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Which of the following fluid solutions is closest to being isotonic in vivo?
A. D5-LR
B. D5-W
C. D10-W
D. D5 0.45%NS
Answer
The answer is B.
While most of us talk about tonicity and osmolarity, what we are REALLY talking about is the effective osmolarity or tonicity. D5W is closest of the list to isotonic with an osmolarity of around 260-272. Don't believe me? [Look it up.] The issue is that the dextrose is quickly metabolized after entering the plasma, leaving only free water (clearly hypotonic). This is described as "physiologic tonicity or osmolarity". Sodium and chloride do not have this problem. So while the fluid of D5W is isotonic in solution, it quickly turns to hypotonic effective volume and you are left with hyponatremia. This hyponatermia is becoming a real problem perioperatively in pediatrics. The REAL question is what fluid should we use in the OR for maintenance? There are a couple of great articles talking about appropriate perioperative fluids. It may not be what you think.[1][2]
Notes
- ↑ Moritz, Michael L., and Juan Carlos Ayus. "Water water everywhere: standardizing postoperative fluid therapy with 0.9% normal saline." Anesthesia & Analgesia 110.2 (2010): 293-295.[1]
- ↑ Bailey, Ann G., et al. "Perioperative crystalloid and colloid fluid management in children: where are we and how did we get here?." Anesthesia & Analgesia 110.2 (2010): 375-390.[2]
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