Surgical Implantation of a Wireless System for Portal Vein Pressure Monitoring in a Chronic Fontan Ovine Model

Summary

This protocol describes the surgical methodology for implantation of a wireless telemetry device involving portal vein cannulation for pressure-sensing catheter placement to enable continuous and long-term collection of portal vein pressures in a Fontan ovine model.

Abstract

The Fontan circulation is characterized by nonpulsatile pulmonary blood flow and chronic systemic venous congestion, which has been associated with several late-term complications, including Fontan-associated liver disease (FALD). Chronic animal models are a valuable tool for studying the pathophysiology underlying such disease processes. However, obtaining hemodynamic data from freely moving large animals poses several challenges. The use of wireless implantable telemetry systems may provide a novel means of acquiring pressure measurements from the hepatic circulation in real time. The following protocol describes the surgical methodology for wireless telemetry device implantation with cannulation of the portal vein in a Fontan ovine model, enabling the continuous and long-term collection of portal venous pressures and facilitating future investigations into the hemodynamic changes associated with chronic liver disease in Fontan patients.

Introduction

Single ventricle is a broad term encompassing various cardiac malformations that result in the anatomic or functional loss of a ventricular cavity. These congenital defects represent some of the most complex heart problems, often requiring multiple surgeries for palliation. The Fontan procedure is a common choice for intervention and typically the culminating operation in a series of staged surgeries^1,2,3. The development of this procedure has led to significant improvements in life expectancy⁴. For instance, 80-90% of patients undergoing Fontan palliation now survive into adulthood^5,6. Consequently, there has been an increasing rate of late hospital utilization by patients with single ventricle physiology^7,8.

Despite the benefits of the Fontan procedure to overall survival, the resulting circulation, characterized by nonpulsatile pulmonary blood flow and chronic systemic venous congestion, is associated with significant morbidity long-term^{9,10,11,12,13}. Fontan-associated liver disease (FALD), arising from congestive hepatopathy, is one of the most commonly described systemic disorders occurring after the operation, affecting approximately one-third of patients during long-term follow-up^13,14,15,16. The development of cardiac cirrhosis can necessitate eventual liver transplantation in a subset of patients¹⁷. It has also been associated with an increased risk for hepatocellular carcinoma^18,19. FALD is therefore an important area of study among researchers endeavoring to improve the outcomes of Fontan patients.

To this end, Fontan animal models are a valuable tool for studying the late-term physiological changes underlying such complications^20,21. However, long-term hemodynamic data collection poses a significant challenge in freely moving large animals. Invasive catheter-based techniques are limited by their transient nature, as well as their associated procedural risks. Moreover, in our experience, we have found that access to the portal venous system can be particularly difficult to achieve when navigating through the reconstructed vasculature of the Fontan circulation. Alternatively, four-dimensional phase-contrast magnetic resonance imaging (MRI) has been utilized as a non-invasive means of assessing hemodynamics within the liver²². However, this process is costly and requires sedation when used on animal models. Furthermore, MRI imaging is unable to provide information on intravascular pressure. Such issues in the postoperative evaluation of chronic Fontan animal models call for the development of a novel approach to data collection.

In this paper, we describe the methodology for the successful implantation and use of a wireless telemetry system to monitor portal vein pressures in a Fontan ovine model. This technique provides a cost-effective and easily accessible means of obtaining long-term hemodynamic data from the portal venous circulation. Application of this technology in preclinical models may be useful for studying Fontan liver pathophysiology and identifying new therapeutic strategies aimed at the treatment or prevention of FALD.

Protocol

This experimental protocol was approved by the Institutional Animal Care and Use Committee of the Nationwide Children's Hospital Abigail Wexner Research Institute (AR20-00121). All procedures adhered to the guidelines outlined in the National Institute of Health's Guide for the Use and Care of Laboratory Animals. This research followed the Animal Research: Reporting of In Vivo Experiments guidelines. Dorset sheep between the ages of 3-13 months and weighing 24.5-40.5 kg were housed in a pathogen-free environment at least one week before surgery, at which time they underwent veterinarian evaluation for surgical clearance. The equipment and reagents used in the study are listed in the Table of Materials.

