Page 537 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Fluid and Diuretic Therapy in Heart Failure 525

administration of furosemide at the same dose can have a concentration or effect and decrease potassium and mag-
dramatic diuretic benefit in some patients because of the nesium losses. Thus stable serum creatinine and potassium
more efficient delivery of the drug. Additional clinical concentrations over two or three reevaluation periods are
situations in which diuretics may fail include treatment likely to be maintained for some time. 141 The overall dos-
of pain with opiates (which stimulate ADH release), age of diuretics in dogs should be limited by using combi-
unanticipated high sodium intake, and acute worsening nation therapy for CHF, including progressive sodium
of heart failure. In these situations the diuretic dosage restriction, ACE inhibitors, spironolactone, and
required to establish diuresis successfully may be substan- pimobendan. 10,61,62,78,95,123 Cats with chronic CHF
tially higher or an alternative route of administration may typically receive furosemide, an ACE inhibitor, and some-
be required. times pimobendan or spironolactone. Cats receiving furo-
Diuretic therapy triggers neurohormonal semide are more prone to develop mild to moderate
responses, 46,149,182 and diuretic monotherapy is not an azotemia and hypokalemia than are dogs, even at dosages
appropriate management strategy for long-term treat- that are 50% lower than daily dosages typically used for
ment of CHF. Diuretic-induced volume depletion invari- dogs. Spironolactone is usually well tolerated in cats but
ably leads to a rebound in renal retention of salt and water may cause anorexia or ulcerative skin lesions. 97
either at the previous or a new steady-state in terms of
sodium balance. This concept, termed the braking phe- EFFECTS OF OTHER
nomenon, is highly important for understanding the basis CARDIOVASCULAR DRUGS ON
for multidrug therapy and why furosemide is typically RENAL FUNCTION
given two or even three times daily. As an example, Angiotensin II is one of the factors responsible for effer-
once-daily dosing of furosemide in human patients is ent arteriolar vasoconstriction and increased filtration
associated with a brisk diuresis for about 6 hours. But fraction in CHF. The ACE inhibitors, such as enalapril,
over 24 hours there may be no net loss in total body may antagonize efferent arteriolar constriction suffi-
sodium or edema because salt and water retention can ciently in some patients to cause an abrupt decrease in
occur for the balance of the day. 70 This effect is mediated glomerular perfusion pressure. This effect is especially
partly by decreased tubular flow rate, salt retention in likely in volume-depleted patients. The result is acute
segments of the nephron unaffected by the diuretic used, renal failure, with serum creatinine concentration often
increased sympathetic activity, and activation of the renin- exceeding 5 mg/dL. Renal failure in this setting generally
angiotensin-aldosterone system (RAAS). 112,141 Thus can be reversed by reducing diuretic dosage, decreasing
control of edema in CHF requires a steady state of the dosage of the ACE inhibitor, and providing judicious
reduced sodium retention, and patients should receive a fluid therapy (see Therapy of Fluid and Electrolyte
consistent dosage of furosemide along with an ACE Imbalances in Congestive Heart Failure section). After
inhibitor, spironolactone, and a sodium restricted diet. volume repletion, the dosage of the ACE inhibitor is
The dosage of diuretics used must be effective but increased over 2 to 4 weeks, and the drug combination
should be carefully controlled to minimize the common is adjusted while monitoring body weight, clinical signs
complications of dehydration, azotemia, electrolyte of CHF, ABP, and serum creatinine concentration.
imbalance, and potentially deafness. The first dosage of Normal autonomic responses to changes in blood
furosemide chosen for a patient with life-threatening pul- pressure and normal heart rate variability are blunted in
monary edema often is high (2 to 5 mg/kg, intravenously CHF. 66,94 This is associated with dominant sympathetic
every 1 to 3 hours) to ensure diuresis. The furosemide activity in cardiac failure. Sympathetic nerve activity can
dosage is promptly reduced if symptomatic improvement increase renin release and affect renal blood flow. 122 Dig-
and a brisk diuresis are observed. These effects can occur italis glycosides such as digoxin appear to exert a neuro-
71
within 1 to 2 hours of administration of furosemide, tropic effect and restore baroreceptor sensitivity and
but a lag period (12 to 24 hours) may be noted between parasympathetic tone, and this effect is independent of
obvious clinical improvement and clearing of radio- the inotropic action of the drug. 81,166 By this or some
graphic pulmonary densities. Owing to the potential other effect, digoxin also can blunt the RAAS in CHF.
for overzealous diuresis and iatrogenic renal failure and Although digoxin has been largely supplanted by the
electrolyte disturbances, the clinician should evaluate inodilator, pimobendan, due to this autonomic effect,
serum biochemistries every 24 to 48 hours until the digoxin therapy maintains a role in patients with atrial
patient is eating and drinking satisfactorily. After a stable fibrillation and end-stage heart failure.
diuretic course of 2 weeks, most dogs and cats maintain Cardiac patients sometimes are treated with aspirin
relatively stable renal function and serum potassium and other antiprostaglandin drugs to prevent blood clots
concentrations unless a decompensating factor (e.g., (cats) or to alleviate signs of osteoarthritis (dogs). These
vomiting, anorexia) intervenes. This is especially true NSAIDs may be deleterious when used in CHF patients.
when ACE inhibitors and spironolactone are prescribed By preventing prostaglandin-induced dilatation of the
concurrently because they reduce aldosterone afferent arteriole, NSAIDs may decrease glomerular