Page 484 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

472 FLUID THERAPY

splanchnic circulation. Although acute volume expansion observed in dogs with PSVA. 65 Primary polydipsia
in cirrhotic human patients increases peripheral blood associated with HE, stimulation of hepatoportal
volume, limited improvement occurs in central blood vol- osmoreceptors, and an impaired renal medullary concen-
ume (i.e., splanchnic, hepatic, and cardiopulmonary cir- tration gradient (e.g., chronic hypokalemia, decreased
culation). However, infusion of albumin in urea synthesis) may contribute to abnormal water balance
combination with administration of terlipressin, a long- in these animals. 95,120,214
acting synthetic AVP analog, can cause splanchnic vaso-
constriction and improved systemic perfusion. 226 Polyuria and Polydipsia
Polydipsia, polyuria, and renal dysfunction may be
Assessment of Ascites associated with liver disease in both dogs and cats. Dogs
Asampleoftheabdominaleffusionshouldbeevaluatedbio- with PSVA may be presented primarily for evaluation of
chemically,cytologically, andbyculture ifcytologysuggests polyuria and polydipsia. 41,95 Mechanisms may include
infection. Ascites arising from liver disease typically is a pure psychogenic polydipsia associated with HE; sensory input
transudate with a total protein concentration of less than signaling splanchnic vasodilatation, decreased hepatic
2.5 g/dL and a specific gravity between 1.010 and 1.015. portal perfusion, or altered osmolality; renal medullary
Cytologically, the fluid has low cellularity with only a few washout caused by low urea concentration; renal tubular
mesothelial cells and neutrophils present. In the jaundiced dysfunction associated with potassium depletion; or
patient, the fluid is yellow and bilirubin crystals may be increased concentrations of endogenous steroids. 120
observed, but the bilirubin concentration of the effusion Evaluation of USG before fluid therapy in dogs with
is lessthan thatofserum. A serum-to-effusion albumin gra- PSVA showed that 47 of 87 had a USG less than
dient greater than 1.1 suggests portal hypertension as a 1.020, and 12 of 87 were hyposthenuric (see
causative mechanism. 170 Body weight and abdominal girth Figure 19-6). Serum electrolyte concentrations were
measurements should be taken as a reference for evaluating not significantly correlated with USG, but subnormal
changes in fluid accumulation. Girth measurements are BUN concentrations occurred in 58 of 123 dogs, and
meaningful only if a consistent method is used. A mark is low normal or subnormal creatinine concentrations were
made on the abdomen with a permanent ink pen, and the found in 83 of 123. These findings suggest that diuresis
owneristaughttomonitorascitesaccumulationbymeasur- contributes to low USG in these patients, as supported by
ing girth circumference using a consistent technique. presence of a supranormal GFR in dogs with PSVA. 65
Subnormal BUN concentrations in dogs with PSVA
ABNORMAL RENAL FUNCTION IN could impair maintenance of the renal medullary solute
LIVER DISEASE gradient necessary for water reabsorption in response

As liver function deteriorates and portal hypertension to AVP. Low serum creatinine concentration probably
worsens, several maladaptive responses threaten renal reflects reduced muscle mass associated with the young
function. Decreased GFR reduces delivery of glomerular age and small size of many affected dogs, hepatic insuffi-
filtrate to the distal diluting segments of the nephron. ciency, and increased water turnover. 34,65,103,162
Coupled with increased resorption in the proximal Similar mechanisms are likely to be operative in dogs
tubule, this increases renal sodium and water reabsorp- with acquired hepatic insufficiency. Of cirrhotic dogs with
tion, impairs renal escape from abnormally increased ascites, 15 of 26 with urinalysis performed before treat-
aldosterone, and favors resistance to atrial natriuretic ment had a USG less than 1.020 (see Figure 19-5). Of
peptides. 72 Systemic counter-regulatory responses that these, only 3 of 26 were hyposthenuric. In the same
normally preserve filtration fraction increase production group. 11 of 42 had low BUN concentrations, and 21
of angiotensin II and further provoke vasoconstriction of 42 had low or subnormal serum creatinine
of the efferent arterioles. Although these events maintain concentrations. In cirrhotic dogs without ascites, 16 of
glomerular capillary pressure, increase filtration fraction, 34 with urinalysis performed before treatment had a spe-
and alter peritubular Starling’s forces favoring fluid reab- cific gravity less than 1.020, and only 1 of 34 was
sorption, they do so at the expense of decreased renal hyposthenuric. In the same group. 20 of 47 had low
blood flow. 90 Functional disruption of solute conserva- BUN concentrations, and 36 of 47 had low normal or
tion in Henle’s loop by loop diuretics (e.g., furosemide) subnormal serum creatinine concentrations.
may further impair the ability of the nephron to dilute or
concentrate urine. Altered Intrarenal Hemodynamics
Subtle changes in intrarenal hemodynamics contribute to
Increased Water Turnover and Glomerular deranged renal function in cirrhosis. Normally, renal
Filtration Rate blood flow is predominantly distributed to cortex

The influence of hepatic insufficiency on BUN and serum (90%) with less blood flow to the outer (9%) and inner
creatinine concentrations is aggravated by increased water medulla (1%). Autoregulation of renal blood flow
turnover and development of a supranormal GFR as maintains proper balance between afferent and efferent