Congenital Heart Disease

Question 1

Describe the major outcomes from echocardiography in congenital heart disease

Answer 1

1. Identify primary structural abnormalities
   
   • Valve malformation, septal defects, outflow tract obstruction
2. Characterise secondary responses
   
   • Atrial enlargement, myocardial hypertrophy
3. Delineate abnormal blood flows
   
   • Valvular regurgitation, increased outflow velocity, shunting
4. Quantify systolic and diastolic function
5. Calculate pressures, flow and resistances

Question 2

Patent Ductus Arteriosus

Describe the role of the dutus arteriosus in fetal circulation.

Discuss the mechanisms the lead to normal ductus closure.

Answer 2

• The ductus arteriosis forms from the left sixth aortic arch and the structure shunts deoxygenated blood from the pulmonary artery into the aorta. Blood flows down the aorta to the placenta where it receives oxygen. The ductus remains open due to high vascular resistance within the pulmonary circulation.
• After parturition and the commencement of breathing, vascular resistance in the pulmonary circulation drops leading to reversal of flow through the ductus.
• Increased local arterial oxygen tension inhibits local protaglandin release
• This leads to constriction of vascular smooth muscle within the wall of the ductus and functinoal closure.
• Patency of the ductus remains for < 4 days and is structurally closed by 7-10 days.

Question 3

Discuss the pathogenesis of PDA formation.

Answer 3

• The normal ductal wall contains a network of loose, circumferential smooth muscle fibres throughout the length
• In PDA prone dogs, varying portion of the wall is composed of elastic fibres only - extension of the non-contractile wall of the aorta into the ductus
• A polygenic mode of inheritance is likely
• The most common form has less muscular fibres towards the aortic end and is funnel shaped, tapering towards the pulmonary artery
• The most severe form is cylindrical and seen in association with persistent pulmonary hypertension leading to bidirectional flow or right to left shunting.

Question 4

Describe the pathophysiological consequences of a left to right shunting PDA

Answer 4

• L-R shunting volume is determined by
  
  • the pressure differential between pulmonary and systemic vasculature
  • The degree of resistance within the PDA
• Shunting leads to pulmonary overcirculation
• Increased pulmonary venous return leads to increased LV stroke volume: Eccentric dilatation occurs
• Increased run off via the PDA leads to decreased systemic diastolic pressures: Increase pulse excursion - hyperkinetic pulses
• Continuous flow through the shunt leads to a continuous murmur: Major clinical exam finding
• Major vasculature dilates - pulmonary artery, proximal aorta, pulmonary vascular bed.
• Left atrial dilatation can occur if shunt volume is significant: Important echo finding
• The right ventricle and right atrium remain normal as they are not exposed to the shunted blood volume.
• With chronicity, increased pulmonary blood flow can exhaust the compensatory mechanisms and overt pulomonary oedema will develop.

Question 5

Describe the mechanisms that lead to development of a R-L shunting PDA.

Answer 5

• For a R-L shunt to develop the PDA must remain open, cylindrical and large.
• The open ductal orrifice at the pulmonary artery allows aortic pressures to be directly transmitted to the pulmonary vasculature
• This precludes the normal post-natal decline in pulmonary vascular resistance
• Due to the increased pulmonary pressures, the right ventricle will hypertrophy
• Reversal of blood flow through the shunt generally occurs within the first 2 weeks of life.

Question 6

What are the pathophysiological consequences of a R-L shunting PDA

Answer 6

• Maintained high pulmonary vascular resistance leads to decreased pulmonary blood flow: hypoxia
• Normal to small left ventricle and left atrium as they do not see the increased blood volume due to increased capacitance in the systemic circulation
• The right ventricle becomes hypertrophied early in life
• The precise pathogenesis of pulmonary hypertension is not known for dogs that show shunt flow reversal later in life
• Histological changes in the pulmonary arterioles include: tuinica media hypertrophy, thickening and fibrosis of the intima and reduction in luminal diameter
• Plexiform lesions develop within the vascular wall - characteristic change
• Due to reduced oxygen tension in the blood reaching the kidneys, EPO release is stimulated. This leads to increased red cell mass to accomodate adequate oxygen delivery. Significant increase in the red cell mass can lead to hyperviscosity, sludging and effective vascular occlusion.

