DETERMINING THE PERCENTAGE OF DEHYDRATION
Table 1 Estimating the Percentage of Dehydration
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The percentage of dehydration can be subjectively estimated based on the presence and degree of loss of body weight, mucous
membrane dryness, decreased skin turgor, sunken eyes, and altered mentation (Table 1).1 These parameters are largely subjective because they can also be affected by decreased body fat and increased age.
The more severe stages of dehydration are also accompanied by signs of hypovolemic shock. Other factors, including hemorrhage
and third spacing of body fluids, can also result in a decrease in intravascular circulating volume, resulting in signs of
hypovolemia. Severe hypovolemia resulting in more than a 15% depletion of effective circulating volume leads to a transcompartmental
fluid shift from the interstitial to the intravascular compartments, which occurs within one hour of fluid loss.6 When fluid loss is so severe that intravascular fluid volume is affected, hypovolemia can result in tachycardia, prolonged
capillary refill time, decreased urine output, and hypotension.
The vascular space is sensitive to changes in the amount of circulating volume. During states of normovolemia, baroreceptors
in the carotid body and aortic arch sense vascular wall tension and send pulsatile continuous feedback via vagal afferent
stimuli to decrease heart rate. In the early stages of hypovolemic shock, the baroreceptors sense a decrease in vascular wall
stretch or tension and blunt the tonic vagal stimulation. This allows sympathetic tone to increase heart rate and contractility
in an attempt to normalize cardiac output. Later, decreased blood flow and sodium delivery to receptors in the juxtaglomerular
apparatus activate the renin-angiotensin-aldosterone axis, stimulating sodium and fluid retention to replenish intravascular
volume.1-3
CORRECTING FLUID IMBALANCES
When clinical signs of hypovolemic shock are present, intravascular fluids must be replaced immediately. Calculated fluid
volumes for patients in shock are 90 ml/kg for dogs and 44 ml/kg for cats.1 A simple guideline to follow is to replace one-fourth of the calculated fluid volume as rapidly as possible and then reassess
perfusion parameters including heart rate, blood pressure, capillary refill time, and urine output. In dogs, a simple method
to calculate one-fourth of the fluid volume for treating shock is to take an animal's weight in pounds and add a zero, giving
you the amount of fluid in milliliters to administer as a bolus.
About 80% of the volume of crystalloid fluid infused will re-equilibrate and leave the intravascular space within one hour
of administration. A constant-rate infusion of a crystalloid fluid is recommended to provide continuous fluid support in patients
that are dehydrated and have ongoing losses. In some cases, the fluid required to restore intravascular and interstitial volume
can cause hemodilution and dilution of oncotically active plasma proteins, resulting in interstitial edema formation. In such
cases, a combination of a crystalloid fluid along with a colloid-containing fluid can help restore oncotic pressure and prevent
interstitial edema.6
Once immediate life-threatening fluid deficits are replaced, provide additional fluid based on the estimated percentage of
dehydration and maintenance needs. Basic dehydration estimates can be calculated based on the fact that 1 ml water weighs
about 1 g and by using the following formula:
Body weightkg × estimated percent dehydration × 1,000 ml/L
This formula helps you determine the amount of fluid deficit in liters. A frequent mistake when replenishing fluid deficits
is to arbitrarily multiply a patient's daily water requirement by a factor of two or three to replenish intravascular and
interstitial deficits. This practice frequently underestimates a patient's fluid needs and does little to treat volume depletion
and interstitial dehydration. Instead, it is better to use the formula above and add the result to daily maintenance fluid
requirements and ongoing losses.
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