Many veterinarians will rather refer to large clinics and facilities cases that will need thoracic surgery. Knowledge of the basics of this kind of surgery make it possible that skilled practitioners can perform some of these basic procedures as the ones to be described without the need to refer them.
These basic thoracic surgical procedures are, in this lecture, Patent Ductus Arteriosus, Persistent Right Aortic Arch and Pericardiectomy. Animals suffering from this conditions show, in each case, certain particularities.
Etiopathogenesis
In PDA the clinical signs depend on how much blood flow is shunted from the aorta to pulmonic artery, that is, a left to right shunt through the ductus that in normal situations becomes closed in 1 to 3 days after birth. In fetal life the duct derives blood from pulmonic artery to aorta1 because at this time blood doesn't oxygenate in the lung but in the placenta.
Clinical signs of dogs brought to the clinic are those of left sided heart failure, meaning tachypnea, dyspnea and cough, in large diameter ducts or none at all in case they are small. When pulmonic hypertension has developed there could be reversal of the shunt and differential cyanosis.
Physical exam shows a machinery murmur very often being most intense in cranial left thorax.
In PRAA clinical signs arise from the compression that the esophagus withstand due to its position among the heart base in ventral, pulmonic artery to the left, aorta to the right and ligamentum arteriosum in dorsal2, different from what is normal in which ascending aorta is formed from the 4th left aortic arch and the pulmonic artery and ligamentum arteriosum are formed from the 6th left aortic arch leaving the oesophagus free staying to the right of them.
The most typical clinical sign is regurgitation which often the owner mistakes as if it was vomiting. Regurgitation appears when the puppy swifts from lactation to solid feeding.
Cranial to the compression site a progressive dilation of the oesophagus is developed, often taking the shape of a diverticulum. This dilation interferes with normal neurologic functions that if gone to far, can last even after surgery is undertaken.
Sometimes this dilation is so large that it can be felt on the base of the neck.
Many of these individuals develop aspiration pneumonia with rales being auscultated in ventral parts of the thorax.
In pericardial effusion, mainly when intrapericardial pressure rises higher that right heart filling pressures, which is called cardiac tamponade, clinical signs are in close relationship with the chronic or acute onset orf the condition.
In the chronic case signs are mostly of right heart failure and in acute onset signs refer to low cardiac output and shock.
Most pericardial effusions are hemorrhagic, be it due to tumors like hemangiosarcomas of the right auricle, heart base tumors, mesotheliomas, or idiopathic.
In a very simplistic view young animals show effusions due to congenital on infectious causes, large middle aged dogs suffer idiopathic hemorrhagic pericarditis and old dogs mostly tumoral in origin.
Anesthesia
Practitioners have to keep in mind that thoracotomies are performed in animals that may have some degree of respiratory compromise. In PDA there can be some degree of lung edema. In PRAA some respiratory disfunction due to aspiration pneumonia can be present and animals to undergo pericardiectomy may have some circulatory abnormalities and tissue oxygenation deficits.
Cardiac arrhythmias are a real threat that have to be prevented using good premedication, induction and maintenance of the anesthesia.
Premedication with butorphanol, 0.2 mg/kg, or Buprenorphine 5-10µgr/ kg IM. Induction with diazepam 0.2 mg/kg + ketamine 5 mg/kg, although propofol can be infused at a dose of 4 to 6 mg/kg. Desensitizing larynx with lidocaine spray is useful and then intubate to maintain anesthesia with halothane 1% - 2% or better use isoflurane 0.25% which is less prone to cause cardiac arrhythmias. Once the thorax is accessed assisted ventilation is mandatory with a maximum inspiratory pressure of 30 cm HO, not more than 20 cm when the thorax remains closed. Breathing frequency between 8 and 14 r.p.m.
Monitoring by means of EKG, pulsoximetry and capnography is very useful and should be pursued.
Post surgical analgesia must be done by intercostal nerve blockade with bupivacaine, also intrapleural, before closing, and opioids like buprenorphine or butorphanol.
Surgical Technique
For the three different conditions described above is necessary to perform a thoracotomy.
In the case of PDA and PRAA it is made by intercostal access. 4th left intercostal space. For pericardiectomy I prefer median sternotomy because both sides of the heart can be reached very easily.
Lateral access needs to cut through different muscular planes, that is, M. Cutaneus trunci, gran dorsalis, serratus ventralis, scalenus, intercostalis superficialis and intercostalis profundus, under which the pleura lies.
