"But the base deficit is..."

[McPoyle]

So... new fellow in the u it this week and everyday I have some service bust my balls about "there base deficit is high" or "let's tank em up and clear the base deficit" or some other nonsense. My question is, I for the life of me cannot find any reasonable explanation for why I should care about the base deficit in regards to the patient's volume status... am I missing something here?


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[dchz]

What's the strong ion difference? pt's pH?

Base deficit is plastered everywhere on trauma literature because trauma surgeons are intensivists.

 

[Nash]

So... new fellow in the u it this week and everyday I have some service bust my balls about "there base deficit is high" or "let's tank em up and clear the base deficit" or some other nonsense. My question is, I for the life of me cannot find any reasonable explanation for why I should care about the base deficit in regards to the patient's volume status... am I missing something here?


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As you know, a negative base excess or base "deficit" is an indicator of an underlying process that is driving an acidosis. It isn't specific for type of acidosis. I've seen plenty of base "deficits" secondary to hyperchloremia. I'll leave the strong ion theory discussion out for sake of simplicity.

Resuscitating a patient based on base "deficit" alone does not make sense, if the patient is otherwise doing well, especially if that deficit is due to iatrogenic chloride administration. Personally, I try to avoid making knee jerk decisions based on singular data points in the ICU.

 

[engineeredout]

I've never been able to get an answer to the following:

What does base deficit tell me that pH and HCO3- doesn't?

 

[engineeredout]

So... new fellow in the u it this week and everyday I have some service bust my balls about "there base deficit is high" or "let's tank em up and clear the base deficit" or some other nonsense. My question is, I for the life of me cannot find any reasonable explanation for why I should care about the base deficit in regards to the patient's volume status... am I missing something here?


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Seems like the same mentality of "The lactate is elevated - give more fluids until it clears"

 

[McPoyle]

That is my feeling too, just get tired of people suggesting fluids for base deficit and for cvp...


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[Nash]

That is my feeling too, just get tired of people suggesting fluids for base deficit and for cvp...


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That's exactly the point. The base deficit is just a number, a rough surrogate for the serum HCO3. It has to be interpreted in the context of the patient. I don't use base deficit, lactate, CVP, or any other single data point to make decisions about resuscitating a patient.

 

[BigRedBeta]

Truthfully, it's information creep. I believe there is coherent data (but probably not enough to say it's evidence based) in the trauma literature with specific regard to hemorrhagic shock and base deficit as a marker of relative hypovolemia. It makes more sense in that regard as you may not be able to have an accurate estimate of intravascular volume loss - either blood lost in the field or into potential spaces. But surgical residents learn it on their trauma rotations, accept it as gospel, then repeat it in other instances and it disperses to other types of patients and physiologies.

 

[sluggs]

The base deficit tells you how much bicarbonate you are short: base deficit x body weight x 2/3 = meq of bicarb you are down.
This does not mean that you should replace the whole thing immediately, or that bicarb is the answer.... you need to look at lactate (not great to treat lactic acidosis with bicarb), anion gap, and the cause of the acidosis. You also need to look at surrogates for the patient's volume status before you start throwing fluid at a patient-- you can have a CHF exacerbation with aki and be fluid long but still be acidotic. On a final note, watch for hyperchloremia as a cause for base deficit and don't use huge quantities of NS if a patient is developing hyperchloremia.

 

[deleted171991]

So... new fellow in the u it this week and everyday I have some service bust my balls about "there base deficit is high" or "let's tank em up and clear the base deficit" or some other nonsense. My question is, I for the life of me cannot find any reasonable explanation for why I should care about the base deficit in regards to the patient's volume status... am I missing something here?


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The base deficit is a pointer to the IQ/knowledge deficit of the one concerned about it. It's usually the same group of people who care about CVP or IVC for volume status, or the kind that treat acidosis with bicarbonate, or will be concerned of the lactate rise in patients on epinephrine (and many other drugs) etc. Knee-jerk medicine.

Btw, the theory is that volume depletion causes a base deficit, i.e. acidosis. Which is, as we all know, usually false (hint: contraction alkalosis). Among the exceptions is the acutely and massively bleeding patient, because ischemia induces overwhelming acidosis (plus the kidney doesn't have time to compensate).

In conclusion, there is no place for BD for volume status guesses, except in trauma (and maybe a few other) patients.

There are very few knee-jerk rules in critical care. Be afraid of anybody who uses them. The best predictor of a good intensivist is thinking in probabilities while looking at the entire picture.

Do yourself a favor and spend elective time with different types of intensivists. If you are MICU-based, rotate through SICU and NeuroICU. If you are mostly in the SICU, spend time with a lot of MICU people. Read the other "side's" books and literature, critically, in an up-to-date evidence-based critical fashion. You will be surprised how much useless dogma you'll find, but also how much you'll learn in the process. Then keep everything to yourself, because nobody likes knowledgeable educated people, especially if lower-ranked.

P.S. Psst! (Trauma) surgeons tend to be among the worst knee-jerk sinners.

 

[deleted171991]

What's the strong ion difference? pt's pH?

The strong ion difference is basically the cation-anion difference in the blood (excluding H+ and HCO3-). In most people, it can be simplified to Na-Cl, and it's usually 38-42. Higher than 42 is alkalosis, lower than 38 acidosis. For example, giving somebody large amounts of abnormal saline (which has an SID of 0, versus the physiologic 38-40) will decrease the SID and cause acidosis.

Now you can rapidly estimate a patient's acid-base status even without a blood gas, just by looking at the electrolytes.

For a more complete approach, use this: https://emcrit.org/wp-content/uploads/acid_base_sheet_2-2011.pdf

 

[RemyMcswain]

The strong ion difference is basically the cation-anion difference in the blood (excluding H+ and HCO3-). In most people, it can be simplified to Na-Cl, and it's usually 38-42. Higher than 42 is alkalosis, lower than 38 acidosis. For example, giving somebody large amounts of abnormal saline (which has an SID of 0, versus the physiologic 38-40) will decrease the SID and cause acidosis.

Now you can rapidly estimate a patient's acid-base status even without a blood gas, just by looking at the electrolytes.

For a more complete approach, use this: https://emcrit.org/wp-content/uploads/acid_base_sheet_2-2011.pdf

Since you opened the can of worms, can you mechanistically explain why the charge difference between cations and anions leads to a change in the concentration of H+ in plasma? I could never understand that fundamental piece of the strong ion theory.

 

[deleted171991]

Since you opened the can of worms, can you mechanistically explain why the charge difference between cations and anions leads to a change in the concentration of H+ in plasma? I could never understand that fundamental piece of the strong ion theory.

Read the beginning of chapter 4 of Stewart's book, where he demonstrates the correlation (on pages 31-32). His explanation is the best.

 

[lymphocyte]

I've never been able to get an answer to the following:

What does base deficit tell me that pH and HCO3- doesn't?

