Surgical Swine Model of Chronic Cardiac Ischemia Treated by Off-Pump Coronary Artery Bypass Graft Surgery

Summary

This protocol presents a surgical large animal model of chronic, single vessel ischemia that results in regional abnormalities but does not create infarct, known as hibernating myocardium. Following establishment of chronic ischemia, animals are treated with off-pump LIMA-LAD coronary artery bypass graft surgery to revascularize the ischemic tissue.

Abstract

Chronic cardiac ischemia that impairs cardiac function, but does not result in infarct, is termed hibernating myocardium (HM). A large clinical subset of coronary artery disease (CAD) patients have HM, which in addition to causing impaired function, puts them at higher risk for arrhythmia and future cardiac events. The standard treatment for this condition is revascularization, but this has been shown to be an imperfect therapy. The majority of pre-clinical cardiac research focuses on infarct models of cardiac ischemia, leaving this subset of chronic ischemia patients largely underserved. To address this gap in research, we have developed a well-characterized and highly reproducible model of hibernating myocardium in swine, as swine are ideal translational models for human heart disease. In addition to creating this unique disease model, we have optimized a clinically relevant treatment model of coronary artery bypass surgery in swine. This allows us to accurately study the effects of bypass surgery on heart disease, as well as investigate additional or alternate therapies. This model surgically induces single vessel stenosis by implanting a constrictor on the left anterior descending (LAD) artery in a young pig. As the pig grows, the constrictor creates a gradual stenosis, resulting in chronic ischemia with impaired regional function, but preserving tissue viability. Following the establishment of the hibernating myocardium phenotype, we perform off-pump coronary artery bypass graft surgery to revascularize the ischemic region, mimicking the gold-standard treatment for patients in the clinic.

Introduction

Coronary heart disease (CHD) affects an estimated 15.5 million people in the United States ¹ and is one of the leading causes of death globally. While the mortality rate associated with CHD has gone down in recent years, the incidence and burden on patients and the health care system remain high ¹. Primary treatment of severe CAD is revascularization, which improves survival and reduces angina^2,3,4. However, cardiac function often remains depressed, especially under increased workload and can progress to heart failure^5,6. Clinical trials of coronary artery bypass surgery (CABG) for chronic ischemia demonstrate improvement in survival and symptoms, but ejection fraction shows only modest improvements of 8-10% ^7,8. Our innovative and well-characterized swine model of chronic myocardial ischemia is a model of clinical CAD with progressive vascular stenosis. We have demonstrated reduced myocardial contractility resulting from gradual reduction in blood flow ⁹. The myocardium does not infarct and can remain viable in this scenario. Recovery is possible, though outcomes are variable even with timely revascularization. Chronically ischemic myocardium that remains viable has been characterized by reduced blood flow and function at rest with retained contractile reserve has been termed HM and treatment requires CABG.Although the revascularization of HM should restore contractile function, experimental and clinical observations demonstrate that the recovery is incomplete^8,10.

HM is characterized by the presence of viable yet dysfunctional myocardium in the presence of reduced regional blood flow¹¹. Despite impaired contractility and metabolic activity at rest, HM is able to demonstrate functional and metabolic reserve under inotropic stimulation¹². HM is suspected in a majority of patients with CAD, and encompasses a broad spectrum of disease. In this protocol, we demonstrate our established swine model of HM bypassed with left internal mammary artery (LIMA) to LAD artery that mimics the clinical scenario. The swine provide an excellent model of heart disease over other large animals as they do not have epicardial bridging collaterals. This allows the stenosis of the LAD alone to result in regional ischemia¹³.

Here, we describe the surgical method of inducing hibernating myocardium in swine by creating single-vessel stenosis at the LAD artery. Once chronic ischemia has been established (8 weeks post implant of LAD constrictor), we describe the method of recreating the clinical treatment for HM in our swine model: an off-pump coronary artery bypass graft. These surgical methods can be used to not only study a clinically relevant model of chronic cardiac ischemia, but also to investigate the effects of bypass surgery on cardiac ischemia as well as test potential alternate or adjunctive therapies for cardiac ischemia.

Protocol

All animal studies were approved by the Institutional Animal Care and Use Committees of the Minneapolis VA Medical Center and the University of Minnesota. Follow current National Institutes of Health guidelines for the use and care of laboratory animals.

