Cardiac Surgery

Chapter 20 Cardiac Surgery



E. Christopher Orton



INTRODUCTION


Cardiac surgery is increasingly an option for management of congenital and acquired cardiac conditions in small animals. Some cardiac surgeries are widely available, whereas open cardiac repairs that require cardiopulmonary bypass (CPB) are currently only performed at a few regional centers. Some cardiac surgeries are performed with curative intent, whereas others are considered palliative only. Cardiac surgeries include closed cardiac surgeries, cardiac surgeries performed during inflow occlusion, and cardiac surgeries performed under CPB.



CLOSED CARDIAC SURGERY



Patent Ductus Arteriosus Ligation


With few exceptions, closure of patent ductus arteriosus (PDA) is indicated in all small animals with this defect. Closure can be accomplished by catheter-based occlusion methods or surgical ligation. Although each has theoretical advantages, both approaches are successful in the hands of an experienced operator and neither approach should be regarded as always superior or preferred. Choosing an approach depends on several factors, including client preference, availability of equipment and expertise, and urgency of the procedure. PDA closure is curative when performed early in life before the onset of severe ventricular remodeling, systolic dysfunction, or functional mitral regurgitation (MR). Surgical ligation of PDA can be accomplished with little or no operative mortality when performed by experienced surgeons.


PDA ligation is undertaken through a left fourth thoracotomy in the dog and a left fifth thoracotomy in a cat (Figure 20-1). The most frequent surgical complication is hemorrhage during ductus dissection. If significant hemorrhage occurs during dissection, the ductus should be closed with pledget-buttressed mattress sutures with or without division of the ductus.


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Figure 20-1 PDA ligation. The vagus nerve courses over the ductus arteriosus and serves as an anatomic landmark for identification of the ductus arteriosus (A). The vagus nerve is isolated at the level of the ductus and gently retracted with one or two sutures (B). Occasionally a persistent left cranial vena cava may overlie the ductus arteriosus. In this case, the vein should be carefully isolated and retracted with the vagus nerve. The ductus arteriosus is isolated by blunt dissection without opening the pericardium. Dissection of the caudal aspect of the ductus is accomplished by passing right-angled forceps behind the ductus parallel to the transverse plane. Dissection of the cranial aspect of the ductus is accomplished by angling the forceps caudally at approximately a 45-degree angle. Dissection is completed by passing the forceps medial to ductus from a caudal to cranial direction (C). Two heavy silk ligatures are passed around the ductus by grasping the ligature with right-angled forceps. The ductus arteriosus is closed by slowly tightening and tying the ligature.



Pulmonic and Aortic Valve Dilation


Pulmonic stenosis (PS) and subvalvular aortic stenosis (SAS) are relatively common congenital heart defects in dogs. Despite the relative importance of PS, the natural history of untreated PS in dogs is not well documented. Dogs with moderate PS may tolerate the defect relatively well for many years. Transpulmonic pressure gradients > 100 mm Hg are considered an indication for intervention, especially if animals are exhibiting activity intolerance, syncope, or have concurrent tricuspid regurgitation. The natural history of untreated SAS in dogs is better understood. Dogs with transaortic pressure gradients > 80 mm Hg are known to be at risk for sudden cardiac death early in life. Gradient reduction by valve dilation is assumed, but not proven, to be palliative for dogs with PS. Valve dilation for SAS does not result in sustained decreases in transaortic pressure gradients. Current evidence suggests little or no palliative benefit from valve dilation or surgical treatment of SAS.


Catheter-based balloon valvuloplasty is preferred to surgical valve dilation of PS because it as a less invasive. Surgical valve dilation of PS is indicated for animals that fail balloon-catheter placement across the PS, or when equipment for cardiac catheterization is not available. Surgical valve dilation of PS is performed through a left fourth thoracotomy (Figure 20-2).


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Figure 20-2 Pulmonary valve dilation. The pericardium over the right outflow tract is opened and sutured to the thoracotomy incision. A buttressed mattress suture is placed in the right ventricular outflow tract and passed through a tourniquet (A). A stab incision is made in the ventricle within the confines of the mattress suture. A dilating instrument is passed into the right ventricular outflow tract and across the pulmonic valve (B). The pulmonic valve is dilated several times. The ventricular incision is closed by tying the mattress suture.



Pulmonary Artery Banding


Pulmonary artery banding is a palliative surgery for ventricular septal defect (VSD) that consists of placement of a constricting band around the pulmonary artery. The intent is a measurable increase in right ventricular systolic pressure that thereby decreases the driving pressure gradient for shunt flow across the defect. The procedure provides protection against both progressive heart failure and pulmonary hypertension, and is a viable option for both cats and dogs with hemodynamically significant VSD. Diagnostic parameters that suggest hemodynamically significant VSD include radiographic evidence of pulmonary over circulation, echocardiographic evidence of left ventricular dilation (increased left ventricular diastolic diameter, left ventricular diastolic volume [index > 150 ml/m2], Doppler-measured shunt flow velocity < 3.5 m/sec, or pulmonic ejection velocity > 2.5 m/sec. Evidence of progressive pulmonary hypertension based on Doppler echocardiography or direct catheter measurement is also a reason to consider surgery. Long term palliation of VSD is possible with this procedure. Possible complications of pulmonary artery banding include acute over tightening of the band or late-term progressive constriction of the band leading to reversal of shunt flow. Worsening of concurrent tricuspid regurgitation is also a possible adverse outcome.


