I am a fan of the blood gas. I think Dr Centor at Medrants and I probably have more medical orgasms per day than any other internists on this earth, when it comes to blood gas evaluation and management. I love them. I love everything about the blood gas and find it to be one of the most important arsenals in the evaluation of an acutely ill patient.
In addition to the increased globulin gap, the blood gas is one of my 20 data points that every internist should seek out if ever in doubt about the status of their acutely ill patient. We are fleas. Be the proudest flea you can be. Order your blood gas with pride, even when the respiratory therapy lady is silently muttering to herself
Why the F does Dr Happy want a gas on this lady?
It's because my differential diagnosis extends beyond the lung to things only internists have ever heard of. Dr Centor and his case examples are an excellent resource for making you discouraged about blood gas evaluation and management. If you want to feel overwhelmed with the minutia of blood gas evaluation and management, make sure you bookmark his site and wait for those nuggets of academia to present themselves. With no offense intended, he is an academic doctor who's goal is to teach his residents and medical students about things none of them will never remember once they are
three years one year out of residency.
That's because the best friend an internist could ever have is a bag of normal saline? What is an internist? Or better yet, What is a hospitalist? We are experts in ordering saline, the best medical therapy ever formulated. I like to call saline the internist's chemo.
We become really good at fixing abnormal blood gases, or at least getting them to a stable base line. If you are ever being pimped by your attending and the question involves a blood gas, the answer will almost always be administer saline first, ask questions later. Medical students, keep this in mind the next time your attending asks you about that anion gap metabolic acidosis.
That may not fly with Dr Centor during his teaching rounds, but it works in the real world where real patients arrive with real medical problems. In fact, Happy's hospital is the largest referral center in my state and saline is almost always the answer. Especially when I get to experience the most radical case of acute metabolic alkalosis I've ever seen.
What are the possible blood gas abnormalities one can experience. Here they are:
- Acute vs Chronic
- Metabolic vs Respiratory
- Acidosis vs Alkalosis
- And for Acidosis you have a gap vs nongap.
There you are. Mixed acid base disorders can be be complicated. Now, if you know what you're doing, you'll be able to figure out 95% of the clinically important blood gas findings 95% of the time. The other 5% we'll have to settle for being unexplainable. Don't worry though, you'll never see those questions on your boards and the patient will never die because you couldn't figure it out. If you remember big picture and saline, you'll do just fine.
So here's the case example that gave me a new respect for what the human body can endure. I had a patient I cared for with normal baseline electrolytes and kidney function one month prior. She presented with several days of nausea and vomiting and this electrolyte panel. Her vitals were normal. She is a non smoker and only 40 years old.
|Na 136||CL 72||BUN 31|
|K 2.5||HCO3 49||Cr 2.6|
The first order I wrote was
Do not call the bicarb critical lab values. Thank you.
After I got that housekeeping issue in order, I was struck by the ferocity of her significant metabolic change, given her normal values the month prior. My presumption here is prerenal azotemia, even though it wasn't the classic 20:1 ratio, given the extent of her contraction alkalosis. Then I looked for an anion gap, as every good internist should do. 136-72-49. The gap of 15 is just slightly above the generally accepted normal gap of 12. That means her delta gap is only three, which doesn't excite me and would actually be normal for a VA patient.
Then I thought to myself, Happy, what do you imagine that blood gas would look like? I imagined a metabolic alkalosis, but given the extreme nature of her metabolic changes, I found myself concerned about what I might find. So I ordered a blood gas. She was awake and talking normally, so the possibility of say perhaps a narcotic or benzodiazepine drug overdose with acute hypercapnic respiratory failure would shock me, especially since it takes several days for the measured bicarbonate in the BMP to catch up to a persistent respiratory acidosis. Here are the pertinent blood gas findings:
|pH 7.56||pCO2 55|
In this case, you can see that her pH is greater than 7.4. That means she has an alkalosis. Her measured bicarb of 49 means this is a metabolic alkalosis. I don't remember the correction formulas, nor do they matter in the real world. I could even look them up on one of my medical iPhone apps, if I cared. But I don't. Since it doesn't matter. Except for Dr Centor and his residents, who are all going to forget them anyway once they graduate. I was there once. I know how it is. However, the truth is, if you
learn understand broad based physiology concepts well and see thousands of patients and you'll do just fine with blood gas physiology, even without the formulas.
pCO2 will always try and move in the same direction as the blood bicarbonate level. If your blood bicarb goes up, your gas CO2 will try to go up as well to compensate. How does one raise their pCO2? By slowing down their breathing. Your body is, in essence, trying to induce a respiratory acidosis (increased pC02) to compensate for the loss of acid and resultant metabolic alkalosis from the vomiting.
Now, if this lady had slowed her breathing down enough, she could have fully compensated her metabolic alkalosis by retaining enough carbon dioxide by slowing down her breathing, in order to drive down her pH back to a normal 7.4. I could make her compensate fully by giving her massive doses of narcotics and benzodiazepines. This would fix her alkalosis, but likely kill her in the process. In fact, I could even draw serial blood gases just to prove it, if I saw patients at an academic hospital.
Obviously, her body won't allow her to stop breathing. So we are left with an uncompensated case of acute extreme metabolic alkalosis. I suspect, given the extreme nature of her metabolic alkalosis, in order to get a normal pH out of her, she probably needs to have a pCO2 in the 80s. For an otherwise young healthy female without lung disease, this is the equivalent of having a heroin overdose. Her body wouldn't let it happen. Now put this same scenario in for an 85 year old and that patient would have been dead two days ago. They simply don't have the reserve to compensate such a radical metabolic change before death consumes.
In my eight years as a hospitalist, three years as a resident and two years of clinical work as a medical student, I have never seen such an extreme metabolic alkalosis. Dr Centor would probably have a hay day with this one, walking you through all the urine electrolytes, repeat blood gases, and even calculating the transtubular potassium gradient.
And then ultimately tell you that the treatment is saline. I'll jump you right to the end. I gave her saline and everything went back to normal. If you want cost effective, no nonsense, straight and to the point health care, you come to Happy. If you want to be an awesome case discussion during morning rounds someday at an academic institution and have one hundred labs drawn, serial blood gases, x-rays up the wazoo, head to your local teaching hospital. I know this to be true, because that's how all of us internists got trained.
The funny thing is, once we all leave the academic institute of higher learning, nobody practices medicine like that anymore.
We can all thank God for that one.