Question 7

Describe the history and clinical findings in dogs with left to right shunting PDA

Answer 7

• The majority of dogs are apparently normal and healthy at 6-8 weeks of life (first vet check). Severely affected puppies may be stunted, have poor body condition or tachypnoea due to CHF.
• Precordial impulse may be exaggerated due to LV enlargement
• A palpable thrill may be present cranially
• A continuous murmur is best heard in the left axilliary region
  
  • Often only a systolic murmur is audible over the mitral region
• Thoracic radiographs can indicate left heart enlargement, dilatation of the pulmonary artery and overcirculation of the pulmonary veins. An aortic bulge may be present.

Question 8

Describe the historical and clinical findings in dogs with a right to left shunting PDA

Answer 8

• Clinical signs are often absent or mild in the first 6-12 months of life.
• Signs include exertional fatigue, hindlimb weakness, shortness of breath, tachypnoea, differential cyanosis, seizures
• Auscultation reveals no murmur or only a soft systolic murmur
• A split or accentuated second heart sound may be present
• Differential cyanosis may only be seen after exertion and is due to shunting occuring distal to the aortic branches that supply the cranial portion of the body.
• Hypoxaemia induced erythrocytosis tends to occur from 12-24 months of age

Question 9

Describe the known natural outcome of uncorrected PDA in dogs and cats

Answer 9

• Minimal studies have reported non-corrected outcomes in these dogs.
  
  • A 1976 paper looked at 100 dogs, 14 of which received no surgical intervention
  • 9/14 (65%) had died within 1 year of examination.
• Aortic dissection is a recently recognized complication of uncorrected PDA in the dog.
• Minimal is known about the natural history in cats
  
  • Cats are more prone to the development of pulmonary hypertension due to pulmonary overcirculation
  • Pulmonary hypertension may develop slowly and may be arrested by prompt intervention.
• In cats, development of pulmonary hypertension can lead to reversal of the shunt flow. CHF is rare but sudden death and complications due to hyperviscosity are common.
• Occasional dogs and cats with mild forms of the disease can live past 10 years of age without intervention.

Question 10

Discuss the options of management of dogs with PDA

Answer 10

• Uncomplicated PDA without CHF - surgical attenuation or coli attenuation results in an excellent outcome with a relatively small morbidity/mortality rate (< 5%)
• Determining the appropriate treatment depends on patient size, PDA morphology, operator experice/availability and owner preference.
• If CHF is present at the time of diagnosis, then standard management for for a short period is recommended - frusemide, pimobendan, +/- benazepril.
  
  • Note: medications will need to be continued after attenuation for several months
• Note: surgical or interventional closure can result in reversal of LV hypertrophy.
• Right to left shunting is a defined contraindication to correction.
• Treatment for dogs with R-L shunting is aimed at reduction of pulmonary hypertension (sildenafil citrate) and management of erythrocytosis (phlebotomy or hydroxyurea)

Question 11

Describe the pathogenesis of atrial and ventricular septal defects

Answer 11

• The cardiac speta and AV valves grow during embryonic development to separate the heart into four chambers
• Septum primium and septum secundum develop to the left and right of the atria respectively. The foramen ovale is a slit that remains to shunt blood from right to left in utero
• The endocardial cushions grow and differentiate to form the upper ventricular septum, lower atrial septum and AV valves
• The tricuspid valve connects to the septum more apically than the mitral valve - the resultant segment between the AV valves is defined as the AV septum
• Abnormal development of the primium or secundum atrial septa or endocardiac cushion can lead to ASD, VSD or malformations of the AV valves or complex lesions such as ToF

Question 12

Discuss the pathophysiology and consequences of ventricular septal defects

Answer 12

• Shunting across small resistive or restrictive VSD dpends primarily on the size of the defect and the relative pressures between the two chambers
• Shunting across large defects depends on the relative resistances in the systemic and pulmonary circulation
• In most instances, left sided pressures exceed right sided pressures and the shunt volume moves from left to right by ~ 80-100 mmHg (100-120 mmHg versus 20-25 mmHg)
  
  • Flow velocities are normally > 4.5 m/s
  • Flow velocity < 4.4 m/s suggests increased right ventricular systolic / pulmonary pressures
• Relative over-circulation of the pulmonary vascular bed occurs
  
  • If the increased volume is large, left sided myocardial failure can result
• Right to left shunting occurs when there is:
  
  • Pulmonary hypertension
  • Pulmonic stenosis
  • Tricuspid dysplasia?
• Right to left shunting causes cyanotic blood to reach the kidney, stimulating EPO production, erythrocytosis and hyperviscosity.

