Fluid & Electrolyte

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Nutrition in all patients is best provided via the enteral route but many surgical patients require parenteral nutrition.
Caloric needs are altered by several factors such as surgical procedures, stress, cold, infection, and trauma.
Starvation in infants is a more precarious situation because of the minimal stores of fat and protein, which are further compromised in prematurity.
Decreased enteral intake and high metabolic demand also increase problems for infants with surgical, cardiac and chronic lung disease.
The hazards of inappropriate nutrition for infants include bone demineralization, rickets, cholestatic jaundice, poor wound healing, impaired lung function and slow growth, which can affect both short and long term outcomes.

Nutritional assessment is based on clinical factors such as history of weight loss, vomiting, diarrhea or feeding intolerance.
Physical examination may show signs of muscle wasting. Height, weight and head circumference normograms should be evaluated for signs of poor growth.
Adequacy of nutrition may also be judged by evaluation of serum proteins
- The half-lives of serum proteins aid interpretation of nutritional status: albumin, 18 days; transferrin, 8 days; pre-albumin, 3 days; and retinol binding protein, 12 hours.

Maintenance fluid requirements for children and adults are calculated based on the lean body weight or body surface area.
During the first week of life, infants are expected to lose 10-15% of body weight and an even greater percentage for premature infants.
Electrolytes are not added initialy; D10W is used for maintenance fluid on DOL 1.
On DOL 2, maintenance fluids are changed to D10W 0.2 NS with 20 mEq KCl/L.
On DOL 2 and beyond, urine output and serum sodium are the most useful parameters to follow in determining the appropriate rate of fluid administration.
Added to maintenance fluid rates should be volume to account for losses.
Environmental losses are higher in radiant warmers compared to a humidified incubator. Infants with phototherapy should have a 50ml/kg/day increase in fluids while on phototherapy. Patients with gastroschisis, ruptured omphalocele, and bladder extrophy have greater evaporative losses requiring a bolus of 20 ml/kg of isotonic fluid at birth and an increase of the maintenance infusion by 20-25% until coverage of the exposed viscera is accomplished.
Surgical patients often have gastrointestinal fluid losses that should be replaced with consideration of both the volume and electrolyte concentration of these losses.
Gastric fluids are typically replaced with D5 0.45 % NS with 20 mEq/liter of KCL, whereas biliary and intestinal losses are replaced with Lactated Ringers solution. Urine output should be monitored to ensure adequate perfusion.

The caloric density of carbohydrates is 4 kcal/g (dextrose, 3.4 kcal/g), lipids 9 kcal/g and protein 4 kcal/g.

Sodium

Sodium is the primary extracellular cation, a major component of the serum osmolarity and is essential for growth as well as fluid homeostasis.
Maintenance requirements for sodium are from 2-4 mEq/kg/day
Requirements may be greater for infants due to renal immaturity and the inability to maximally reabsorb sodium.
Hyponatremia is most frequently a result of water retention due to excess antidiuretic hormone secretion. Conversely, hypernatremia is most frequently due to dehydration.

Potassium

Potassium is the primary intracellular cation and is essential for proper cardiac and neurologic function.
Daily requirements are 1-2 mEq/kg/day to account for cellular proliferation and to replace obligatory renal losses.
Consequently, for decreased renal function, careful adjustment and often cessation of potassium supplementation may be needed.
Potassium is most safely administered by the enteral route; intravenous infusion should generally be 0.5 mEq/kg/hour with no greater than 1 mEq/kg/hr.
Potassium is inflammatory to veins and therefore should be given at concentrations of no more than 60 mEq/L in peripheral lines and 120 mEq/L in central lines
Potassium requires careful monitoring for acute and chronic renal failure, abnormal acid base status, abnormal glucose status and during the use of certain drug therapies such as digoxin, amphotericin, high dose beta agonists, insulin drips and diuretics such as furosemide.

Chloride

Chloride is an anion that is provided in parenteral solutions to balance the cations such as potassium and sodium.
An overabundance of chloride can lower serum pH, causing a low anion gap metabolic acidosis.

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