64 BMP Masterclass
64.1 What this covers
- Sodium
- Potassium
- Chloride
64.2 Learning objectives
- Sodium
- Potassium
- Chloride
- Bicarbonate
- BUN
- sCr
- Glucose
64.3 Bottom line / summary
- You’re a physician in the early days of medicine and you need to come up with 7 tests to interrogate renal function…
- How did medicine arrive at the tests we did?
- When is this misleading?
- Is Saline Nephrotoxic?
- Does a high or low bicarbonate indicate that there must be a metabolic alkalosis or acidosis present?
64.4 Approach
- rebuild an understanding of electrolyte balance from the clinical application to the scientific / physiology (as opposed vice versa)
- generate a better understanding of the gaps of our routine labs in assessment of the body’s ability to maintain homeostasis
- surrogate for osmolality / tonicity of the serum.
- of course this goes wrong if there is another major source of osmolality, such as alcohol / glucose / contrast
- important in the functioning of nerve cells (particularly the resting membrane potential)
64.5 Red flags / when to escalate
- TODO: List red flags that require urgent escalation.
64.6 Common pitfalls
- TODO: Capture common errors or missed steps.
64.7 References
- https://curiousclinicians.com/2020/08/19/episode-7-is-saline-nephrotoxic/
- https://doi.org/10.1681/ASN.2011070720
64.8 Source notes
64.8.1 BMP Masterclass
65 BMP Masterclass
65.0.1 Learning goals:
- rebuild an understanding of electrolyte balance from the clinical application to the scientific / physiology (as opposed vice versa)
- generate a better understanding of the gaps of our routine labs in assessment of the body’s ability to maintain homeostasis
You’re a physician in the early days of medicine and you need to come up with 7 tests to interrogate renal function… what do you choose? How did medicine arrive at the tests we did?
65.1 Sodium
- surrogate for osmolality / tonicity of the serum.
When is this misleading? - of course this goes wrong if there is another major source of osmolality, such as alcohol / glucose / contrast
65.2 Potassium
- important in the functioning of nerve cells (particularly the resting membrane potential)
65.3 Chloride
- allows for calculation of the anion gap (characterize what anions on present in the blood, which is especially important to differentiate the different types of causes of metabolic acidosis)
- Post-SMART trials, now understood to additionally be inherently renal-toxicity (vasoconstriction, predisposes to vasoconstriction)
65.4 Bicarbonate
Does a high or low bicarbonate indicate that there must be a metabolic alkalosis or acidosis present? - Yes: under Stewart conception of acid base, the serum bicarbonate allows you to assess whether there is a metabolic acidosis or alkalosis present (though, it does not tell you if it is primary or compensatory - this requires a blood gas)
Keep in mind: Acid removal…
- 30 - 60 mg of fixed acid is removed in kidney
- 14,000 mg of volatile acid (CO2 via bicarbonate buffer system) is removed by the lungs
65.5 BUN
What information does the BUN give you that the creatinine doesn’t? - it is reabsorbed in the tubules, so the BUN / creatinine ratio can tell you about how much the kidneys are trying to hold on to volume. - nitrogen balance (e.g. in a catabolic state such as with high dose steroids, BUN will go up. Also, in a GI bleed where
65.6 sCr
Allows for estimation of glomerular filtration rate, as it is not reabsorbed (it is somewhat actively secreted). What are the major roles of kidney’s work to maintain hemostasis that are not interrogated by sCr?
- tubule function (e.g. ability to reabsorb electrolytes)
- function of filtration unit (GFR = amount filtered, not type of things filtered… e.g. is albumin getting through?)
65.7 Glucose
- allows us to interrogate the insulin-glucagon axis functioning, as well as additional information on osmolality (e.g. in HHS where your sodium is likely low but overal osmolality is high due to all the glucose)
65.8 Calcium
- role in clotting
- role in nerve functioning
65.9 Albumin
- surrogate for oncotic pressure in the vessels.
66 Other Acid Base
66.1 Metabolic Alkalosis
Chloride depletion alkalosis formerly known as contraction alkalosis - JASN
- Key point: volume depletion is not required to maintain the alkalosis (just chloride depletion). So, metabolic alkalosis after diuretics should not be used as sole indication of adequate diuresis. Keep going (add acetazolamide if problematic)
Urine chloride measurement = surrogate for if chloride responsive or no.
66.2 Metabolic Acidosis
66.2.1 Anion gap - GOLDMARK
66.2.2 Non-anion gap
66.2.2.1 Urine anion gap
Surrogate for NH4+ excretion, which is the way that the urine gets rid of excess H+