Question 13

Describe the pathophysiology and compensatory response of the heart to a significant volume ASD

Answer 13

• Flow across an ASD occurs primarily during ventricular diastole.
• Flow direction is dependent on the relative diastolic resistance to inflow - RV normally more compliant than the LV
• Flow typically moves from left to right initially
• Increased flow to the RA leads to RA dilatation, RV eccentric hypertrophy and pulmonary over-circulation
• LA receives the shunted blood (as with PDA and VSD) but it is rapidly transmitted to the RA.
• LA remains a normal size unless there is concurrent AVSD or MVD
• Relative increases in pulmonic outflow velocity can create a low grade murmur.
• Combination of delayed pulmonic valve closure and early aortic valve closure can create a split S2
• Right heart failure and pulmonary vascular injury leading to pulmonary hypertension can result.

Question 14

Describe the major clinical findings in dogs or cats with clinically significant ASD.

Note the expected changes with the commonly utilised diagnostic tests.

Answer 14

• Note: the majority of ASD are clinically silent and inconsequential
• Soft 2-3/6 may be heard - due to relative pulmonic stenosis.
• Split S2 may be heard
• ECG changes:
  
  • may indicate right heart enlargement (axis shift)
  • partial or complete right BBB
• Radiographs:
  
  • Right heart enlargement
  • Enlargement of the main pulmonary arteries
  • Pulmonary hypervascularity
• Echo changes - best assessed with Doppler studies
  
  • Laminar or mildly turbulent trans atrial flow during diastole
  • Increased RVOT velocity
  • Concurrent problems can also be assessed - eg. MV disease

Question 15

Describe the major clinical findings in dogs or cats with clinically significant VSD.

Note the expected changes with the commonly utilised diagnostic tests.

Answer 15

• Note: The majority of VSDs are small and clinically inconsequential
• Typical harsh holosystolic murmur over the right ventral, mid-cranial precordium.
• Split S2 may occur but is generally not distinguishable owing to the murmur
• A diastolic murmur may be present if there is distortion of the aortic root / aortic insufficiency
  
  • Can be confused for a continuous murmur
• ECG findings (with moderate to large shunts)
  
  • Mild to moderate LA or LV enlargement
  • RV conduction defects can occur - widening and notching of the Q wave
  • RV enlargement can occur when there is concurrent PS or pulmonary hypertension
• Radiographs: changes are all in proportion with shunt volume
  
  • Pulmonary hypervascularity
    
    • Main, lobar and peripheral pulmonary arteries are prominent
  • LA and LV enlargement
• Echo findings:
  
  • Variable LA and LV enlargement
  • 2D right parasternal images to identify membranous VSD
  • Subarterial VSD best identified in short axis views at the level of the aorta
  • Doppler studies helpful to view shunt volume and estimate pressure differential
  • May see concurrent defects including aortic regurgitation
  • Note: general rule - if VSD is < 40% of the aortic root diameter, the lesion is likely to be well tolerated.

Question 16

Describe the pathophysiological consequences of pulmonic regurgitation.

Comment on the relevance or regurgitation when there is concurrent cardiac abnormalities

Answer 16

• PR is common with mild to moderate regurgitation inconsequential if seen in isolation
• PR causes RV volume overload and eccentric dilatation is significant
• The pulmonary arteries may enlarge due to the increased RV stroke volume
• PR is most likely to become clinically significant if there is an increase in pulmonary vascular resistance or with concomminant PS or tricuspid insufficiency
• Severe PR with PS can cause a too and fro diastolic/systolic murmur.
• Radiographs may show enlargement of the pulmonary artery and RV, while Doppler echocardiography can readily demonstrate PR and 2D echo can identify the valve abnormality.

Question 17

Describe the causes of isolated aortic regurgitation and identify the common concurrent cardiac abnormalities

What are the pathophysiologcial consequences of aortic regurgitation?

