Page 228 - Clinical Small Animal Internal Medicine

196 Section 3 Cardiovascular Disease

respiratory compromise and prescribed dose of diuret- (1–10 μg/kg/min) or dobutamine (2.5–10 μg/kg/min) are
VetBooks.ir ics. There is no “one size fits all” formula for diuretic administered via constant rate infusion (CRI) and titrated
administration and clinicians should constantly reassess
upwards to effect. Both agents are diluted in small vol-
their therapy during the first 18–24 hours. The authors
discourage the routine use of IV fluid administration at umes of 5% dextrose to limit the total volume of fluids
administered to dogs with acute CHF. During adminis-
the same time diuretics are being administered. Rather, tration, blood pressure, mentation, muscle tone, urine
they recommend offering water ad libitum during treat- output, and other indices of improved tissue perfusion
ment of acute CHF to allow patients to semi‐self‐regu- such as body temperature or blood lactate guide dose
late their hydration status. Given the potency of the loop titration. For both dopamine and dobutamine, the rate‐
diuretics, patients will be unable to consume enough limiting factor during administration is tachycardia,
water to replace the fluids being lost. In cases of dire tachyarrhythmias, or excessive vasoconstriction. Efficacy
CHF or instances where patients obsessively drink likely is best within the first 48 hours of administration,
repeated quantities of water when offered, water restric- after which the effects wane as beta‐receptors are satu-
tion might be needed. rated or downregulated.
While diuretics are generally considered a highly effec- Pimobendan is a calcium sensitizer and phosphodiet-
tive means of relieving congestion, inadequate response erase inhibitor. Its positive inotropic action is due to its
to diuretics or the inability to administer sufficient doses ability to increase the binding of calcium ions to the
due to renal disease necessitates additional strategies to actin‐ myosin‐troponin complex without a substantial
improve renal perfusion and decrease intravascular pres- increase in intracellular calcium concentration. This
sure though positive inotropes and vasodilators. mechanism provides improved cardiac contractility
without substantial concomitant increases in myocardial
oxygen or energy demands. Pimobendan (0.25 mg/kg PO
Positive Inotropes
BID or 0.15 mg/kg IV once in dogs followed by start of
Acute heart failure is accompanied by decreased cardiac oral dosing 12 hours after injection) can used in place of
output. Decreased tissue perfusion, particularly to the either dopamine or dobutamine when constant rate infu-
kidneys, activates sodium‐ and water‐retaining mecha- sion is not practical. Pimobendan is well tolerated in dogs
nisms, such as the RAAS and SNS, leading to signs of with acute CHF and oral dosing continues into the chronic
congestion. Poor perfusion of the proximal convoluted phase of CHF once the acute episode of CHF is resolved.
tubule can decrease response to furosemide as active Digoxin is a digitalis glycoside with modest positive
secretion of furosemide into the nephron’s intraluminal inotropic effect as well as ability to reduce or slow con-
space is required for its diuretic effect. Positive inotropes duction through the AV node. Its use in acute heart fail-
increase myocardial contractility and cardiac output and ure is limited by its variable oral bioavailability, long
are particularly indicated in instances of low‐output (for- half‐life, and narrow therapeutic window. Use of digoxin
ward) heart failure wherein signs of cardiac shock, such for acute heart failure has been largely replaced by
as weakness, pallor, hypotension, and hypothermia, are pimobendan except in instances of atrial fibrillation or
evident. Positive inotropes are often used in dogs with supraventricular tachycardia wherein oral digoxin
DCM, and less commonly in dogs with MMVD. Because (0.005–0.0075 mg/kg PO BID) is used to help slow the
most forms of feline cardiomyopathy are diastolic in ventricular heart rate. The long‐half life of digoxin pre-
nature, positive inotropes are rarely used in cats with cludes rapid achievement of therapeutic serum levels,
heart failure. Positive inotropes act via increased binding and digoxin therapy is usually combined with other anti-
of calcium ions to the myocardial actin‐myosin‐troponin arrhythmic agents such as diltiazem (see Chapter 21).
complex, either by increasing intracellular calcium con- Toxicity includes arrhythmias and gastrointestinal signs
centration or by increasing the sensitivity of the actin‐ such as vomiting, anorexia, and diarrhea. The authors
myosin‐troponin complex to calcium binding. discourage the use of IV digoxin administration due to a
Commonly administered positive inotropes include high incidence of toxicity.
sympathomimetic agents such as dopamine or dobu-
tamine, calcium sensitizers such as pimobendan, and Vasodilators
intracellular calcium agents such as digoxin. Dopamine
is a precursor of norepinephrine while dobutamine is a Venous and arterial vasodilators reduce preload and
synthetic catecholamine, and both agents bind to myo- afterload, respectively. In cases of acute CHF, venous
cyte beta‐adrenergic receptors in order to increase myo- vasodilation is often used in conjunction with diuretics
cardial contractility and heart rate. At high doses, both to help rapidly relieve congestion. Arterial vasodilation
agents also act on peripheral alpha‐1 receptors to cause reduces peripheral vascular resistance/afterload and
unwanted arterial vasoconstriction. In dogs, dopamine encourages improved cardiac output.