Sorry for bumping this thread, but I just finished a deep-dive on acid-base for our residents. I used the NEJM series as my launching pad, which features one article on the Boston bicarb approach, one article on the Copenhagen base excess approach, and one article on the Stewart strong ion approach. All three articles, and the discussion in the correspondences, are fantastic.

Base excess, especially standard base excess, has two advantages.

First, it accounts for more buffering systems, of which bicarb is only one (and probably better described as a CO2 transport mechanism than a buffer). This is important when other buffering systems get deranged, like Hb or phosphate, as they do in sick patients. It's been convincingly shown that the Boston approach will miss acid-base disturbances that the other two approaches don't (though this often goes away if you correct for albumin).

Second, compensatory changes are very easy to calculate with base excess. It's also a cinch to dose bicarb, which I'm doing more in light of BICAR-ICU. (And perhaps one day dose HCl if I could get pharmacy to stop freaking out). Why remember six equations when you can do better with four (and really only two)?

In the end, I tell the residents to pretend using the Boston approach because that's what everybody else does, understand the Stewart approach because it really helps explain things lIke how normal saline causes acidosis or why albumin matters, and embrace the standard base excess approach because it's tremendously simple.

Berend K. Acid-base pathophysiology after 130 years: confusing, irrational and controversial. J Nephrol 2013; 26:254-65.

Dubin A, Menises MM, Masevicius FD, et al. Comparison of three different methods of evaluation of metabolic acid-base disorders. Crit Care Med. 2007;35(5):1264-1270.

Rastegar A. Clinical utility of Stewart’s method in diagnosis ans management of acid-base disorders. Clin J Am Soc Nephrol 2009; 4:1267-74.

 

[Hamhock]

In the end, I tell the residents to pretend using the Boston approach because that's what everybody else does, understand the Stewart approach because it really helps explain things lIke how normal saline causes acidosis or why albumin matters, and embrace the standard base excess approach because it's tremendously simple.

This.
This.
This.

HH

 

[Tipsy McStagger]

Sorry for bumping this thread, but I just finished a deep-dive on acid-base for our residents. I used the NEJM series as my launching pad, which features one article on the Boston bicarb approach, one article on the Copenhagen base excess approach, and one article on the Stewart strong ion approach. All three articles, and the discussion in the correspondences, are fantastic.

Base excess, especially standard base excess, has two advantages.

First, it accounts for more buffering systems, of which bicarb is only one (and probably better described as a CO2 transport mechanism than a buffer). This is important when other buffering systems get deranged, like Hb or phosphate, as they do in sick patients. It's been convincingly shown that the Boston approach will miss acid-base disturbances that the other two approaches don't (though this often goes away if you correct for albumin).

Second, compensatory changes are very easy to calculate with base excess. It's also a cinch to dose bicarb, which I'm doing more in light of BICAR-ICU. (And perhaps one day dose HCl if I could get pharmacy to stop freaking out). Why remember six equations when you can do better with four (and really only two)?
View attachment 250872
In the end, I tell the residents to pretend using the Boston approach because that's what everybody else does, understand the Stewart approach because it really helps explain things lIke how normal saline causes acidosis or why albumin matters, and embrace the standard base excess approach because it's tremendously simple.

Berend K. Acid-base pathophysiology after 130 years: confusing, irrational and controversial. J Nephrol 2013; 26:254-65.

Dubin A, Menises MM, Masevicius FD, et al. Comparison of three different methods of evaluation of metabolic acid-base disorders. Crit Care Med. 2007;35(5):1264-1270.

Rastegar A. Clinical utility of Stewart’s method in diagnosis ans management of acid-base disorders. Clin J Am Soc Nephrol 2009; 4:1267-74.

I feel like I still have a fundamental piece missing from my understanding of Stewart. Maybe you can help me clear things up? At the basic level I am still not quite clear on what is causing the acidosis/alkalosis in his methodology. Lets take the simple example here:

H2O ⇌ H+ OH-
HCl ⇌ H+ CL-
Let's say we add hydrochloric acid to pure water. Being strong ions we get full dissociation.

Stewart is asserting that it is not the addition of H+ to the solution but rather Cl- that is driving the acidosis. HCl being a SID 0 solution. However, H2O is also a SID 0 solution so based on what he's saying I don't see how it's explaining the decreased pH of the solution if we aren't using [H+] as the primary determinant of acidity? Or is he saying that the [H+] is in fact driving acidity but we are not able to directly change it by simply adding/removing H+ or HCO3-?

I'm sneaking up on understanding where he's coming from , and I dig it as it's really helping me understand acid/base on a deeper level, but I'm stuck at what I think is probably an elementary aspect of his approach and it's limiting my understanding.

Thanks a bunch!

 

[anbuitachi]

Whats wrong with giving patients bicarb when the pH is 7 and the lactate is high?

 

[Hamhock]

Whats wrong with giving patients bicarb when the pH is 7 and the lactate is high?

What's the benefit? When I know this, your question is easier to answer.

Independent of the putative benefit (situation dependent), bicarb in the situation you are describing is often given IVP from an "amp"; by folks who aren't really sure what's in that "amp".

Given IVP bicarb 8.4% gives a tremendous sodium load from a +300 osm solution that results in fluid shifts and not sustained acid-base changes (depending on ventilation; and to a lesser extent, renal function). It additionally often distracts or clouds the providers diagnostic plan.

As a "bridge" in crisis or for severe metabolic acidosis (regardless of lactate; that's a whole other discussion) with AKI, sure....but that's rarely what people are talking about when they say "lactate is high".

HH

 

[anbuitachi]

What's the benefit? When I know this, your question is easier to answer.

Independent of the putative benefit (situation dependent), bicarb in the situation you are describing is often given IVP from an "amp"; by folks who aren't really sure what's in that "amp".

Given IVP bicarb 8.4% gives a tremendous sodium load from a +300 osm solution that results in fluid shifts and not sustained acid-base changes (depending on ventilation; and to a lesser extent, renal function). It additionally often distracts or clouds the providers diagnostic plan.

As a "bridge" in crisis or for severe metabolic acidosis (regardless of lactate; that's a whole other discussion) with AKI, sure....but that's rarely what people are talking about when they say "lactate is high".

HH

Yea i realize it doesn't treat anything underlying, i mainly use it as a temporizing measure. i dont work in the ICU, only OR, and I give bicarb to pH of 7 even if lactate is high, mainly to have drugs work better (and despite what studies show, im pretty convinced pressors work better cause i see teh results immediately). i guess i use it as a bridge

 

[Beeftenderloin]

Yea i realize it doesn't treat anything underlying, i mainly use it as a temporizing measure. i dont work in the ICU, only OR, and I give bicarb to pH of 7 even if lactate is high, mainly to have drugs work better (and despite what studies show, im pretty convinced pressors work better cause i see teh results immediately). i guess i use it as a bridge

As a temporizing measure for a dangerously low pH, fine. But the “pressors working better” is really just fluid shift from the massive solute load you’re giving the patient. An amp of 8.4% bicarb has an osmolarity of around 2000, it’s like giving an equal amount of ~7% hypertonic saline.