Answer 17

• Innocent trace or mild volume aortic regurgitation is reasonably common
• Aortic regurgitation can occur with aortic valve dysplasia (bicuspid or quadricuspid aortic valves)
• AR can also occur with dilatation of the aorta - annuloaortic ectasia.
• AR will be seen secondary to endocardiosis of the aortic valve
• AR is commonly identified in combination with other congential abnormalities such as SAS, VSD, ToF.
• When a diastolic murmur is audible, the AR is often severe
• When severe, hyperkinetic pulses will be identifed due to increased stroke volume and reduced diastolic arterial pressures due to increased run-off back into the LV
• Eccentric to mixed LV dilatation occurs.
• When severe, AR results in left sided CHF
  
  • Arterial vasodilators may help delay the onset of CHF

Question 18

Describe the pathogenesis of TVD highlighting known genetic abnormalities

Answer 18

• TVD has been shown to have a genetic basis in the labrador retriever and others
  
  • Labrador - autosomal dominant mutation with incomplete penetrance, mapped to chromosome 9
• Wide variety of abnormalities have been describe
  
  • Note: some may be more variations on normal than true TVD
• Usual and most common consequence of TVD is valvular insufficiency
• Concurrent ASD or PFO may be present.

Question 19

Describe the pathophysiological consequences of TVD

Answer 19

• TVD results in a right sided volume overload.
  
  • Right atrial dilatation
  • Right ventricle eccentric hypertrophy
• With increasing severity, supraventricular arrhythmias such as APCs or AFib may develop
• If there is TVD with pulmonic stenosis, then concentric hypertrophy may develop
• Significant stenosis can diminish cardiac output and low-output signs may predominate - syncope, hypotension, collapse - especially with exertion.

Question 20

Describe the pathophysiological changes seen with mitral valve dysplasia.

Answer 20

• The consequences of mitral valve dysplasia is alligned with the severity of the defect, whether insufficiency predominates or if significant stenosis exists.
• Significant LV insufficiency results in left atrial enlargement and eccentric hypertrophy of the left ventricle.
• CHF commonly develops at a young age.
• MV stenosis can limit cardiac output leading to signs of forward flow failure
• Resultant and chronic increases is LA pressure can lead to pulmonary hypertension and even secondary right sided failure (more common in cats)
• MVD can also cause dynamic LVOT obstruction despite small regurgitant volumes.
  
  • This can lead to LV concentric hypertrophy, increased LV systolic pressures and worsening of the MV regurgitant leak
• Supraventricular arrhythmia is common with marked atrial enlargement and can result in a sudden clinical deterioration.

Question 21

Describe the various pathological changes that can contribute to or be associated with pulmonic stenosis in dogs

Answer 21

• Pulmonic stenosis can be due to a subvalvular fibrous ring (double chambered right ventricle - infundibular stenosis), valvular lesions or narrowing of the supravalvular region .
• Valvular lesions can include:
  
  • Valve thickening
  • Leaflet fusion
  • Hypoplasia of the valvular annulus
  • Distal leaflet tethering to the pulmonary artery
• Note that subvalvular PS has been associated with anomalous coronary artery development especially in English Bulldog and Boxer dogs
  
  • Coronary circulation derived from single ostium in the right aortic sinus of Valsalva
  • Both left and right arteries develop from a single large coronary artery.
  • The left coronary artery encircles the RVOT just below the pulmonic valve.
  • This coronary artery malformation has been seen without PS

Question 22

Describe the pathophysiological consequences of clinically significant pulmonic stenosis.

Answer 22

• Pulmonic stenosis causes an increase in right ventricular systolic pressures (increased outflow resistance)
• Right ventricualr concentric hypertrophy develops to normalise wall stress
• The increased ejection velocities contribute to a post-stenosis dilatation in the main pulmonary artery as the wall absorbs the excessive kinetic energy
• Concentric hypertrophy reduces diastolic compliance and filling - this can lead to increased right atrial pressures
• Ventricular hypertrophy and increased pressures can result in tricuspid insufficiency, further increasing RA pressure
• As the RA pressure approaches 15 mmHg, jugular distension, ascites and pleural effusion may start to develop - ie. Right sided CHF
• Pressure overload in the RV stimulates mechanoreceptors and can trigger a reflex bradycardia / peripheral vasodilatation
  
  • May contribute to syncope or signs of forward flow failure.
• With increased RV thickening, infundibular outflow tract obstruction may worsen. LVOT obstruction can also occur with advanced changes.

Question 23

Describe the clinical findings in dogs with pulmonic stenosis.

Answer 23

• Various breeds are predisposed:
  
  • Beagle, Samoyed, English Bulldog, Mini schnauzer, Cocker Spaniels and others
• Most dogs are clinically normal for the first 12 months - Initial findings include a variably intense systolic murmur over the left heart base.
  
