A Large Animal Model for Pulmonary Hypertension and Right Ventricular Failure: Left Pulmonary Artery Ligation and Progressive Main Pulmonary Artery Banding in Sheep

Summary

This manuscript describes the surgical technique and experimental approach to develop severe right ventricular pressure overload to model their adaptive and maladaptive phenotypes.

Abstract

Decompensated right ventricular failure (RVF) in pulmonary hypertension (PH) is fatal, with limited medical treatment options. Developing and testing novel therapeutics for PH requires a clinically relevant large animal model of increased pulmonary vascular resistance and RVF. This manuscript discusses the latest development of the previously published ovine PH-RVF model that utilizes left pulmonary artery (PA) ligation and main PA occlusion. This model of PH-RVF is a versatile platform to control not only the disease severity but also the RV's phenotypic response.

Adult sheep (60-80 kg) underwent left PA (LPA) ligation, placement of main PA cuff, and insertion of RV pressure monitor. PA cuff and RV pressure monitor were connected to subcutaneous ports. Subjects underwent progressive PA banding twice per week for 9 weeks with sequential measures of RV pressure, PA cuff pressures, and mixed venous blood gas (SvO₂). At the initiation and endpoint of this model, ventricular function and dimensions were assessed using echocardiography. In a representative group of 12 animal subjects, RV mean and systolic pressure increased from 28 ± 5 and 57 ± 7 mmHg at week 1, respectively, to 44 ± 7 and 93 ± 18 mmHg (mean ± standard deviation) by week 9. Echocardiography demonstrated characteristic findings of PH-RVF, notably RV dilation, increased wall thickness, and septal bowing. The longitudinal trend of SvO₂ and PA cuff pressure demonstrates that the rate of PA banding can be titrated to elicit varying RV phenotypes. A faster PA banding strategy led to a precipitous decline in SvO₂ < 65%, indicating RV decompensation, whereas a slower, more paced strategy led to the maintenance of physiologic SvO2 at 70%-80%. One animal that experienced the accelerated strategy developed several liters of pleural effusion and ascites by week 9. This chronic PH-RVF model provides a valuable tool for studying molecular mechanisms, developing diagnostic biomarkers, and enabling therapeutic innovation to manage RV adaptation and maladaptation from PH.

Introduction

Decompensated right ventricular (RV) failure is the predominant cause of morbidity and mortality for patients with pulmonary hypertension (PH). RV failure is responsible for over 50% of hospitalizations in patients with PH and is a common cause of death in this patient population^1,2. Although current medical treatments for PH can provide temporizing measures, they do not reverse the progression of the disease. As such, the only long-term treatment is lung transplantation. To explore and test novel medical treatments and interventions for PH and RVF, a clinically relevant animal model is needed to recapitulate the disease's complex pathophysiology. In particular, there is a great clinical need to develop RV-targeted therapeutics for PH patients to improve RV function. To date, most published animal studies of PH and RV dysfunction have relied on small mammals such as mice and rats³. On the other hand, there have only been a handful of large animal models to study the disease and RV pathophysiology from abnormal afterload^4,5,6,7. In addition, none of the previously published large animal models include descriptions of experimental procedures for controlled titration of disease severity that differentially leads to compensated versus decompensated RV failure phenotypes. An animal model of PH that can be titrated to induce acute and chronic RV failure with varying degrees of compensation is needed to study disease mechanisms and to develop, test, and translate novel diagnostics and therapeutics for PH and RVF into clinical practice. Such a model in a large animal is especially valuable for the development of mechanical circulatory support devices⁸.

Here, a chronic, large animal PH-RVF model using left pulmonary artery (PA) ligation and progressive main PA banding in adult sheep is presented^9,10. The ligation of the left PA (LPA) increases the pulmonary vascular resistance and decreases PA capacitance^11,12. The progressive PA banding approach allows for precise titration of disease severity and RV adaptation. This platform can also be readily utilized for longitudinal investigation of disease progression toward RV decompensation. The procedures and processes required to execute this model are presented as a resource for investigators interested in a large animal platform to develop novel treatments for PH and RVF.

