Page 91 - Small Animal Internal Medicine, 6th Edition

CHAPTER 3 Management of Heart Failure 63

recommended for heart failure caused by diastolic dysfunc- Other positive inotropic therapy might also be indicated
tion or ventricular outflow obstruction. for acute CHF caused by poor myocardial contractility or
VetBooks.ir dilator with direct action on vascular smooth muscle; unfor- when there is persistent hypotension. Treatment for 1 to 3
Sodium nitroprusside is a potent arteriolar and venous
days with an IV sympathomimetic (catecholamine) or phos-
tunately, it has become prohibitively expensive in the United
pressure, forward cardiac output, and organ perfusion when
States. Nitroprusside is given by IV infusion because of its phodiesterase (PDE) inhibitor drug can help support arterial
short duration of action. Blood pressure must be closely myocardial failure or hypotension is severe.
monitored when using this drug. The dose is titrated to Catecholamines enhance contractility via a cyclic adeno-
maintain mean arterial pressure at about 80 mm Hg (at least sine monophosphate (cAMP)-mediated increase in intracel-
++
> 70 mm Hg) or systolic blood pressure between 90 and lular Ca . They can provoke arrhythmias and increase
110 mm Hg. Nitroprusside CRI usually is continued for 12 pulmonary and systemic vascular resistance (potentially
to 24 hours. Dosage adjustments may be necessary because exacerbating edema formation). Their short half-life (<2
drug tolerance develops rapidly. Profound hypotension is the minutes) and extensive hepatic metabolism necessitate con-
major adverse effect. Cyanide toxicity can result from exces- stant IV infusion. β-receptor downregulation and uncou-
sive or prolonged use (e.g., longer than 48 hours). Nitroprus- pling limit their effectiveness within a few days of
side should not be infused with other drugs and should be administration. Concurrent use of a β-blocker also blunts
protected from light. the effect of the catecholamines. Dobutamine (a synthetic
Hydralazine is an alternative to nitroprusside. It is a analog of dopamine) has lesser effect on heart rate and after-
pure arteriolar dilator. Hydralazine is useful for refrac- load and is preferred over dopamine. Dobutamine stimulates
tory pulmonary edema caused by mitral regurgitation β 1 -receptors, with only weak action on β 2 - and α-receptors.
(MR) because it can reduce regurgitant flow and lower left Lower doses (e.g., 3-7 µg/kg/min) have minimal effects on
atrial (LA) pressure. It should be used only cautiously in heart rate and blood pressure. The initial infusion rate should
patients with DCM. An initial oral dose of 0.5 to 1 mg/ be low; this can be gradually increased over hours to achieve
kg (or 0.05 to 0.1 mg/kg IV or IM) can be repeated every greater inotropic effect and maintain systolic arterial pres-
2 to 3 hours until the systolic blood pressure is between sure between 90 and 120 mm Hg. Heart rate, rhythm, and
90 and 110 mm Hg or clinical improvement is obvious. If blood pressure must be monitored closely. Although dobu-
blood pressure cannot be monitored, an initial PO dose of tamine is less arrhythmogenic than other catecholamines,
1 mg/kg can be repeated in 2 to 4 hours if sufficient clinical higher infusion rates (e.g., 10-20 µg/kg/min) can precipitate
improvement has not been observed. The addition of 2% supraventricular and ventricular arrhythmias. Adverse
nitroglycerin ointment may provide beneficial venodilating effects are more likely in cats; these include nausea and sei-
effects. zures at relatively low doses.
An ACEI or amlodipine, with or without nitroglycerin Dopamine at low doses (<2-5 µg/kg/min) also stimulates
ointment, is an alternative to hydralazine/nitroglycerin. vasodilator dopaminergic receptors in some regional circu-
However, their onset of action is slower and effects are less lations. Low to moderate doses enhance contractility and
pronounced, but this regimen can still be helpful. Usually cardiac output, but high doses (10-15 µg/kg/min) cause
an ACEI is introduced after the patient has been stabilized peripheral vasoconstriction and increase heart rate, O 2 con-
and appetite is returning. Amlodipine generally is reserved sumption, and the risk of ventricular arrhythmias. An initial
for dogs with refractory CHF caused by MR, or for patients IV infusion of 1 µg/kg/min can be titrated upward to desired
with hypertension. clinical effect. The infusion rate should be decreased if sinus
Nitroglycerin (and other orally or transcutaneously tachycardia or other tachyarrhythmias develop.
administered nitrates) acts mainly on venous smooth muscle Other acute IV inotropic therapy could include the bipyr-
to increase venous capacitance and reduce cardiac filling idine PDE inhibitors such as amrinone and milrinone
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pressure. The major indication for nitroglycerin is acute car- increase intracellular Ca by inhibiting PDE III, an intracel-
diogenic pulmonary edema. Nitroglycerin ointment (2%) is lular enzyme that degrades cAMP. However, they are unlikely
applied to the skin, usually of the groin, axillary area, or ear to offer a substantive advantage over pimobendan. These
pinna; however, the efficacy of this in heart failure is unclear. drugs also cause vasodilation because increased cAMP pro-
An application paper or glove is used to avoid contact with motes vascular smooth muscle relaxation. Hypotension,
the skin of the person applying the drug. tachycardia, and gastrointestinal (GI) signs can occur when
giving high doses. These drugs can exacerbate ventricular
Inotropic Support arrhythmias. The effects of amrinone are shortlived (<30
The inodilator pimobendan is used for CHF caused by minutes) after IV injection in normal dogs, so CRI is required
chronic MR and DCM, as well as a number of other causes for sustained effect. Peak effects occur after 45 minutes of
of CHF. Its onset of action is fairly rapid, even with oral CRI in dogs. Amrinone is sometimes used as an initial slow
administration. The initial dose is usually given as soon as IV bolus followed by CRI; half the original bolus dose can
practicable, with subsequent doses continued as part of long- be repeated after 20 or 30 minutes. Milrinone has a much
term HF management (see p. 69 and Table 3.3). The IV form greater potency than amrinone, but there is scant informa-
of pimobendan is not yet available in the United States. tion on the IV form in small animals. These agents could be