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 solutions can be administered as a constant-rate infusion at 0.5–1 mL/kg/hour in dogs, or 0.25–1 mL/kg/hour in cats. Newer HES solutions may be administered at higher rates (2 mL/kg/hour) 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 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 x body weight (kg) + 70 = mL of maintenance fluids per 24 hours
(70 x body weight (kg))0.75 = maintenance fluids per 24 hours
The second formula is necessary for patients <2 kg or >70 kg. 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%.
Monitoring Fluid Therapy
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.
Patients should be regularly monitored for complications associated with fluid administration, which may be grouped by category:
catheter-related sepsis and fever
extravasation of fluids
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 subcutaneous 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.
There are three categories of shock: hypovolemic, cardiogenic, 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, extravascular, and interstitial spaces, separated by a capillary membrane and an endothelial glycocalyx layer.
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 endpoints 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.