Protocol

The Institutional Animal Care and Use Committee at Vanderbilt University Medical Center approved the protocol. The described procedures were conducted in accordance with the US National Research Council's Guide for the Care and Use of Laboratory Animals, 8th edition. The overview and the timeline of the experimental procedure are provided in Figure 1. Supplementary Table 1  describes the sheep’s sex, weight, breed, source of sheep, and other relevant information that may be helpful for reproducibility purposes.

1. One day before surgery, preparation of the animal

1. Withhold food for 24-40 h prior to the surgical procedure to decompress the animal's rumen.
2. Apply a 50 µg/h fentanyl patch to a sheared area on the sheep's dorsum 12 h before the procedure. Clean the area with chlorhexidine to remove lanolin oil residues prior to patch application. Cover and protect the patch with an elastic tubular dressing.

2. Day of the surgery, pre-operative steps in the preparation room

1. Administer tiletamine/zolazepam intramuscularly (2.2-5 mg/kg) and deliver 1%-3% of isoflurane mixed with 80%-100% of oxygen via a face mask to induce anesthesia.
2. Position the sheep supine on the preparation table and secure its legs.
3. Intubate with a 10 mm endotracheal tube and start mechanical ventilation under volume-control mode (tidal volume, TV = 10 mL/kg, respiratory rate, RR = 15 breaths per minute).
4. Shave the surgical field from the sheep's neck to its upper abdomen, as detailed below.
   1. Shave the sheep's anterior neck to expose the skin overlying the jugular veins for central venous catheterization (see step 3.7).
   2. Shave the anterolateral thorax bilaterally in preparation for thoracotomy (see step 4.1).
   3. Shave the left side of the torso from the chest to the back (i.e., as dorsally as the table will allow with the subject in the supine position), and from breast to rear flank caudally, in preparation for implantation of subcutaneous ports (see steps 4.12-4.15).
5. Insert a 20 G angiocatheter in the auricular artery for arterial pressure monitoring and blood gas sampling.
6. Place a silicone tube with an inner diameter of 3/8"-1/2" for rumen decompression. The orogastric tube will remain in the rumen throughout the entire procedure.
7. Transport the animal from the pre-operative prep room to the surgical suite.

3. Day of surgery, pre-operative steps in the operating suite

1. Reconnect the sheep to the ventilator in the surgical suite, and continue ventilation at the same setting in step 2.3 (isoflurane 1%-3%, TV = 10 mL/kg, RR = 15 breaths per minute)
2. Connect the pulse oximetry (SpO₂), arterial blood pressure, temperature, end-tidal capnograph, and electrocardiogram (ECG) sensors to the anesthetic monitor.
3. Connect the sensors for vital signs to the animal.
   1. Place the pulse oximeter on the tongue of the animal.
   2. Place the temperature probe into the rectum.
   3. Connect 3-lead electrocardiogram probes: Place the red lead on the left rear leg, the white lead on the right front leg, and the black lead on the left anterior leg.
   4. Connect the three-way stopcock's male luer end to the auricular artery angiocatheter and connect the opposite female luer end to the pressure transducer for arterial line monitoring using an appropriately sized pressure tubing.
      1. Align the transducer to the level of the operating table.
      2. Open the three-way stopcock on the transducer.
      3. Scroll the main knob of the vitals monitor to highlight the arterial blood pressure channel, and then press the knob to select the channel.
      4. Select ZERO IBP to zero the transducer.
   5. Connect the male luer connection of the capnography monitor line to the female luer connection on the ventilator tube to monitor end-tidal CO₂.
4. Set up the IV pumps for continuous fluid administration and inotropic or vasopressor support.
   1. Perforate the septum on the saline bag with the IV administration set. Make sure that the IV tubing is clamped prior to perforating the bag to prevent spillage.
   2. Align and fit the IV administration set tubing into the IV roller pump, and check whether the direction specified on the pump matches the direction of fluid administration.
      NOTE: Ensure that the IV administration set is compatible with the IV pump.
   3. Turn on the pump and specify PRIME to remove all the air in the line.
5. Position the sheep for the operative procedure.
   1. From the supine position, rotate the sheep to a partial right lateral decubitus position.
   2. Secure the right front foot downward and secure the left front foot while retracting it cephalad and lateral with rope or atraumatic straps.
   3. Perform transthoracic echocardiography for baseline assessment of the ventricular anatomy and function. Ultrasonography is also useful to determine the optimal intercostal space that facilitates surgical access to both the main pulmonary artery and the left pulmonary artery.
6. Clean the surgical field free of dirt and other contaminants using soap or scrub brush. Prep the neck and the chest with chlorhexidine or betadine solution and drape the surgical field in a sterile fashion.
7. Using ultrasound guidance or anatomic landmarks, access the left or right internal jugular vein using a finder needle or angiocath. Using Seldinger technique, insert a 7-French triple-lumen central venous catheter into the internal jugular vein for intravenous access and central venous pressure monitoring.
   1. Use the proximal port for pressure monitoring and distal port for fluid and drug administration.
8. Administer 20 mg/kg of cefazolin and 5 mg/kg of enrofloxacin intravenously. Repeat the dosing of cefazolin every 2-4 h during the procedure.
9. Administer a 500 mL bolus of normal saline solution to augment the preload before surgery. Begin a maintenance intravenous fluid rate of 15 mL/kg/h.

