Fluid therapy is a mainstay component of patient care. Understanding the concepts of fluid compartments, fluid balance, fluid types, fluid plans, and fluid monitoring are all important. As the role of veterinary technicians continues to grow, the need for them to understand all aspects of fluid therapy becomes more important.

Fluid Compartments

Physiologically speaking, the body’s fluid compartments are divided between intracellular (ICF), or within the cell, and extracellular (ECF), or the intravascular and interstitial, compartments. Each fluid compartment comprises total body water (TBW); the ICF compartment is ~66% TBW and the ECF compartment is ~33%.

Figure 1

Fluid Balance

Fluid imbalances occur from any condition(s) that alters a patient’s ability to adequately compensate and restore its own fluid requirements. Intravenous fluids are used in the critically ill to maintain intravascular volume and adequate perfusion. Definitions that are part of fluid balance include hypovolemia and dehydration. Hypovolemia is loss of intravascular fluid volume (i.e., hemorrhage, vomiting/diarrhea, urinary loss). Dehydration is loss of interstitial and intracellular fluid volume (i.e., decreased water intake in relation to water lost).

Fluid Types

The mainstay fluid type for treating critical patients are crystalloid fluids. Crystalloids can be further categorized as isotonic, hypertonic, and hypotonic solutions.

Isotonic Crystalloids

Isotonic crystalloids are the primary fluid type for treating critical patients, as they have the most similar composition to the patient’s ECF compartment. Crystalloids are a water-based solution composed of osmotically active small molecules that are permeable to capillaries. Crystalloid solutions include 0.9% NaCl, lactated Ringer’s (LRS), Normosol-R, and Plasmalyte-A. It’s important to note that ~75% of crystalloid solutions leave the intravascular space within 1 hour of administration. The maintenance rate of crystalloids in the small animal patient is 40–60 ml/kg/day. The shock volume of crystalloids is 60–90 ml/kg in the canine patient and 45–60 ml/kg in the feline patient. Shock doses of crystalloids should start in aliquots, such as ¼ or ½ the shock dose. The replacement rate of crystalloids is based off the dehydration deficit, which is calculated as: percent dehydration × kg × 1000 = volume to replace. The dehydration deficit should be replaced over 12–24 hours and is a rate in addition to the maintenance fluid requirement.

Hypertonic Crystalloids

Hypertonic fluids are solutions that have a higher osmolality compared to an animal’s normal fluid composition. The use of hypertonic solutions can be lifesaving in the emergency and critical care setting, as they cause a free water shift from the ICF space to the ECF space. The most commonly used hypertonic solution is 7–7.5% NaCl. Hypertonic saline is dosed at 3–5 ml/kg and given IV over 10–15 minutes. Although short lived (typically 30 minutes), the transient cardiovascular effects of administration may provide enough time for other therapies, such as crystalloids, to take full effect. Use of hypertonic saline is also desirable as you can deliver a much smaller volume to obtain the wanted restoration of intravascular volume effect. Hypertonic saline is often and ideally given in combination with isotonic crystalloids. Hypertonic saline is an excellent choice for rapid, small-volume resuscitation.

Hypotonic Crystalloids

Hypotonic fluids are solutions that have a lower osmolality compared to an animal’s normal fluid composition. Hypotonic fluids replace free water deficits by distributing throughout the intracellular and extracellular fluid compartments, with less remaining extracellularly. The larger distribution of hypotonic fluids makes them a safer choice for patients that can’t tolerate intravascular volume or have a sodium derangement (i.e., hypernatremia). Hypotonic fluids should never be administered as a bolus; they are ineffective at expanding intravascular volume, can rapidly affect osmolality, cause water shifts from the ECG space to ICF space, and can lead to life-threatening cerebral edema. Hypotonic crystalloids include 5% dextrose in water (D5W) and 0.45% NaCl.

Synthetic colloids are also a fluid option for critically ill patients. Colloid solutions contain large molecules suspended in crystalloid solutions that help maintain intravascular volume because they don’t as readily cross the blood vessel barrier. Because molecules in colloids are bigger, they tend to remain within the intravascular space longer, thus, better sustaining intravascular volume. The most commonly available synthetic colloids are derivatives of hydroxyethyl starches; these solutions include Hetastarch, Tetrastarch, Pentastarch, and VetStarch. In recent years, there has been controversy and debate about the use of synthetic colloids in fluid resuscitation. There has been more recent research and evidence in the human field about the adverse effects of synthetic colloid use. The main concerns of colloid use in human medicine are they may cause acute kidney injury and coagulation derangements. While there is currently no veterinary evidence regarding the adverse effects of colloid use, many clinicians are giving more patient consideration prior to implementing colloid use. Synthetic colloids can be given as a bolus at 5–10 ml/kg or used as a CRI so long as the dose doesn’t exceed 20 ml/kg/day.

