Page 713 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

700 SPECIAL THERAPY

Similarly, polyuric animals with severe CKD accumulate by use of fluid monitoring equipment (e.g., in-line blood
orally administered fluids associated with tube feeding volume monitor, venous oxygen saturation, continuous
and parenteral fluids used to supplement hydration or weight, bioimpedance spectroscopy).* In-line blood vol-
to manage episodes of decompensation. Hypervolemia umemonitorsareespeciallyusefultoassesstheefficacyand
and circulatory overload develop under both the safety of ultrafiltration (Figure 29-10). 167,168
circumstances as expressed by chemosis, pleural effusion, A lack of change in blood volume during ultrafiltration
peripheral or pulmonary edema, congestive heart failure, indicates the rate of fluid removal from the vasculature is
and hypertension. Once established, overhydration may precisely matched by a fluid transfer from the extravascu-
not resolve with cessation of fluid delivery or diuretic lar fluid load. If blood volume does not decrease after
administration, leaving no medical therapies to manage starting ultrafiltration, a faster rate of fluid removal could
these disorders. Restoration of fluid balance is an impor- be attempted to increase the efficiency of fluid removal.
tant indication for hemodialysis and a consistent compo- As the vascular refill rate lags behind the ultrafiltration
nent of the dialysis prescription. rate, the relative change in blood volume becomes nega-
During hemodialysis, fluid can be extracted from the tive in proportion to the deficit in vascular refilling. The
patient by the process of ultrafiltration. The volume change in blood volume stabilizes when the forces for vas-
and rate of fluid removal must be prescribed for each dial- cular refilling match ultrafiltration. Moderate fluid loads
ysis session based on the estimated volume excess and can be removed at a steady 5% to 8% decrease in relative
deviation from the animal’s ideal dry body weight. Ideal blood volume without overt clinical consequences. More
dry body weight is a progressively derived value deter- intensive ultrafiltration at a stable 10% to 12% decrease in
mined as the body weight at which additional fluid blood volume is tolerated by some animals with readily
removal would produce hypotension or signs of transferable fluid loads, but greater decreases in blood
hypovolemia. 83,84 Ideal dry weight usually is predicted volume are likely to lead to clinically evident
from recent historical weight measurements before the hypovolemia. The rate of change in blood volume during
onset of illness, or it is estimated from the postdialysis ultrafiltration helps predict the animal’s ability to surren-
weight when blood pressure was controlled or there der the fluid burden and attain dry weight. 84 Steep
was no demonstrated fluid accumulation. Ideal dry changes in relative blood volume at greater than 10%
weight should not be considered a static parameter but per hour (especially at the initiation of the treatment)
should be redefined regularly to compensate for ongoing forecast an excessive ultrafiltration rate that is unlikely
changes in the animal’s lean body mass and body fat. Fail- to plateau at a safe level (see Figure 29-10). If ultrafiltra-
ure to update the targeted ideal dry weight can trigger a tion is stopped transiently, a rapid positively directed
prescription for excessive or inadequate ultrafiltration, change in blood volume indicates the fluid load has not
leading to hypovolemia or progressive overhydration, been corrected completely, whereas no change suggests
respectively, as the patient gains or loses nonfluid mass. 83 the animal is at dry weight. A positive change in blood
Progressive deviation from dry weight also can be volume may be seen when the dialysate sodium is greater
recognized by routine assessment of body condi- than the animal’s serum sodium concentration causing a
tion. 33,34,112,117 The determination of dry weight can shift of fluid into the animal or after administration of
be elusive on the basis of clinical parameters alone and intravenous or oral fluids or mannitol (see Figure 29-7).
often is facilitated by more objective techniques including Animals often tolerate ultrafiltration better at the
blood volume assessment and bioimpedance beginning of the treatment than at the end, and the rate
spectroscopy. 187,188 of fluid removal can be profiled to achieve greater fluid
The rate and volume of ultrafiltration achieved is con- losses at the beginning and scaled back later in the session
tingent on the hemodynamic stability of the animal. All to achieve the same treatment goal. Sodium profiling can
available hemodialyzers have sufficient ultrafiltration per- be used to offset the hypovolemic and hypotensive effects
formance to remove fluid from the vascular space faster of aggressive ultrafiltration to maximize fluid removal.
than its rate of redistribution (refill) from the interstitium Sodium loading during the hypernatremic stages of the
and intracellular compartments. This imbalance can pro- modeling profile expands intravascular volume and
mote hypovolemia, hypotension, and circulatory collapse facilitates redistribution of fluid from the interstitium
if ultrafiltration is not prescribed and monitored carefully. and intracellular compartments (see Figure 29-7). The
The process of ultrafiltration is precisely regulated by the administration of small doses of 6% hydroxyethyl starch
dialysis machine, but small errors or deviations in (hetastarch; at 1 to 2 mL/kg) can facilitate achievement
the tolerance of these systems can cause unscheduled of the ultrafiltration target by maintaining intravascular
volume losses in small animals during the course of a dialy- volume, supporting vascular refilling, and preventing
sis session. Slow rates of ultrafiltration between 5 and hypotension. The net volume of fluid subsequently
10 mL/kg/hr generally are tolerated by dogs and cats, removed will far exceed the volume administered and
but faster rates must be prescribed cautiously and adjusted
according to the animal’s vital signs and blood pressure or *References 34, 59, 83, 85, 93, 146, 188, 189.