  • ~35% of dogs with severe disease will show signs attributable to forward failure or right sided CHF
• A diastolic murmur may be audible if there is significant valvular insufficiency
• A systolic click may be heard if there is a fused but mobile valve
• Jugular venous distension or a jugular pulse may be evident - generally only once CHF develops

Question 24

Descibe the various testing modalities used to diagnosis pulmonic stenosis including their diagnostic utility.

Answer 24

• Electrocardiography (non-specific)
  
  • Right heart enlargement
    
    • Deep S Wave and right axis deviation
• Thoracic radiographs (non-specific right heart changes)
  
  • Right heart enlargement
  • Prominent pulmonic artery - post-stenotic dilatation
  • Reduced prominence of the pulmonary vasculature
  • Enlarged caudal vena cava
• Echocardiography (gold standard non-invasive test)
  
  • Concentric hypertrophy of the right ventricle
  • Stenosis / narrowing at the subvalvular, valvular or supravalvular region
  • Variable right atrial enlargement
  • Pulmonic valves may be fused, thickened or domed into the pulmonary artery
  • Doppler echo is essential to help identify the location of the stenosis
  • Careful assessment of the coronary vessels is warranted in Bulldogs and Boxers

Question 25

Note the severity grading of pulmonic stenosis.

Discuss when treatment is recommended.

When is definitive treatment contra-indicated?

Answer 25

• Pulmonic stenosis severity is directly causative of increased blood velocity out the RVOT. Pressures across the stenosis can be estimated by measuring Doppler velocites or via cardiac catheterisation techniques.
• PS is considered mild, moderate and severe at the following pressure gradients (echo velocity measurements)
  
  • Mild: < 50 mmHg (3.5 m/s)
  • Moderate: 50-80 mmHh (3-5-4.5 m/s)
  • Severe > 80 mmHg (> 4.5 m/s)
• Treatment with balloon valvoplasty is recommended for severe PS and generally recommended for moderate PS (> 50-60 mmHg)
  
  • Translation from human studies shows Balloon valvoplasty in moderate pressure gradients may reduce the risk of clinical signs, CHF and myocardial fibrosis.
• Treatment with balloon valvoplasty is contraindicated with an anomalous left coronary artery
• Type B morphology may have less adequate outcomes after balloon dilatation
• The benefits of balloon dilatation in the setting of RCHF is less well defined, but theoretically still beneficial

Question 26

Briefly explain the known genetic basis for aortic stenosis

Describe the various pathological changes that contributie to aortic stenosis.

Answer 26

• An autosomal dominant mutation in the PICALM gene has been associated with SAS in Newfoundland dogs - but this data has been disputed
• Autosomal recessive trait reported in the Dogue de Bordeaux
• The obstruction likely develops between 4-8 weeks of age
• An abnormal aorto-septal angle (aortopathy) may play a role in development or progression (golden retriever)
• Pathology:
  
  • Grade 1: small whitish raised nodule on the endocardial surface of the septum below the aortic valve
  • Grade 2: Narrow ridge or whitish thickened endocardium partial extending about the LVOT
  • Grade 3: Fibrous band or collar just below the aortic valve. May invole the aortic or mitral valves.
• Note: Grade 1 lesions may not be detected by cardiac catheter studies. Grade 2 lesions typically cause a low grade murmur and minimal systolic pressure gradients
• Numerous concurrent cardiac defects have been described in conjunction with SAS including MVD, PDA, aortic arch anomalies.
• In the bull terrier breed, myxomatous degeneration and cartilage metaplasia of the valve can be seen
• Post-stenotic dilatation of the aortic arch is variable and not often directly related to the degree of stenosis.

Question 27

Describe the variations in gross pathological changes seen to cause aortic stenosis (other than a fibrous ring)

Answer 27

• Thickenend mitral valve leaflets in apposition to a septal plaque of endocardial fibrosis - consequence of dynamic obstruction
• Broad fibromuscular ridge arising from the IVS
• Malformed, malpositioned or misaligned papilliary muscles
• Thickened chordae tendinae
• Distorted or elongated mitral valve leaflets

Question 28

Describe the pathological consequences and the cardiac changes secondary to aortic stenosis

Answer 28

• Concentric LV hypertrophy develops - more or less in proportion with the severity of the obstruction
• Structural and functional abnormalities of the coronary circulation and LV circulation are well documented
  
  • Diminished baseline flow in the coronary arteries during diastole
  • Reversal of coronary artery flow during systole
• Focal areas of myocardial infarction and fibrosis - common in the papilliary muscles and subendocardium. Seen in association with abnormal coronary vessels
  
  • Coronary artery intimal proliferation of smooth muscle and connective tissue
  • Coronary artery medial degeneration

Question 29

Describe the pathophysiological consequences of clinically significant sub aortic stenosis.