The incision has to start almost under the vertebral bodies and run all the way to the costochondral joint, avoiding the internal thoracic artery which runs closely under the costochondral joint and parallel to sternum. Once the chest cavity is opened we reflex the cranial lung lobe to caudal with the help of wet towels, counting them and making it sure that we get all of them before surgical closure.
To close the chest wall preplace single sutures, absorbable or not, every 10 to 15 mm, depending on the size of the animal and tightening them all. In case a thoracostomy tube is to be left we shall place it before closing and exiting through a different hole.
To free the esophagus from the ligamentum arteriosum the mediastinum dorsal to the heart has to be exposed. Identify the aorta, pulmonic artery, ligamentum arteriosum, vagus and phrenic nerves. Make double sutures of the ligamentum before resecting it just in case it could remain permeable. Once done a tube with inflatable cuff has to be passed back and forth so fibrous bands can be disrupted.
Keep in mind that in occasionally this pathology occurs concurrently with the presence of persistent left cranial vena cava, hemizygous vein, constrictive subclavian artery. All of them have to be transected after ligation.
The earliest in life this surgery is performed the best chances of regaining neurological function of the esophagus.
Surgical treatment of PDA consists in its surgical ligation and sometime transection. Nowadays PDA's not very large in diameter can be approached by cardiac catheterization and coil placing inside the duct but in many instances this is far of the reach of many clinicians and economical possibilities ot the owners.
In the surgical setting is necessary to identify, isolate and elevate the vagus nerve and recurrent laryngeal that runs right on top of the duct or just caudal to it. Dissect very carefully underneath the duct. This can be very dangerous because of the risk of ductal rupture mainly on the side of pulmonic artery because of its acute angle that forms with the pulmonic artery wall. I much prefer to perform the so called Jackson technique in which the suture, double, is passed from dorsal to the aorta to cranial to the ductus and the other end from dorsal to the aorta to caudal to the ductus. Once passed, pull from both ends, the suture will come underneath the ductus, cut the suture in two separate pieces tightening them one on the aortic side and one on the pulmonic side of the ductus. I don't usually transect the ductus as it is a dangerous maneuver.
Suture material can be silk 2/0, or similar.
When performing pericardiectomy median sternotomy is preferable. It is much easier to reach the thorax, much easier to close and less chance for seroma formation. Besides, it is much easier to see and get the pericardium making a wide resection of it.
For median sternotomy we'll use an oscillating saw, cutting sternebrae for as many as five of them, starting from caudal. It is safer to go as deep as 2/3 of the bone thickness and complete the cut with hand osteotome being careful of the lung and heart underneath.
Pericardium shall be resected from under phrenic nerves at each side of the pericardium as close to the cardiac base as possible.
For sternal closure strong sutures can be used in dogs weighing less than 15 kg. For over that weight use steel sutures in a figure of eight fashion.
To leave in place or not thoracostomy tube depends mainly on the possibility to survey the patient closely after surgery, because the chance that a dog could remove by himself the catheter exists.
When the surgical field has been free of infection or contaminated material and in conditions that economics is a limiting factor I don't place thoracostomy tubes. In case we do we keep the animal for at least 48 hours and then remove it in case there is not any air or liquid escape.
References
1. Mark D. Kittleson: Patent ductus arteriosus. In: Kittleson MD and Kienle RD. Cardiovascular medicine of the small animals, Multimédica,2000; 220-221
2. Hedlund Ch. S.: Surgery of the digestive system. In: Theresa W. Fossum: Small Animal Surgery, Intermédica editorial, 1999; 283
3. Darke P., Bonagura J.D., Kelly D.F.: Pericardial diseases. In: Color Atlas of Veterinary Cardiology, Mosby-Wolfe,1996; 123
4. Dodam J.R.: Ventilators, Ventilation and Small Animal Anesthetic Equipment. In: Veterinary Proceedings Small Animal and Exotics, The North American Veterinary Conference, 2000 (14); 24
5. Hedlund Ch.S.: Surgery of the digestive system. In: Theresa W. Fossum: Small Animal Surgery, Intermédica editorial, 1999; 285
6. Eyster G.E., Probst M.R.: Basic Cardiovascular Surgery and Procedures. In: Phillip R. Fox: Canine and Feline Cardiology, Churchill Livingstone, 1988; 617