Thoracoscopic partial pericardectomy in the dog

Pericardial effusion presents clinicians with a challenge when diagnosing the underlying cause, since the prognosis can be favorable in certain cases. Partial pericardectomy can be performed via thoracoscopy; and in select cases, this minimally invasive procedure can provide long-term relief of clinical signs.

There are many potential causes of pericardial effusion, but the most common is intrapericardial neoplasia, particularly right atrial hemangiosarcoma and aortic body tumor. Other less-common neoplastic causes of pericardial effusion include ectopic thyroid neoplasia, lymphosarcoma, mesothelioma, rhabdomyosarcoma and metastatic neoplasia. Idiopathic pericardial effusion (IPE) is diagnosed when no other cause of effusion is found. Rare causes of pericardial effusion include congestive heart failure, left atrial tear or cardiac trauma, coagulopathy, infection (although coccidiodiomycosis is more common in some geographic locations), foreign body and chronic uremia.

As pericardial fluid accumulates, it will compress the heart, known as tamponade, resulting in poor ventricular filling. The right side of the heart has lower pressures than the left side and is primarily affected by tamponade. In chronic cases, this leads to signs of right heart failure. When pericardial fluid forms acutely, the pericardium is minimally distensible and tamponade can be severe even with relatively small volumes of fluid, resulting in reduced stroke volume and systemic hypotension.

Presenting signs

Middle-aged to older large-breed dogs, including retrievers and German Shepherds are most commonly presented. Brachiocephalics may be over-represented with chemodectomas. The history can include acute collapse or a longer period of clinical signs such as lethargy, weakness, inappetance, exercise intolerance, tachypnea and abdominal distension. Physical examination often reveals pallor, slow capillary refill time, weak arterial pulses, weakness, tachypnea, tachycardia and muffled heart sounds. If the pericardial effusion has formed acutely, the patient can be profoundly weak or recumbent with circulatory collapse. In chronic pericardial effusion, there often are signs of right heart failure, including jugular vein distension, ascites and possibly tachypnea due to pleural effusion. Pulmonary edema is not expected.

Diagnostics

Thoracic radiographs reveal mild to severe enlargement of the cardiac silhouette, which is usually globoid with loss of the typical contours. In some cases, there may be enlargement of the caudal vena cava and/or pleural effusion. Metastatic pulmonary lesions may be detected by thoracic radiographs or CT scan. An ECG often shows sinus tachycardia. Arrhythmias are possible.

There will be low-voltage QRS complexes (<1mV) in about 50 percent of cases. Electrical alternans (EA) describe a regular variation in QRS-T wave height or morphology resulting from the heart swinging back and forth within the pericardial sac. Electrical alternans may be present in less then 20 percent of cases, but the presence of EA and the above clinical signs are strongly suggestive of pericardial effusion. Echocardiography is very sensitive for diagnosing pericardial effusion (Photo 1), and is useful for detection of intrapericardial masses, and is always recommended.

Photo 1: This is an echocardiogram image showing the long axis view from the right parasternum. Pericardial effusion is surrounding the heart. PE = pericardial effusion, RV = right ventricle, RA = right atrium, LA = left atrium, LV = left ventricle

Most masses are easier to detect with the presence of the pericardial effusion. It is not always possible to differentiate idiopathic pericardial effusion from neoplastic causes since there is often no discrete mass with mesothelioma or lymphoma. Occasionally, hemangiosarcoma lesions too small for echocardiographic detection can be the source of the hemorrhagic effusion.

Analysis and cytology of the pericardial fluid can be performed but is unlikely to provide a definitive diagnosis except when an infective cause is present. Intrapericardial tumors often exfoliate poorly, except for lymphosarcoma, which is very rare. Neoplastic mesothelial cells are hard to differentiate from reactive cells. Biochemical parameters, pH and PCV have been studied but are not reliably able to differentiate neoplastic from IPE in an individual case. The pericardial fluid is usually hemorrhagic, the PCV can be high in IPE and hemangiosarcoma, chemodectomas typically have lower PCV. There is recent data that a serum biomarker, cardiac troponin-I, may differentiate cardiac hemangiosarcoma from IPE in dogs. Other types of neoplasia have not been evaluated with this biomarker.

Treatment

Pericardiocentesis should be performed. Dogs with cardiac tamponade may need intravenous fluid support, diuretics are contraindicated. If there was no mass noted during the work-up, then the case is monitored to see whether pericardial effusion recurs. Approximately 50 percent of IPE cases may resolve after one or two pericardial taps. If the effusion forms again, repeat taps can be performed, but after three episodes surgical options are considered. If IPE is confirmed, then partial pericardectomy is potentially curative. If an intrapericardial mass was noted on the right atrium or right atrial appendage, it is most likely a hemangiosarcoma. A proximal aortic mass (heart-base mass) without right atrial involvement is most likely a chemodectoma. Partial pericardectomy can be considered in chemodectoma cases; a medial survival of 730 days post-partial pericardecomy has been reported. Removal of a chemodectoma is usually not possible since they are generally locally extensive at the time of diagnosis. Hemangiosarcoma has a worse prognosis even if thoracotomy is performed with surgical debulking of the mass and follow-up chemotherapy.

