember we’re generally talking about sick people who should get ABGs, that is a small fraction of those who get u/a s or cbc s. U’a ‘s are commonly drawn on people not even admitted to the hospital so it is clearly a larger subset and one whom the question of when to obtain the test certainly still holds. Same with cbcs, which can be argued are intermediate in the pain spectrum.

I brought the u/a up as an example of what indications we should have to draw routine tests and when they should be done as a matter of course. My point is there is a place for both. ABGs are clearly not a matter of course – Pain is a certainly factor in the decision to obtain ABGs but I would still submit that bicarb less than 17 or greater than 30, FIO2 of greater than 50% and severe shortness of breath are still reasons to draw blood gases in everyone. Based on current literature, I may be wrong but I don’t think many institutions even have indications for when to draw them – they are “entertainment”‘ (even in respiratory cases where people substitute saturations. That doesn’t sound right to me. The question goes right to the heart of when and how to order tests, any tests.

I’m open to counter arguments.