section name header

Basics ⬇

Outline

BASICS

Definition!!navigator!!

Occurs when the mitral valve allows blood to leak into the left atrium during systole and creates a systolic murmur with its point of maximal intensity over the heart base radiating caudodorsally.

Pathophysiology!!navigator!!

  • Mitral regurgitation can occur with structurally normal valve cusps or when dysplastic, inflammatory, or degenerative disease, prolapse or rupture of a chorda tendinea is present
  • Ventricular dilation, e.g. due to cardiomyopathy or severe aortic regurgitation, leads to mitral regurgitation
  • During systole, blood regurgitates into the left atrium, causing increased left atrial pressure and a left atrial and ventricular volume overload
  • As the regurgitation becomes more severe, increases in left atrial pressure produce pulmonary hypertension, pulmonary edema, and clinical signs of CHF
  • Pulmonary artery dilation and rupture can be a sequela to pulmonary hypertension

Systems Affected!!navigator!!

Cardiovascular

Genetics!!navigator!!

N/A

Incidence/Prevalence!!navigator!!

The prevalence of mitral regurgitation murmurs in a middle-aged and older population of apparently healthy horses in the UK was 2.9% and in Thoroughbreds was 9% in flat and 19% in jump racing.

Signalment!!navigator!!

Athletic breeds and older horses.

Signs!!navigator!!

General Comments

Often an incidental finding. mitral regurgitation may not necessarily affect performance. The severity of signs is dependent on the nature and severity of valvular pathology.

Historical Findings

  • Often poor performance
  • Sometimes CHF

Physical Examination Findings

  • Grade 2–6/6, band-shaped, crescendo or musical holosystolic or pansystolic murmur with point of maximal impulse in the mitral to aortic valve area (left fifth to fourth intercostal space) and radiating dorsally to the left heart base
  • Other less common findings—supraventricular premature depolarizations, atrial fibrillation, accentuated third heart sounds, tachypnea, cough, and CHF

Causes!!navigator!!

  • Physiologic mitral regurgitation, often related to cardiac adaptation in response to athletic training
  • Prolapse
  • Degenerative changes of the mitral leaflets
  • Nonvegetative valvulitis
  • Ruptured chordae tendineae
  • Infective endocarditis
  • Congenital malformation

Risk Factors!!navigator!!

  • Athletes have a high prevalence of physiologic mitral regurgitation
  • Increasing age
  • Large ponies and horses had a higher prevalence of left-sided valvular regurgitation (i.e. mitral and/or aortic regurgitation) than small ponies

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

Physiologic ejection murmur—this murmur is typically grade 1–3/6 and localized to the aortic valve.

CBC/Biochemistry/Urinalysis!!navigator!!

May have leukocytosis and hyperfibrinogenemia and elevated SAA with infective endocarditis.

Other Laboratory Tests!!navigator!!

  • Increased concentrations of cardiac troponin I may be present with concurrent myocardial disease
  • Positive blood culture may be obtained from horses with infective endocarditis

Imaging!!navigator!!

Echocardiography

  • Diffuse or nodular thickened mitral valve leaflets
  • Prolapse of a mitral leaflet (usually an accessory leaflet) into the left atrium
  • Ruptured chorda tendinea, flail mitral leaflet, or bacterial endocarditis are detected infrequently
  • Left atrium—enlarged and dilated, with a rounded appearance
  • Left ventricle—enlarged and dilated, with a rounded apex
  • Thinning of the left ventricular free wall and interventricular septum
  • Subjective ventricular hyperkinesis, dilation, and increased fractional shortening suggest left ventricular volume overload
  • Dilatation of the pulmonary veins and, later, the pulmonary artery in severely affected horses
  • Doppler echocardiography reveals jet(s) of regurgitation in the left atrium. The size and extent of the jet semiquantitate severity

Thoracic Radiography

  • Left-sided cardiac enlargement and dorsal displacement of the trachea
  • Pulmonary edema with left-sided CHF

Other Diagnostic Procedures!!navigator!!

ECG

Ventricular and/or supraventricular premature depolarizations or atrial fibrillation may be present.

Pathologic Findings!!navigator!!

  • Where the regurgitation is physiologic, no pathologic findings are expected
  • Focal or diffuse thickening or distortion of 1 or more mitral leaflets may be present
  • Ruptured chordae tendineae, flail mitral leaflets, infective endocarditis, or congenital malformations of the mitral valve infrequently are detected
  • Jet lesions are detected in the left atrium
  • Left atrial and ventricular enlargement in cases with significant regurgitation
  • Dilatation of the pulmonary artery and veins with pulmonary hypertension
  • In horses with CHF, peripheral edema, pleural effusion, pericardial effusion, chronic hepatic congestion, and, occasionally, ascites may be detected
  • With acute severe left heart failure, frothy pulmonary edema may be found within the airways

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

  • Management by intermittent monitoring in horses with mitral regurgitation that is mild or moderate in severity
  • Palliative care in horses with severe mitral regurgitation

Appropriate Health Care!!navigator!!