 

[anbuitachi]

As a temporizing measure for a dangerously low pH, fine. But the “pressors working better” is really just fluid shift from the massive solute load you’re giving the patient. An amp of 8.4% bicarb has an osmolarity of around 2000, it’s like giving an equal amount of ~7% hypertonic saline.

im going to disagree with that one. 1 amp of bicarb has osmolarity of 2000 mosm per LITER. 1 Amp of bicarb is 50 mL, so 100 mosm. pushing 1-2 amps of bicarb is unlikely to quickly improve your pressures due to fluid shift, because otherwise, me dumping 1L of fluid thru my central line would bump up my pressure as well since i'm directly increasing vascular volume.

 

[ProtonPump]

I feel like I still have a fundamental piece missing from my understanding of Stewart. Maybe you can help me clear things up? At the basic level I am still not quite clear on what is causing the acidosis/alkalosis in his methodology.

Let me see if I can help you out. Stewart is saying that H+ and OH- are a weak acid and a weak base respectively, that is to say that in solution the dissociation constant of water favors the formation of H20 over the dissociated products H+ and OH-. The pH of pure water 7.0 fits in with the weak acid/base designation. Moreover if you Neutralize a strong acid (e.g. HCl) with a strong base (e.g. NaOH) you form a salt(in this case sodium chloride) and water(which is composed of a weak acid and a weak base) the reaction is driven forward by the strong acid (Cl-) neutralizing the strong base (Na+):
H2O ⇌ H+ OH-
HCl+NaOH= NaCl+H20

Stewart is also saying that HCO3- is a weak base, in the same way as above the carbonic acid dissociation constant favors the formation of H20 and CO2:

H20+CO2⇌ H2CO3

The law of electroneutrality requires all of the charges in a solution to balance meaning that the charge difference of
HCl
NaOH
H20
H2CO3
NaHCO3
are all zero.
NaCl also has a charge difference of zero in or outside of solution.

The strong Ion difference however only considers strong ions therefore H20 and H2CO3 (which is a weak acid) you can not calculate a SID because there is no strong Ion present (this is different than having a SID O).

The SID of a pure solution of:

HCl= -154
NaOH= +154
NaHCO3=+154
NaCl=O

Now the normal plasma SID is around 40 and pH 7.40. Solutions which lower the SID (e.g. HCl or NaCl (0.9%))will acidify plasma. Solutions which increase SID (e.g. NaOH or NaHCO3) will alkalinize plasma. Remembering that we are now speaking about acidify and alkalinize in the physiological sense in the plasma and not the chemical sense as pH 7.0 is chemically neutral (even though it is also a physiological acidosis).

 

[jdh71]

dork

 

[Akarat]

im going to disagree with that one. 1 amp of bicarb has osmolarity of 2000 mosm per LITER. 1 Amp of bicarb is 50 mL, so 100 mosm. pushing 1-2 amps of bicarb is unlikely to quickly improve your pressures due to fluid shift, because otherwise, me dumping 1L of fluid thru my central line would bump up my pressure as well since i'm directly increasing vascular volume.

Background: Nephrology fellow who is applying for critical care this year. Let me show you how just two ampules of NaHCO3 can increase the blood pressure via some calculations...

First off, the intravascular volume is influenced by the tonicity (otherwise known as effective osmolality which is not the same as osmolality, If you want an analogy, think of a hypervolemic patient with HF. The patient is hypervolemic but has a low effective arterial blood volume. What really counts is the effective arterial blood volume and not the total body water. Same for osmolality. What really counts is the tonicity or effective osmolality and not just osmolality).

In this example, let's use a 60 kg man with an estimated total body water of 50% (total body water estimation for men usually ranges from 50-65% of total body weight). Therefore, this 60 kg man has a total body water of 30 kg or 30 L (1 kg = 1 L).

The distribution of total body water in a human body can be estimated as 1/3 being in the extracellular fluid (ECF) space and 2/3 being in the intracellular fluid (ICF) space. Therefore, this man's ECF volume is 10 L and his ICF volume is 20 L.

The usual effective osmolality can be calculated as: Effective osm = 2(serum Na concentration) + (glucose/18)
The BUN is not an effective osmole as its concentration rapidly equalizes between the ECF and ICF compartments. Effective osmoles are substances that cannot equalize between two compartments due to a semi-permeable membrane separating the ECF and ICF compartments. Note that what you usually calculate is the osmolality of the serum and NOT the effective osmolality when faced with a scenario in which you're trying to see if there's a osmolar gap or not.

In order to keep this calculation as simple as possible, I will assume that only the serum Na concentration contributes to the effective osmolality. Therefore, the effective osmolality per liter is 280 mOsm/kg H2O (2 x 140 mmol/L). Given that the total body water of this man is 30 L, this man's total effective osmoles is 8400 mOsm (280 mOsm/kg H2O x 30 L; again, remember that 1 L = 1 kg and so the units can be interchangeable and cancel appropriately)

So, to recap:
This man has a total body water of 30 L (20 L in the ICF compartment and 10 L in the ECF compartment)
This man has a an effective osmolality of 280 mOsm/kg H2O, and his total body effective osmoles is 8400 mOsm

Now, let's give this man two ampules of NaHCO3.
An ampule of NaHCO3 has 8.4% NaHCO3. The volume of an ampule is 50 mL. Therefore, in one ampule of 8.4% NaHCO3, there are 4.2 g of NaHCO3. Going back to general chemistry, the molecular weight of NaHCO3 is 84.01 g/mol. Calculating moles and converting to millimoles (mmol) will get you that there are approximately 50 mmol of NaHCO3 in one 8.4% ampule of NaHCO3. Because NaHCO3 has a molar ratio of 1 mol Na for every 1 mol of HCO3, there are 50 mmol of Na and 50 mmol of HCO3 in one 8.4% ampule.
Therefore, two 8.4% ampules of NaHCO3 contains 100 mmol Na and 100 mmol of HCO3
Because Na and HCO3 can be thought of as effective osmoles, the total effective osmoles in two 8.4% ampules of NaHCO3 is 200 mOsm

Now... Going back to the patient...

Giving the patient two 8.4% ampules of NaHCO3 will increase the total effective osmole content to 8600 mOsm (200 + 8400)
The total body water will increase by 100 mL because two 8.4% ampules of NaHCO3 has a total volume of 100 mL

Now, we must calculate the new effective osmolality of the body. The new effective osmolality is equal to 8600 mOsm / 30.1 L
The new total body water is 30.1 L because, again, the patient originally had 30 L of total body water and we added 100 mL with two ampules
The new effective osmolality is 285.7 mOsm/kg H2O

Keep in mind that the effective osmolality in the ICF compartment is the same as the effective osmolality in the ECF compartment. If they were not the same, then fluid would shift (via osmosis) until the effective osmolality of the two compartments become equal.