1. Hibernation Surgery

1. Animal Preparation
   1. To create a model of hibernating myocardium, use 5-week old, female, Yorkshire pigs, which weigh in the range of 8-10 kg with 9.2 kg as the average weight.
   2. For pre-surgical health monitoring, weigh the animal within 3 days prior to surgery.
   3. Fast the animal for 12 h before starting anesthesia. Ensure that water is available at all times for fasting animals.
   4. Give SR Buprenorphine 0.18 mg/kg IM 24 h prior to the surgery.
2. Induction
   1. Sedate the animal with 6.6 mg/kg tiletamine-zolazepam and xylazine cocktail given intramuscularly. Allow 5-15 min for the drug cocktail to take effect.
   2. Prepare the ear vein aseptically by wiping three times with 70% isopropyl alcohol for IV catheter placement.
      NOTE: If an ear vein is not adequate, another peripheral vein may be used (i.e. cephalic).
   3. Apply ophthalmic ointment to both eyes to prevent corneal drying.
   4. If necessary, induce anesthesia by administering 1-2 mg/kg of propofol IV or masked with 3 L/min of oxygen and 3% isoflurane, to allow for tracheal intubation.
      ​NOTE: The animal is considered appropriately anesthetized when the blink reflex or jaw tone is absent.
   5. Intubate the animal with an appropriately sized endotracheal tube.
3. Surgery
   1. Once the animal is anesthetized, shave the left thoracic area for the surgery.
   2. Initiate the mechanical ventilation at 10-15 breaths per min. Set oxygen at 1-4 L/min, and isoflurane between 1.0 and 3.0% as needed to maintain a surgical plane of anesthesia.
   3. Place the monitoring devices on the animal. Monitoring devices should include ECG, blood pressure, temperature, and pulse oximeter.
   4. Connect the IV catheter to an IV drip with 0.9% normal saline at a rate of 5-10 mL/kg/h.
   5. Aseptically prepare the left lateral chest with an FDA approved surgical scrub.
   6. Evaluate the depth of anesthesia every 5-15 min. Record the baseline vital signs (heart rate, ECG, SpO₂, NIBP, ETCO₂ and temperature) every 15 min.
      NOTE: The animal is considered appropriately anesthetized when the blink reflex or jaw tone is absent.
   7. With the animal laying on its right side, place sterile drapes the left side of the chest from the spine to the scapula centering the third rib space in the sterile field.
      NOTE: The sterile field involves left front shoulder including the axillary area continuing to the sternum to the sixth rib space cranially.
   8. Gently scrub the field with an FDA approved surgical method such as chlorhexidine gluconate or an iodine surgical scrub for 2 min using a target pattern.
   9. Rinse the field with water and wash it two more times with an FDA approved method as above for 2 min. Rinse the surgical field with water in between washes, then pat dry with sterile toweling or 4 x 4 cm gauze sponges.
   10. Spray the area with an iodine solution and drape the entire animal (except the surgical field) with sterile drapes. Use steam or gas sterilization to sterilize the instruments, gowns, and drapes for the procedure.
   11. Administer an intercostal nerve block using 0.3 mL of 0.5% bupivicaine at the 2nd, 3rd and 4th ribs.
   12. Give 125 mg of cefazolin IV before opening the chest.
       NOTE: 1.5 mg/kg succinylcholine IV may be given as a paralytic agent to reduce muscle movement per surgeon's request during the opening of the chest cavity.
   13. Expose the heart via a left thoracotomy. Prior to heart manipulation, give a 2 mg/kg IV bolus of lidocaine to prevent arrhythmia. Make the lateral 3rd space thoracotomy incision with monopolar electrocautery. The incision should extend from the cranial aspect of the scapula to the axillary area for approximately 10 cm. Use a cautery to incise the skin and underlying musculature to allow the entry into the chest cavity.
   14. Place a finochietto rib retractor and enter the pleural space at the third intercostal space to retract the lung.
   15. Sharply incise the pericardium and retract to the side using 2-0 polyglactin stay sutures, exposing the LAD artery and vein. Retract the left atrial appendage with a gauze-covered spatula retractor.
   16. Using iris scissors, open the adventitia over the LAD.
   17. Bluntly dissect an approximately 0.25 cm portion of the LAD artery approximately 1 cm distal to the LAD and circumflex artery junction. Use a right-angle clamp to encircle the LAD.
   18. Place two 4-0 polyester sutures around the artery without placing any tension.
   19. Place a plastic c-shaped constrictor with an internal diameter of 1.5 mm on the LAD proximal to the first diagonal without occluding the vessel.
   20. Secure it with two 4-0 polyester ties encircling the LAD.
   21. Loosely close the pericardium using 2-0 polyglactin suture in a simple interrupted pattern.
   22. Perform a breath hold to remove air from the chest while the intercostal muscle layer is closed. Carefully monitor the airway pressure manometer on the anesthesia machine to maintain the pressure in a proper range of 20-30 cm of water and release when complete.
       NOTE: This eliminates the need for a chest tube in the post-operative period.
   23. Close the muscle and skin layers in the standard fashion using 2-0 and 3-0 absorbable suture.
   24. Use skin adhesive over the incision for skin closure.
   25. Administer a total dose of 1 mg/kg bupivacaine intramuscularly at multiple sites along the length of the incision at the end of the procedure. Administer meloxicam 0.2 mg/kg SQ before transport to the recovery area.
   26. Wean the animal off the ventilator as the skin layer is closed.
   27. Leave the animal connected to the anesthesia machine until it can spontaneously breathe and protective reflexes begin to return.
   28. Do not remove the endotracheal tube until the animal can swallow to protect its airway.
   29. Apply a sterile, non-adherent dressing with triple-antibiotic ointment over the incision.
4. Post-Surgery
   1. Record the heart rate, respiration rate, body temperature and mucous membrane color every 15 min until the animal can hold a sternal position unassisted.
   2. Do not leave the animal unattended until it can lift and hold its head up and can stand unassisted.
   3. Give meloxicam (0.2 mg/kg) subcutaneously.
   4. Administer sustained release buprenorphine 6 h following the initial pre-surgical dose.
   5. Leave the dressing on the incision for up to 3 days, if it remains clean and dry. Replace the dressing if it becomes soiled.
   6. Monitor the pain level, general well-being and the condition of the incision for 5 days following the procedure.
      NOTE: A piglet grimace scale is now available to assess the level of pain in these animals post-operatively¹⁴. Use half dose meloxicam as needed once per day for breakthrough pain.
   7. Single-house the animal during the recovery period of 5 day to limit activity and allow the wound to heal.
   8. Allow 8 weeks to fully develop the hibernating myocardium phenotype.

