A Chronic Cardiac Ischemia Model in Swine Using an Ameroid Constrictor

The purpose of this protocol is to demonstrate the placement of a delayed constricting device (an ameroid constrictor) around a coronary artery in a swine model. This device creates an ischemic area of the heart that is useful for studying new diagnostic imaging techniques and new methods of treatment.

The overall goal of this surgical procedure for placing a delayed constrictor device around a coronary artery in a pig is to develop a cardiac ischemia model that is useful for developing and evaluating new methods of treatment for heart disease. This method for inducing cardiac ischemia in swine is valuable in the field of cardiovascular research, as it allows researchers to study the efficacy of new diagnostic imaging techniques and new methods for treatment of cardiovascular disease. Main advantage of the delayed constrictor is that it is made in various sizes to accommodate the anatomy and the size of the vessel to be constricted.

Additionally, the surgery is a relatively simple one and there is no post-operative maintenance of the device. Demonstrating the procedure will be Kenneth Jeffries and Arthur Zetts. Both are staff members from the Animal Surgery and Resources Core.

After anesthetizing, catheterizing and shaving the pig, move it into the operating room. There, position it right side down on the operating table and connect the endotracheal tube to the anesthesia machine. Mechanically ventilate the pig, starting at a tidal volume of 10 milliliters per kilogram.

Then, adjust the ventilator to an airway pressure of about 15 centimeters of water, but never exceed 30 or the lungs will be damaged. Set the respiratory rate between 10 and 20 breaths per minute and adjust as needed to keep the end tidal carbon dioxide levels at 28 to 35 millimeters of mercury. Next, extend the left front leg forward and secure it to the operating table.

Then, attach all the probes needed to monitor body temperature, electrocardiogram, pulse oximetry and so forth. Now, prepare the surgical site using three alternating scrubs of 2%chlorhexidine and 70%alcohol. Use a circular motion, beginning with center and move outwards.

After scrubbing, drape the surgical site and proceed with the surgery. Begin the surgery by exposing the heart through a left thoracotomy. Using a number 10 scalpel, make an eight to nine centimeter incision, parallel to the fourth and fifth intercostal spaces.

Next, used curved Metzenbaum scissors and Brown-Adson forceps to cut through the latissimus dorsi and serratus ventralis. As needed, use electrocautery to maintain hemostasis. Now, enter the pleural cavity through the intercostal muscles between the fourth and fifth ribs by using Metzenbaum scissors to cut along the anterior aspect of the fourth rib.

Just before entering the cavity, temporarily turn off the ventilator. Then, open the cavity with a reduced chance of accidentally damaging the lungs. Once the cavity is opened, turn the ventilator back on.

Next, use a small Finochietto Retractor to spread the ribs and expose the heart. Then, use Debakey forceps to grasp and lift the pericardium. Now, use Potts scissors to make a small hole in the pericardium, allowing air to enter the pericardial space.

Continue the incision with the Potts scissors over the junction of the LAD and LCX arteries. Next, use Babcock forceps to retract the left atrial appendage. Then, with Debakey forceps and small right-angle forceps, dissect the LCX artery from the surrounding tissue, prior to or proximal to, the first obtuse marginal branch.

Next, place two vessel loops under the dissected LCX artery, one at each end, and prepare to position an Ameroid constrictor. Select a constrictor that will closely encircle the vessel without initially constricting the vessel. Hold the Ameroid constrictor with small right-angle forceps and lift the vessel loops to gently guide the LCX artery through the opening of the constrictor.

Then, gently rotate the constrictor so the opening is facing up and remove the vessel loops. Now, reapproximate the pericardium and close it with 4-0 polypropylene sutures. Then, place three to four polyester sutures, size five, around the ribs, which will be tied later to reapproximate the ribs.

Then, to reestablish negative thoracic pressure, insert and 12 French chest tube into the pleural space between the layers of closure, generally two intercostal spaces caudal to the thoracotomy site. Before proceeding, observe the lung for atelectasis. If any signs of under-inflation are observed, reinflate the lungs manually, up to a pressure of 30 millimeters of mercury.

Now, close the ribs using the pre-placed sutures. Then, close the chest using size one polypropylene sutures for the intercostal layer and use size zero polypropylene sutures for the serratus and fascial layer. Then, use 2-0 polypropylene sutures for the subcutaneous layer, followed by 3-0 polypropylene sutures for the subcuticular layer.

Finally, staple or suture the skin closed. Now, attach a three-way stopcock to the end of the chest tube and evacuate the chest air using a 40 to 60cc syringe, until a negative pressure is achieved. Then, roll the animal onto its sternum, or opposite side, to remove all the air from the chest.

Once the chest maintains a negative pressure, place a size 3-0 polypropylene purse string suture around the chest tube site. While applying tension to the purse string, maintain negative pressure on the chest tube while withdrawing it from the thoracic cavity. Then, tighten and secure the purse string.

Finally, administer bupivacaine around the incision site for long acting local anesthesia. Ameroid placement surgeries were performed over a two years period on 25 Yorkshire pigs, weighing between 12 and 15 kilograms. Twenty survived to the follow-up procedure, two fibrillated and died shortly after closing, two were euthanized for severe heart failure and pulmonary edema, and one had an anesthetic death during a follow-up radiograph.

Survivors underwent a second thoracotomy and were followed out for up to 16 weeks. At 28 days post-ameroid placement, animals were imaged by MRI to measure their heart function and the size of the ischemic area. The ameroid was either titanium encased or plastic encased.

Once mastered, this technique can be done in 60 to 90 minutes if it is performed properly. While attempting this procedure, it's important to remember to perform careful yet adequate dissection around the artery. Too little dissection may cause kinking of the artery, whereas overly aggressive tissue handling may cause excess scarring around it.

After its development, this technique provided a way for investigators in the field of cardiovascular research to explore new methods for treating heart disease.