The dog has a brief repeat echocardiogram on the day of surgery prior to the induction of anesthesia. If significant pericardial fluid is present, some is removed by pericardiocentesis to reduce cardiac compromise under anesthesia. A small amount of pericardial effusion is not a problem and actually aids picking up the pericardium intraoperatively. Anesthesia is induced using a cardiovascular sparing protocol, and the patient is closely monitored throughout, including measurement of blood pressure, electrocardiogram, pulse oximetry and end tidal CO2.

Photo 2: The pericardium has been grasped and lifted ready for the first cut.

Partial pericardectomy can be performed via an open intercostal or median sternotomy approach, or via thoracoscopy. Thoracoscopic partial pericardectomy has advantages over an open thoracotomy approach since it results in less postoperative pain, fewer wound complications and a quicker return to function. Hospital stays are shorter, and client costs are usually lower. Disadvantages of thoracoscopy include the need for special equipment. In addition, it is often not possible to biopsy a mass via the scope without risk of hemorrhage, and visualization dorsal to the heart is limited. Thoracoscopy involves the use of a monitor, camera, light source, cautery, trochars and thoracoscopic instruments. Intermittent positive pressure ventilation is required, and a primary and assistant surgeon are needed to perform the procedure.

The patient is placed in dorsal recumbency and rolled slightly to the right side. A wide area is clipped and prepared for surgery. The owner is always made aware that, should the need arise, conversion to a median sternotomy approach will be made. Usually three ports suffice for the procedure, but additional ports can easily be placed if needed. Selective intubation of the right main bronchus can be performed but is not necessary. A trochar is placed in the paraxiphoid transdiaphragmatic position, directed into the left hemithorax. The camera is placed into this port and the left hemithorax is visualized.

Additional ports are placed under direct observation at about the seventh and third left intercostal spaces. Endoscopic graspers are used to pick up pericardium. When a good grasp of the pericardium has been achieved, the pericardium is lifted away from the heart and endoscopic scissors connected to the cautery unit are used to make the first cut in the pericardium (Photo 2). Effusion will spill out at this time. Suction can be used to clear the field if needed. The graspers and cutters are used via the two instrument ports, switching position of the instruments as needed to remove a portion at least 3-cm square from the ventral pericardium. Every cut is visualized to ensure that the lungs and phrenic nerve are not in danger of trauma. The camera can be moved to one of the intercostals ports and the paraxiphoid port used for an instrument. This is occasionally helpful to complete the procedure. Bleeding is usually adequately controlled with the use of cautery through the scissors as each cut is made (Photo 3). Mediastinal vessels can be a source of hemorrhage. Endoscopic clips can be used if needed to aid in control of hemorrhage. Conversion to an open procedure is rarely needed for bleeding that cannot be controlled. The piece of pericardium is removed via the port with the grasper. The port can be withdrawn and the incision enlarged slightly if needed for retrieval. The pericardium is submitted for histopathology, particularly in the absence of a heart base or right atrial mass. A red rubber catheter is placed via the paraxiphoid port and kept in place when the ports are removed to allow evacuation of any fluid and air. The port incisions are closed routinely. The chest is evacuated again at the end of the procedure, then the red rubber catheter is removed.

Photo 3: Each cut in the pericardium is visualized. The scissors are attached to an electrocautery. The epicardial surface is visible. The pericardium can be lifted away from the heart.

Postoperative management includes routine monitoring and pain management. Most cases are able to go home the next day. Some pleural fluid is expected after surgery since the underlying source of effusion has not been removed, but the amount of fluid in the pleural space is unlikely to cause clinical signs.

Prognosis

The prognosis depends upon the etiology of the pericardial effusion. The prognosis for hemangiosarcoma is poor. Median survival time with surgical removal of the hemangiosarcoma and chemotherapy has been reported as 175 days, versus 42 days with surgery alone. Aortic body tumors have a relatively good prognosis following pericardectomy. Growth is often slow with only a 10-percent to 20-percent metastatic rate. Evidence of metastatic disease does not significantly alter survival time. Aortic body tumors are reported to have a median survival of 730 days post-pericardectomy versus 42 days without.

The prognosis for IPE is good to excellent. The prognosis for mesothelioma is generally poor. It can be difficult to differentiate IPE from mesothelioma even with histopathology because there can be exuberant mesothelial proliferation present in cases of IPE. Mesothelioma can be seen in small-breed dogs, whereas most IPE is seen in large breeds. Survival times of more than 120 days post-pericardectomy is associated with a higher probability of IPE than mesothelioma.

Constrictive pericardial disease is a rare condition in which pericardial fibrosis results in impaired diastolic function. The clinical signs are of systemic congestion and low cardiac output, and the treatment involves thoracotomy for pericardectomy and potentially epicardial stripping.

Carl Sammarco, BVSc, MRCVS, Dipl. ACVIM (cardiology) joined Red Bank Veterinary Hospital, Tinton Falls, N.J. in 2001. He previously served as a lecturer/assistant clinical professor at the University of Pennsylvania, where he completed a residency in cardiology in 1994 .

Jill Sammarco, BVSc, MRCVS, Diplomate ACVS joined Red Bank Veterinary Hospital in 2003. She completed a residency in surgery at the University of Pennsylvania in 1995 and is a veterinary graduate of the University of Liverpool in England.