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					           Managing the dog with heart disease: When is surgery indicated?

                             Sonya G. Gordon, DVM, DVSc, DACVIM
                             Theresa W. Fossum, DVM, PhD, DACVS
                                   David A. Nelson, DVM, CCP

        Prior to the availability of cardiopulmonary bypass (CPB) surgical options for the treatment
of many cardiac diseases were severely limited. Although still in its relative infancy, clinical
veterinary CPB is currently available at a number of specialty centers. It is the availability of CPB
that will act as a catalyst for the continued development of successful definitive and palliative
surgical techniques for the treatment of acquired and congenital heart defects in the dog and
potentially in other species.
        Historical data suggests that as many as 11% of dogs have evidence of cardiac disease at the
time of presentation to their primary care practitioner and an additional 8% have signs suggestive of
cardiac disease. Congenital heart disease comprises approximately 2% of all canine heart disease.
Left to right shunting patent ductus arteriosus (PDA) is reportedly the most common canine
congenital heart defect and represents approximately 1/3 of these cases. Sub-aortic stenosis (SAS) is
the second most common defect with estimates of approximately 22%. Valvular pulmonic stenosis
(PS) ranks a distant 3rd. These 3 diseases account for as much as 90% all canine congenital heart
disease. The remainder of relatively common congenital heart defects includes: tricuspid and mitral
dysplasia, ventricular and atrial septal defects, cortriatriatum dexter and right to left shunts such as
Tetralogy of fallot and reverse PDA. However, the vast majority of canine heart disease is acquired
in nature. Valvular endocardiosis or chronic degenerative (myxomatous) atrioventricular valve
disease (CVD) is the leading cause of canine heart disease and failure, representing approximately
70-75% of all canine heart disease cases. Idiopathic dilated cardiomyopathy is the second most
common acquired cardiac disease. The remainder of acquired heart disease is composed of
disorders such as: pericardial disease, intracardiac tumors, heartworm infection and conduction
system abnormalities.
        PDA’s have traditionally been corrected via surgical ligation but more recently numerous
centers elect to occlude PDA’s via percutaneous, transarterial coil embolization. Neither of these
procedures requires nor benefit from CPB; however, the treatment of virtually all other congenital
heart diseases will benefit from the availability of CPB in clinical veterinary medicine. Severe
valvular pulmonic Stenosis (PS) carries a guarded long-term prognosis and many of these patients
have undergone percutaneous balloon valvuloplasty (BV) in attempts to palliate clinical signs and
decrease mortality. Although many patients with severe valvular PS benefit from BV, the presence
of significant hypoplasia of the pulmonic annulus or concurrent congenital defects such as a single
right coronary artery makes BV less likely to be successful or completely contraindicated
respectively. Some of these patients may benefit from alternative palliative procedures that could be
performed under partial or complete CPB. Subaortic stenosis in its severe form carries a guarded
long-term prognosis and previous attempts to palliate these patients with BV or other minimally
invasive procedures have failed. One group reported no improvement in survival for dogs with SAS
undergoing open surgical correction under CPB. However, alternative and perhaps more aggressive
surgical interventions may yet be beneficial for dogs with severe SAS and are only made possible
by CPB. Additionally, many of the other congenital heart defects listed previously may benefit from
palliative or definitive surgical correction performed using bypass techniques.



