Heart: dilated cardiomyopathy (DCM) (DCM)

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• Common acquired disease, especially large and giant breeds in middle age, some spaniels.
• Cause : systolic (contractility) failure of left ventricle.
• Signs : exercise intolerance, weight loss, dyspnea.
• Diagnosis : radiography, echocardiography.
• Treatment : manage cardiac failure and control arrhythmias.
• Prognosis : guarded, survival usually months rather than years but varies with breed.

• Exercise intolerance.
• Breathlessness.

• Recent weight loss .
• Cough.
• Abdominal distension.
• Syncope.

• Sudden death (particularly in Boxers  and Dobermanns  ).

• Usually >5 years old however cases younger than this recorded.

• Reports vary acccording to breed:
• Male (Dobermann  , Irish Wolfhound  ) - tend to go downhill more rapidly and die younger than females.

• Female (Irish Wolfhound  ).

• Dobermann .
• Irish Wolfhound .

• Great Dane .
• Boxer .
• Cocker Spaniel .
• Old English Sheepdog .

• German Shepherd dog .
• Labrador Retriever .
• English Springer Spaniel .
• Weimaraner .
• Golden Retriever .
• Newfoundland .

• Medical therapy including ACE inhibitors  and pimobendan  may be expensive in large breeds.

• Same as congestive heart failure .
• Sudden death due to dysrhythmia .

Pathogenesis

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• Idiopathic, see also secondary dilated cardiomyopathy.
• Nutritional (taurine  /carnitine deficiency  ).

• Pure bred dogs are over-represented implying a familial or genetic component.
• May be autosomal dominant inheritance in some Newfoundland and Irish Wolfhound families.

• L-carnitine deficiency   within the cardiomyocytes (some Boxers); and possibly other breeds may respond to carnitine supplementation.
• Carnitine transports long chain fatty acids across mitochondral membranes  energy for myocardium.
• Deficiency may exist in up to 40% of dogs with cardiomyopathy but most of these have normal plasma carnitine levels.
• Can only be assessed by endocardial biopsy so treatment may be initiated on a trial basis however the supplement may be expensive.
• Taurine   is the most abundant free amino acid in the heart.
• Normal plasma level 63 nmol/l (range 44-224 nmol/l).
• Taurine deficiency most likely to occur in dogs on vegetarian diet, but also seen in some dogs on normal proprietary diets.
• Low plasma taurine levels recorded in Golden Retrievers  , American Cocker Spaniels   and Newfoundlands   with cardiomyopathy (<60 nmol/l).
• American Cocker Spaniels  may be taurine  and carnitine deficient. Respond to supplementation.

• Anthracycline cytotoxic drugs  , eg doxorubicin  , monensin, salinomycin.

• Hyperpyrexia or hypothermias .

• Toxoplasma gondii   , parvovirus  , borrelia burgdorferi .
• Lymphoma  infiltration of myocardium.

• Breed.
• Family.

• Exertion.
• Stress.
• Sudden increase in salt intake.

• Familial basis (breed).
• Age.

• Metabolic or biochemical lesion affecting cardiac myocyte  failure of contractility/increased work for myocardium  :
  • Increased end-diastolic left ventricular volume and pressure  dilation and non-compensatory eccentric hypertrophy of left ventricle  compromised papillary muscle function and passive dilation of atrioventricular annulus causing mitral regurgitation  increased left atrial pressures  left atrial enlargement  increased pulmonary venous (capillary wedge) pressure and pulmonary edema.
  • Poor cardiac output (CO) due to decreased stroke volume (SV)  increased sympathetic drive  increased heart rate (HR), especially if atrial fibrillation present.
  • Reduced cardiac output with arteriolar constriction to increase vascular resistance (mediated through sympathetic tone, active renin-angiotension-aldosterone system (RAAS) and increased vasopressin release); also venoconstriction  increased venous return  increased stretch of myocardium (preload).
  • Arteriolar constriction increases afterload (left ventricle wall stress) and myocardial workload  further myocardial dysfunction, further dilation.
• Signs of poor systolic function and vasoconstricted state:
  • Poor pulse.
  • Pale mucous membranes.
  • Delayed capillary refill time.
  • Cold extremities, etc.
• Left atrial stretch may  supraventricular dysrhythmias, eg atrial fibrillation .
• Increased myocardial wall stress, poor coronary perfusion of myocardium and diseased myocytes may result in ventricular dysrhythmias .
• Right-sided failure may also result due to myocardial involvement or secondary to the left congestive failure  ascites, hepatomegaly, hepatojugular reflex, pleural effusion  , pericardial effusion .
• Salt and water retention by kidney due to poor renal perfusion  increased sympathetic drive and activation of the RAAS (increased aldosterone) leads to increased venous return (preload) and venous pressure, further stretch of myocardium and predisposition to edema.

