Fluid therapy: Hold the salt (Proceedings)
In this lecture I will outline how to formulate a plan for fluid therapy for supportive care of sick animals using the three classic goals of fluid therapy:
1. Provide for maintenance needs until the animal becomes self-sufficient with oral intake
2. Repair of existing deficits of water and electrolytes3. Replacement of concurrent losses due to ongoing disorders such as vomiting during the planned fluid therapy period.
These are the goals that need to be assessed & met for most sick animals needing fluid therapy. Therapy should be planned and executed over a period of several hours to 1 full day, depending on the needs of the patient and your available resources. At the end of this planned period the physical and laboratory findings are reassessed and the next treatment period is planned.
Two-thirds of lean body weight in adult mammals is water. Of this, roughly 2/3 is present as cellular water, and 1/3 is extracellular fluid (ECF). 1/4 of the extracellular water, or about 50 cc per kg of lean body weight, is plasma. The single most important determinant of plasma volume is the ECF compartment volume, as plasma volume remains a relatively constant fraction (1/4) of that compartment. The ECF compartment size is controlled by manipulation of the total body sodium content. As the body's content of sodium is increased or decreased, osmoregulation and thirst adjust water balance to maintain osmolality in the normal range.
Maintenance calculations are based on requirements to maintain homeostasis in a normal animal is resting in a cage at room temperature with no activity and no oral food or water. The requirement for water and electrolytes is based on the animal's mass and is not affected by health problems – therefore it is always the same and may be obtained from a table or chart.
The easiest way to administer water and electrolytes needed for maintaining the animal's physiology is to use a commercial maintenance solution. These products have approximately 30-40 mEq/l of sodium, 13-16 mEq/l of potassium, and variable amounts of bicarbonate precursor, calcium, magnesium, and other electrolytes. Although the sodium content is still higher than necessary, these fluids are reliably too low in potassium and need to be supplemented to around 28 mEq/l. If the solution contains 28 mEq/l of potassium, and is administered at the appropriate maintenance rate for water, this will deliver 0.05 - 0.1 mEq/kg/hour of potassium to the patient, meeting its maintenance needs for that electrolyte.
Veterinarians often use replacement solutions (for example, lactated Ringer's solution [LRS]) to provide maintenance water. This provides the patient with > 20 x maintenance for sodium – in fact, one liter of LRS solution provides the amount of sodium present in 100 pounds of dry dog food formulated to A.A.F.C.O. standards. Inappropriate use of replacement solutions in animals with heart disease or other causes of congestion may be a fatal mistake. Many critically ill patients – often immobile and inflamed - are prone to edema and will readily retain these fluids when used for maintenance water. Always think about whether the patient can tolerate the high sodium load it will receive if you use replacement solutions to provide maintenance water!!!