Milrinone. A clinical trial in 29 dogs with moderate to severe congestive heart failure

Management of Chronic Congestive Heart Failure Caused by Myxomatous Mitral Valve Disease in Dogs: A Narrative Review from 1970 to 2020

Simple Summary

Myxomatous mitral valve disease (MMVD) is the most common acquired cardiovascular disease in dogs. The progression of the disease and the increasing severity of valvular regurgitation cause a volume overload of the left heart, leading to left atrial and ventricular remodeling and congestive heart failure (CHF). The treatment of chronic CHF secondary to MMVD in dogs has not always been the same over time. In the last fifty years, the drugs utilized have considerably changed, as well as the therapeutic protocols. Some drugs have also changed their intended use. An analysis of the literature concerning the therapy of chronic heart failure in dogs affected by this widespread degenerative disease is not available; a synthesis of the published literature on this topic and a description of its current state of art are needed. To the authors’ knowledge, a review of this topic has never been published in veterinary medicine; therefore, the aim of this study is to overview the treatments of chronic CHF secondary to MMVD in dogs from 1970 to 2020 using the general framework of narrative reviews.

Abstract

The treatment of chronic congestive heart failure (CHF), secondary to myxomatous mitral valve disease (MMVD) in dogs, has considerably changed in the last fifty years. An analysis of the literature concerning the therapy of chronic CHF in dogs affected by MMVD is not available, and it is needed. Narrative reviews (NRs) are aimed at identifying and summarizing what has been previously published, avoiding duplications, and seeking new study areas that have not yet been addressed. The most accessible open-access databases, PubMed, Embase, and Google Scholar, were chosen, and the searching time frame was set in five decades, from 1970 to 2020. The 384 selected studies were classified into categories depending on the aim of the study, the population target, the pathogenesis of MMVD (natural/induced), and the resulting CHF. Over the years, the types of studies have increased considerably in veterinary medicine. In particular, there have been 43 (24.29%) clinical trials, 41 (23.16%) randomized controlled trials, 10 (5.65%) cross-over trials, 40 (22.60%) reviews, 5 (2.82%) comparative studies, 17 (9.60%) case-control studies, 2 (1.13%) cohort studies, 2 (1.13%) experimental studies, 2 (1.13%) questionnaires, 6 (3.40%) case-reports, 7 (3.95%) retrospective studies, and 2 (1.13%) guidelines. The experimental studies on dogs with an induced form of the disease were less numerous (49–27.68%) than the studies on dogs affected by spontaneous MMVD (128–72.32%). The therapy of chronic CHF in dogs has considerably changed in the last fifty years: in the last century, some of the currently prescribed drugs did not exist yet, while others had different indications.

Chronic heart disease and its management

Diseases of the cardiovascular system are a common cause of morbidity and mortality among geriatric canine and feline patients. Furthermore, diseases of the heart and vasculature often complicate the management of other abnormalities. This article contains a brief overview of the effects of aging on the circulatory system. It also contains a discussion of the cardiovascular disorders most frequently encountered in geriatric animals, namely, the feline myocardial disorders and feline hypertension and degenerative valvular disease, dilated cardiomyopathy, and cardiac in dogs.

Electrophysiologic and proarrhythmic effects of intravenous inotropic agents

Intravenous inotropic agents promote increased myocardial contractility via elevation of myocyte calcium concentrations, a mechanism that is also known to promote the development of cardiac arrhythmias. The purpose of this article is to review the electrophysiologic effects and relative potential for proarrhythmia associated with dobutamine, dopamine, and the phosphodiesterase inhibitors amrinone and milrinone. Dobutamine increases sinoatrial node automaticity and decreases atrial and atrioventricular (AV) node refractoriness and AV nodal conduction time. The drug also decreases ventricular refractoriness in both healthy and ischemic myocardium. Dobutamine has been shown to increase heart rate in a dose-related fashion in animals and in humans. In humans, dobutamine has been reported to induce ventricular ectopic activity (VEA) in 3% to 15% of patients, although VEAs are often asymptomatic, requiring no intervention. Ventricular tachycardia (VT) associated with dobutamine appears to occur rarely. Patients with underlying arrhythmias or heart failure or those receiving excessive doses of dobutamine are at greatest risk for proarrhythmia. Dopamine increases automaticity in Purkinje fibers and has a biphasic effect on action potential duration. Dopamine has been reported to induce atrial or ventricular arrhythmias in animals. In humans, dopamine may be associated with dose-related sinus tachycardia but has also been reported to cause VEA, which is usually asymptomatic. Dopamine-associated VT appears to occur rarely. Dopamine produces greater elevations in heart rate or frequency of ventricular premature beats at a given value of cardiac index than does dobutamine. The phosphodiesterase inhibitors amrinone and milrinone increase conduction through the AV node and decrease atrial refractoriness. Intravenous administration of these drugs may result in sinus tachycardia in some patients and has been reported to cause VEA, which is often asymptomatic, in up to 17% of patients. VT has also been reported in association with short-term use of intravenous phosphodiesterase inhibitors. In summary, intravenous inotropic agents may be associated with proarrhythmic effects in some patients. The primary arrhythmias reported are sinus tachycardia and VEA, although other supraventricular or ventricular arrhythmias have been reported less commonly. However, clinically significant proarrhythmic effects associated with these agents appear to occur rarely, and, at conventional doses, intravenous inotropic agents are relatively safe with respect to proarrhythmic effects.