Now, we will calculate the new ICF compartment volume.

The new effective osmolality of the ICF compartment is 285.7 mOsm/kg H2O and the total effective osmole content of the ICF compartment is 5600 mOsm
The total effective osmole content of the ICF was obtained originally by using 280 mOsm/kg H2O multiplied by the ICF compartment volume, 280 x 20 = 5600). The addition of effective osmoles (as in this case with the two ampules of NaHCO3) DOES NOT change the total effective osmoles of the ICF compartment because, remember, effective osmoles cannot easily pass through a semi-permeable membrane.
Therefore, the new ICF compartment volume is 19.6 L (5600 mOsm / 285.7 mOsm/kg H2O).

The ICF compartment has lost 0.4 L or 400 mL with the addition of two 8.4% ampules of NaHCO3. Where did the water go? It went into the ECF compartment.

The new ECF compartment volume is now 10.5 L
The original ECF compartment volume was 10 L. Giving the two ampules adds a volume of 100 mL. The remaining 400 mL comes from the fluid shift from the ICF compartment to the ECF compartment due to the change in effective osmolality.


Thus... Giving two 8.4% ampules of NaHCO3 is approximately equal to bolusing the patient 500 mL of normal saline. It's not too difficult to now see how giving ampules of NaHCO3 can increase the blood pressure.


I hope that made sense.

 

[DNA888]

Background: Nephrology fellow who is applying for critical care this year. Let me show you how just two ampules of NaHCO3 can increase the blood pressure via some calculations...

First off, the intravascular volume is influenced by the tonicity (otherwise known as effective osmolality which is not the same as osmolality, If you want an analogy, think of a hypervolemic patient with HF. The patient is hypervolemic but has a low effective arterial blood volume. What really counts is the effective arterial blood volume and not the total body water. Same for osmolality. What really counts is the tonicity or effective osmolality and not just osmolality).

In this example, let's use a 60 kg man with an estimated total body water of 50% (total body water estimation for men usually ranges from 50-65% of total body weight). Therefore, this 60 kg man has a total body water of 30 kg or 30 L (1 kg = 1 L).

The distribution of total body water in a human body can be estimated as 1/3 being in the extracellular fluid (ECF) space and 2/3 being in the intracellular fluid (ICF) space. Therefore, this man's ECF volume is 10 L and his ICF volume is 20 L.

The usual effective osmolality can be calculated as: Effective osm = 2(serum Na concentration) + (glucose/18)
The BUN is not an effective osmole as its concentration rapidly equalizes between the ECF and ICF compartments. Effective osmoles are substances that cannot equalize between two compartments due to a semi-permeable membrane separating the ECF and ICF compartments. Note that what you usually calculate is the osmolality of the serum and NOT the effective osmolality when faced with a scenario in which you're trying to see if there's a osmolar gap or not.

In order to keep this calculation as simple as possible, I will assume that only the serum Na concentration contributes to the effective osmolality. Therefore, the effective osmolality per liter is 280 mOsm/kg H2O (2 x 140 mmol/L). Given that the total body water of this man is 30 L, this man's total effective osmoles is 8400 mOsm (280 mOsm/kg H2O x 30 L; again, remember that 1 L = 1 kg and so the units can be interchangeable and cancel appropriately)

So, to recap:
This man has a total body water of 30 L (20 L in the ICF compartment and 10 L in the ECF compartment)
This man has a an effective osmolality of 280 mOsm/kg H2O, and his total body effective osmoles is 8400 mOsm

Now, let's give this man two ampules of NaHCO3.
An ampule of NaHCO3 has 8.4% NaHCO3. The volume of an ampule is 50 mL. Therefore, in one ampule of 8.4% NaHCO3, there are 4.2 g of NaHCO3. Going back to general chemistry, the molecular weight of NaHCO3 is 84.01 g/mol. Calculating moles and converting to millimoles (mmol) will get you that there are approximately 50 mmol of NaHCO3 in one 8.4% ampule of NaHCO3. Because NaHCO3 has a molar ratio of 1 mol Na for every 1 mol of HCO3, there are 50 mmol of Na and 50 mmol of HCO3 in one 8.4% ampule.
Therefore, two 8.4% ampules of NaHCO3 contains 100 mmol Na and 100 mmol of HCO3
Because Na and HCO3 can be thought of as effective osmoles, the total effective osmoles in two 8.4% ampules of NaHCO3 is 200 mOsm

Now... Going back to the patient...

Giving the patient two 8.4% ampules of NaHCO3 will increase the total effective osmole content to 8600 mOsm (200 + 8400)
The total body water will increase by 100 mL because two 8.4% ampules of NaHCO3 has a total volume of 100 mL

Now, we must calculate the new effective osmolality of the body. The new effective osmolality is equal to 8600 mOsm / 30.1 L
The new total body water is 30.1 L because, again, the patient originally had 30 L of total body water and we added 100 mL with two ampules
The new effective osmolality is 285.7 mOsm/kg H2O

Keep in mind that the effective osmolality in the ICF compartment is the same as the effective osmolality in the ECF compartment. If they were not the same, then fluid would shift (via osmosis) until the effective osmolality of the two compartments become equal.

Now, we will calculate the new ICF compartment volume.

The new effective osmolality of the ICF compartment is 285.7 mOsm/kg H2O and the total effective osmole content of the ICF compartment is 5600 mOsm
The total effective osmole content of the ICF was obtained originally by using 280 mOsm/kg H2O multiplied by the ICF compartment volume, 280 x 20 = 5600). The addition of effective osmoles (as in this case with the two ampules of NaHCO3) DOES NOT change the total effective osmoles of the ICF compartment because, remember, effective osmoles cannot easily pass through a semi-permeable membrane.
Therefore, the new ICF compartment volume is 19.6 L (5600 mOsm / 285.7 mOsm/kg H2O).

The ICF compartment has lost 0.4 L or 400 mL with the addition of two 8.4% ampules of NaHCO3. Where did the water go? It went into the ECF compartment.

The new ECF compartment volume is now 10.5 L
The original ECF compartment volume was 10 L. Giving the two ampules adds a volume of 100 mL. The remaining 400 mL comes from the fluid shift from the ICF compartment to the ECF compartment due to the change in effective osmolality.


Thus... Giving two 8.4% ampules of NaHCO3 is approximately equal to bolusing the patient 500 mL of normal saline. It's not too difficult to now see how giving ampules of NaHCO3 can increase the blood pressure.


I hope that made sense.

Excellent! It's always fun to quantify. Total Nephrology move. Was this method of learning taught in your fellowship training? I found similar solutions here - https://www.physiology.org/doi/full/10.1152/advan.00094.2018 - and was totally blown away by it.

Can you recommend any resources for acid-base, fluids, and electrolytes?

 

[Beeftenderloin]

Background: Nephrology fellow who is applying for critical care this year. Let me show you how just two ampules of NaHCO3 can increase the blood pressure via some calculations...

