Just after drafting this response, I noticed Alchemist and Christian’s brilliant responses. I eco them below, maybe in a bit different words, I guess no harm of the repeat.
2 – 6 is the maintenance listed in most of references and guidelines. However depending on level of prematurity, hydration status and oliguric versus polyuric phases seen due to prematurity and renal impairment/recovery, you might need more or less.
Initially right after birth, preterm infants have translucent skin that contributes to dehydration. Those who are extremely premature, might require review to their sodium and fluid status every 6 hrs, this can result into increasing the fluid intake in response to hypernatremia. Thus the first day PN is almost sodium free, with probably very small amount of sodium (1 mmol/kg) as sodium glycerol phosphate.
This is also to account for the fact that we still give sodium chloride in the form of saline or ½ saline flushes at times of line insertion, to keep arterial lines patent and to flush IV medications. Thus ½ saline is preferred and small flush volumes should be targeted. This might help control the iatrogenic hypernatremia and hyperchloremia early in life.
Neonates who go through renal impairment become oliguric and develop 3rd spacing. During this phase they are hyponatremic, and you manage that by just giving the maintenance, while restricting the fluid intake in response to the increase in the oedema weight and sodium readings, as well as clinical findings of overload.
When they recover from the renal impairment phase, they start passing a lot of urine, and a lot of sodium with that, they might as well be responding to diuretics initiated during the oedema phase. During that time it might be helpful to respond to low sodium supplementations as much as needed. Calculation of deficiencies gives an idea of how much is needed.
Other instances when the needs are high is when there are stoma losses, as well as sodium losing congenital diarrhoeal diseases.
Now back to how much in PN. Mostly we just supplement the needed daily supplement of 2 – 6 mmol/kg/day, further corrections done outside PN is the optimal, and we can do that when the Total fluid intake allow given that we do not compromise nutrition for that. If you will need a lot of fluid to give side drips, or when there is extreme fluid restriction, correcting in the PN is unavoidable.