Medicine
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Sterile Pericarditis in Aachener Minipigs As a Model for Atrial Myopathy and Atrial Fibrillation
Chapters
Summary September 24th, 2021
We describe a sterile pericarditis model in minipigs to study atrial myopathy and atrial fibrillation (AF). We present surgical and anesthetic techniques, strategies for vascular access, and a protocol to study the inducibility of AF.
Transcript
Atrial myopathy can result in atrial fibrillation, which is the most common arrhythmia in humans. The sterile pericarditis model is a reliable, large animal model that resembles the pathogenesis of atrial myopathy. This model provides a rapid induction of atrial myopathy within a matter of weeks.
Also, repeated electrophysiology studies can easily be performed in a follow up period without the need of repeated catheterizations. Because of the similarity in anatomy and physiology compared to humans, the minipig model presented here can be used to study the pathophysiology of atrial myopathy and atrial fibrillation, but can also be used in pre-clinical drug discovery. Begin by preparing the pressure conducting system.
Add 5, 000 international units of heparin to an IV bag with 500 milliliters of 0.9%saline. Place the animal in the supine position, then extend the leg and locate the femoral artery using ultrasound with the vascular probe in the carotid setting. Disinfect the inguinal zone with chlorhexidine.
Puncture the femoral artery using ultrasound guidance and insert a 3 French sheath using the Seldinger technique. Fixate the sheath with the suture and connect it to the transducer to flush. Monitor the arterial blood pressure in real time.
Make a five centimeter incision in the groove at the medial border of the sternocleidomastoid muscle. Then bluntly dissect until the internal jugular vein is reached. Remove fibrous tissue around the vein and place a Prolene 6 by 0 squared suture around the desired catheterization site to gain vessel control.
Cannulate the internal jugular vein with a 3 French triple-lumen CVC using the Seldinger technique. Then tighten the Prolene 6-0 suture around the catheter. Fixate the handle of the catheter to the sternocleidomastoid muscle.
Tunnel the three catheter lumina separately and attach the ends firmly to the skin. Put on the needle free injection port, then close the incision site in two layers. Make a median incision manubrium of the sternum to three centimeters below the xiphoid process until the sternum becomes apparent.
Bluntly dissect caudally from the xiphoid process. Put a finger or blunt dissecting scissors on the visceral side of the sternum and remove connective tissue following the visceral sternal surface as far as possible. Use the sternum saw to cleave the sternum.
Then use the sternum spreader to enlarge the access to the thoracic cavity, avoiding damaging the pleura. Open the pericardium carefully and use suspension sutures to keep it out of the surgical field. After testing the extension and retraction mechanism of the lead's fixation screw, put the lead tip on a curved forceps and curve the stylet by 60 degrees, if necessary.
Put a compress on the left ventricle and gently pull it aside to gain a view of the left atrium. Upon visualization of the left atrium, firmly put the lead tip on its wall as close as possible to the pulmonary veins and as far as possible from the ventricle. Screw it in by extending the helix into the atrial tissue, preferably with a slight inclination.
Work quickly and release the pressure on the left ventricle immediately. Measure the sensing and pacing threshold and impedance of the lead using a programmable electrical stimulator or pacemaker program. Ensuring that there is no ventricular over capture when pacing at high voltages.
Place a pacemaker lead on the right atrium completely analogous to the placement of the left atrial lead. Ensuring both leads leave the thorax at the midline. The left atrial lead must be tunneled through the abdominal subcutaneous fat from the xiphoid process to the left flank and the right atrial lead to the right flank.
Make a pacemaker pocket in the subcutaneous fat at the left and right flank of the pig. Connect a pacemaker capable of performing 50 Hertz burst pacing with the left atrial lead, and a pacemaker from a different manufacturer with the right atria lead, then place them inside the pockets. Expose the atria again by gently pulling aside the ventricles, then cover the ventricles with gauze.
Spray sterile talcum over the epicardial surface of both atria using the dispenser. Leave one layer of sterile gauze on the epicardial surface of the atria. Check the position of the pacemaker leads one last time before starting closure.
Close the pericardium using monocryl 3-0 and the sternum using the stainless steel wire, then close subcutaneous and the skin using Vicryl zero and monocryl 3-0, respectively. After the sternal wound has healed, weigh the pig again for follow up, then place the animal in a restraining sling and bring it to the operating theater. Attach ECG and saturation monitoring and place the programmer heads over their corresponding pacemakers.
Interrogate the pacemaker. Check the pacemaker settings for the occurrence of spontaneous AF.A ventricular warning is normal when using a dual chamber pacemaker. Determine impedance and sensing and pacing thresholds.
Determine the atrial effective refractory period approximated by the shortest cycle length at which a one-to-one ratio capture is maintained during burst pacing. Determine the conduction time between left and right atrial leads by measuring the time between the initiation of the pacing spike and the atrial depolarization on the right atrial lead. Perform three protocols as described in the text manuscript.
Noting the AF duration and the AF inducibility for each protocol, then allow the animal to awaken or continue with other procedures. A gradual increase in the voltage threshold and impedance of the left atrial lead were observed over time, indicating increased fibrosis. Detrimental pacing and 50 Hertz burst pacing protocols were more successful than the AERP plus 30 milliseconds pacing protocol.
AF inducibility began to increase two weeks after surgery up to approximately 25%The AERP plus 30 millisecond protocol was the least effective, showing approximately 10%AF inducibility. Detrimental pacing and 50 Hertz burst pacing increased AF inducibility to approximately 40%This atrial electrogram of the left atrial pacemaker shows induction of an episode of AF after five seconds of 50 Hertz burst pacing. Whereas in this one, there is no AF induction.
Masson's trichrome staining of left atrial tissue showed higher levels of interstitial or paravascular fibrosis in the sterile pericarditis animals compared to shams. Blinded quantification of the percentage of blue fibrotic tissue area relative to the total myocardial area showed that sterile pericarditis induces more paravascular and interstitial fibrosis in atrial tissue than sham surgery. The most important thing is to obtain good positioning of the pacemaker leads with adequate voltage threshold and without ventricular capture.
It is very difficult to correct problems with pacemaker leads after the initial surgery because the extensive fibrosis observed in this model prohibits replacement of the leads. This protocol focuses on the surgical and electrophysiological studies, but the model can also be used for histology and cardiac imaging, including CT and MRI. Also, compared to rodents, the larger quantity of atrial tissue allows for detailed transcriptomic and proteomic studies.
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