2. Revascularization or Off Pump Bypass

1. Animal Preparation
   1. Weigh the animal within 3 days prior to surgery.
   2. Fast the animal for 12 h prior to surgery. Ensure that water is available at all times for fasting animals.
   3. Give sustained release buprenorphine 0.18 mg/kg intramuscularly 24 h prior to the surgery.
2. Induction
   1. Sedate the animal with 6.6 mg/kg tiletamine-zolazepam/xylazine cocktail intramuscularly. 5-15 min after the sedation, prepare the ear vein aseptically for catheter placement.
      ​NOTE: If an ear vein is not adequate, another peripheral vein may be used (i.e. cephalic).
   2. Apply ophthalmic ointment to both eyes.
   3. Induce anesthesia with 1-2 mg/kg of propofol IV.
   4. Intubate the animal with an appropriately sized endotracheal tube.
3. Surgery
   1. Shave the animal's sternum, left lateral chest and femoral triangles for the surgery.
   2. Set mechanical ventilation to 10-15 breaths per minute, oxygen at 1-4 L/min, and isoflurane at 1.0-3.0% as needed to maintain a surgical plane of anesthesia.
   3. Position the monitoring equipments (heart rate, ECG, SpO₂, direct blood pressure, ETCO₂ and temperature) on the animal.
   4. Connect the IV catheter to an IV drip with normal saline or LRS.
   5. Aseptically prepare the skin with povidone iodine scrub or other FDA approved method.
   6. Evaluate the animal for the depth of anesthesia.
      NOTE: The animal is considered deeply anesthetized when the eye or jaw reflex is absent.
   7. Position the animal dorsally, prepare the skin as previously described and drape the animal with sterile towels.
   8. Perform a cut down to the femoral artery and cannulation. Use monopolar electrocautery to create an approximately 3 cm skin incision over the femoral artery to expose it for cannulation.
   9. Perform a femoral nerve block after the incision is made, to allow precise placement of drug. Connect the femoral cannula to a transducer to monitor blood pressures during the surgery.
   10. Use electro-cautery to incise from the manubrium to the xiphoid process, as well as to incise the muscle, adipose, and connective tissue to the level of the sternum.
   11. Divide the sternum with an oscillating saw.
       NOTE: Using an oscillating saw, rather than a standard sternal saw avoids potential myocardial injury from pericardial adhesions as a result of the thoracotomy for constrictor placement on the LAD.
   12. Administer lidocaine (2 mg/kg IV loading dose, 50 µg/kg/min continuous IV infusion) to prevent arrhythmias. Dilute lidocaine in a 500 mL bag of normal saline at a concentration of 1 mg/mL.
   13. Divide the posterior sternal plate with straight Mayo scissors.
   14. Use a chest retractor to expose the thoracic cavity.
   15. Take down the adhesions with monopolar electro-cautery.
   16. Using the same retractor, with gentle elevation of the left sternal border by an assistant, dissect the left internal mammary artery free from the chest wall.
   17. Control the arterial branches with hemoclips.
   18. Heparinize the animal with 100-300 units/kg of heparin IV.
   19. After 3 min, divide the artery with the distal end over sewn. Prepare the proximal end for grafting.
   20. Open and retract the pericardium. Confirm that the length of the LIMA is adequate to reach the appropriate anastomotic site on the LAD.
   21. Perform the LIMA to LAD anastomosis with 7-0 polypropylene suture using an off pump technique with a cardiac stabilizer, spring form clip, and appropriately sized intraluminal devices.
   22. Place a chest tube between the ribs on the left lateral chest, externalized and attached to vacuum to remove air from the chest cavity for 15-30 min.
   23. Approximate the sternum with #5 polyester sutures using a figure eight pattern.
   24. Close the muscle, subcutaneous and skin layers in the usual fashion as described in the Hibernation surgery. Remove the chest tube while performing a Valsalva maneuver to remove any residual air from the chest cavity and close the incision with one skin suture.
   25. Apply skin adhesive over the skin to protect the sternal incision. Gently remove the femoral catheter and repair the artery using 7-0 polypropylene suture in a purse string pattern. Close the skin using 2-0 polyglactin suture in a subcuticular pattern.
   26. Administer a total dose of 1 mg/kg bupivacaine intramuscularly at multiple sites along the length of the incision at the end of the procedure.
   27. Administer meloxicam 0.2 mg/kg subcutaneously before transport to the recovery area.
   28. Follow the post-operative protocol in the hibernation procedure.