• Weeks-months from onset of signs.
• Dobermann : rapid, usually <6 weeks survival after diagnosis without ACE inhibition; sudden death common 50% dead in 3 months, 10% live >1 year. Survival may be improved by addition of pimobendan  to medication regime.
• Boxer : variable, may be sudden death from dysrhythmia, may present with congestive heart failure .
• Irish Wolfhound : variable dysrhythmias and progressive cardiac dilatation over several years or sudden death.

• Survival depends on breed.
• Cocker Spaniels may survive >12 months.
• Giant breeds may survive for months to years after diagnosis.

Diagnosis

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• Cough.
• Ascites.
• Weight loss .
• Exercise intolerance.
• Dyspnea.
• Syncope.
• Sudden death.

• Breathlessness/cough.
• Exercise intolerance.
• Recent weight loss/inappetence/anorexia.
• Syncope.
• Restless (especially at night); 'heavy breathing'.
• 'Fading' over several months.

• Dobermann : severe dyspnea, cough (often productive, pink/white froth).

• Boxer : syncope, sudden death.

• Tachycardia: sinus tachycardia or a dysrhythmia such as:
  • Atrial fibrillation.
  • Supraventricular tachycardia.
  • Ventricular premature complexes.
  • Ventricular tachycardia (paroxysmal or sustained).
• Boxer : severe ventricular dysrhythmias.
  Treat immediately.
• Giant breeds : atrial fibrillation .

• Pulse deficit - generally weak, may be variable with arrhythmia.
• Signs of left backward failure: tachypnea, hyperpnea, coughing, adventitious respiratory sounds.
• Signs of right backward failure: ascites, hepatomegaly, distended jugular veins.
• Loss of body condition.
• Systolic (forward) failure:
  • Weak pulse.
  • Pallor of mucous membranes.
  • Delayed capillary refill time.
  • Cold extremities.
  • Weak precordial impulse.
  • Soft heart sounds.
• Low grade systolic murmur of mitral +/or tricuspid regurgitation (<3-6)   .
• Pronounced diastolic gallop rhythm (loud S3, S4) = uncontrolled CHF.
• Dobermann :
  • Collapsed.
  • Cold extremities.
  • Subnormal rectal temperature.
  • Tachycardia.
  • Dysrhythmias.
  • Dyspnea.
  • Acute fulminant pulmonary edema.
    Act immediately to save life.
• Boxer : classic cardiomyopathy with congestive heart failure; collapse with ventricular tachycardia.

• 2-D echocardiography  :
  • Left atrial and left ventricle enlargement with with normal or occasionally thin ventricles.
  • Rounded apex.
  • Hypokinetic left ventricle   .
  • Massive left atrial dilatation often seen in Irish Wolfhound.
• M-mode echocardiography :
  • Increased E point to septal separation if left ventricular dilation .
  • Left ventricular internal dimensions  increased compared with normal for weight of dog - breed measurements used preferably.
  • Decreased fractional shortening (<20%), due to increased end-systolic dimension.
  • Left ventricular freewall and interventricular septum normal or slightly thicker than normal for weight and breed despite appearing relatively thin walled .
  • Increased LA:Ao ratio with significant left auricular appendage dilation and increased maximum left atrial diameter.
  • Decreased left ventricular ejection time and increased pre-ejection period.
  • Boxer with ventricular dysrhythmias may have little abnormality.
• Dobermanns have different normal M-mode values. Overlap between normal and occult DCM - EPSS most specific measurement for DCM. LVIDs >38 mm - significant; LVIDd >46 mm - significant.

Döppler echocardiography

• Low aortic and pulmonic velocities caused by poor systolic function.
• Mitral regurgitation and/or tricuspid regurgitation.

• See thoracic radiography .
• Gross generalized cardiomegaly   .
• Left atrial and left ventricular enlargement.
• Pulmonary venous congestion and pulmonary edema .
• Pleural effusion.
• Abdominal radiographs  may show ascites and hepatomegaly .
• Left atrial enlargement with little other chamber pathology on x-ray (Dobermann and Boxer).
• May be normal heart size in Boxers and minimal enlargement in Dobermanns.

• See ECG overview .
  Dysrhythmia may be present but not diagnostic - many different forms
  Life-threatening ventricular dysrhythmias (hemodynamically significant arrhythmia; sustained or paroxysmal ventricular tachycardia; short coupling interval between ectopics and preceding R wave; multifocal VPCs; R on T phenomenon) require immediate antiarrhythmic treatment
• May show criteria for chamber enlargement, especially left atrial/left ventricular.
• Holter monitor useful for detection of intermittent dysrhythmias.

