Incorrectly determining that a patient is dehydrated when it is hypovolemic can cost the animal its life. This leads some professionals to stress the need for caution when prescribing fluids to small animals and for a possible review of a veterinarian’s fluid therapy protocol.
“My primary concerns with the way fluids are administered to patients include incorrect volumes being given and improper selection of fluid type,” says Wayne E. Wingfield, DVM, Dipl. ACVS, Dipl. ACVECC, a professor emeritus of emergency and critical care medicine at Colorado State University.
“Secondly, too much emphasis on use of subcutaneous fluids is given. Failure to properly assess electrolytes and lack of understanding of acid base in relation to electrolytes and fluid needs is also a problem,” says Dr. Wingfield, also a consultant for Abbott Animal Health.
The lack of a step-by-step protocol for administering fluids is one reason missteps occur, says Elisa Mazzaferro, DVM, Ph.D., Dipl. ACVECC, of Wheat Ridge Animal Hospital in Wheat Ridge, Colo.
“I see a lot of animals in need of fluid therapy, often referred to me by other practices,” Dr. Mazzaferro says. “Sometimes practices stabilize the animal before transferring it, other times they immediately refer. When treating an animal in need of fluids, a complete and thorough physical exam is necessary.”
The average adult animal is composed of 40 percent intracellular and 20 percent extracellular water. A juvenile animal has more total body water (TBW), accounting for about 70 to 75 percent of body weight.
Obese animals’ TBW is closer to 50 percent, something that needs to be considered when making fluid therapy choices.
“Veterinarians can get confused during the initial exam when determining hypovolemia versus dehydration,” says Lesley King, MVB, Dipl. ACVECC, Dipl. ACVIM, director of the veterinary hospital intensive care unit at the University of Pennsylvania.
“They take the symptoms they find and make a determination as to what they’re dealing with, often without doing bloodwork. A [packed cell volume] and total protein along with blood pressure and central venous pressure should be measured upon presentation.”
In addition to bloodwork and exam findings, the body weight and blood lactate concentration should be used to make initial fluid therapy determinations, says Lauren Sullivan, DVM, a resident in small animal emergency and critical care at Colorado State. After finding initial markers, Dr. Sullivan says, use urine output and the results of central venous pressure to construct the target parameters.
Shock can be subtle and easy to miss in animals, Sullivan says.
“In hypovolemic animals, compensatory shock may present as tachycardia, strong or bounding pulses, bright mucous membranes, quick capillary refill time and a normal to high blood pressure,” she says. “For animals displaying subtle changes in their perfusion parameters, consider hypovolemia as a source of those changes. I often try a small bolus of IV crystalloids (10-20 ml/kg) and then reassess the perfusion parameters. Clinical signs of shock can differ depending on if the patient is a dog or cat, experts say. While dogs can be tachycardic, cats can be bradycardic.”
The wrong fluid can delay hydration resolution. Using hypotonic solutions for volume resuscitation is a bad choice. This happens most frequently in patients more than 10 percent dehydrated and showing signs of hypovolemia, the doctors say.
“Hypotonic solutions translocate from the vascular space very quickly, making them less effective for resuscitation,” says Amy Butler, DVM, Dipl. ACVECC, an assistant professor at Ohio State University. “There’s a big difference between hypovolemia and dehydration. Hypovolemia is low blood volume and can be diagnosed if the patient is showing signs of shock. Shock is a syndrome of clinical signs, including pale mucous membranes, altered mentation, cold extremities, poor pulse quality and prolonged CRT.”
Mazzaferro says Normosol-R is her fluid of choice when treating an animal in shock and says warming fluids can help stabilize a hypovolemic patient. Hypotonic solutions include 0.45 percent saline, 5 percent dextrose in water, 0.45 percent saline with 2.5 percent dextrose and Normosol-M.
“The shock dose of isotonic crystalloids for dogs is 80 to 90 ml/kg and for cats is 40 to 60 ml/kg,” Dr. Butler says. “This volume should be calculated, but only one-fourth to one-third of the calculated amount should be given in aliquots over 10 to 20 minutes. At the end of each aliquot, the patient should be reassessed to determine if signs of shock are still present and if additional volume is indicated.”
Not calculating actual fluid deficit in dehydrated animals is a problem, experts say. Several formulas can be used for this, but make sure to avoid doubling maintenance levels.
“Dehydration is calculated by multiplying the estimated dehydration by the body weight [in kg],” Butler says. “This will give the actual dehydration in liters. For example, a 5-kg cat that is 8 percent dehydrated has a fluid deficit of 400ml. Many veterinarians, when presented with a dehydrated animal, simply place the patient on twice maintenance fluids. As a rough rule of thumb, twice maintenance will only replace 5 percent dehydration over 24 hours, which may be insufficient for the very dehydrated patient.”
Choose the Fluid
Serum electrolytes, sodium, potassium and chloride should be used to guide the fluid type, authorities say.
“Patients with severe hypovolemia or low oncotic pressure may need colloids in addition to crystalloids,” Butler says. “Patients with known cardiac disease or a history of congestive heart failure may require more cautious fluid therapy as they are less able to handle large volumes.”