First off, the intravascular volume is influenced by the tonicity (otherwise known as effective osmolality which is not the same as osmolality, If you want an analogy, think of a hypervolemic patient with HF. The patient is hypervolemic but has a low effective arterial blood volume. What really counts is the effective arterial blood volume and not the total body water. Same for osmolality. What really counts is the tonicity or effective osmolality and not just osmolality).

In this example, let's use a 60 kg man with an estimated total body water of 50% (total body water estimation for men usually ranges from 50-65% of total body weight). Therefore, this 60 kg man has a total body water of 30 kg or 30 L (1 kg = 1 L).

The distribution of total body water in a human body can be estimated as 1/3 being in the extracellular fluid (ECF) space and 2/3 being in the intracellular fluid (ICF) space. Therefore, this man's ECF volume is 10 L and his ICF volume is 20 L.

The usual effective osmolality can be calculated as: Effective osm = 2(serum Na concentration) + (glucose/18)
The BUN is not an effective osmole as its concentration rapidly equalizes between the ECF and ICF compartments. Effective osmoles are substances that cannot equalize between two compartments due to a semi-permeable membrane separating the ECF and ICF compartments. Note that what you usually calculate is the osmolality of the serum and NOT the effective osmolality when faced with a scenario in which you're trying to see if there's a osmolar gap or not.

In order to keep this calculation as simple as possible, I will assume that only the serum Na concentration contributes to the effective osmolality. Therefore, the effective osmolality per liter is 280 mOsm/kg H2O (2 x 140 mmol/L). Given that the total body water of this man is 30 L, this man's total effective osmoles is 8400 mOsm (280 mOsm/kg H2O x 30 L; again, remember that 1 L = 1 kg and so the units can be interchangeable and cancel appropriately)

So, to recap:
This man has a total body water of 30 L (20 L in the ICF compartment and 10 L in the ECF compartment)
This man has a an effective osmolality of 280 mOsm/kg H2O, and his total body effective osmoles is 8400 mOsm

Now, let's give this man two ampules of NaHCO3.
An ampule of NaHCO3 has 8.4% NaHCO3. The volume of an ampule is 50 mL. Therefore, in one ampule of 8.4% NaHCO3, there are 4.2 g of NaHCO3. Going back to general chemistry, the molecular weight of NaHCO3 is 84.01 g/mol. Calculating moles and converting to millimoles (mmol) will get you that there are approximately 50 mmol of NaHCO3 in one 8.4% ampule of NaHCO3. Because NaHCO3 has a molar ratio of 1 mol Na for every 1 mol of HCO3, there are 50 mmol of Na and 50 mmol of HCO3 in one 8.4% ampule.
Therefore, two 8.4% ampules of NaHCO3 contains 100 mmol Na and 100 mmol of HCO3
Because Na and HCO3 can be thought of as effective osmoles, the total effective osmoles in two 8.4% ampules of NaHCO3 is 200 mOsm

Now... Going back to the patient...

Giving the patient two 8.4% ampules of NaHCO3 will increase the total effective osmole content to 8600 mOsm (200 + 8400)
The total body water will increase by 100 mL because two 8.4% ampules of NaHCO3 has a total volume of 100 mL

Now, we must calculate the new effective osmolality of the body. The new effective osmolality is equal to 8600 mOsm / 30.1 L
The new total body water is 30.1 L because, again, the patient originally had 30 L of total body water and we added 100 mL with two ampules
The new effective osmolality is 285.7 mOsm/kg H2O

Keep in mind that the effective osmolality in the ICF compartment is the same as the effective osmolality in the ECF compartment. If they were not the same, then fluid would shift (via osmosis) until the effective osmolality of the two compartments become equal.

Now, we will calculate the new ICF compartment volume.

The new effective osmolality of the ICF compartment is 285.7 mOsm/kg H2O and the total effective osmole content of the ICF compartment is 5600 mOsm
The total effective osmole content of the ICF was obtained originally by using 280 mOsm/kg H2O multiplied by the ICF compartment volume, 280 x 20 = 5600). The addition of effective osmoles (as in this case with the two ampules of NaHCO3) DOES NOT change the total effective osmoles of the ICF compartment because, remember, effective osmoles cannot easily pass through a semi-permeable membrane.
Therefore, the new ICF compartment volume is 19.6 L (5600 mOsm / 285.7 mOsm/kg H2O).

The ICF compartment has lost 0.4 L or 400 mL with the addition of two 8.4% ampules of NaHCO3. Where did the water go? It went into the ECF compartment.

The new ECF compartment volume is now 10.5 L
The original ECF compartment volume was 10 L. Giving the two ampules adds a volume of 100 mL. The remaining 400 mL comes from the fluid shift from the ICF compartment to the ECF compartment due to the change in effective osmolality.


Thus... Giving two 8.4% ampules of NaHCO3 is approximately equal to bolusing the patient 500 mL of normal saline. It's not too difficult to now see how giving ampules of NaHCO3 can increase the blood pressure.


I hope that made sense.

Don’t take this the wrong way, but I think I love you.

I’ve had this all quantified out for me once before but I could never quite commit all the steps to memory, I always just remembered that an amp of Bicarb is basically equivalent to a few hundred cc fluid Bolus and that’s why it helps the BP when you slam one in. But a lot of folks I work with don’t buy that. And me simply telling them that’s the case isn’t enough.

Now I have a beautiful, clear, concise explanation for why that is that I can easily reference.

Thank you.

 

[Akarat]

Haha, thanks guys.

For a good physiology book, I recommend:
https://www.amazon.com/Fluid-Electrolyte-Acid-Base-Physiology-Problem-Based/dp/0323355153/ref=sr_1_fkmr0_1?keywords=Fluid,+Electrolyte+and+Acid-Base+Physiology,+Fifth+Edition&qid=1564189739&s=gateway&sr=8-1-fkmr0

To answer your question DNA888, my nephrology fellowship program did not teach me the method that I demonstrated in my earlier post. I learned the method from the aforementioned book.

 

[anbuitachi]

Background: Nephrology fellow who is applying for critical care this year. Let me show you how just two ampules of NaHCO3 can increase the blood pressure via some calculations...

First off, the intravascular volume is influenced by the tonicity (otherwise known as effective osmolality which is not the same as osmolality, If you want an analogy, think of a hypervolemic patient with HF. The patient is hypervolemic but has a low effective arterial blood volume. What really counts is the effective arterial blood volume and not the total body water. Same for osmolality. What really counts is the tonicity or effective osmolality and not just osmolality).

In this example, let's use a 60 kg man with an estimated total body water of 50% (total body water estimation for men usually ranges from 50-65% of total body weight). Therefore, this 60 kg man has a total body water of 30 kg or 30 L (1 kg = 1 L).

The distribution of total body water in a human body can be estimated as 1/3 being in the extracellular fluid (ECF) space and 2/3 being in the intracellular fluid (ICF) space. Therefore, this man's ECF volume is 10 L and his ICF volume is 20 L.

