Inflow Occlusion Techniques

 Cessation of venous return to the heart with circulatory arrest

 Indicated for surgeries with only a brief period of open heart

 Simple (compared to bypass), minimal cardiopulmonary and metabolic derangements

 Can be combined with hypothermia for additional time

 Limited time

 Motion of surgical field (even with fibrillation), lack of fallback if more time is required

 2 minutes or less in normothermic patient. Up to 4 minutes in hypothermic patient (32°C)

 Hypothermia makes defibrillation more difficult

 Requires careful anesthesia

 Atracurium (0.1 mg/kg), fentanyl citrate (2 mcg/kg), isoflurane/sevoflurane

 Resuscitation drugs and internal defibrillator paddles are required

 Fibrillation can be achieved with application of current from 9V battery-constant IV lidocaine infusion (50–80 mcg/kg/min) required before occlusion and continued as long as needed

 Positive inotrope support as needed.

 Occlusion can be achieved from either a right or left thoracotomy or median sternotomy (easier from a right 4th or 5th intercostal approach)

 Rummel (tape) tourniquets or vascular clamps are required

 Avoid injuring right phrenic nerve

 Occlude cranial vena cava, caudal vena cava, and azygous vein

 Procedures requiring inflow occlusion:

 Patch graft valvuloplasty (pulmonic stenosis)

 Open membranotomy and septal myectomy for aortic stenosis

 Double chambered right ventricle repair

 Cor Triatriatum

 Most valvuloplasties are now performed via balloon dilation by interventional cardiologists

 Most other open repairs require cardiopulmonary bypass, which is beyond the scope of these lectures

Pulmonic Stenosis

 Common congenital heart defect in dogs (20%)

 English bulldogs, chihuahuas, terriers have a high predilection

 English bulldogs and boxers may have an anomalous left coronary artery, which complicates balloon dilation

 Rare in cats

 Narrowing or obstruction of pulmonic valve

 Usually valvular, but may be supra- or sub-valvular

 80% of dogs with PS have dysplastic valves

Pathophysiology

 Pressure overload and concentric hypertrophy of the right ventricle

 Hypertrophy of RVOT may compound the stenosis due to a dynamic obstruction in addition to the static valvular stenosis

 Asymptomatic with mild to moderate stenosis

 Exercise intolerance, syncopal episodes, right sided heart failure, and sudden death in severe cases

Diagnosis

 High-pitched systolic ejection murmur heard best at left heart base

 Jugular distension, systemic venous hypertension, hepatomegaly, positive hepatojugular reflux, and ascites

 Thoracic radiographs: varying degrees of right ventricular hypertrophy and main pulmonary artery enlargement

 Echocardiography is usually diagnostic

 Cardiac catheterization rarely performed anymore

 Systolic pressure gradient above 80 mm Hg across defect indicates increased risk for progressive heart failure or sudden death

Treatment

 Dilation valvuloplasty (balloon or surgical)

 Contraindicated in bulldogs or boxers with anomalous coronary artery

 Pulmonary valvulotomy or valvulectomy (little benefit over dilation)

 Patch graft valvuloplasty

 Indicated for severe stenosis

 Failed balloon dilation

 Contraindicated in bulldogs or boxers diagnosed with anomalous coronary artery based on echocardiography or cardiac catheterization

 Significant mortality (10–15%), increased morbidity

Prognosis

 Treatment is palliative

 Reduced risk of developing heart failure

 Reduced risk of sudden death, but still occurs

Double-Chambered Right Ventricle

 Uncommon congenital defect

 Primarily large breed dogs

 Fibromuscular diaphragm at the junction of the inflow and outflow portions of the right ventricle

 Often described as sub-valvular pulmonic stenosis

Pathophysiology:

 Hypertophy of the inflow, but not the outflow portion of the right ventricle

 Often accompanied by progressive tricuspid regurgitation

 Risk of progressive right sided heart failure and sudden death

Diagnosis

 Similar to pulmonic stenosis

 Echocardiography reveals hypertrophy of the right ventricle with an abrupt transition to a normal appearing RVOT

Indications for Surgery

 Same as for pulmonic stenosis, although dogs appear to tolerate less of a pressure gradient (> 50 mm Hg)

Surgical Procedure

 Ventriculotomy across defect and patch graft

Subvalvular Aortic Stenosis

 25% of cardiac malformations in dogs

 Primarily large breed dogs

 Golden retrievers, German shepherd, Boxer, and Newfoundland predisposed

 Typically a sub-valvular fibrous membrane

 Varying degrees of muscular septal hypertrophy and fibrosis of LVOT

 Varying degrees of aortic insufficiency

Pathophysiology

 Pressure overload of left ventricle

 Left ventricular concentric hypertrophy

 Risk for sudden death with moderate to severe stenosis

 Median survival with moderate to severe disease is 18–40 months of age

Diagnosis

 Often asymptomatic

 Systolic ejection murmur at the left heart base

 Weak femoral pulses

 Thoracic radiographs often unrewarding

 Echocardiography for definitive Dx

 Systolic gradients > 50 mm Hg are moderate to severe

Indications for Intervention

 Indicated with gradiant > 80 mm Hg

 Questionable survival benefit

 Balloon valvuloplasty

 Transventricular valve dilation

 Open membranectomy and septal myotomy (bypass)