There is a long winded explanation that involves osmolarity and electroneutrality of different fluids that is not important for reasons why you choose a fluid. I will leave it out. This is still long winded but gives far more insight than you'd get from a chapter on fluid. I would not share this information with your surgery team; surgeons dont know why they give LR, they just do.
Essentially,
Lactated Ringers is used by surgeons. It is isotonic as is Normal Saline, but it has the interesting property of not causing an acidosis. Inherently, you understand that if you get a contraction alkalosis you should therefore be able to induce an expansion acidosis. Lactated Ringers is unfortunately named because the lactate has actually nothing to do with its use. You can dump 20L of LR into a person during resusciation in surgery and not change their pH status. The draw back to LR is that is contains a ton of Potassium. This is not a big deal in someone who has a good heart and clean kidneys (i.e. the "medically cleared surgical candidate"), but can be devestating in a renal patient. Surgeons use LR because they can give large amounts without altering the pH of the patient, while medicine doctors stay away from LR because of the Potassium.
Normal Saline is used by medicine doctors. This is because medicine rarely, if ever, will give more than 10 Liters of fluid in a day. At small volumes of NS, there is no pH alteration. But, if you give fluid like you need to in surgery, you will produce an acidosis. Medicine keeps to small fluid volumes and is in fear of the potassium of LR. This is especially true in the renal, heart, and liver patients (i.e. every hospitalized medicine patient). medicine doctors use NS because the volume is low so no acidosis will develop and they fear potassium. Surgeons stay away from NS because large volumes produce acidosis.
D5W is a hypotonic fluid. Its usually a bad idea. It would take 10L to get the nutrional value of a can of coke. 10L of hypotonic solution will dilute their blood, cause a nasty (and rapid) hyponatremia, and give them a seizure. Basically, never use D5W unless in special situations such as DKA.
Rather than confuse people and make them decide, its simple to say "Surgeons LR, Medicine NS...always"
Bottom Line:
(1) LR used by surgeons. Large volumes do not cause acidosis, potassium load is usually able to be handled by most surgical candidates (if they were medically sick, they wouldnt operate).
(2) NS used by medicine. Large volumes would cause acidosis, but medicine does not give large volumes. There is no potassium in NS to worry about.
(3) D5W is a foolish fluid to use. It is hypotonic, has a nutritional content of 0, so use it only in special cases like DKA where the goal is to drive down the serum osmolarity.
