Maintenance Fluid Plan
Maintaining intravascular fluids after resuscitation from hypovolemic shock and during systemic inflammatory response syndrome disease conditions causing increased capillary permeability can be a challenge. Hydroxyethylstarch solutions or Oxyglobin® can be administered as a constant-rate infusion at 0.5–1 mL/kg/hr in dogs, or 0.25–1 mL/kg/hr in cats. Newer HES solutions may be administered at higher rates (2 mL/kg/hr) without impacting coagulation. The dosage is adjusted to maintain an adequate mean arterial pressure and CVP. The amount of crystalloids administered with colloids must be reduced by 40%–60% of what would be administered if crystalloids were used alone. The maintenance fluid plan should address three ongoing requirements: replacement of lost interstitial volume (rehydration), maintenance fluids (for normal homeostasis), and 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 throughout 4–12 hr with standard isotonic, balanced electrolyte replacement fluids.
Maintenance fluid requirements (40 mL/kg/day for larger animals and 60 mL/kg/day for smaller animals) are added to the rehydration rate. 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, or vomitus volume. Insensible losses, which can be increased with fever, or higher metabolic demands can increase the maintenance rate by 15–20 mL/kg/day.
All animals receiving fluids should have a physical examination, including assessment of hydration and body weight, with urine production checked at least twice per day, more frequently in the critically ill. Overzealous administration of crystalloids can manifest as increased respiratory rate and effort, crackles or wheezes on auscultation, serous discharge from the nares, chemosis, jugular vein distention or pulsations, shivering, edema, hypertension (>140–150 mmHg systolic), increased CVP (>8–10 cm H2O), significant increase in body weight (>12%–15%), and 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 animals, although pulmonary artery catheters are rarely placed. Monitoring electrolytes and PCV/total solids may provide an objective measurement of fluid balance.
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 subcutaneous fluid administration. Tapering the volume infused IV throughout 24–48 hr allows the renal medulla to reestablish the osmotic gradient and helps prevent excessive fluid loss through diuresis.