Na correction

[pgg]

From the ABA 1990 ITE (Book A, question 76)

Postoperatively a 70-kg patient has a serum sodium value of 130 mEq/L. To increase this value to 140 mEq/L would require the administration of how much sodium in mEq?

(A) 140
(B) 250
(C) 420
(D) 600
(E) 740

The ABA's key says the answer is A.


My understanding is that a sodium deficit is calculated based on total body water (60% of mass in adult males), so

• 70 kg x .6 = 42 kg (L) of water
• 42 L x (140 - 130) mEq/L = 420 mEq

A couple of sources agree with using TBW:

• http://www.fpnotebook.com/Renal/Lab/TtlBdySdmDfct.htm
• p 637 of Longnecker

Longnecker actually includes an example very similar to this questions:

If the patient's weight is 70 kg, and the serum sodium is 120 mEq/L, then the desired change is 10 mEq/L.

Total body deficit (TD) of sodium is the sodium deficit x TBW:

NaD x (weight in kg x 0.6) = TD

The formula is applied as follows:

10 x (70 x 0.6) = 420 mEq

Is the key wrong or is there something else here I'm not getting - ie, is there a difference between the total body Na deficit and what you need to administer to raise the plasma concentration?

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[Jeff05]

i think it's the same thing. there are definitely wrong answers in the keys for those ancient exams.

 

[Noyac]

I believe the answer is 420. For what thats worth.

 

[InductionAgent]

If anyone's still interested in this question:

Sodium is found predominantly in the extracellular fluid. The intracellular fluid contains only minimal amounts of sodium and will not have a substantial sodium deficit with a serum sodium of 130 mEq/L.

Extracellular fluid comprises 20% of body weight. (1/3 of the 60% comprised by total body water). Thus the ECF of a 70kg patient is 14kg or 14L

Therefore, this patient's sodium deficit is (140-130)mEq/L x 14L = 140mEq. I think the answer key is correct.

 

[paiute]

If anyone's still interested in this question:

Sodium is found predominantly in the extracellular fluid. The intracellular fluid contains only minimal amounts of sodium and will not have a substantial sodium deficit with a serum sodium of 130 mEq/L.

Extracellular fluid comprises 20% of body weight. (1/3 of the 60% comprised by total body water). Thus the ECF of a 70kg patient is 14kg or 14L

Therefore, this patient's sodium deficit is (140-130)mEq/L x 14L = 140mEq. I think the answer key is correct.

I agree.

 

[xjohns1]

the aba answer is wrong. your calculation is correct. the replies to your op that indicate calculation should be based on extracellular fluid are incorrect. the total body water must be used. i looked into this question and the reason one looks at the total body water is because the osmotic effect of the administered nacl is distributed throughout the total body water, even though nacl itself is largely restricted to the ecf. as the sodium is replaced, osmotic water movement out of the cells lowers the plasma sodium by dilution. this comes from the electrolyte text by burton rose and seems to me the definitive answer.

From the ABA 1990 ITE (Book A, question 76)

Postoperatively a 70-kg patient has a serum sodium value of 130 mEq/L. To increase this value to 140 mEq/L would require the administration of how much sodium in mEq?

(A) 140
(B) 250
(C) 420
(D) 600
(E) 740

The ABA's key says the answer is A.


My understanding is that a sodium deficit is calculated based on total body water (60% of mass in adult males), so

• 70 kg x .6 = 42 kg (L) of water
• 42 L x (140 - 130) mEq/L = 420 mEq

A couple of sources agree with using TBW:

• http://www.fpnotebook.com/Renal/Lab/TtlBdySdmDfct.htm
• p 637 of Longnecker

Longnecker actually includes an example very similar to this questions:


Is the key wrong or is there something else here I'm not getting - ie, is there a difference between the total body Na deficit and what you need to administer to raise the plasma concentration?

 

[InductionAgent]

the aba answer is wrong. your calculation is correct. the replies to your op that indicate calculation should be based on extracellular fluid are incorrect. the total body water must be used. i looked into this question and the reason one looks at the total body water is because the osmotic effect of the administered nacl is distributed throughout the total body water, even though nacl itself is largely restricted to the ecf. as the sodium is replaced, osmotic water movement out of the cells lowers the plasma sodium by dilution. this comes from the electrolyte text by burton rose and seems to me the definitive answer.

Some correction for the osmotic effect of the administered NaCl could be justified, but not to the extent of using total body water in the calculation. Using total body water to calculate your sodium dose implies that the entire water content of the intracellular space is going to be drawn into the extracellular fluid by the administered NaCl. Perhaps some NaCl will also be diuresed, so another correction factor might be in order.

But I would not use total body water to calculate my sodium dose. It makes sense neither in theory nor in practice. If I were actually treating the patient in the stem, first off - I wouldn't be all that up in arms about a sodium of 130mEq. I sure as hell would not give more than 140 mEq of hypertonic saline without checking a sodium level, nor would I bolus him with 2.72 L of normal saline (to get his 420mEq) unless he seemed hypovolemic.

So whatever your textbooks say on this one, have fun with them.

 

[usnavdoc]

Some correction for the osmotic effect of the administered NaCl could be justified, but not to the extent of using total body water in the calculation. Using total body water to calculate your sodium dose implies that the entire water content of the intracellular space is going to be drawn into the extracellular fluid by the administered NaCl. Perhaps some NaCl will also be diuresed, so another correction factor might be in order.

But I would not use total body water to calculate my sodium dose. It makes sense neither in theory nor in practice. If I were actually treating the patient in the stem, first off - I wouldn't be all that up in arms about a sodium of 130mEq. I sure as hell would not give more than 140 mEq of hypertonic saline without checking a sodium level, nor would I bolus him with 2.72 L of normal saline (to get his 420mEq) unless he seemed hypovolemic.

So whatever your textbooks say on this one, have fun with them.

Miller uses ECV with this calculation. As does Hall question book. 0.2% adults 0.4% neonates

 

[xjohns1]

Miller uses ECV with this calculation. As does Hall question book. 0.2% adults 0.4% neonates

you guys are killing me with this. miller, 7th edition, p. 1708:

dose (meq)=weight x (140 - Na+) x 0.6

Hall, 3rd ed., p. 171, q. 402

dose of Na+ = body weight x 0.6 x (desired Na+ - current Na+)

the 0.6 is fraction of body weight that is total body water, not ecv.