Pulmonary artery banding is performed through a left fourth thoracotomy (Figure 20-3). The appropriate degree of pulmonic constriction is based on pulmonary artery pressure distal to the band and systemic arterial pressures. Pulmonary artery pressure is measured intraoperatively by a catheter introduced through a small purse-string suture in the pulmonary artery. Optimal banding is where pulmonary artery pressure distal to the band is decreased to less than 30 mm Hg (assuming significant pulmonary vascular remodeling is not present), and when the increase in systemic arterial pressure just begins to plateau. As a general rule, optimal banding requires a two-thirds reduction in the diameter of the pulmonary artery although this will vary depending on the degree of pulmonary artery dilation.


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Figure 20-3 Pulmonary artery banding. The pericardium is opened and sutured to the thoracotomy incision. The pulmonary artery is separated from the aorta by sharp and blunt dissection. A large cotton or Teflon tape is passed around the pulmonary artery just distal to the pulmonic valve. The tape is tightened to reduce circumference of the pulmonary artery.



Systemic–to–Pulmonary Artery Shunt


Creation of a systemic–to–pulmonary artery shunt is a palliative surgery for tetralogy of Fallot. The functional goal of a systemic–to–pulmonary artery shunt is to increase pulmonary blood flow without creating an overwhelming left-to-right shunt. The desired result is a measured increase in pulmonary blood flow that lessens hypoxemia by lessening the shunt-to-pulmonary flow ratio. Systemic–to–pulmonary shunt is indicated for animals that have resting cyanosis, debilitating activity intolerance or persistent polycythemia (polycythemia vera > 70%) that requires frequent phlebotomy. Most veterinary experience is based on various modifications of the classic Blalock-Taussig shunt. The original Blalock-Taussig shunt consisted of dividing the left subclavian artery and performing an end-to-side anastomosis of the distal end of the divided artery to the pulmonary artery. In animals, the left subclavian artery generally does not have sufficient length to reach the pulmonary artery without kinking. Several modifications of the classic procedure have been devised including a synthetic vascular graft matched in size to the subclavian artery, harvesting the left subclavian artery as a free autogenous graft, or using autogenous jugular vein. Animals can receive significant palliation from any of the previous methods so long as pulmonary blood flow is increased to an appropriate degree.


A modified Blalock-Taussig shunt is performed through a left fourth thoracotomy (Figure 20-4). A continuous thrill should be palpable on the pulmonary artery and hypoxemia should be lessened immediately after surgery.


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Figure 20-4 Systemic–to–pulmonary artery shunt. The pericardium is opened and sutured to the thoracotomy incision. Tangential vascular clamps are placed on the pulmonary artery and ascending aorta, and incisions are made in each vessel (A). The autogenous or synthetic graft is interposed between the aorta and pulmonary artery by two end-to-side anastomosis using simple continuous suture patterns with polypropylene or polytetrafluoroethylene suture. The vascular clamps on the pulmonary artery and aorta are released (B).



Pericardiectomy


Pericardial disease can result from neoplasia, bacterial or mycotic infection, foreign body, or idiopathic causes. Pericardial disease can take the form of acute or chronic pericardial effusion, constrictive pericarditis, or constrictive-effusive pericarditis. These conditions can result in pathophysiologic syndromes of acute cardiac tamponade, chronic cardiac tamponade, or pericardial constriction. Pericardiectomy is indicated for the management of chronic pericardial effusions, particularly when the effusion recurs after pericardiocentesis. Pericardiectomy is either palliative or curative depending on the underlying cause of pericardial effusion. Pericardiectomy is the only viable treatment for animals with constrictive or constrictive-effusive pericarditis.


Pericardiectomy can be performed via either a right or left thoracotomy, or a median sternotomy. Median sternotomy has the advantages of providing access to both ventricles and requiring less cardiac manipulation, and thus is preferred by many surgeons (Figure 20-5). Excision of the pericardium ventral to the phrenic nerves (i.e., subphrenic pericardiectomy) is adequate in most cases. In animals with constrictive pericarditis, the pericardium may have to be separated from the epicardium by blunt and sharp dissection. Additionally, epicardial decortication may be necessary to relieve constrictive physiology in animals with pericarditis. Epicardial decortication entails careful separation of a fibrous layer from the myocardium by sharp dissection. Decortication should not be attempted over the atria or portions of the ventricles containing major coronary vessels.


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Figure 20-5 Pericardiectomy. Pericardiectomy may be performed via median sternotomy to provide access to both ventricles without extensive cardiac manipulation. In animals with constrictive pericarditis, adhesions may be present between the pericardium and epicardium.

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Aug 15, 2016 | Posted by in SMALL ANIMAL | Comments Off on Cardiac Surgery

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