Page 401 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Monitoring Fluid Therapy and Complications of Fluid Therapy 391

Administration of a colloid has been recommended salivation, or inadequate resuscitation as causes. In addi-
when the TS is less than 4.0 g/dL (less than 40 g/L) tion, alterations in vasopressin (antidiuretic hormone or
to avoid a clinically relevant decrease in colloid osmotic ADH) activity and intravascular sodium, may result in
pressure (COP), 50 which may predispose to tissue and marked hyposthenuric polyuria or concentrated oliguria.
pulmonary edema, especially when additional crystalloid [Chapter Hypernatremia, Hyponatremia]. Calculating
fluids are to be administered. The refractometer reading appropriate fluid requirement is important because
for hetastarch is 4.5 g/dL (45 g/L) and that for administering an excessive volume of fluids will result
pentastarch is 7.5 g/dL (75 g/L). After these colloids in diuresis, medullary washout, and electrolyte
are administered, the TS measurement is difficult to inter- disturbances (especially potassium). Ongoing diuresis
pret and cannot be extrapolated to a COP measurement. may require a prolonged hospital stay for correction of
Response to administration of colloids must be assessed resulting fluid and electrolyte imbalances. Measurement
by direct COP measurement, CO determination, or of urine volume can be accomplished by:
improvement in clinical signs. 1. Collection of urine when the animal voids
2. Use of a metabolic cage
URINE PRODUCTION 3. Intermittent or continuous urinary bladder
During hypovolemia and dehydration, renal blood flow is catheterization
decreased. When blood volume is decreased by hemor- 4. Placing preweighed towels or pads under the animal
rhage, the decreased pressures result in activation of the and weighing them after voiding. Any increase in
sympathetic nervous system, including renal sympathetic towel or pad weight over baseline, unless otherwise
nerves. Sodium and water are conserved by constriction soiled, is assumed to be the result of urine. The vol-
of the glomerular arterioles, decreased glomerular filtra- ume of urine voided can be estimated by assuming
tion rate (GFR), increased tubular reabsorption of salt 1000 mL equals 1000 g (1 kg or 2.2 lb). This tech-
and water, and activation of the renin angiotensin aldoste- nique underestimates urine produced because some
rone system. Decreased arterial pressure also results in urine may remain in the cage.
secretion of antidiuretic hormone (ADH). 28 Together, Weighing the animal several times a day will assist in
these actions serve to replenish the intravascular space estimating fluid loss or gain. If the animal’s weight
and return blood volume toward normal. As a conse- declines despite fluid therapy, it is assumed that ongoing
quence of these effects, a very small volume of hypertonic losses such as high urine output, vomiting, diarrhea, sali-
urine is produced. In addition to renal blood flow and vation, or evaporative losses caused by fever or hyperther-
GFR, urine volume also is dependent on the mia are in excess of fluids administered. A weight loss of
concentrating ability of the kidneys. If underlying renal 0.1 to 0.3 kg body weight per 1000 kcal energy require-
tubular dysfunction is present, increased urine volume ment (approximate caloric requirement for a caged 20-kg
may not reflect adequate renal perfusion and GFR. When dog) per day is anticipated in an anorexic animal. Third-
renal function is otherwise normal, however, urine pro- space losses must be assessed by other means because
duction and specific gravity are useful parameters to mon- weight loss will not be evident. After urine flow has been
itor when assessing intravascular volume. Urine output established, regardless of the underlying problem, ongo-
has been referred to as the “poor man’s cardiac output.” ing fluid requirements are calculated as follows:
Intravenous fluid therapy also will expand the intravascu- 1. Divide the day into six 4-hour intervals, four 6-hour
lar space and consequently increase urine volume. intervals, or three 8-hour intervals.
2. Determine urine produced during each time interval,
Assessment of Urine Output and add the estimated insensible and ongoing losses
Careful monitoring is necessary to ensure that urine pro- for that period.
duction is maintained by adequate fluid replacement 3. Determine ongoing losses in vomitus, diarrhea, and
(Chapter 22). Normal urine production is between 0.5 saliva for the period selected.
and 2 mL/ kg/hr but varies with the concentrating abil- 4. Determine insensible loss at 20 mL/kg/day. In addi-
ity of the kidneys. The goal is to maintain urine output of tion, for each degree Celsius above 38.5 C, add 10%

1 to 2 mL/kg/hr with a urine specific gravity of approxi- of the normal daily maintenance fluid requirement
mately 1.026 (dog) and 1.035 (cat). However, if there is (i.e., if the normal daily requirement is 1 L and tem-
loss of concentrating ability (e.g., renal tubular injury, perature is 40.5 C, then 200 mL should be added).

Escherichia coli pyelonephritis), urine output can be Divide this total amount by 6, 4, or 3 depending on
extremely high (25 to 40 mL/kg/hr), and specific grav- the interval selected above.
ity may be in the hyposthenuric or isosthenuric range, 5. This volume of fluid, in addition to the amount deter-
hence the importance of measuring of urine output mined by urine produced and ongoing losses, is to be
and specific gravity. If urine output is decreased, the delivered during the next period. Daily weight is
patient should be assessed for possible third-space loss, advised in all hospitalized patients because unnoticed
capillary leak, increased temperature, vomiting, diarrhea, polyuria or inadequate intravenous or oral fluids for an