4. Operative procedure

1. Perform a muscle-sparing mini-thoracotomy (length < 8 cm) at the left fourth intercostal space to obtain mediastinal exposure. Choose mini-thoracotomy to expedite postoperative recovery.
   1. After dividing the skin, split the underlying muscle (pectoralis major) longitudinally along its fibers, which run slightly oblique to the intercostal space. Place a self-retaining retractor to spread the muscle layer and expose the chest wall.
   2. Divide the serratus anterior and the underlying intercostal muscle in the selected intercostal space, taking care to stay immediately cephalad to the rib.
   3. Enter the pleural space and then continue to fully release the intercostal muscles posteriorly toward the spine and anteromedially toward the sternum to prevent inadvertent rib fracture or dislocation at the sternum. Avoid injury to the mammary vessels medially.
   4. Place the self-retaining retractors to open the rib space and the overlying soft tissue. Use a small or medium Finochietto retractor to separate the ribs and a Tuffier retractor (5 cm retractor blade) to sit perpendicular to the Finochietto within the intercostal space, which retracts the soft tissue within the intercostal space to improve exposure.
2. Incise the pericardium anterior to the phrenic nerve without injuring it and create a pericardial well with 2-0 silk sutures to expose the main PA and RV. Identify the left atrial appendage within the exposure as a landmark for the level of the PA bifurcation.
   1. Assess the exposure and ensure whether the appropriate intercostal space has been entered. Ideally, the proximal PA and the left atrial appendage are readily visible directly below the incision, suggesting the optimal intercostal space has been selected to provide exposure to both the main PA and LPA.
   2. If exposure is deemed inadequate to safely reach both the main PA and LPA, do not hesitate to open an additional intercostal space to accomplish all the necessary steps of the operation; however, this will not be necessary with appropriate incision selection.
3. Dissect around the main PA and isolate it with an umbilical tape. Ensure adequate posterior dissection for the eventual occluder placement and PA flow probe as distal as possible on the main PA.
   1. Place a sterile flow probe into a bowl of water or saline on the sterile field to calibrate the data acquisition software. Handoff the electrical plug on the other end to a non-sterile designee to connect the probe to the meter.
      1. Refer to the supplementary documents for details of connecting and calibrating PA flow probe and meter.
   2. Apply a generous amount of sterile ultrasound gel in the groove of the PA flow probe.
   3. Fit the silicone liner into the groove of the PA flow probe and apply an additional layer of ultrasound gel onto the liner.
   4. Place the PA flow probe onto the PA and acquire PA flow readings on the flow meter and the data acquisition interface.
      1. Placement of the PA flow probe may cause partial occlusion of the PA that can decrease left ventricular preload and mean arterial pressure. Pay careful attention to the hemodynamics during PA flow acquisition.
      2. Check on the flow meter screen to ensure that the PA flow signal strength is 5 bars. If the meter displays fewer than 5 bars, ensure adequate contact between the flow probe and the main PA. Apply additional ultrasound gel if needed.
4. Complete intra-pericardial dissection of LPA and encircle it with an umbilical tape.
   1. Use a small sponge stick or thin malleable retractor for caudal retraction of the left atrial appendage.
      NOTE: Exposure to the LPA is facilitated by caudal retraction of the left atrial appendage, cephalad retraction of the main PA, and lateral retraction of the pericardium just anterior to where the LPA exits the pericardium.
5. Place a heavy-duty silicone vascular occluder around the main PA (Figure 2A,B, circle). Occluder size can be adjusted based on PA diameter; ensure the fit is snug. Use a 0 silk suture on a Keith needle to secure the ends of the vascular occluder together with a U stitch. Once secured around the main PA, slide the occluder distally along the main PA.
