Page 286 - Feline Cardiology

Chapter 19: Congestive Heart Failure 293

troponin C. Inhibition of phosphodiesterase III reduces the first couple of days. Therefore, when feasible, it is the
degradation of cAMP, an important second messenger authors’ recommendation to postpone ACE inhibitor
involved in contraction and relaxation. Pimobendan therapy until the patient is at home and is eating and
increases myocardial contractility to a degree that is drinking. If the cat is currently receiving an ACE inhibi-
comparable to dobutamine, but without increasing tor, it can be continued, provided that the cat is able to
myocardial oxygen consumption (Fitton and Brogden tolerate oral medications, is eating and drinking, and
1994). It also improves diastolic function and reduces does not have moderate azotemia, because stopping it
diastolic filling pressure, which is an important thera- could cause a rebound RAAS activation due to high
peutic target in cats with heart failure. Pimobendan levels of angiotensin I from the ACE inhibition.
moderately reduces systemic vascular resistance, reduces Sometimes in cats with moderate dehydration and azo-
pulmonary vascular resistance (∼30%), dilates coronary temia due to acute aggressive diuretic therapy, the ACE
arteries, and increases renal blood flow (van Meel and inhibitor may be temporarily postponed during the first
Diederen 1989; Kanno et al. 2007). Unlike dobutamine, few days of the initial acute heart failure treatment.
pimobendan improves myocardial efficiency by reduc- Sedation may be necessary for severely anxious,
ing afterload, increasing myocardial contractility, and distressed hospitalized patients, or in fractious animals
increasing myocardial perfusion, without increasing that require emergency care. An attractive sedation pro-
myocardial oxygen consumption. Pimobendan also tocol is the combination of an opioid (butorphanol
reduces several inflammatory negative inotropic cyto- 0.02 mg/kg or hydromorphone 0.05 mg/kg) and mid-
kines including tumor necrosis factor-α, interleukin-1, azolam (0.2 mg/kg), which can be given as a single sub-
and inducible nitric oxide synthase. cutaneous injection. If midazolam is not available,
Pimobendan is used in both acute and chronic heart low-dose acepromazine (0.01–0.05 mg/kg) may be sub- Congestive Heart Failure
failure in dogs. There is early positive anecdotal experi- stituted. Sublingual buprenorphine (0.01 mg/kg orally)
ence in cats with heart failure, but further studies are is another sedation option for cats resisting injections.
needed to determine for which diseases it should be used. This is an attractive option for owners to administer at
At this time, it is recommended to be used in cats with home to the cat prior to the car trip to the doctor’s office
heart failure secondary to myocardial failure (i.e., DCM) in cats that become severely distressed in the car or cats
and possibly in cardiac diseases causing severe volume that are intractable once in the hospital. The last resort
overload such as degenerative valve disease (uncommon is box anesthesia with inhaled isoflurane, because it will
in cats) or in heart diseases that have a component of decrease blood pressure and contractility and depress
systolic dysfunction. Use in cats with unclassified or spontaneous respirations (see Chapter 27).
restrictive cardiomyopathy is not routine or well
described; these cats may benefit from the vasodilating Treatment of Refractory Congestive Heart
properties of the drug in the presence of elevated diastolic Failure
filling pressures seen in heart failure (Sturgess and Ferasin Treatment of refractory CHF includes the following:
2007). At this time, it is not recommended to use a posi-
tive inotropic agent in cats with hypertrophic cardiomy- • Furosemide dose should be maximized (4 mg/kg PO
opathy, but further studies are needed. In an uncontrolled q 8 h) in cats with refractory heart failure that con-
retrospective study of 161 cats treated with pimobendan, tinue to develop pulmonary edema or pleural effusion
cats with larger end-diastolic diameter treated with despite medical therapy.
pimobendan and standard therapy lived longer (P = 0.03) • Addition of a second diuretic such as hydrochloro-
and cats with greater concentric hypertrophy of the left thiazide (1–2 mg/kg PO q 12–24 h) or torsemide (0.1–
ventricular free wall lived a shorter time when treated 0.3 mg/kg PO q 12 h) may be needed in cats with
with pimobendan and standard therapy (P = 0.02); refractory heart failure despite maximal furosemide
whether these results reflect natural disease progression doses. These cats are very fragile and are prone to
or drug effect is unknown (MacGregor et al. 2010). develop significant dehydration and azotemia, which
ACE inhibitors are used for chronic heart failure may negatively impact overall quality of life.
therapy, but acute hemodynamic effects appear to be Substitution or supplementation of one or more oral
mild and intermittent (IMPROVE Study Group 1995). doses per week of furosemide with subcutaneous
ACE inhibitors may cause functional azotemia in some injections is another option in cats receiving maximal
cats secondary to decreased efferent arteriolar glomeru- oral furosemide doses.
lar tone, which may be potentiated by decreased renal • A low-salt cardiovascular diet (0.15–0.2 g sodium/
blood flow during aggressive diuretic therapy. 1000 kcal) should be fed to cats with refractory heart
Symptomatic benefits of ACE inhibitors tend to occur failure, if palatable. A professionally formulated
after a couple of weeks to months of therapy, not within home-cooked low-salt diet is an option in finicky cats.