Magnesium is an often overlooked electrolyte, Sullivan says, but may be deficient in many critically ill animals.
“Lack of dietary intake, renal or gastrointestinal losses can all lead to hypomagnesemia in dogs and cats,” Sullivan explains. “The best way to determine magnesium status in the body is to monitor ionized magnesium. Although this capability is not available at every practice, one common manifestation of hypomagnesemia is refractory hypokalemia. In patients with hypokalemia that are not responding to appropriate potassium supplementation, consider hypomagnesemia as a possible culprit.
“In these cases, supplement magnesium in the base fluids. If possible, monitoring ionized magnesium is the best way to determine the need for supplementation.”
Calculating the rate that fluid should be administered is another step in the fluid therapy process. Dr. King is a fan of the fluid infusion pump.
“The pump tells you if the line is occluded, if it’s out of fluid, and it provides extremely accurate rates of infusion,” King says. “Pumps have changed fluid therapy more than any other evolution.”
Check Fluid Loss
Monitoring a patient’s declining need for fluids is as important as monitoring fluid loss, experts say.
“Patients with excessive ongoing losses, such as puppies with parvovirus infections, will have huge fluid losses that cannot be matched simply by increasing a maintenance rate,” Butler says. “Monitoring urine output by weighing pads or towels, or estimating the amount lost, will help to determine how much fluid is being lost.”
The measured fluid loss can be given back over one to four hours depending on how much and how rapidly the losses are occurring, experts say.
“Fluid losses may arise from vomiting, diarrhea, third spacing into the gastrointestinal tract/pleural space/peritoneal cavity, or polyuria,” Sullivan says. “If the ongoing losses are not accounted for, the fluid rate will not be high enough and an animal may become dehydrated or hypovolemic.”
–Skipping bloodwork before fluid administration
–Misdiagnosing as dehydration instead of hypovolemia
–Giving too much or too little fluid
–Using the wrong type of fluid
–Using subcutaneous fluid when intravenous fluids are needed
–Not keeping up with continued fluid loss such as vomiting and diarrhea
–Not giving needed fluids to anemic animals
Keep this in mind when performing fluid therapy:
How Much Fluid
With fluids, most criticalists use (30 x BW in kg) + 70 per day, or 132 x BW0.75 per day for dogs and 70 x BW0.75 per day for cats. All of these give the answer in ml per day. Other formulas, such as 2 ml/kg/hr, 40-60 ml/kg/day and 1 ml/lb/hr are all used.
“Multiple fluid rate formulas can be used,” says Amy Butler, DVM, Dipl. ACVECC, of Ohio State University. “There is no one correct maintenance fluid calculation. Every patient will have different fluid requirements.”
Butler says the requirements of a patient suffering from renal insufficiency are likely to be higher than those of a patient with hyperadrenocorticism, who in turn would have higher requirements than an otherwise healthy patient. Commitment to one formula isn’t as important as continued assessment of the patient after fluid administration.
“When calculating a fluid rate, the dehydration maintenance rate and ongoing losses are the first considerations,” Butler says. “A rate of 100-180 ml/kg/day is often used for young animals. Patients with renal or endocrine disease will likely need higher maintenance fluid rates.”
Time to Reassess
Continuing to monitor the patient on fluids is important when deciding the next move in fluid therapy.
“The patient should be assessed at least once daily to make sure the fluid therapy plan is meeting the patient’s needs,” Butler says. “Increased skin turgor, dry mucous membranes, thirst or weight loss are signs that the patient is not receiving enough fluids. Very frequent urination in a patient with normal kidneys, weight gain, chemosis and pulmonary crackles are signs that the patient is receiving too much fluid.
“The bottom line is to pick a fluid rate based on calculation, but then titrate to effect.”
Watch for Anemia
Not giving IV fluids to anemic patients is another fluid therapy misstep.
“Many clinicians are afraid to give IV fluids to anemic animals out of fear that they will bring down the PCV,” Butler says. “Even if the plasma volume is expanded and PCV decreases, the total number of red blood cells remains the same. If the patient is showing signs of poor perfusion or shock, giving IV fluids will improve tissue perfusion and tissue oxygen delivery.”
Dehydrated patients and those in shock should be stabilized before anesthesia or a surgical procedure.
"Aliquots of the shock dose should be given until signs of shock have improved,” Butler says. “Anesthesia decreases the body’s compensatory responses to hypovolemia, so it is important that hypovolemia be resolved prior to surgery. If the patient is severely dehydrated, perfusion will be compromised, especially once those compensatory mechanisms are removed under anesthesia.
“Provide adequate fluid rates during anesthesia using 5-10 ml/kg of isotonic crystalloids. However, patients with excessive losses may require more fluids.”
In severe cases, administration of too much fluid can cause pulmonary or interstitial edema, but not giving enough fluid also has consequences.
“Too little fluid can result in tissue hypoperfusion,” says Wayne E. Wingfield, DVM, Dipl. ACVS, Dipl. ACVECC, of Colorado State University. “Continued dehydration, renal failure and hypovolemic shock, even death, can occur.”