Page 541 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Fluid and Diuretic Therapy in Heart Failure 529

half of the total ascitic volume is drained. The high depends on the cause and severity of CHF, and in many
protein content of hepatic lymph and the dynamic cases furosemide, nitroglycerine, and pimobendan are the
equilibrium between the third-space and plasma only drugs needed for initial control of CHF in dogs. In
compartments argue against complete drainage of the more advanced cases, systemic arterial dilation therapy
peritoneal space. 141 Pulmonary edema sufficient to cause may be beneficial. Although vasodilator therapy has the
respiratory failure and respiratory muscle fatigue is an potential to induce systemic hypotension, such treatment
indication for artificial ventilation. generally is safe in dogs when baseline ABP is greater than
Diuresis is initiated and maintained with parenterally 95 mm Hg. Sodium nitroprusside (1 to 5 mg/kg/min
administered furosemide. An initial intravenous bolus intravenously by CRI), enalapril (0.5 mg/kg orally every
of 2 to 5 mg/kg can be followed by serial intravenous 12 hours), and hydralazine (1 to 2 mg/kg orally every 12
or intramuscular boluses of 1 to 4 mg/kg every 6 to hours) all exert vasodilator effects in the hospital setting.
8 hours or more frequently when necessitated by insuffi- Each drug can increase stroke volume and reduce pulmo-
cient clinical response. The use of constant rate infusion nary edema, especially in the setting of severe mitral
(CRI) of furosemide also may be used to treat dogs regurgitation arising from valvular endocardiosis.
and cats with life-threatening pulmonary edema. In Afterload reduction can also increase stroke volume when
healthy dogs and in human patients with CHF, furose- there is left ventricular dysfunction, as in dogs with
mide CRI increases urine output and minimizes electro- dilated cardiomyopathy. 11 The choice of vasodilator in
lyte disturbances when compared with repeated bolus dogs depends on the urgency of the situation. In florid
injections. 2,31 Our approach for a CRI is to initially pulmonary edema, nitroprusside can be infused to a spe-
administer an intravenous bolus of furosemide, estimate cific endpoint, such as a systolic ABP of 85 to 90 mm Hg.
the furosemide dosage required for the next 24 hours, In less urgent cases, or when intravenous therapy is
and then infuse this volume by syringe pump. Supple- impractical, enalapril or hydralazine can be administered
mental boluses also can be given if required during the orally to provide afterload reduction. After stabilization,
CRI. A novel approach for treatment of severe CHF in enalapril or another ACE inhibitor is initiated (or
human patients has been advocated by Licata et al. 92 They continued) as part of the home treatment plan.
administered small-volume, hypertonic saline combined Nitroprusside and hydralazine rarely are used in cats,
with furosemide and demonstrated enhanced diuresis in and most cats with CHF are treated with furosemide,
refractory CHF. This therapy has not been studied in nitroglycerin, and eventually an ACE inhibitor.
animals with spontaneous disease but deserves consider- Pimobendan (1.25 mg/cat every 12 hours) can be used
ation, especially in hyponatremic patients. Another as an extralabel treatment in cats with severe CHF and
approach that may be adopted in veterinary practice hypotension.
involves addition of intravenous synthetic human brain Cardiac output, ABP, and tissue perfusion are
natriuretic factor (h-BNP) or nesiritide to the hospital supported when necessary by providing inotropic sup-
treatment protocol. 107 Although expensive, nesiritide is port. In dogs or cats with severe systemic hypotension
labeled for human use and appears to be effective in dogs. (ABP <80 mm Hg), inotropic support with
The h-BNP increases urine output and decreases the dobutamine (2.5 to 10 mg/kg/min) or dopamine (2
effects of aldosterone in furosemide-treated dogs, 17 to 10 mg/kg/min) is indicated (in conjunction with
increases urine volume in normal dogs and those with pimobendan). Catecholamines most often are
experimental CHF, 18,167 and has limited electrophysio- administered to dogs with CHF caused by dilated car-
logic effects on the canine heart. 41 diomyopathy. Occasionally, this approach is used in
Both preload reduction and afterload reduction are patients with severe mitral regurgitation or pulmonary
beneficial to the failing left ventricle. The inotropic drug embolism. Cats with any form of cardiomyopathy may
pimobendan (0.2 to 0.3 mg/kg PO q12h) also exerts develop cardiogenic shock characterized by bradycar-
vasodilator properties via phosphodiesterase-3 inhibi- dia, hypothermia, and hypotension. Treatment with
tion. Nitrates such as nitroglycerin ointment and sodium dobutamine can be life saving in affected cats. Infusions
nitroprusside increase concentrations of the vasodilator should be titrated to a systolic ABP of 90 to 120 mm
nitric oxide in vascular smooth muscle, leading to relaxa- Hg and can be combined with slow external warming
tion of arterioles and systemic veins. 113 Two percent in an oxygen incubator. When treatment with
nitroglycerin ointment ({1/4} to 1 inch of the 2% oint- catecholamines is impractical, oral administration of
ment, topically every 12 hours) acts primarily as a sys- the calcium sensitizer pimobendan may be effective.
temic venodilator, and this treatment is well tolerated Intravenous administration of a related compound,
by both dogs and cats although efficacy has not been levosimendan, may become a treatment alternative in
demonstrated in a clinical study. The anticipated the future. Both pimobendan and levosimendan are
venodilation should work in concert with furosemide considered “inodilators” owing to potent positive ino-
to decrease venous and capillary hydrostatic pressures. tropic effects combined with vasodilatation, which
The need for arteriolar dilators in the hospital setting unloads the left ventricle. 95,164