Results

Following the initial hibernation surgery, stenosis greater than 70% should be able to be observed by clinical imaging techniques such as angiography or cardiac MRI (Figure 1A). 8 weeks following the surgical application of the LAD constrictor, analysis of regional function by ECHO or cardiac MRI reveals reduced function under increased workload (Figure 2). This can be tested by stimulating the heart with dobutam...

Discussion

Here, we show that our swine model of HM accurately mimics the clinical experience of patients with single vessel disease and preserved left ventricular function. Prior to the revascularization, animals with single vessel HM exhibit minimal impairment in global function as measured by ejection fraction, but show significant reduction in regional wall thickening. Following the revascularization, CMR imaging at one or three months of recovery demonstrates preserved viability and graft patency but persistent regional dysfun...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Bair Hugger	3M	Model 505	Patient Warming system
SR Buprenorphine 10 mg/mL	Abbott Labs	NADA 141-434	Post operative Analgesic
Surgical Spring Clip	Applied Medical	A1801	Clamp end of LIMA after takedown
Arterial Line Kit	Arrow	ASK-04510-HF	Femoral catheter for blood pressure monitoring
1000mL 0.9% Sodium chloride	Baxter	2B1324X	IV replacement fluid
250 mL 0.9% saline	Baxter	UE1322D	Replacement IV Fluid
500mL 0.9% Sodium chloride	Baxter	2B1323Q	Drug delivery, Provide mist for Blower Mister
Flo-thru 1.0	Baxter	FT-12100	used to anastomos LIMA to L
Flo-thru 1.25	Baxter	FT-12125
Flo-thru 1.5	Baxter	FT-12150
Flo-thru 2.0	Baxter	FT-12200
Cloroprep	Becton Dickenson	260815	Surgical skin prep
Meloxicam	Boehringer Ingelheim Vetmedica, Inc.	0010-6013-01	NSAID for analgesia
Hypafix	BSN Medical	4210	Secure wound dressing and IV catheters
IV Tubing for Blower Mister	Carefusion	42493E	Adapts to IV Fluids for Blower/Mister
Bovie Cautery hand piece	Covidien	E2516	Hemostasis
Chest Tube	Covidien	8888561043	Evacuates air from chest cavity
Monopolar Cautery	Covidien	Valleylab FT10	Hemostasis
Telpha pad	Covidien	2132	Sterile wound dressing
4-0 Tevdek II Strands	Deknatel	7-922	Suture to secure constrictor around LAD
Propofol	Diprivan	269-29	Induction agent
long blade for laryngoscope	DRE	12521	Allows for visualization of trachea for intubation
ECG Pads	DRE	1496	Monitor heart rhythm
laryngoscope	DRE	12515
Anesthesia Machine + ventilator	DRE Drager- Fabius Tiro	DRE0603FT	Deliver Oxygen and inhalant to patient
5 Ethibond	Ethicon	MG46G	Suture
0 Vicryl	Ethicon	J208H	Suture
2-0 Vicryl	Ethicon	J317H	Suture
3-0 Vicryl	Ethicon	VCP824G	Suture
7-0 Prolene	Ethicon	M8702	Suture
Dermabond	Ethicon	DNX12	Skin adhesive
Ligaclips	Ethicon	MSC20	Surgical Staples for LIMA takedown
Sterile Saline 20 mL	Fisher Scientific	20T700220	Flush for IV catheters
Telazol 100 mg/mL	Fort Dodge	01L60030	Pre operative Sedative
Triple Antibiotic Ointment	Johnson & Johnson	23734	Topical over wound