Answer 29

• SAS causes obstruction to LV outflow - increased LV systolic pressure - concentric hypertrophy
• High velocity flow across the stenosis contributes to the audible murmur and post-stenotic dilatation
• LA hypertrophy develops due to impaired diastolic function and reduced compliance of the LV
• Mild aortic regurgitation is common
• Jet lesions to the aortic valvular endothelium predisposes dogs with SAS to infective endocarditis
• CHF can develop due to a combination of:
  
  • myocardial failure
  • diastolic dysfunction
  • mitral regurgitation - volume overload
  • atrial arrhythmia
• Suddent death may be due to malignant arrhythmia due to myocardia ischaemia
• Exercise induced collapse may be secondary to increased LV pressure - mechanoreceptor activation - inappropriate/reflex bradycardia and vasodilation

Question 30

Discss the signalment and clinical examination findings in dogs with subaortic stenosis

Answer 30

• Breed pre-disposition:
  
  • Newfoundland, Boxer, German Shepherd, Golder Retriever, Rottweiler. Bull Terrier - aortic valvular stenosis
• Assymptomatic dogs have soft to moderate intensity murmur
• Puppies with mild to moderate disease may appear normal, whereas puppies with severe disease may be smaller and fail to thrive when compared to littermates
• Severe signs include exertional fatigue, syncope or overt left sided CHF.
• Sudden death without premonitory signs is common
• Loud systolic murmur at the left heart base, possibly radiating to the right or ventral cervial region (up the carotid arteries)
• Mitral regurgitant murmur may also be present - loudest over the left apical beat

Question 31

Describe the abnormalities seen on routine cardiac diagnostic tests

Answer 31

• Electrocardiography:
  
  • often normal
  • In severe cases, left heart enlargement - tall R wave
  • ST segment depression and T wave abnormalities
    
    • With repolarisation changes or myocardial ischaemia
    • May be precipitated by exercise
  • Holter recording is more sensitive ventricular arrhythmia or exercise induced ST segment changes
• Thoracic radiographs:
  
  • Normal or LV hypertrophy
  • ]Post-stenotic aortic dilatation - loss of waist in lateral view, widened mediastinum in the VD view
  • LA enlargement - usually mild unless there is clinically significant mitral insufficiency
• Angiography - can further define the abnormalities
• Echo - 2D and Doppler - non-invasive gold standard
  
  • 2D: LV concentric hypertrophy, subvalvular obstruction, narrowing of the LV orifice, post-stenotic aortic dilatation.
    
    • Endocardial and pupilliary muscle hyperechogenicity in severe disease
  • Doppler: Increased velocity and turbulence blood flow through the stenotic region (+/- valvular regurgitation)
    
    • Doppler measurements may over or under-estimate severity based on cardiac output. Indexing to orifice area may provide a better estimate of severity

Question 32

Describe the normal Doppler measurements for the LVOT.

Define the increase required for a diagnosis and subsequent grading of SAS

Answer 32

• Normal LVOT velocites vary depending on the view used to obtain the measurement. Right apical view upper normal is 1.7-1.8m/s. Subcostal views have an upper limit of ~ 2.0 m/s
• Boxers and Bull Terriers have narrowed outflow tract dimensions leading to mildly increased velocities as the “breed normal”
• Mild SAS usually diagnosed with LVOT velocity from 2.3 m/s to 3.5 m/s.
• Moderate: 3.5-5.5 m/s
• Severe SAS: LVOT of 5.5 m/s or higher

Note that the SAS phenotype develops over the first ~ 12 months as a puppy grows. An increased aortoseptal angle predisposes to the phenotype in Golden Retriever dogs.

Question 33

Discuss the various management options for dogs with clinically significant subarotic stenosis.

