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Dehydration Is Not Synonymous
With Hypovolemia!
Freddie, a seven-year-old Chesapeake Bay Retriever, presents with a hemoabdomen secondary to the hemangiosarcoma in his spleen. Blue, an eight-year-old Standard Poodle, presents in an Addisonian crisis. Bentley, a five-year-old Boston Terrier, presents with acute vomiting because he got into the garbage. The common thread that holds these cases together? They are all hypovolemic. Although it indicates low total body water, it does not equate to dehydration and requires a different treatment approach.
Hydration status is a measure of interstitial fluid content and is determined by evaluating skin turgor and moisture (or lack thereof) of the mucous membranes. Volume status is a measure of tissue perfusion and is evaluated by checking heart rate, capillary refill time, mucous membrane color, and blood pressure. Hypovolemic patients are tachycardic, have prolonged capillary refill times, tend to have pale mucous membranes, and are often (but not always) hypotensive. If hypovolemia is severe, one may see obtundation, weak, thready pulses, and lack of venous distension when the veins are held off. Hypovolemia may, of course, exist concurrently with dehydration but you must replace volume before rehydrating.
One should not treat hypovolemia by determining the patient's hydration status and then calculating (based on % dehydration and body weight) your fluid administration over the next 6-12 hours. Treatment of hypovolemia should be finished within 1-2 hours of presenting to the hospital. This type of resuscitation routinely requires rapid administration of large volumes of fluids intravenously. These are known as "shock boluses" of replacement crystalloids: 90 mL/kg/hour for dogs and 60 mL/kg/hr for cats. Typically, one should give a portion of the total volume and then reassess to determine if more volume is truly necessary.
But Blue, the Addisonian mentioned above, is also hypoproteinemic, so giving her large volumes of only crystalloids will drop her colloid osmotic pressure, likely resulting in generalized edema that may lead to its own set of problems. To prevent this, one will need to reach for a synthetic colloid (i.e., hetastarch) and bolus it at the same time you are giving crystalloids. In dogs, one may give 5 mL/kg increments (2-3 mL/kg in cats) up to a total of 20 mL/kg. There are other colloids (such as other synthetic colloids, and human and canine albumin) one may use if needed, but typically these are reserved as second stringers.
There are exceptions to virtually every rule. Patients with known cardiac disease and animals with a new or unevaluated heart murmur may not tolerate large fluid volumes. One may still deliver fluid boluses but use LRS − this fluid qualifies as a replacement fluid, and contains less sodium than others. Use smaller volumes in each bolus. In patients with pulmonary contusions or non-cardiogenic pulmonary edema, aggressive fluid therapy may increase the edema and lead to worsening respiratory distress.
Indiscriminate use of the terms dehydration and hypovolemia risks confusion and therapeutic errors. The pathophysiology of both dehydration and volume depletion must be understood if these conditions are to be properly recognized and appropriately treated whether they occur separately or together.
For more information or to talk to one of
our specialists, please call 402-614-9000
or email info@midwestvetspecialists.com
Christopher G. Byers, DVM, DACVECC, DACVIM (SAIM)
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