This method can help answer critical questions in the cardiovascular field, such as the consequences of chronic heart pacing. It can also help in the development of novel techniques, and introducing them into practice. The main advantage of this model is the minimally invasive approach for the transvenous lead implantation.
This model utilizes simple surgical techniques, and the results are highly reproducible. To begin, locate the external jugular vein, and mark its position on the skin of the anesthetized animal. Sterilize the entire surgical area using povidone iodine, and place a sterile drape to cover the surgical site with a hole over the marked jugular area.
Make an incision on the skin parallel to the marked jugular vein. Locate the external jugular vein, and isolate a length of one centimeter from the adjacent fibrous tissue and the vascular bundle. Find the carotid artery for orientation, and to prevent its injury.
Create a pocket in the subcutaneous tissue to accommodate the pacemaker. Use scissors for blunt dissection to prevent excessive bleeding and tissue damage. Secure the vessel using a rubber tie on both ends of the isolated vessel segment, and occlude the blood flow.
Using the standard cutdown technique, cut approximately one third of the circumference of the vessel wall with a sterile blade. Use a vessel pick to widely open the cut, and introduce a single passive pacing lead into the lumen. Under fluoroscopic guidance, advance the lead's tip to the apex of the right ventricle.
Preshape a stylet into a curve, and use it to guide the lead to pass the tricuspid annulus towards the apex of the right ventricle. Ensure that the tip of the lead is unsupported by the stylet so that the lead remains flexible and atraumatic when touching the tissue. Then test the pacing parameters.
The ventricular lead sensed signal and impedance must be stable and the pacing threshold should be low. There should be no fasciculation of the adjacent muscles. Secure the lead position by stitching it over a protective rubber sleeve to the underlying fibrous tissue.
Remove the rubber ties and seal the vessel lumen around the lead and distally using silk ties. Connect the pacemaker to the pacing lead, and secure the IS-1 connector using a screw. Bury the pacemaker and the extra length of the lead into the preformed subcutaneous pocket.
Flush the pocket with povidone iodine. Suture the skin wound using a monofilament thread. Set the desired pacing program, and perform a final check of the pacing parameters.
After the post-operative routine is carried out, interrogate the pacemaker, and set the backup pacing mode by selecting minimal base rate in the Parameters menu. Record the pacing lead impedance continuously by starting data collection in the Pacemaker Programmer Diagnostics menu. In the Pacemaker Programmer select Test Menu, and under the Sensing tab, record the myocardial potential by measuring the ventricular sense amplitude.
Measure both unipolar and bipolar myocardial potentials. Use the non-invasive pacing study function to measure the pacing threshold. Assess the pacing threshold for various stimulus durations, and express it in volts.
Use the intracardiac electrograms or surface ECG for the determination of the loss of capture before the pacing stimulus output becomes sub-threshold. Repeat this regularly with weekly interrogation. A total of six animals were included in the study with successful pacing system implantation with the pacing lead tip positioned into the right ventricular apex.
At the time of implantation, the mean sensed myocardial potential was 5.6 volts. The lead impedance was 675 ohms, and the measured pacing threshold was 0.8 volts with the stimulus duration set to 0.4 milliseconds. After the follow-up at three in six months with intermittent pacing, the mean sensed myocardial potential was approximately 7.4 millivolts and 6.3 millivolts respectively.
The mean lead impedance measured was about 869 ohms and 725 ohms respectively. While the pacing threshold changed to 1.2 and 1.4 volts respectively/All the parameter changes were not statistically significant over this period, and the bipolar and unipolar parameters followed similar trends. No significant changes were observed in white blood cells, hemoglobin and platelet counts during the first week after the procedure.
Under microscopic evaluation, the pacing lead silicon surface was covered by fibrous tissue, but no cells were found. After training, the surgery can be done in 30 minutes. Gentle handling of the tissues is necessary during the procedure, and fluoroscopic guidance must be ensured while manipulating the lead to avoid injury.
The procedure provides a good understanding of implanting the pacing system. The developed model then serves optimally for studying chronic heart pacing and investigating the resultant cardiomyopathy.