1. Anesthesia and preoperative set-up

1. Prepare the animals for surgery as described previously²³.
2. Administer sedation by injecting a combination of ketamine (4 mg/kg) and diazepam (0.5 mg/kg) via the internal jugular (IJ) vein.
3. Introduce an 8-9 mm single-lumen endotracheal tube into the trachea to maintain the airway, and advance an orogastric tube to facilitate decompression of the stomach and rumen.
4. Establish venous access using a 16-18 G single-lumen catheter placed in the right IJ or lateral saphenous vein to enable continuous fluid delivery, continuous rate infusion (CRI) of propofol, and drug delivery as required.
5. Position a 22-24 G arterial line in the auricular artery to allow continuous monitoring of blood pressure.
6. Throughout the procedure, anesthesia should be sustained using inhaled isoflurane at concentrations of 1-3% in 100% oxygen and/or propofol at 20-45 mg/kg/h.
7. Provide antibiotic prophylaxis by administering cefazolin at a dose of 25 mg/kg approximately 30 min before the surgical incision, with repeat dosing every 8 h intraoperatively as required.
8. Place the sheep in a left lateral decubitus position on the surgical table.
9. Shave the sheep in a wide perimeter from the thigh to the lower rib cage in the cephalocaudal direction and from the spine to the mid abdomen just above the bedside in the dorsoventral direction (Figure 1A).
10. Prepare the telemetry device as previously described²³. Zero the device to atmospheric pressure on a flat surface while it is in its original packaging prior to implantation.

2. Exposure of the portal vein

1. Make an approximately 20 cm subcostal incision 3 cm caudal to the right lower rib cage.
2. Use electrocautery to divide the external oblique, internal oblique, and transversus abdominis muscles.
3. Lift up and sharply incise the peritoneum with scissors to enter the abdomen, taking care to avoid causing an enterotomy.
4. Retract the liver cephalad, and the rumen, stomach, small intestine, and colon away in the medial and caudal directions to reveal the portal triad within the hepatoduodenal ligament.
5. Often, a lymph node can be identified situated on the portal vein. Ligate the lymphatic vessels above and below this node and excise the node (Figure 1B).
6. Using a combination of blunt and sharp dissection, continue to dissect around the portal vein towards the liver cephalad and the pancreas caudally to create space for the placement of a vascular side clamp.

3. Creation of the device pocket

1. Create a subcutaneous pocket for placement of the telemetry device body by making a separate 6 cm transverse incision through the skin on the right side of the lower abdomen, approximately 15-20 cm caudal and parallel to the subcostal incision.
2. Carefully separate the subcutaneous fat and connective tissue to form a 6 × 4 cm pocket over the external oblique muscle, employing both electrocautery and blunt dissection..
3. Insert the telemetry device into the subcutaneous pocket and secure it in place using 2-0 silk suture.
4. Form a subcutaneous tunnel extending from the device pocket to the subcostal incision, and pass the pressure catheter corresponding to the blood pressure (BP) channel through this pathway.
5. Place the unused ECG leads and pressure catheter of the telemetry implant within the subcutaneous pocket alongside the device body (Figure 1C).