6.0 mm ID endotracheal tube	Mallinckrodt	86049	Establish airway for Revasc,MRI and Termination
1" medical tape	Medline	MMM15271Z	Secure wound dressing and IV catheters
4.0 mm ID endotracheal tube	Medline	DYND43040	Establish airway for Hibernation
4.5 mm ID endotracheal tube	Medline	DYND43045	Establish airway for Hibernation
5.0 mm ID endotracheal tube	Medline	DYND43050	Establish airway for Hibernation
6.5 mm ID endotracheal tube	Medline	DYND43065	Establish airway for Revasc,MRI and Termination
7.0 mm ID endotracheal tube	Medline	DYND43070	Establish airway for Revasc,MRI and Termination
Bair Hugger Blanket -  Large size, underbody	Medline	AUG55501	Patient Warming system
Basic pack	Medline	DYNJP1000	Sterile drapes and table cover
Bone Wax	Medline	ETHW31G	Hemostasis of cut bone
Suction  tubing	Medline	DYND50223
Suction Container	Medline	DYNDCL03000
1 mL Syringe	Medtronic/Covidien	1188100777	Administer injectable agents
12 mL Syringe	Medtronic/Covidien	8881512878	Administer injectable agents
20 mL Syringe	Medtronic/Covidien	8881520657	Administer injectable agents
3 mL Syinge	Medtronic/Covidien	1180300555	Administer injectable agents
6  mL Syringe	Medtronic/Covidien	1180600777	Administer injectable agents
60 mL Syringe	Medtronic/Covidien	8881560125	Administer injectable agents
Blower Mister Kit	Medtronic/Covidien	22120	Clears surgical field for vessel anastomosis
Roncuronium	Mylan	67457-228-05	Neuromuscular blocking agent
# 40 clipper blade	Oster	078919-016-701	Remove hair from surgery sites
Hair Clipper	Oster	078566-011-002	Remove hair from surgery sites
Bupivicaine	Pfizer	00409-1161-01	Local Anesthetic
Cephazolin	Pfizer	00409-0805-01	Antibiotic
Heparin	Pfizer	0409-2720-03	anticoaggulant
Lidocaine 2%	Pfizer	00409-4277-01	Local Anesthetic/ antiarrthymic
Succinylcholine 20 mg/mL	Pfizer	00409-6629-02	Neuromuscular blocking agent
Anesthesia Monitor	Phillips  Intellivue	MP70	Supports ventilation with inhalant
Artificial Tears	Rugby	0536-1086-91	Lubricate eyes to prevent corneal drying
Buprenorphine 0.3 mg/mL	Sigma Aldrich	B9275	Pre operative Analgesic for survivial procedures
Isoflurane	Sigma Aldrich	CDS019936	General Anesthestic- Inhalant
36” Pressure monitoring tubing	Smith’s Medical	MX563	Connect art. Line  to transducer
48” Pressure monitoring tubing	Smith’s Medical	MX564	Connect art. Line  to transducer
Jelco 18 ga IV catheter	Smiths medical	4054	IV access in Revasc, MRI and Term
Jelco 20 ga IV catheter	Smiths medical	4059	IV access in the MRI
Jelco 22 ga  IV catheter	Smiths medical	4050	IV access in Hibernation Procedure
OPVAC Synergy II	Terumo Cardiovascular System	401-230	Heart positioner and Stabilizer
Sternal Saw/ Necropsy Saw	Thermo Fisher	812822	Used to open chest cavity
Delrin Constrictor	U of MN	Custom made	Creates stenosis of LAD
Oxygen Tank E cylinder	various	various	Used for Blower Mister if anesthesia machine doesn't have auxiliary flow meter
Pressure Transducer	various	Must adapt to anesthesia monitor	Monitor direct arterial pressures
Xylazine 100 mg/mL	Vedco	468RX	Pre operative Sedative/ analgesic