Proceedings 8th IVECCS                            66                          San Antonio, TX, 2002
        Acquired cardiac disorders such as pericardial disease and intracardiac tumors have long
benefited from surgical intervention. With the advent of clinically available CPB, definitive tumor
resection, or debulking, particularly of those masses previously considered inoperable, may, become
possible.
        Mitral regurgitation secondary to degenerative myxomatous changes of the left
atrioventricular valve (CVD) is reportedly responsible for approximately 75% of all cardiovascular
disease in the dog and represents the most important cause of cardiovascular morbidity and
mortality in the dog. Chronic valvular disease is easily diagnosed in its early (asymptomatic) stages
by the presence of a characteristic murmur. Clinical progression is typically slow but can be
punctuated by acute exacerbations and although not all dogs with CVD develop clinical signs of
heart failure, all dogs experience disease progression, however the absolute number of dogs with
CVD who progress to heart failure is currently unknown. There is no proof that any medication
used during the asymptomatic phase of CVD can delay its progression. In fact, recent evidence
suggests that the most common medication recommended for treatment of these patients, an
angiotensin converting enzyme inhibitor (ACEI) with modest afterload reduction properties
(enalapril), offers no benefit over placebo in delaying the onset of CHF in Cavalier King Charles
spaniels with CVD. Therefore, current recommended therapy for dogs with CVD focuses on
relieving clinical signs of congestive heart failure (CHF) when they develop. Despite traditional
therapy for CHF including diuretics, an ACEI, +/- digoxin, these patients have a poor long-term
prognosis with an average survival of 4-6 months following the onset of CHF. Primary mitral
insufficiency (MI) of people is a disease that bears many similarities to canine CVD. The preferred
treatment for MI in people is timely open-heart surgery under CPB to repair or replace the faulty
valve. Definitive therapy of this nature has only recently become available to veterinary patients and
parallels the availability of CPB. Thus an obvious target for the veterinary cardiovascular surgeon is
the disease most responsible for congestive heart failure in the dog, CVD.
        The following set of guidelines is suggested by the cardiovascular group at the Texas A&M
College of Veterinary Medicine for canine patients whose owners wish them to be considered as
candidates for surgical treatment of CVD.
           •   Absence of significant concurrent disease such as end stage renal disease.
               Heartworm disease, hepatic failure, metastatic neoplasia, septicemia, etc.
           •   Confirmed diagnosis (echocardiogram) of CVD without the presence of concurrent
               severe congenital heart disease such as SAS, PS or ventricular septal defects etc.
           •   History of cardiogenic pulmonary edema (congestive heart failure) with a clinical
               and radiographic response to diuretic (furosemide) therapy. The patient must require
               maintenance diuretic therapy.
           •   Indirect systolic blood pressure by Doppler or Dynamap should be greater than 100
               mmHg when the patient is conscious and stable.
           •   Preservation of left ventricular systolic function. Systolic function will be considered
               preserved in dogs with CVD if the internal dimension of the left ventricle (LVIDs)
               measured by 2-D guided m-mode echocardiography is within normal limits. These
               measurements should be carried out in accordance with published criteria for canine
               m-mode acquisition. This measurement is based on the weight of the patient and
               normal estimates are reported in numerous veterinary publications. The following
               formula can be used as a guide. 0.69 BW (kg)0.41=mean normal LVIDs (cm).
           •   With the exception of digoxin, the patient cannot require long term inotropic support.



Proceedings 8th IVECCS                            67                         San Antonio, TX, 2002
        Pre-surgical screening evaluations that must be performed within 30 days of presentation, at
the referral hospital or Texas A&M and include:
                    1. Complete blood count including manual platelet count
                    2. Biochemistry panel
                    3. Urinalysis
                    4. Canine blood typing
                    5. Thoracic radiographs
                    6. Indirect blood pressure
                    7. Abdominal ultrasound
                    8. Echocardiogram
                    9. Further work-up may be required if clinical or diagnostic findings suggest
                        concurrent diseases such as hypothyroidism, hyperadrenocorticism, urinary
                        tract infection etc.

        Digital or VHS video of an echocardiogram can be provided for evaluation prior to referral.
Recorded echocardiograms should include standard 2-D imaging planes from the right and left
parasternal windows as well as m-mode measurements of the left ventricle and left atrium/aorta.

      Dogs with some forms of congenital heart disease, other acquired cardiac disease and dogs
with CVD that do not meet the above criteria will be considered on an individual basis.

Selected References
Orton EC et al. Influence of open surgical correction on intermediate-term outcome in dogs with
subvalvular aortic stenosis: 44 cases (1991-1998). JAVMA, Vol 209, No. 7 October 1, 1996. pp
1255.

Kvart C et al. Efficacy of enalapril for prevention of congestive heart failure in dogs with
myxomatous valve disease and asymptomatic mitral regurgitation. J Vet Intern Med. 2002;16:80.

The COVE Study Group. Controlled clinical evaluation of enalapril in dogs with heart failure:
Results of the Cooperative Veterinary Enalapril Study Group. J Vet Intern Med. 1995; 9:243




Proceedings 8th IVECCS                           68                        San Antonio, TX, 2002

				
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