4. Portal vein cannulation

1. Returning to the main surgical site, place a purse-string stitch using 6-0 polypropylene suture on the surface of the portal vein at the site of cannulation and secure the suture with a plastic tourniquet.
2. Fill the gel tip of the pressure catheter with non-compressible, high-viscosity gel to prevent coagulation inside the tip, ensuring that there are no air bubbles.
3. Administer a dose of intravenous heparin (100 units/kg) 3 min before vascular clamping and cannulation.
4. Apply a vascular side clamp along the longitudinal axis of the portal vein surrounding the purse-string suture.
5. Using a #11 scalpel blade, make a precise incision into the vessel at the center of the purse-string stitch, then gently enlarge the opening with the tip of a curved hemostat.
6. Insert the pressure-sensing catheter into the portal vein, then tighten the tourniquet and remove the side clamp.
7. Advance the catheter approximately 3 cm towards the liver, then tighten and tie the purse-string suture.
8. In a separate location along the portal vein, place another purse-string stitch using 6-0 polypropylene.
9. Insert an 18 G angiocatheter needle into the center of this purse-string suture and connect to an arterial line transducer to obtain direct portal vein pressures.
10. Recalibrate the telemetry device by entering the difference in portal vein pressures obtained from the angiocatheter needle and device catheter as the offset value in the telemetry software program.
11. Remove the needle and tie the purse-string suture.
12. Apply a few drops of skin glue at the insertion site of the device catheter to ensure it remains stable within the portal vein.

5. Closure and end of procedure

1. Reapproximate the abdominal wall muscle and fascia using 0-0 absorbable suture with the pressure-sensing catheter exiting from the abdomen through the incision.
2. Administer intramuscular injections of bupivacaine liposome suspension (5.3 mg/kg) for postoperative pain control.
3. Close the skin at both incisions with deep dermal and subcuticular sutures using 3-0 and 4-0 absorbable suture, respectively.
4. Wean the animal off sedation and recover as described previously²³.

Results

Surgical outcomes
A total of 6 sheep underwent wireless telemetry device implantation with cannulation of the portal vein (Table 1). No animals experienced any major postoperative complications associated with device implantation. All 6 sheep went on to undergo a single-stage Fontan procedure, which involved total cavopulmonary connection with detachment of both the SVC and inferior vena cava (IVC) from the right atrium, direct end-to-side anastomosis of the SVC to pulmonary artery (...

Discussion

We have developed a novel surgical method for the implantation of a wireless telemetry device to facilitate continuous and long-term collection of portal vein pressures in a chronic Fontan ovine model. The device was successfully implanted in six sheep with no major complications. Data collected using this technique can inform future research studying the association between localized hemodynamic changes within the hepatic circulation and the development of FALD.