Vasopressor Therapy

Vasopressors may be indicated if there’s no cardiovascular improvement with crystalloids or colloid fluid resuscitation methods. Vasopressors are potent vasoconstrictors used to redirect blood flow from the peripheral to central circulation, with the goal of increasing blood pressure. It’s important to note that vasopressors should be considered after the patient has been fully fluid resuscitated (received their full shock dose of IV crystalloids). Vasopressor drugs include norepinephrine, dopamine, dobutamine, and vasopressin.

Albumin/Oncotic Pull

Albumin is the predominant protein within the intravascular space and is responsible for maintaining vascular integrity and colloid oncotic pressure (COP). Oncotic pull is what keeps fluids within the vascular space; without albumin, there would be increased intravascular permeability, resulting in fluids leaving the intravascular space and causing third spacing and edema. Critical patients often suffer from hypoalbuminemia, which can result in hypotension or interstitial or pulmonary edema. Nutritional support, synthetic colloids and intravenous albumin are therapies that can be used to raise albumin levels. There are both canine and human albumin solutions commercially available; administration of either is done similarly to a blood product transfusion (administered slowly with frequent monitoring for transfusion reaction). It should be noted that using fresh frozen plasma (FFP) to raise albumin isn’t considered an appropriate therapy; it requires 40–50 ml/kg FFP to raise albumin by 1 g/dL.

Fluid Delivery

In order to provide fluid therapy, venous access must first be obtained. Proper IV catheter placement and maintenance are crucial for IV fluid therapy to be successful.

In critically ill patients, it is most ideal to place a large gauge, short-length IV catheter in the cephalic vein. IV catheter size is preferably 18-gauge in medium to large sized dogs and 20-gauge in small dogs and cats. Smaller gauge IV catheters create increased resistance to fluid flow, which can be counterproductive if administering high volume or multiple fluid types. The cephalic vein is preferred as it allows for easier access, reduces the likelihood of contamination from GI or urinary losses, allows for better visualization as the vessel is closer to the heart, and reduces potential issues with patient mal-positioning or occlusion. Other venous access options include central venous access, either via the jugular vein (cranial vena cava) or the lateral saphenous (caudal vena cava). Jugular central catheters have the added benefit of being used for total parenteral nutrition (TPN) or to measure central venous pressure (CVP). Jugular catheters are contraindicated in patients with coagulopathy or head trauma. Alternatively, a peripherally inserted central catheter (PICC) line can be placed. Both central catheters typically have double or triple lumen ports and are placed following sterile technique.

Fluid Therapy Plan

To develop a fluid therapy plan, things like disease process, life stage, hydration status, and ongoing losses are patient considerations that should be included in determining maintenance and replacement fluid therapy.

Disease states that require more aggressive fluid therapy include hypovolemia, renal disease, vasodilatory states, and certain toxicities. Disease states that require more conservative fluid therapy include cardiovascular compromise and electrolyte derangements. Pediatric patients have higher fluid requirements, whereas geriatric patients have lower fluid requirements. Dehydrated patients will require additional fluid support on top of maintenance fluid requirements. Patients with ongoing losses (i.e., hemorrhage, profound vomiting/diarrhea, excessive urinary loss) will need to have that volume replaced.

Fluid Monitoring

When it comes to monitoring fluid therapy, a variety of parameters are assessed. First, assess the patient; this is done by evaluating physical parameters (heart rate, pulse quality, MM color/moisture, CRT, extremity temperature, attitude/mentation, blood pressure, skin turgor). Next, assess ongoing losses, which can include vomiting, diarrhea, urinary, third-spacing, or wound loss. Then, assess the patient’s inputs versus outputs, which can be done by evaluating urine output and serial body weights. Additionally, lab values (i.e., PCV/TP, lactate, acid-base status) can give insight on a patient’s fluid status.

Patients should also be monitored for signs of fluid overload, or over-hydration. Signs include serous nasal discharge, chemosis, subcutaneous edema, ascites, increased respiratory rate, coughing, and restlessness.

Conclusion

Fluid therapy is an essential component of emergency and critically ill patient care. By having a better understanding of body fluid composition, fluid types, and disease process, veterinary technicians can be more involved in the fluid therapy plan. Veterinary technicians are also the primary caregivers when it comes to venous access and monitoring of fluid therapy.

References

1.  Battaglia AM, Steele AM, eds. Small Animal Emergency and Critical Care for Veterinary Technicians. St. Louis, MO: Elsevier; 2016.

2.  Norkus CL, ed. Veterinary Technician’s Manual for Small Animal Emergency and Critical Care. Chichester, West Sussex, UK: Wiley-Blackwell; 2018.

3.  Scalf R, ed. Study Guide to the AVECCT Examination. San Antonio, TX: AVECCT; 2014.

4.  Silverstein DC, Hopper K. Small Animal Critical Care Medicine. St. Louis, MO: Elsevier; 2015.