Page 714 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Hemodialysis and Extracorporeal Blood Purification 701

HCT Any decrease in venous oxygen saturation should prompt
31
immediate assessment of the patient and possible adjust-
29 ment to the ultrafiltration goals.
27 Ultrafiltration and diffusive solute removal are inde-
pendent processes controlled by separate functions of
25
the delivery system. Animals with life-threatening fluid
23
overload and severe azotemia are at risk for excessive sol-
21
ute removal and dialysis disequilibrium syndrome if the
19 treatment is protracted to resolve the overhydration.
A Conversely, they remain at increased cardiopulmonary
BV% risk if the overhydration is not corrected during low
5
intensity treatments. Both of these contrasting dialysis
0
requirements and risks can be managed safely by prescrib-
5
ing periods of ultrafiltration without hemodialysis
10
throughout the treatment or by scheduling independent
15
periods of ultrafiltration before or after the azotemia has
20
been treated to an appropriate URR. During ultrafiltra-
25
tion without dialysis, the machine is placed in bypass
30 mode to stop dialysate flow to the dialyzer (and diffusive
B
solute removal), while blood flow and transmembrane
Sat% pressure gradients are maintained to continue ultrafiltra-
100
90 tion. This technique permits slower and more complete
80 fluid removal without producing unsafe rates of diffusive
70
60 hemodialysis. Isolated ultrafiltration can be used in
50 nonuremic patients to treat fluid congestion associated
40 with heart failure and pulmonary edema refractory to
30
20 diuretics.* Resolution of the fluid burden from patients
10 with congestive heart failure may improve hemodynamic
0
function, clinical well-being, pulmonary function, drug
dependency, and exercise capacity. 5,111,154 Similar
Time (hours) 1 2 3 4 5 6 7
C indications exist in animals, and this aspect of extracorpo-
Figure 29-10 Change in hematocrit (HCT, A), relative blood real therapy should be evaluated further. Ultrafiltration
volume (DBV%, B), and venous oxygen saturation (Sat%, C) requirements for individual treatments can be increased
assessed by an in-line monitor in a dog with acute uremia during to offset administered loads of blood products, drugs,
hemodialysis and continuous ultrafiltration. The figure illustrates the and alimentation solutions. Ultrafiltration becomes espe-
decreases in relative blood volume and venous oxygen saturation cially important in oliguric animals with no excretory
associated with hypovolemia induced by ultrafiltration. The late capacity and no tolerance for additional volume. The vol-
increase in oxygen saturation reflects the supplemental ume of essential fluid-containing therapies should be bal-
administration of oxygen (arrow).
anced by equivalent or proportional fluid removal during
the dialysis session to balance the anticipated fluid input.
improve the efficiency of the ultrafiltration prescription. Net fluid balance at the end of the dialysis treatment is the
Progressive hypovolemia from excessive ultrafiltration is difference between the delivered ultrafiltered volume and
detectable with in-line blood volume monitors well the volume of the priming solution administered at the
before development of hemodynamic signs, permitting beginning of the treatment and the amount of rinse-back
adjustment of the ultrafiltration rate to avert hemody- fluid used to return blood to the animal. Air can be used
namic complications. Changes in blood pressure and as a rinse-back medium to displace the extracorporeal
heart rate are rarely sensitive or early predictors of blood rather than fluid to maximize net fluid removal.
hypovolemia under these conditions. Ultrafiltration contributes marginally to total solute
Venous oxygen saturation also is a sensitive indicator of removal during the treatment by convective transfer.
hemodynamic stability. Sudden or progressive decreases Convective solute removal does not change the plasma
reflect directional decreases in cardiac output secondary concentration of solutes as occurs with dialysis because
to hypovolemia and can foreshadow impending hypoten- the transfer occurs with plasma water at the existing con-
sive events. Venous oxygen saturation can be measured centration. Dialysis dose predicted by URR, simple urea
continuously with an in-line hematocrit monitor kinetic models, and measurement of postdialysis serum
or observed visibly as darkening (desaturation) of blood
in the extracorporeal circuit (Figure 29-10). 167 *References 5, 89, 111, 138, 139, 154, 184.