While animal models can provid...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
0.9% Sodium Chloride solution	Baxter Healthcare Corporation	Pharmacy	Intraoperative fluid resuscitation and wound rinse
16 G intravenous catheter	BD	382259	For fluid and drug administration and portal vein pressure confirmation
22 G intravascular catheter	BD	381423	For arterial  blood pressure monitoring
70% isopropyl alcohol	Aspen Vet	11795782	Topical cleaning solution
ACT cartridge	Abbot Diagnostics	03P86-25	Activated clotting time
Backhaus towel clamp	Medline	MDS1411111	To affix sterile drape
Banamine	Hospira Pharmaceuticals	Pharmacy	Postoperative pain control: concentration 50 mg/mL, dose 2.2 mg/kg
Blood pressure cuff	Royal Philips	9.89803E+11	Non-invasive blood pressure monitoring
Bupivacaine hydrochloride	Hospira Pharmaceuticals	Pharmacy	Local anesthetic: concentration 2.5 mg/mL, dose 2.5 mg/kg
Buprenorphine	Hospira Pharmaceuticals	Pharmacy	Postoperative pain control: concentration 0.3 mg/mL, dose 0.03 mg/kg
Castroviejo needle holder	Medline	MDS0750386	Needle holder when suturing blood vessels
Cautery cleaner pad	Cardinal Health	300-2SS	To clean cautery pencil tip
Cautery pencil	Medline	ESRK3002L	For dissection using electrocautery
Cefazolin	Hospira Pharmaceuticals	Pharmacy	Antibiotic prophylaxis
Cetacaine	Cetylite	220	Topical anesthetic spray for intubation
Chloraprep	BD	930825	Topical antiseptic
Debakey atraumatic forceps	Medline	MDS1130630F	For tissue handling
Debakey Satinsky vascular clamp	Medline	MDS1438413	To isolate the point of access into the portal vein
Diazepam	Hospira Pharmaceuticals	Pharmacy	Sedative: concentration 5mg/mL, dose 0.5 mg/kg
ECG leads	3M	2570	ECG monitoring
Endotracheal tube, size 8-9	Covidien	86452, 86114, or 86454	To secure airway
Hartmann hemostatic forceps	Medline	MDS1221109	To clamp blood vessels and hold small sutures
Heparin	Hospira Pharmaceuticals	Pharmacy	Anticoagulant: 1,000 USP units/mL
Isoflurane	Baxter Healthcare Corporation	Pharmacy	Anesthetic: dose 1-3%
Kantrowitz forcep (right angle)	Medline	MDS1243528	For blunt dissection around portal vein
Ketamine	Hospira Pharmaceuticals	Pharmacy	Sedative: concentration 100 mg/mL, dose 4mg/kg
Laparotomy drape	Medline	DYNJP3008	Sterile drape
Lubricating jelly	Medline	MDS0322273Z	Endotracheal tube lubricant
Mayo Hegar needle holder	Medline	MDS2418420F	Needle holder when suturing soft tissue
Mayo scissors	Medline	MDS0816121	To cut suture
Metzenbaum curved scissors	Medline	MDS3223226	For sharp dissection
Needles and syringes	Cardinal Health	309604	For intravenous and subcutaneous drug administration
Optixcare	Aventix	OPX-4252	Corneal lubricant
Orogastric tube	Jorgensen Lab, Inc.	J0348R	For stomach and rumen decompression
Perma-Hand silk suture	Ethicon	C016D	For blood vessel ligation and attachment of the telemetry device subcutaneously
PhysioTel Digital wireless telemetry device	Data Sciences International	L21 model	Wireless telemetry device implant
Pierce microforceps	Medline	MDG384908	Small needle handling
Plastic tourniquet and suture snare	Medtronic	79013	To facilitate hemostasis during vessel cannulation
Pressure bag	Carefusion	64-10029	For arterial blood pressure monitoring
Pressure transducer kit	Edwards Lifesciences	VSYPX12N	For arterial  blood pressure monitoring
Prolene 6-0 suture	Ethicon	8307H	Purse string stitch for vessel cannulation
Propofol	Fresenius Kabi	Pharmacy	Anesthetic: concentration 10 mg/mL, dose 20-45 mg/kg/hr
Pulse oximeter lingual clip	Nellcor	PO736	For pulse oximetry
Scalpel #10 blade	Medline	MDS15310	For skin incisions
Scalpel #11 blade	Medline	CISION11CS	For incision into the portal vein
Schnidt tonsil forceps	Medline	MDS5018719	For blunt dissection through subcutaneous tissue
SoftCarry stretcher	Four Flags Over Aspen	SSTR-4	For animal transportation
Sterile bowl	LSL Industries	5232	To hold saline solution
Sterile cotton X-ray detectable gauze sponge	Medline	NON21430LF	Fluid absorption
Sterile disposable OR towel	Medline	MDT2168201	Sterile drape
Topical skin adhesive	Ethicon	DHVM12	To fix telemetry device catheter in place at the site of insertion into the portal vein
T-port	Medline	DYNDTN0001	Intravenous catheter tubing connector
Urine drainage bag	Covidien	3512	Connects to orogastric tube to collect gastric fluids
Veterinary trocar with stylet	Braintree Scientific, Inc.	TRO-STY 7B-12	To guide telemetry device catheter through subcutaneous tissue
Vicryl 0 suture	Ethicon	J346H	Closure of abdominal wall fascia
Vicryl 2-0 suture	Ethicon	VCPB269H	Closure of subcutaneous soft tissue
Vicryl 3-0 suture	Ethicon	VCPB416H	Closure of deep dermal layer
Vicryl 4-0 suture	Ethicon	J494H	Closer of subcuticular layer
Warming blanket	Jorgensen Lab, Inc.	J1034B	To maintain animal's body temperature
Yankauer bulb tip suction	Medline	DYND50138	Sterile waste management