• Pre-renal azotemia  with increased urea  and creatinine .
• May show hepatic venous congestion with right-sided failure with increased ALT  and SAP .
• Plasma taurine - to identify etiology.
  Evaluation of myocardial carnitine level requires myocardial biopsy.

Hematology

• Stress hemogram (increased WBC  , neutrophilia  , monocytosis, eosinopenia  , lymphopenia  ).

Cardiac catheterization :

• Raised end diastolic left ventricular pressure in severe cases.
• Increased capillary wedge pressure in severe cases.
• +/- mitral regurgitation.
• Poor contractility between systolic/diastolic frames.

Histopathology

• Endomyocardial biopsy: research tool - identifies inflammatory, infiltrative cardiomyopathy.
• Some pathologists claim that wavy fibers identified on HSE stains of myocardium are diagnostic of DCM.

• Increased end-diastolic and systolic dimensions compared with normal for breed and weight.
• Reduced fractional shortening and ejection fraction.

• Left atrial and left ventricular dilation with relatively thin flaccid walls.
• Myocardium grossly normal and valves may show signs of concurrent myxomatous degeneration as DCM is predominantly a disease of older animals.

• Variable - some cases have features of myocarditis.
• Scattered areas of fibrosis, myofibrillar hypertrophy and myofibrillar necrosis.
• Wavy fibers found in 64/70 dogs suspected of having DCM (one report).
  Some pathologists believe that wavy fibers are a processing artifact.

• Causes of cough.
• Causes of breathlessness.
• Causes of exercise intolerance/syncope:
  • Infectious tracheobronchitis ('kennel cough'  ), especially in Dobermann.
  • Chronic bronchitis .
  • Degenerative mitral valve disease .
  • Degenerative tricuspid valve disease.
  • Pneumonia .
  • Bronchopneumonia .
  • Pulmonary infiltrate with eosinophils  ('allergic lung disease').

Treatment

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• Control signs of congestion: depends on class of heart failure .

• Venodilation improves venous capacitance  diverts blood away from the cardiopulmonary circuit  helps control pulmonary edema  improves venous return  decreases right-sided congestive signs (hepatomegaly, ascites).
• Early stages: aim to prevent further myocardial cell loss by reducing myocardial work rather than stimulating pump function.
• Later stages: use positive inotropic agents to stimulate severely depressed myocardium (Class IV CHF).
  Response may be limited by contractile reserve.

Standard therapy for Class III and IV CHF due to DCM:

• Diuretics.
• ACE inhibitors.
• Pimobendan.
• Digoxin (additional, if atrial fibrillation present).

Diuretics

• Furosemide  (2-4 mg/kg BID-TID PO). In Class IV CHF can give <8 mg/kg IV for rapid diuresis and ventricular action.

Ace inhibitor
See ACE inhibitors overview .

Pimobendan  

• An inodilator.
• Increases the sensitivity of the myofibrils to calcium   improvement in systolic (contractile function).
• Phosphodiesterase III inhibitor thereby causing vasodilation   afterload reduction.
• May increase survival time particularly in Dobermanns (270 days in one study).
• Improves cerebral blood flow; dogs appear more alert.
• Dose: 0.2-0.6 mg/kg/day divided into 2 equal doses given 12h apart.
• Give pimobendan 1h before food.

Digoxin

• Digoxin  dose based on body surface area (0.22 mg/m2  ).
• Reduce dose if pre-renal azotemia or overt renal dysfunction, ascites, cachexia or low plasma protein levels.
• Digoxin slows AV nodal conduction, hence slows ventricular response to atrial fibrillation; may act as a mild positive inotrope; may improve baroreceptor sensitivity and function; neurohormonal modulator for CHF.

Optional additional treatments:

• Indicated in dogs with myocardial L-carnitine deficiency (up to 40% of dogs with dilated cardiomyopathy).
• American Cocker Spaniels and some Boxers with dilated cardiomyopathy have been shown to respond to L-carnitine supplementation.
• 50 mg L-carnitine/kg TID PO.
  Supplementation can be very expensive in large dogs

• Recommended in Cocker Spaniels and dogs on a vegetarian diet.
• 50 mg/kg taurine q8h PO. One recent report found that 250 mg taurine PO q12h was sufficient to "normalize" whole blood taurine levels in Newfoundland dogs.

Digoxin excretion is dependent on renal function.

• Diuretics can exacerbate pre-renal azotemia.
• ACE inhibitors reduce angiotensin II levels which has the potential to decrease renal function particularly if renal function is already impaired. (In practice, problem uncommon ).