The usual effective osmolality can be calculated as: Effective osm = 2(serum Na concentration) + (glucose/18)
The BUN is not an effective osmole as its concentration rapidly equalizes between the ECF and ICF compartments. Effective osmoles are substances that cannot equalize between two compartments due to a semi-permeable membrane separating the ECF and ICF compartments. Note that what you usually calculate is the osmolality of the serum and NOT the effective osmolality when faced with a scenario in which you're trying to see if there's a osmolar gap or not.

In order to keep this calculation as simple as possible, I will assume that only the serum Na concentration contributes to the effective osmolality. Therefore, the effective osmolality per liter is 280 mOsm/kg H2O (2 x 140 mmol/L). Given that the total body water of this man is 30 L, this man's total effective osmoles is 8400 mOsm (280 mOsm/kg H2O x 30 L; again, remember that 1 L = 1 kg and so the units can be interchangeable and cancel appropriately)

So, to recap:
This man has a total body water of 30 L (20 L in the ICF compartment and 10 L in the ECF compartment)
This man has a an effective osmolality of 280 mOsm/kg H2O, and his total body effective osmoles is 8400 mOsm

Now, let's give this man two ampules of NaHCO3.
An ampule of NaHCO3 has 8.4% NaHCO3. The volume of an ampule is 50 mL. Therefore, in one ampule of 8.4% NaHCO3, there are 4.2 g of NaHCO3. Going back to general chemistry, the molecular weight of NaHCO3 is 84.01 g/mol. Calculating moles and converting to millimoles (mmol) will get you that there are approximately 50 mmol of NaHCO3 in one 8.4% ampule of NaHCO3. Because NaHCO3 has a molar ratio of 1 mol Na for every 1 mol of HCO3, there are 50 mmol of Na and 50 mmol of HCO3 in one 8.4% ampule.
Therefore, two 8.4% ampules of NaHCO3 contains 100 mmol Na and 100 mmol of HCO3
Because Na and HCO3 can be thought of as effective osmoles, the total effective osmoles in two 8.4% ampules of NaHCO3 is 200 mOsm

Now... Going back to the patient...

Giving the patient two 8.4% ampules of NaHCO3 will increase the total effective osmole content to 8600 mOsm (200 + 8400)
The total body water will increase by 100 mL because two 8.4% ampules of NaHCO3 has a total volume of 100 mL

Now, we must calculate the new effective osmolality of the body. The new effective osmolality is equal to 8600 mOsm / 30.1 L
The new total body water is 30.1 L because, again, the patient originally had 30 L of total body water and we added 100 mL with two ampules
The new effective osmolality is 285.7 mOsm/kg H2O

Keep in mind that the effective osmolality in the ICF compartment is the same as the effective osmolality in the ECF compartment. If they were not the same, then fluid would shift (via osmosis) until the effective osmolality of the two compartments become equal.

Now, we will calculate the new ICF compartment volume.

The new effective osmolality of the ICF compartment is 285.7 mOsm/kg H2O and the total effective osmole content of the ICF compartment is 5600 mOsm
The total effective osmole content of the ICF was obtained originally by using 280 mOsm/kg H2O multiplied by the ICF compartment volume, 280 x 20 = 5600). The addition of effective osmoles (as in this case with the two ampules of NaHCO3) DOES NOT change the total effective osmoles of the ICF compartment because, remember, effective osmoles cannot easily pass through a semi-permeable membrane.
Therefore, the new ICF compartment volume is 19.6 L (5600 mOsm / 285.7 mOsm/kg H2O).

The ICF compartment has lost 0.4 L or 400 mL with the addition of two 8.4% ampules of NaHCO3. Where did the water go? It went into the ECF compartment.

The new ECF compartment volume is now 10.5 L
The original ECF compartment volume was 10 L. Giving the two ampules adds a volume of 100 mL. The remaining 400 mL comes from the fluid shift from the ICF compartment to the ECF compartment due to the change in effective osmolality.


Thus... Giving two 8.4% ampules of NaHCO3 is approximately equal to bolusing the patient 500 mL of normal saline. It's not too difficult to now see how giving ampules of NaHCO3 can increase the blood pressure.


I hope that made sense.

Makes some sense. But ECF is mostly interstitial fluid isnt it. Vascular volume is less. Does the NaHCO3 not equilibrate with interstitial fluids? How much of the 400ml is intravascular? Also how quickly does this occur? Is it faster for bicarb to pull in fluid, or is it faster to dump 1L of plasmalyte

 

[Akarat]

Makes some sense. But ECF is mostly interstitial fluid isnt it. Vascular volume is less. Does the NaHCO3 not equilibrate with interstitial fluids? How much of the 400ml is intravascular? Also how quickly does this occur? Is it faster for bicarb to pull in fluid, or is it faster to dump 1L of plasmalyte

The ECF volume can be split into two compartments, the interstitial compartment and the intravascular compartment. The interstitial compartment is composed of 3/4 of the ECF volume while the intravascular compartment is composed of 1/4 of the ECF volume.

In the above example, this would mean that out of 500 mL of fluid that is moved into the ECF compartment, only 125 mL would stay in the intravascular compartment once equilibrium is reached. I do not know how quickly this process occurs, though. It might not seem like a lot but keep in mind that if 1 L of normal saline is given, only 250 mL of the volume would remain in the intravascular space.

As a side note, if 500 mL of D5W was given, only 1/12 of the volume would remain in the intravascular volume (approximately 42 mL). This is because D5W is a hypotonic solution, and so the volume will distribute according to how the total body water is distributed. For clarity, it is 1/12 because given a volume of fluid, 1/3 is distributed to the ECF and of that volume that is distributed into the ECF only 1/4 is distributed into the intravascular space (i.e., 1/3 x 1/4 = 1/12). If an isotonic or hypertonic solution is given, the volume will not distribute into the ICF because the effective osmoles in the solution are unable to cross into the ICF compartment (hence why they are considered effective osmoles rather than just osmoles), and so there is no osmotic force for water to move into the ICF compartment.

This is why you don't volume resuscitate someone with D5W. If you think that it is underwhelming how only 250 mL of 1 L of normal saline stays in the intravascular compartment, think about how only 1/12 of 1 L of D5W (approximately 83 mL...) would stay in the intravascular space.

 

[MirrorTodd]

Side question: how do you guys prevent yourselves from pulling a Home Simpson style throat throttle with the overbearing surgeons trying to micromanage everything from outside the unit? (Rhetorical question cause I already know the problem with the way our SICU is run, but I just had to get that off my chest.)

 

[deleted171991]

Background: Nephrology fellow who is applying for critical care this year. Let me show you how just two ampules of NaHCO3 can increase the blood pressure via some calculations...

