musings of a salt whisperer
In February, for the first time, Facebook sent more readers to PBFluids than Google. I believe this is from the ASN Kidney News now linking to my posts on their Facebook page. It’ll be interesting to see if the trend continues or if it was a one month aberration.
In response to the increasing importance of Facebook to my readership I have created a PBFluids facebook page. A new experiment with this medical blog unfolds. Exciting.
Nephron Power has a fine post on the top ten reasons the kidney’s are the best organs. Here is my riff:
The lungs serve to maintain the composition of the extracellular fluid with respect to oxygen and carbon dioxide, and with this their duty ends. The responsibility for maintaining the composition of this fluid in respect to other constituents devolves on the kidneys. It is no exaggeration to say that the composition of the body fluids is determined not by what the mouth takes in but what the kidneys keep: they are the master chemists of our internal environment. Which, so to speak, they manufacture in reverse by working it over some fifteen times a day. When among other duties, they excrete the ashes of our body fires, or remove from the blood the infinite variety of foreign substances that are constantly being absorbed from our indiscriminate gastrointestinal tracts, these excretory operations are incidental to the major task of keeping our internal environments in the ideal, balanced state.
When I talk about toxic alcohols causing anion gap metabolic acidosis I emphasize that these patients have large anion gaps. When you see an anion gap of 16 or 20 think uremia and lactic acid, not methanol. The cases I have seen have almost all had gaps greater than 25 and typically they run in the 30s.
But I have never heard of or seen a gap as big the one that came into the ICU this week-end:
To summarize the data from above. A patient was admitted with an anion gap of 65 that went up to 70 in the next 6.5 hours. I can visualize the ER doc reading the first chemistries, freaking out and re-ordering them, assuming that if the anion gap is greater than the chloride it must be a lab error, hence the repeat labs at 150 minutes after the first set.
Think about that, the anion gap was larger than the chloride concentration.
What kind of alien infestation causes numbers like that?
So when working up a large anion gap one tries to explain the anion gap. The lactic acid was only 34.5 mmol/L. So this patient has the highest lactic acid ever, yet it only covers half the gap. She also had serum ketones that remained positive at a 1:8 dilution and a creatinine of 14.
So is that it? Lactic acidosis, ketosis and uremia for an anion gap of 70? I sent off a D-lactic acid and a 5-oxoproline level, cause what the hell, when’s the next time I’m going to see an anion gap of 70. A toxic alcohol screen was sent.
To compete the picture the ABG was: 6.95/13.4/187 with a measured bicarb of 3. With numbers this crazy a trip to the Henderson-Hasselbalch formula is probably not a bad idea:
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| MedCalc has a sweet HH calculator |
So 6.97 is pretty close to the 6.95 measured, so no lab error at least in the ABG. Running Winter’s formula (1.5 x HCO3) + 8 ±2 gives a predicted pCO2 of 13, so no respiratory component to the metabolic acidosis.
The next step is to look for toxic alcohols while waiting for the assay to come back from toxicology lab at Children’s Hospital of Michigan. The osmolal gap calculation should be greater than 10 in the presence of methanol, isopropyl alcohol or ethylene glycol. The measured osmolality was 327.
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| MedCalc also has a sweet osmolar gap calculator |
So no significant osmolal gap rules out a toxic alcohol. This was confirmed by the toxicology screens that eventually came back. Given the anuric renal failure, profound intractable acidosis and unknown anions still unaccounted for we initiated CVVHD. You can see the effect that had on her bicarbonate and creatinine.
Michael Hyatt has an a good read about the costs of blogging and advises against getting bogged down in details about design, custom software and hosting. Find a way to cut through that BS inorder to start writing.
He recommends WordPress.com, premium version for $12-17 a year. He doesn’t say why he would choose that over Blogger (the host of PBFluids).
Today I received my bill for blogging for the next year here at PBFluids:
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| Blogging. Total bargain. |
$10. That’s all.
Here is Dr. Ves on Medical Blogging. He is actually quoting Seth Godin and Tom Peters. This best sums up my feelings on blogging:
It doesn’t matter if anyone reads it. What matters is the humility that comes from writing it. What matters is the metacognition of thinking about what you’re going to say.
No single thing in the last 15 years professionally has been more important to my life than blogging. It has changed my life, it has changed my perspective, it has changed my intellectual outlook, it’s changed my emotional outlook.
And it’s free.
Medicine is magical and magical is art
The Boy in the Bubble
And the baby with the baboon heart
On the first Friday of every month I give a lecture to the residents at St. John Hospital and Medical Center. I like to do an electrolyte lecture but for March the chief resident asked me to talk about cardiorenal syndrome. In researching the lecture I came across this article by Claudio Ronco.
The article defines cardiorenal syndrome as any condition with simultaneous kidney and heart failure. He then goes on to subdivide cardiorenal syndrome into 5 types:
This is terrible. Cardiorenal syndrome used to signify the unique cause of acute kidney injury where the decrease in function is due to apparent volume depletion in a patient that obviously overloaded. It named the only scenario where acute kidney injury responded to diuresis. It was unique and specific. Ronco comes along and says, yes I like your version of cardiorenal syndrome so I will make it type 1 in my new all purpose definition of cardiorenal syndrome. Now whenever there is cardiac dysfunction and simultaneous kidney dysfunction we can just call it cardiorenal syndrome.