Efficacy of inotropic support of the failing heart

The clinical literature on the subject of inotropic therapy of heart failure, particularly use of digitalis glycosides, is full of contradictions. Most of this disparity can be accounted for if not reconciled by taking the methodology of the clinical trials into consideration. Because drug interventions may produce subtle effects requiring a subjective determination, the questions being asked in these studies cannot be answered without removing as many sources of bias as possible from the patient management and data analysis. If a study has not been adequately randomized, double-blinded, and placebo-controlled, the clinical findings will be inconclusive at best. Systolic myocardial dysfunction plays a pivotal role in the pathogenesis of CHF in many patients and is a prerequisite for the use of cardiotonic drugs. Although the clinical signs of heart failure may be relieved initially by diuretics and vasodilators, compensation may require the addition of a positive inotrope, particularly in advanced cases. In veterinary medicine, the choice of positive inotrope is limited to digoxin, digitoxin, dobutamine, or amrinone. Digoxin possesses superior pharmacokinetics and is the cardiac glycoside of choice for use in the dog. Dobutamine and amrinone are more potent inotropes, but since they must be administered by continuous intravenous infusion, their use is limited to critical care therapy. At the present time, only digoxin can be administered orally for sustained long-term maintenance therapy. Milrinone, a more potent derivative of amrinone, also offers this option, but it has not been available since its brief trial debut as an investigational drug. None of the nonglycoside alternatives couples the benefits of positive inotropic and negative chronotropic effects. Consequently, digoxin remains the mainstay for chronic inotropic support of the heart. Atrial fibrillation with a rapid ventricular response rate is the prime indication for digoxin. In the last few years, evidence from methodologically sound clinical trials on humans has also restored faith in the efficacy of digoxin for treating heart failure in patients with normal sinus rhythm. From these studies, the profile of a digitalis responsive heart failure patient has emerged. Digoxin is most likely to be efficacious when heart failure is associated with chronic, severe ventricular systolic dysfunction, which has resulted in ventricular dilatation. The most reliable clinical marker is the presence of a third heart sound (gallop rhythm). Although the patients in the worst heart failure generally have the shortest survival time, they may also have the most dramatic short-term clinical benefit. However, once cardiac reserve is exhausted in the terminal stages of failure, cardiotonic stimulation ceases to be effective.(ABSTRACT TRUNCATED AT 400 WORDS)

The efficacy and safety of milrinone for treating heart failure in dogs

Milrinone has been studied in a variety of situations. In experimental dogs it has been documented to increase contractility to a similar degree as beta-receptor agonists and to produce mild arteriolar dilation in dogs. In canine patients with heart failure, milrinone produces demonstrable improvement in echocardiographic ventricular function and hemodynamic variables. In addition, it improves clinical signs in these patients, improving their quality of life. Milrinone is superior to digoxin as evidenced by the improvement in clinical signs noted in dogs that were unresponsive or no longer responding to digoxin administration. There is no doubt that milrinone improves short-term prognosis and in so doing prolongs life. Many of the patients that the author has observed would not have gone home without the benefits of milrinone. Milrinone's effects on long-term survival cannot be assessed, but its effects on survival time are certainly not dramatic enough to be evident without a comparison population. Therefore, milrinone administration should be considered palliative, as is administration of all other cardiovascular medications for heart failure. In addition to its beneficial effects, milrinone also appears to be relatively safe when compared with the alternative of digoxin administration. Fatal events attributable to milrinone administration are rare, and those directly attributable to enhanced ventricular arrhythmia can generally be avoided by monitoring an electrocardiogram after initial milrinone administration commences. Milrinone does not increase the incidence of sudden death in Doberman Pinschers. It is possible that a small number of dogs with mitral regurgitation may develop mitral chordal rupture. For this reason and possibly others, milrinone probably will not be indicated in early heart failure due to mitral regurgitation when heart failure is readily responsive to diuretic administration. The risk-to-benefit ratio turns markedly in the favor of milrinone administration in the dog with mitral regurgitation that is partially or completely refractory to other cardiovascular drugs. Milrinone appears to be a more effective and safer positive inotrope for long-term treatment of dogs with congestive heart failure than drugs currently available. The author and all the investigators involved in the milrinone clinical trials hope that it will soon be available for use by the veterinary community.