First off, the intravascular volume is influenced by the tonicity (otherwise known as effective osmolality which is not the same as osmolality, If you want an analogy, think of a hypervolemic patient with HF. The patient is hypervolemic but has a low effective arterial blood volume. What really counts is the effective arterial blood volume and not the total body water. Same for osmolality. What really counts is the tonicity or effective osmolality and not just osmolality).

In this example, let's use a 60 kg man with an estimated total body water of 50% (total body water estimation for men usually ranges from 50-65% of total body weight). Therefore, this 60 kg man has a total body water of 30 kg or 30 L (1 kg = 1 L).

The distribution of total body water in a human body can be estimated as 1/3 being in the extracellular fluid (ECF) space and 2/3 being in the intracellular fluid (ICF) space. Therefore, this man's ECF volume is 10 L and his ICF volume is 20 L.

The usual effective osmolality can be calculated as: Effective osm = 2(serum Na concentration) + (glucose/18)
The BUN is not an effective osmole as its concentration rapidly equalizes between the ECF and ICF compartments. Effective osmoles are substances that cannot equalize between two compartments due to a semi-permeable membrane separating the ECF and ICF compartments. Note that what you usually calculate is the osmolality of the serum and NOT the effective osmolality when faced with a scenario in which you're trying to see if there's a osmolar gap or not.

In order to keep this calculation as simple as possible, I will assume that only the serum Na concentration contributes to the effective osmolality. Therefore, the effective osmolality per liter is 280 mOsm/kg H2O (2 x 140 mmol/L). Given that the total body water of this man is 30 L, this man's total effective osmoles is 8400 mOsm (280 mOsm/kg H2O x 30 L; again, remember that 1 L = 1 kg and so the units can be interchangeable and cancel appropriately)

So, to recap:
This man has a total body water of 30 L (20 L in the ICF compartment and 10 L in the ECF compartment)
This man has a an effective osmolality of 280 mOsm/kg H2O, and his total body effective osmoles is 8400 mOsm

Now, let's give this man two ampules of NaHCO3.
An ampule of NaHCO3 has 8.4% NaHCO3. The volume of an ampule is 50 mL. Therefore, in one ampule of 8.4% NaHCO3, there are 4.2 g of NaHCO3. Going back to general chemistry, the molecular weight of NaHCO3 is 84.01 g/mol. Calculating moles and converting to millimoles (mmol) will get you that there are approximately 50 mmol of NaHCO3 in one 8.4% ampule of NaHCO3. Because NaHCO3 has a molar ratio of 1 mol Na for every 1 mol of HCO3, there are 50 mmol of Na and 50 mmol of HCO3 in one 8.4% ampule.
Therefore, two 8.4% ampules of NaHCO3 contains 100 mmol Na and 100 mmol of HCO3
Because Na and HCO3 can be thought of as effective osmoles, the total effective osmoles in two 8.4% ampules of NaHCO3 is 200 mOsm

Now... Going back to the patient...

Giving the patient two 8.4% ampules of NaHCO3 will increase the total effective osmole content to 8600 mOsm (200 + 8400)
The total body water will increase by 100 mL because two 8.4% ampules of NaHCO3 has a total volume of 100 mL

Now, we must calculate the new effective osmolality of the body. The new effective osmolality is equal to 8600 mOsm / 30.1 L
The new total body water is 30.1 L because, again, the patient originally had 30 L of total body water and we added 100 mL with two ampules
The new effective osmolality is 285.7 mOsm/kg H2O

Keep in mind that the effective osmolality in the ICF compartment is the same as the effective osmolality in the ECF compartment. If they were not the same, then fluid would shift (via osmosis) until the effective osmolality of the two compartments become equal.

Now, we will calculate the new ICF compartment volume.

The new effective osmolality of the ICF compartment is 285.7 mOsm/kg H2O and the total effective osmole content of the ICF compartment is 5600 mOsm
The total effective osmole content of the ICF was obtained originally by using 280 mOsm/kg H2O multiplied by the ICF compartment volume, 280 x 20 = 5600). The addition of effective osmoles (as in this case with the two ampules of NaHCO3) DOES NOT change the total effective osmoles of the ICF compartment because, remember, effective osmoles cannot easily pass through a semi-permeable membrane.
Therefore, the new ICF compartment volume is 19.6 L (5600 mOsm / 285.7 mOsm/kg H2O).

The ICF compartment has lost 0.4 L or 400 mL with the addition of two 8.4% ampules of NaHCO3. Where did the water go? It went into the ECF compartment.

The new ECF compartment volume is now 10.5 L
The original ECF compartment volume was 10 L. Giving the two ampules adds a volume of 100 mL. The remaining 400 mL comes from the fluid shift from the ICF compartment to the ECF compartment due to the change in effective osmolality.


Thus... Giving two 8.4% ampules of NaHCO3 is approximately equal to bolusing the patient 500 mL of normal saline. It's not too difficult to now see how giving ampules of NaHCO3 can increase the blood pressure.


I hope that made sense.

Holy macaroni, what a brainfart! I mean I applaud you for it, but I honestly hope you don't explain things like this in life, including in your notes, because you'll be talking alone most of the time (and your critical patients will die in the meanwhile). Here's the tl;dr (which should have been your post):

One ampule of NaHCO3 8.4% (1 mEq/mL) contains 50 mEq. By comparison, NaCl 0.9% contains 154 mEq/L. So 3 amps of bicarb are more or less like giving 1L of normal saline (both will equilibrate with the about 40L of total body water for a 70 kg patient) . The End.

Even a surgeon will understand.

Sincerely,
Dr. Pareto

 

[Akarat]

Holy macaroni, what a brainfart! I mean I applaud you for it, but I honestly hope you don't explain things like this in life, including in your notes, because you'll be talking alone most of the time (and your critical patients will die in the meanwhile). Here's the tl;dr (which should have been your post):

One ampule of NaHCO3 8.4% (1 mEq/mL) contains 50 mEq. By comparison, NaCl 0.9% contains 154 mEq/L. So 3 amps of bicarb are more or less like giving 1L of normal saline (both will equilibrate with the about 40L of total body water for a 70 kg patient) . The End.

Even a surgeon will understand.

Sincerely,
Dr. Pareto

In terms of the ECF volume, the changes are about the same (an increase from 10 L to ~10.7 L if the 60-kg man example that I used earlier is considered) with three ampules of 8.4% NaHCO3 vs one liter of normal saline. However, in terms of the ICF volume, the difference between the two is that the ICF volume compartment loses volume when 8.4% NaHCO3 is given while the ICF volume compartment doesn't lose volume when one liter of normal saline is given.

I won't bore you with the math.

 

[deleted171991]

In terms of the ECF volume, the changes are about the same (an increase from 10 L to ~10.7 L if the 60-kg man example that I used earlier is considered) with three ampules of 8.4% NaHCO3 vs one liter of normal saline. However, in terms of the ICF volume, the difference between the two is that the ICF volume compartment loses volume when 8.4% NaHCO3 is given while the ICF volume compartment doesn't lose volume when one liter of normal saline is given.