Make reference to the evidence available for each management recommendation

Answer 33

1. Exercise restriction: may reduce the risk of syncope or sudden death.
   
   • Not always practical for young energetic dogs
2. Beta-adrenergic blockade - atenolol:
   
   • Theoretic benefits of:
     
     • reducing myocardial oxygen demand
     • Reduced heart rate - increased diastolic time - improved coronary artery perfusion
     • Reduced heart rate may help to limit vigorous exercise
3. Balloon valvoplasty (or open surgery)
   
   • technically challenging with a significant to high risk profile
   • Shown to significantly reduce pressure gradients initially
   • Not shown to improve survival when compared to atenolol therapy
4. Eason 2014 showed no improvement in survival in dogs with severe LVOT obstruction (PG of 121 and 127 for non-treated and treated groups) with atenolol treatment.
   
   • Median (all cause mortality) survival was 5.1-5.9 years

Question 34

Define the specific abnormalites that comprises the complex congenital cardiac disease, Tetralogy of Fallot

Answer 34

1. RVOT obstruction due to infundibular obstruction
   
   • Pulmonic valve may also be hypoplastic
2. RV hypertrophy
3. Typically large perimembranous VSD
4. Rightward positioned aorta (dextroarota)

Question 35

Describe the various gradings of “conotruncal septal defects”

Also note other associated abnormalities and mode of inheritance

Answer 35

• Conotruncal septal defects are inherited autosomal recessive traits with variable expression
• The pathogenesis likely related to centrocranial deviation of the ventricular outlet septum

1. Grade 1: subclinical - malformation of the conus septum fusion lines, aneurysm of the ventricular septum, absnece of the papilliary muscle of the conus
2. Grade 2: PS or VSD in addition to the grade 1 lesions
3. Grade 3: ToF - PS, VSD and dextropositioned aorta

Associated abnormalities include:

• tortuous ascending aorta, pulomonary atresia, abnormalities in the aortic arch, hypoplasia of the supraventricular crest

Question 36

Describe the pathophysiological consequences of tetralogy of fallot

Answer 36

• Right ventricular outflow tract obstruction leads to concentric hypertrophy of the right ventricle
• Due to increased RV pressures, there is right to left shunting through the large VSD
• Pulmonary arterial volumes are reduced together with pulmonary venous return
• The left atrium and ventricle are under-developed
• Systemic hypoxaemia (due to R-L shunting) leads to decreased hb saturation, renal hypoxaemia, EPO production and secondary erythrocytosis
• Systemic collateral circulation to the lung increases via the bronchial artery system - these vessels can directly supply the pulmonary capilliaries or anastomose with the largery pulmonary arteries.

Question 37

Describe the unique components of management for tetralogy of fallot (when compared to single defect management alone)

Answer 37

• ToF involves pulmonic outflow tract obstruction and a large right to left shunting VSD (with secondary right ventricular hypertrophy
• Congestive heart failure is an uncommon outcome
• sudden death due to hyperviscosity, hypoxaemia or arrhythmia is more common
• Balloon dilatation of the pulmonic stenosis can be “gently’ performed - reduced RV pressures due to much reduced stenosis will cause left to right shunting through the VSD and left sided CHF
• VSD closure and PS dilatation can be performed but is rarely contemplated in veterinary medicine
• Surgical creation of a left to right shunt or stenting of a patent ductus arterious may improve pulmonary circulation. Care must be taken not to overload the small left ventricle.
• Maintain hydration and periodic phlebotomy to achieve a PCV of ~62-68%
• Beta blockers may be of theoretical benefit to reduced RVOT obstruction
• Vasocontrictive medications such as phenylephrine can help reduce R-L shunting and boost systemic blood pressure.

Question 38

List the various clinically relevant arterial vascular anomalies described in the thorax.

Answer 38

1. Patent ductus arteriosus
2. Persistent truncus arteriosus
   
   • Large VSD and single great vessel exiting the heart
   • Clinical signs largely depend on whether pulmonary circulation is increased or restricted.
3. Aorticopulmonary septal defect
   
   • Failure of the truncus to differentiate leading to mixing of aortic and pulmonary blood
   • Clinically similar to L-R shunting PDA initially.
   • Pulmonary hypertension may develop within 12 months of life causing shunt reversal
4. Coronary artery anomalies
   
   • single right coronary artery ostium is most common abnormality - left coronary artery encircles the pulmonary trunk
5. Vascular ring anomalies:
   
   • Persistent right aortic arch (PRAA) - GSD predisposed
   • retro-esophageal subclavian arteries
   • Double aortic arch
6. Systemic to pulmonary arterial vascular malformations
   
   • These cause left sided volume overload secondary to pulmonary over-circulation.
   • Multiple malformations should be suspected if coil closure is to be attempted.