It doesn’t have to be this way look at the example of hepatorenal syndrome. The syndrome does not refere to just any situation with simultaneous renal and liver dysfunction. It is a very specific diagnosis that only occurs with chronic liver disease and ascites. The patients must be oliguric, there is no non-oliguric HRS. Patients must be sodium avid and unresponsive to fluids and albumin. Additionally the patients cannot have laboratory or imaging evidence for an alternative cause of renal failure. Because of this definition hepatorenal syndrome identifies a very specific disorder, with a specific pathophysiology and unique prognosis and treatment options.
Ronco takes the beautiful and evocative name cardiorenal syndrome, strips it of all specificity and then tries to restore it by tacking on five different types. The fifth type 5 is the one that makes my brain explode. Sepsis, really? Acute kidney injury from sepsis that happens in the same patient who also suffers from sepsis induced cardiomyopathy should now be considered to have cardiorenal syndrome? Ronco is a man who has spent his life studying sepsis and acute renal failure, I can’t believe he is actually referring to that condition as CRS type 5.
I’m not buying what Ronco’s selling. Cardiorenal syndrome begins and ends with type 1 for me.
FYI: Here is the lecture (Keynote, PDF). It still needs some work. I’d like to add a section on ultrafiltration and I need to include the NEJM article on furosemide that was published yesterday.
I’m a big fan of the Renal Fellow Network but one of the consequences of the Post-Nate structure, with a large cohort of authors is variable quality. One of the new horses is a first year fellow at Stanford, Graham Abra. He is doing a great job. Take a look at the work he has done this year. Great stuff. His post on alimentary azotemia is about as good a post as I have ever read on RFN. I can’t wait for him to finish his work on Kt/V.
Keep on writing Graham, you’re hitting it hard.
The interesting concept, and the original question, is why does the creatinine rise when the GFR falls. Looking at the clearance formula if we decrease the GFR to 45 mL/min and keep the creatinine excretion fixed at 1,400 mg per day, the only way to balance the equation is to increase the serum creatinine.
The only way for the kidney to excrete the daily creatinine load is to allow the creatinine to rise. The increase in serum creatinine allows the kidney to clear the daily creatinine load.
But this doesn’t really answer why the creatinine rises with a falling GFR. The secret comes from the efficiency of ultrafiltration as the source of clearance. Excluding secretion in other parts of the nephron clearance is provided by filtration at the glomerulus.
Substances filtered at the glomerulus are found in the ultrafiltrate at the same concentrations they are found in the plasma. So a liter of ultrafiltrate will have 140 mEq of sodium and 4 mEq of potassium. These examples should make it clear that ultrafiltration is much more efficient for excreting substances found at high concentration. Americans consume about 180 mmol of sodium a day (4140 mg), this can be cleared with less than 1.5 liters of ultrafiltration. Potassium intake is around 50 mmol per day, clearing this much potassium requires 12 liters of ultrafiltrate. Note: sodium and potassium handling do not depend on ultrafiltration because of extensive reabsorption and secretion that largely overwhelm the effect of ultrafiltration.
Let’s look at the patient at steady state with, 1,400 mg of creatinine production, a GFR of 100 and a creatinine of 0.97. He suddenly loses half his renal function and now has a GFR of only 50 mL/min. Looking at the clearance formula, the only things that changes at first is the GFR. For the first moments after the loss of GFR the serum Cr will still be 0.97. We can solve for amount of creatinine excreted by the kidney at the GFR:
So with a GFR of 50 and a serum creatinine of 0.97, only 698mg, or just under half, of the creatinine created is excreted by the kidneys. It is impossible for the kidneys to clear the daily creatinine load with a GFR of 50 and a serum Cr of 0.97. The 702 mg of creatinine that are not excreted, remain behind and serves to increase the serum the creatinine. If the patient has 60% body water, his total body creatinine initially was 407 mg of creatinine (0.97 mg/dl x 420 dL body water) and the additional retained creatinine will raise his serum creatinine to 2.6 (407mg + 702mg divided by the same 420 dL).
The next day, armed with the higher serum creatinine of 2.6, the same GFR allows the body excrete 1,929 mg of creatinine, more than the daily creatinine load. The resulting creatinine is then 1.4 mg/dl. Ultimately if you carry this calculation forward the creatinine will stabilize at 2.16 mg/dl.
Understanding the equations and calculations is not as important as understanding that higher serum creatinines allow more creatinine to be cleared by ultrafiltration, in fact the only way for the kidney to excrete the same daily creatinine load at lower GFRs is by allowing the serum creatinine to rise.
Think of a rising creatinine as not so much a complication of renal failure but as an adaptation to renal failure.
Kidney transplant error: Wrong patient got kidney at USC University Hospital – latimes.com
Come on guys. At least pretend you’e trying to be competent.