The maintenance fluid plan should address three ongoing requirements:
replacement of lost interstitial volume (rehydration)
maintenance fluids (for normal homeostasis)
replacement of ongoing losses
The volume of rehydration fluids required is determined by reassessing hydration parameters after resuscitation, using the following formula:
% dehydration × body wt (kg) × total body water (0.6)
This volume is commonly administered over 4–12 hours with standard isotonic, balanced electrolyte replacement fluids.
Maintenance fluid requirements are added to the rehydration rate. Maintenance fluids are calculated with one of the following formulas:
30 × body weight (kg) + 70 = mL of maintenance fluids per 24 hours (dogs and cats)
132 × body weight (kg)0.75 = maintenance fluids per 24 hours (dogs)
80 × body weight (kg)0.75 = maintenance fluids per 24 hours (cats)
The first formula is appropriate for patients weighing between 2 and 70 kg; for patients outside of that weight range, the species-specific formulas should be used.
Maintenance fluid should include all parenteral and enteral medications, fluids (including flushes), and nutrition administered. With prolonged parenteral fluid administration, usually throughout a course of days, serum sodium may increase, and maintenance fluids (eg, half-strength saline or 5% dextrose in water) may be needed to replace free water deficits.
Ongoing or increased fluid losses vary substantially and must be estimated and replaced. Ongoing losses can be estimated by measuring urine and fecal output, nasogastric tube suction, bandage weight, or vomitus volume. Patients should be weighed regularly to determine changes in body weight.
Insensible losses, which can be increased with fever, wounds, higher metabolic demands, and other factors, can increase the maintenance rate by 15–20 mL/kg/day. Increased fluid requirements in patients with fever have not been well established in veterinary medicine; in human medicine, an increase 1°C (1.8°F) above normal body temperature may increase fluid requirements by 10–12%.
Maintaining intravascular fluids after resuscitation from hypovolemic shock and during systemic inflammatory response syndrome disease conditions causing increased capillary permeability can be a challenge.
Hydroxyethyl starch (HES) solutions can be administered IV CRI at 0.5–1 mL/kg/h in dogs or 0.25–1 mL/kg/h in cats. Newer HES solutions (eg, Vetstarch) may be administered at higher rates (2 mL/kg/h) without impacting coagulation. The dosage is adjusted to maintain an adequate mean arterial pressure and central venous pressure (CVP). The amount of crystalloids administered with colloids must be decreased by 40–60% of what would be administered if crystalloids were used alone.
Monitoring Fluid Therapy in the Maintenance Fluid Plan
All patients receiving fluids should have a physical examination, including assessment of hydration and body weight, with urine production checked at least every 12 hours, and more frequently in the critically ill.
Iatrogenic fluid overload with overadministration of crystalloids can manifest as the following clinical signs:
increased respiratory rate and effort
crackles or wheezes on auscultation
serous discharge from the nares
chemosis
jugular vein distention or pulsations
shivering
edema
hypertension (systolic arterial blood pressure > 140–150 mm Hg)
increased CVP (> 8–10 cm H2O)
notable increase in body weight (> 12–15%)
rapid and/or dramatic decrease in PCV and total solids
In animals with urinary catheters, urine output can be monitored and compared with fluid administration volumes. Monitoring CVP, pulmonary capillary wedge pressures, and cardiac output variables may be helpful in selected patients; however, pulmonary artery catheters are rarely placed. Monitoring electrolytes (particularly sodium, chloride, potassium, and calcium) and PCV/total solids may provide an objective measurement of fluid balance.
Patients should be regularly monitored for complications associated with fluid administration, which can be grouped by category:
Delivery method:
catheter-associated phlebitis
catheter-related thrombosis
catheter-related sepsis and fever
extravasation of fluids or medications
catheter-related foreign bodies (rare)
Incorrect fluid prescription:
fluid intolerance (overload), which may result in ascites, edema, etc
insufficient fluid administration, which may result in dehydration or shock
exacerbation of noncompressible hemorrhage
exacerbation of acid-base abnormalities
exacerbation of electrolyte abnormalities
exacerbation of tonicity abnormalities
development or exacerbation of renal injury (colloids)
exacerbation of coagulopathies
exacerbation of anemia
hyperglycemia (with dextrose-containing fluids)
When parenteral fluid administration is to be discontinued, the animal should be able to maintain hydration by voluntary drinking and eating or tolerate enteral supplementation (through a feeding tube) or SC fluid administration. Tapering the volume infused IV throughout 24–48 hours allows the renal medulla to reestablish the osmotic gradient and helps prevent excessive fluid loss through diuresis.
Key Points
There are four categories of shock: hypovolemic, cardiogenic, obstructive, and distributive.
The ability to create an effective fluid plan depends on understanding the different body fluid compartments and dynamics.
There are three major fluid compartments, the intravascular, interstitial, and intracellular spaces, separated by a capillary membrane, an endothelial glycocalyx layer, and a cell membrane, respectively.
Crystalloids are water-based solutions with small-molecular-weight particles, freely permeable to the capillary membrane, and are interstitial volume replacement solutions.
Colloids are water-based solutions with a molecular weight too large to cross the capillary membrane and are intravascular volume replacement solutions.
Determination of appropriate resuscitation end points and resuscitation technique is vital to creating a fluid resuscitation plan.
Patients should be regularly assessed for fluid requirements and complications.
Patients not responding to IV fluid resuscitation should be assessed for reasons for poor response.
For More Information
2024 AAHA Fluid Therapy Guidelines for Dogs and Cats. American Animal Hospital Association.
