Page 394 - Clinical Small Animal Internal Medicine

362 Section 5 Critical Care Medicine

not correlated reliably with alterations in fluid balance As mentioned earlier, the composition of maintenance
VetBooks.ir based on weight. This may be due to a variety of reasons fluids is different from that of replacement fluids. In
general, maintenance fluids have a lower sodium con-
and is most probably a result of the patient’s underlying
disease process.
Serum sodium can be used as an indicator of free water centration and a higher potassium concentration than
replacement fluid. Maintenance fluid requirements will
balance in most situations. It should be kept in mind that vary with the size, age, and metabolic needs of the
serum sodium does not reflect total body sodium but patient, taking physiologic derangements into considera-
instead is a reflection of the water balance within the tion. Smaller and younger patients tend to have higher
body. Serum sodium is of no value in determining the maintenance fluid requirements. These requirements
presence of hypovolemia or the need for rapid volume may also be increased with pyrexia and panting. Diet
expansion. The most common types of volume lost are composition may also affect daily fluid requirements due
isotonic or hypotonic fluid loss. In the case of hypotonic to the amount of water provided by the food source as
fluid loss, free water is lost in excess of sodium. As a well as the relative nutrient composition of the diet. In
result, serum sodium concentration elevates. To correct critically ill or sedentary animals, maintenance fluid
free water deficit, a solution providing free water such as requirements may be decreased.
0.45% NaCl or D5W is used. Interstitial fluid deficits The calculation of maintenance fluid requirements can
should be corrected before free water deficits are be done in several ways. One commonly used shortcut is
addressed. In addition, when correcting free water defi- 40–60 mL/kg/day. When using this formula, it should be
cits, it is imperative to consider the duration of the free remembered that larger dogs will usually have mainte-
water deficit. When free water deficits are long‐standing, nance needs near the lower end of the dose range while
the brain will synthesize idiogenic osmoles. These are small dogs will likely have requirements toward the
osmotically active particles that oppose the movement of higher end of the range. Another way to consider main-
fluid out of the brain in response to hypertonicity. tenance requirement is to link it to daily energy require-
Idiogenic osmoles require several days to be metabolized ments. With this approach, the maintenance requirement
so it is best to correct long‐standing free water deficits is roughly 1 mL of water per 1 kilocalorie (kcal) of energy
slowly over a number of days. Overzealous correction of expended. Maintenance energy requirements can be cal-
long‐standing hypernatremia can result in cerebral culated using the following formula:
edema. If hypernatremia is acute then it can be corrected 073
.
more rapidly. 140kcal body weight kg

To replace the free water deficit, the volume of the
deficit can be calculated using the following equation: Alternatively, the basal energy requirements can be cal-
culated and then multiplied by a factor of 2 to approxi-
Free water deficit Weight kg mate maintenance energy requirements. In critical
Na present Na/ 1 patients, water requirements may be decreased and
normal therefore the use of maintenance energy requirement
Example A10kgdog with serum sodium calculation may overestimate what is required. In this
:
concentration of 180mEq / L instance, basal energy requirements may be more reflec-
tive of true water requirements. Basal energy require-
Water deficit L 10 180 145 1 ments (BER) can be calculated using the following
/
formula in the dog:
24
.
.
97 body weightkg 0 655

In this example, the patient had a free water deficit of
2.4 L. This volume will need to be replaced after the In the cat, BER is roughly 50–60 kcal/kg/day.
interstitium is replete and additional fluids are indicated Once the degree of dehydration has been estimated
to meet insensible losses as well as ongoing losses that and the maintenance fluid requirement and free water
may be quantifiable. deficit calculated, consideration must also be given to
ongoing losses. This may be due to loss of volume from
the GI tract, urinary tract or third space losses. Also
Maintenance Fluids
remember to take into consideration the possibility of
Maintenance fluids are designed to meet sensible and iatrogenic loss of volume. An example of this would
insensible losses. In a normal animal, these needs are be when residual gastric volumes are discarded follow-
met through the intake of food and water. In the patient ing aspiration of a gastric tube. The volume of esti-
where this is not possible, or intake is inadequate to meet mated ongoing losses is then added in to the fluid
fluid requirements, a maintenance fluid may be used. prescription.