  • Most affected horses require no treatment and can be monitored on an outpatient basis
  • Horses with moderate to severe regurgitation may benefit from long-term vasodilator therapy, particularly with ACE inhibitors
  • Treat horses with severe regurgitation and CHF with positive inotropic drugs, vasodilators, and diuretics on an inpatient basis, if possible, and monitor response to therapy

Nursing Care!!navigator!!

N/A

Activity!!navigator!!

  • Most horses with mitral regurgitation are safe to continue in full athletic work until the regurgitation becomes severe or ventricular arrhythmias develop
  • Monitor horses with moderate to severe regurgitation by ECG during high-intensity exercise to ensure they are safe to continue in ridden work. These horses can be used for lower level athletic activities until they begin to develop CHF
  • Horses with significant ventricular arrhythmias or pulmonary artery dilatation are no longer safe to ride

Client Education!!navigator!!

  • Regularly monitor the cardiac rhythm; any irregularities should prompt ECG
  • Carefully monitor for exercise intolerance, respiratory distress, prolonged recovery after exercise, increased resting respiratory or heart rate, or cough; if detected, seek a cardiac reexamination

Surgical Considerations!!navigator!!

N/A

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Treat affected horses in CHF with furosemide, torsemide vasodilators such as benazepril and quinapril, or the inodilator pimobendan
  • Antimicrobials are indicated with infective endocarditis

Contraindications!!navigator!!

  • ACE inhibitors are contraindicated in pregnancy
  • Diuretics, ACE inhibitors, and other vasodilators must be withdrawn before competition to comply with the medication rules of the various governing bodies of equine sports

Precautions!!navigator!!

ACE inhibitors can cause hypotension; thus, do not give a large dose without time to accommodate to this treatment.

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Frequently monitor the intensity of the cardiac murmur, cardiac rhythm, and respiratory system
  • Horses with moderate regurgitation should be reexamined echocardiographically every year. Mild cases should be reexamined every other year and horses with severe regurgitation should be reexamined echocardiographically more frequently, particularly if the horse continues to be ridden
  • Exercising ECG is indicated in horses with moderate to severe mitral regurgitation and should be repeated regularly and during exercise comparable with the horse's usual workload

Prevention/Avoidance!!navigator!!

N/A

Possible Complications!!navigator!!

Chronic and/or severe regurgitation—atrial fibrillation; CHF, pulmonary artery rupture

Expected Course and Prognosis!!navigator!!

  • Many affected horses have a normal performance life and life expectancy
  • Prognosis for horses with mitral valve prolapse and mild regurgitation is excellent; in many, the amount of regurgitation remains unchanged for years
  • Progression of regurgitation associated with degenerative valve disease usually is slow; if the regurgitation is mild, these horses also have a good prognosis
  • Horses with ruptured chordae tendineae, flail mitral valve leaflets, infective endocarditis, or cardiomyopathy have a more guarded prognosis because regurgitation usually becomes more severe and results in shortened performance life and life expectancy
  • Affected horses with CHF usually have severe underlying valvular heart and/or myocardial disease and a guarded to grave prognosis for life
  • Most affected horses being treated for CHF respond to supportive therapy and improve. This improvement usually is short lived, however, and most are euthanized within 2–6 months of initiating treatment

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

Old horses are more likely to be affected.

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

  • Affected mares should not experience any problems with pregnancy unless the regurgitation is severe
  • The volume expansion of late pregnancy places an additional load on the already volume-loaded heart and may precipitate the onset of CHF in mares with severe regurgitation
  • Pregnant mares affected with CHF should be treated for the underlying cardiac disease with positive inotropic drugs and diuretics; ACE inhibitors are contraindicated because of potential adverse effects on the fetus

Synonyms!!navigator!!

Mitral insufficiency.

Abbreviations!!navigator!!

Suggested Reading ⬆ ⬇

Afonso T, Giguere S, Rapoport G, et al. Pharmacodynamic evaluation of 4 angiotensin-converting enzyme inhibitors in healthy adult horses. J Vet Intern Med 2013;27:1185–1192.

Afonso T, Giguere S, Rapoport G, et al. Cardiovascular effects of pimobendan in healthy mature horses. Equine Vet J 2016;48:352–356.

Reef VB, Bonagura J, Buhl R, et al. Recommendations for management of equine athletes with cardiovascular abnormalities. J Vet Intern Med 2014;28:749–761.

Stevens KB, Marr CM, Horn JN, et al. Effect of left-sided valvular regurgitation on mortality and causes of death among a population of middle-aged and older horses. Vet Rec 2009;164:6–10.

Young LE, Rogers K, Wood JL. Heart murmurs and valvular regurgitation in Thoroughbred racehorses: epidemiology and associations with athletic performance. J Vet Intern Med 2008;22:418–426.

Author(s) ⬆

Author: Celia M. Marr

Consulting Editors: Celia M. Marr and Virginia B. Reef

Acknowledgment: The author acknowledges the prior contribution of Virginia B. Reef.