I won't bore you with the math.

The loss in ICF volume is like 1-2%. Nothing earth-shaking. I wouldn't even bother to calculate it in real life (probably about 0.850 x 2/3 divided by 24 in your 60 kg person).

This sounds like when nephrologists freak out from people giving a liter of LR to hyperkalemic patients (instead of holy water normal saline). It's the same drop in the ocean.

I would be way more concerned for all the bad effects of bicarb, such as intracellular acidosis. Hence I don't give it at a pH higher than 7.1-7.15 (unless there was a clear GI/renal bicarb loss from the body), but that's a different discussion.

Again, I respect your thoroughness, but the average non-intensivist (or even intensivist) won't. Let's not mention trainees. People tend to have a very short attention span for stuff like this, and you need an elevator pitch. Just my 2 cents.

P.S. I used to think like you.

 

[Katheudontas parateroumen]

Background: Nephrology fellow who is applying for critical care this year. Let me show you how just two ampules of NaHCO3 can increase the blood pressure via some calculations...

First off, the intravascular volume is influenced by the tonicity (otherwise known as effective osmolality which is not the same as osmolality, If you want an analogy, think of a hypervolemic patient with HF. The patient is hypervolemic but has a low effective arterial blood volume. What really counts is the effective arterial blood volume and not the total body water. Same for osmolality. What really counts is the tonicity or effective osmolality and not just osmolality).

In this example, let's use a 60 kg man with an estimated total body water of 50% (total body water estimation for men usually ranges from 50-65% of total body weight). Therefore, this 60 kg man has a total body water of 30 kg or 30 L (1 kg = 1 L).

The distribution of total body water in a human body can be estimated as 1/3 being in the extracellular fluid (ECF) space and 2/3 being in the intracellular fluid (ICF) space. Therefore, this man's ECF volume is 10 L and his ICF volume is 20 L.

The usual effective osmolality can be calculated as: Effective osm = 2(serum Na concentration) + (glucose/18)
The BUN is not an effective osmole as its concentration rapidly equalizes between the ECF and ICF compartments. Effective osmoles are substances that cannot equalize between two compartments due to a semi-permeable membrane separating the ECF and ICF compartments. Note that what you usually calculate is the osmolality of the serum and NOT the effective osmolality when faced with a scenario in which you're trying to see if there's a osmolar gap or not.

In order to keep this calculation as simple as possible, I will assume that only the serum Na concentration contributes to the effective osmolality. Therefore, the effective osmolality per liter is 280 mOsm/kg H2O (2 x 140 mmol/L). Given that the total body water of this man is 30 L, this man's total effective osmoles is 8400 mOsm (280 mOsm/kg H2O x 30 L; again, remember that 1 L = 1 kg and so the units can be interchangeable and cancel appropriately)

So, to recap:
This man has a total body water of 30 L (20 L in the ICF compartment and 10 L in the ECF compartment)
This man has a an effective osmolality of 280 mOsm/kg H2O, and his total body effective osmoles is 8400 mOsm

Now, let's give this man two ampules of NaHCO3.
An ampule of NaHCO3 has 8.4% NaHCO3. The volume of an ampule is 50 mL. Therefore, in one ampule of 8.4% NaHCO3, there are 4.2 g of NaHCO3. Going back to general chemistry, the molecular weight of NaHCO3 is 84.01 g/mol. Calculating moles and converting to millimoles (mmol) will get you that there are approximately 50 mmol of NaHCO3 in one 8.4% ampule of NaHCO3. Because NaHCO3 has a molar ratio of 1 mol Na for every 1 mol of HCO3, there are 50 mmol of Na and 50 mmol of HCO3 in one 8.4% ampule.
Therefore, two 8.4% ampules of NaHCO3 contains 100 mmol Na and 100 mmol of HCO3
Because Na and HCO3 can be thought of as effective osmoles, the total effective osmoles in two 8.4% ampules of NaHCO3 is 200 mOsm

Now... Going back to the patient...

Giving the patient two 8.4% ampules of NaHCO3 will increase the total effective osmole content to 8600 mOsm (200 + 8400)
The total body water will increase by 100 mL because two 8.4% ampules of NaHCO3 has a total volume of 100 mL

Now, we must calculate the new effective osmolality of the body. The new effective osmolality is equal to 8600 mOsm / 30.1 L
The new total body water is 30.1 L because, again, the patient originally had 30 L of total body water and we added 100 mL with two ampules
The new effective osmolality is 285.7 mOsm/kg H2O

Keep in mind that the effective osmolality in the ICF compartment is the same as the effective osmolality in the ECF compartment. If they were not the same, then fluid would shift (via osmosis) until the effective osmolality of the two compartments become equal.

Now, we will calculate the new ICF compartment volume.

The new effective osmolality of the ICF compartment is 285.7 mOsm/kg H2O and the total effective osmole content of the ICF compartment is 5600 mOsm
The total effective osmole content of the ICF was obtained originally by using 280 mOsm/kg H2O multiplied by the ICF compartment volume, 280 x 20 = 5600). The addition of effective osmoles (as in this case with the two ampules of NaHCO3) DOES NOT change the total effective osmoles of the ICF compartment because, remember, effective osmoles cannot easily pass through a semi-permeable membrane.
Therefore, the new ICF compartment volume is 19.6 L (5600 mOsm / 285.7 mOsm/kg H2O).

The ICF compartment has lost 0.4 L or 400 mL with the addition of two 8.4% ampules of NaHCO3. Where did the water go? It went into the ECF compartment.

The new ECF compartment volume is now 10.5 L
The original ECF compartment volume was 10 L. Giving the two ampules adds a volume of 100 mL. The remaining 400 mL comes from the fluid shift from the ICF compartment to the ECF compartment due to the change in effective osmolality.


Thus... Giving two 8.4% ampules of NaHCO3 is approximately equal to bolusing the patient 500 mL of normal saline. It's not too difficult to now see how giving ampules of NaHCO3 can increase the blood pressure.


I hope that made sense.

Has got to be one of the best posts on SDN ever haha. Felt like I was back in college chemistry! Basic science at its best haha. But I do see how @FFP is how we can think of it clinically as well. Awesome stuff

 

[MoMoGesiologist]

This sounds like when nephrologists freak out from people giving a liter of LR to hyperkalemic patients (instead of holy water normal saline). It's the same drop in the ocean.

Everyone at my hospital thinks this. They think are so smart switching LR to NS in hyperK pts. Then they think I must be dumb or not pay attention or dangerous when I prefer LR to NS. I’ve stopped arguing...

 

[deleted171991]

Everyone at my hospital thinks this. They think are so smart switching LR to NS in hyperK pts. Then they think I must be dumb or not pay attention or dangerous when I prefer LR to NS. I’ve stopped arguing...

You can't fix stupid.

That's why I don't believe in healing by committee. Too many stupid/undereducated people in the committee. (And I'm the kind of guy who's the first to bow to superior knowledge or intellect.)