Digoxin assay

• 5-7 days after start of digitalization (6-8 hours post-pill) to check adequate plasma levels and no evidence of digoxin toxicity .

• If atrial fibrillation present and if heart rate not controlled (>160 beats per minute) need careful introduction of diltiazem  or beta-blocker, eg propranolol  to slow heart rate.
• Maintain on lowest dose of frusemide  which controls congestive signs (usually ~ 2 mg/kg BID).
• Doses >4 mg/kg TID PO not more effective (ceiling dose).
  Use either ACE inhibitors  BID instead of SID, or add additional diuretic, or reduce salt intake.
• Maintain on digoxin  dose which gives therapeutic plasma levels.

• Renal function   including electrolytes.
• Digoxin assay .

Prevention

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• Avoid breeding with affected animals or families of affected animals.

Sequelae

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• Boxer: may survive months.
• Giant breeds can usually maintain quality of life for 3-6 months.
• Dobermann: generally <6 weeks, rarely >6 months survival after onset of signs.
  Pimobendan therapy may prolong survival.
• Some Cocker Spaniel: >2-8 years.
• Giant breeds: may survive several years from onset of dysrhythmias but poor prognosis once congestive signs develop.

• Risk of sudden death even with anti-dysrhythmic therapy.

• Control of congestive signs:
  • Decreased cough/breathlessness.
  • Ascites.
  • Restlessness.
  • Increased appetite/weight gain.
  • Exercise tolerance.
• Decreased ventricular rate, ie atrial fibrillation to <160 bpm.
• Control of ventricular dysrhythmia.
• Normal or slightly elevated blood urea  and creatinine  levels.

• Standard .
• Relentless progression of disease process, eg congestive signs, dysrhythmia.
• Complications due to concurrent disease, especially renal disease, hypothyroidism .
• Digoxin toxicity or other intolerance of medication, eg anorexia, diarrhea.

Sources

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• Recent references from PubMed.
• Calvert C A & Wall M (2001) Effects of severity of myocardial failure on heart rate variability in Doberman Pinschers with and without echocardiographic evidence of dilated cardiomyopathy. JAVMA 219 , 1084-1088.
• Meurs K M, Miller M W & Wright N A (2001) Clinical features of dilated cardiomyopathy in Great Danes with results of a pedigree analysis - 17 cases (1990-2000). JAVMA 218 , 729-732.
• Calvert C A, Pickus C W, Jacobs G J & Brown J (1997) Signalment survival and prognostic factors in Doberman Pinschers with endstage cardiomyopathy. JVIM 11 , 323-326.
• Kittleson M D, Keene B et al (1997) Results of the multicenter spaniel trial - Taurine and L-carnitine responsive dilated cardiomyopathy in American cocker spaniels with decreased plasma taurine concentration. JVIM 11 , 204-211.
• Tidholm A, Svensson H & Sylven C (1997) Survival and prognostic factors in 189 dogs with dilated cardiomyopathy. JAAHA 33 , 364-368.
• Tidholm A, Svensson H & Sylven C (1997) A retrospective study of 189 dogs with dilated cardiomyopathy. JAAHA 33 , 544-550.
• Kramer G K, Kittleson M D, Fox P R et al (1995) Plasma taurine concentrations in normal dogs and dogs with heart disease. JVIM 9 , 253-258.
• Keene B W, Panciera D P et al (1991) Myocardial L-carnitine deficiency in a family of dogs with dilated cardiomyopathy. JAVMA 198 , 647-650.

• Willis R, Dukes-McEwan J et al (2003) The role of taurine in dilated cardiomyopathy in Newfoundland dogs. VCS Proceedings, Spring meeting, Birmingham.
• Lombard C W et al (2000) Clinical experience with pimobendan. VCS Proceedings, Spring meeting, Birmingham.
• Luis Fuentes V, Kleeman R et al (1998) The effect of a novel inodilator pimobendan on heart failure status in Cocker Spaniels and Dobermanns with idiopathic dilated cardiomyopathy. Proc BSAVA Congress. p284.

• Serena Brownlie BVM&S PhD CertSAC MRCVS , Broadacres, Bedford Road, Little Houghton, Northampton NN7 1AW, UK.
• Dr Bruce Keene , Department of Companion Animal and Special Species Medicine, North Carolina State University, 4700, Hillsborough Road, Raleigh NC 27606, USA.
• Dr Mark Rishniw DVM DipACVIM , VRT Box 34, Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853, USA.
• Ruth Willis BVM&S VVC MRCVS, Cardiorespiratory Referral Service, Broadleys Veterinary Hospital, Craig Leith Road, Stirling, FK7 7LE, UK.

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