6. Encircle the proximal main PA with a ½" Penrose drain to facilitate dissection and reserve space to place a flow probe at subsequent re-operative surgery. Trim the Penrose drain to fit loosely around the PA and secure the Penrose to itself with a running 4-0 Prolene suture (Figure 2B).
7. Establish an RV pressure line for monitoring of RV pressures (Figure 2B, white arrow).
   1. Select a location for the RV pressure line in the RV outflow tract-free wall. Place a 5-0 monofilament, nonabsorbable polypropylene purse-string suture with pledgets surrounding the selected location and seat a vascular snare. Make the pledgets from a sterile surgical glove.
   2. Prepare the RV pressure line: cut off the male end of sterile 36'' pressure tubing at a 30° angle to facilitate insertion through the myocardium. Use a 2-0 silk tie to mark the pressure line at an optimal depth for placement within the RV.
   3. Using an 11-blade scalpel, make a small cardiotomy in the RVOT free wall within the previously placed purse-string suture. Control bleeding with manual pressure or by tightening the snare on the purse-string suture.
      NOTE: Obtain a baseline biopsy of the RV free wall at this step by sampling RV tissue within the purse-string suture. This biopsy site can then serve as the entry point for the RV pressure line.
   4. Insert and secure the cut end of the pressure tubing into the RV outflow tract (RVOT). Tie down the purse-string and then secure the purse-string to the pressure tubing to secure the pressure line.
8. Extend the RVOT tubing by connecting an additional pressure tubing to the RVOT pressure line.
9. Hand off the additional pressure tubing to a non-sterile designee to connect the tubing to a pressure transducer and monitor for the measurement of the baseline RV pressure. Set up the pressure transducer as follows.
   1. Connect IV administration set's male luer end to transducer's female luer end.
   2. Connect pressure tubing's female luer end to transducer's male luer end.
   3. Spike the IV administration set into a heparinized saline bag (2 IU/mL).
   4. Fit the saline bag into a pressure bag and pump the pressure bag to 250-300 mmHg as indicated on the gauge.
   5. Fully prime the line by releasing the valve on the transducer, ensuring proper de-airing.
   6. Follow Supplementary Methods for transducer calibration.
10. After carefully dissecting around the LPA, encircle it with an umbilical tape. Ligate the LPA by tying down the umbilical tape. Note the animal's hemodynamic response to ligation if relevant to the study. Increase the minute ventilation to compensate for the increased dead-space ventilation created upon LPA ligation. These ventilator adjustments mitigate respiratory acidosis.
11. Slowly inject up to 3 mL of saline into the main PA occluder to ensure there is no leakage while monitoring RV pressure from the RVOT pressure line. Once the RV response is confirmed, withdraw the instilled saline.
12. Bring the RVOT pressure line and PA occluder tubing out of the chest one intercostal space below the thoracotomy incision.
13. Form two subdermal pockets along the fascial layer on the left dorsum of the sheep as far posteriorly toward the spine as feasible within the sterile field. These serve as the sites for indwelling ports (Figure 2C).
14. Using a chest tube puller, tunnel the RVOT pressure line and occluder tubing from the chest incision out to the left dorsum port sites.
15. Secure both the occluder tubing and RV pressure line to the port's barb connections. Anchor the occluder and pressure tubing around the port connectors with additional ties. Use the provided barbed connector fitting to protect the connection (Figure 1C). Seat the ports within the pre-formed subdermal pockets.
16. Anchor the ports in three locations around its rim to the underlying fascia with 3-0 polypropylene sutures to prevent port migration. Reapproximate the subcutaneous tissue, dermis, and skin in layers with polyglactin 910 sutures. Reconfirm the pressure readings through percutaneous access of the ports. Flush the RVOT port with 5 mL (1000 IU/mL, 5000 units) of heparin sodium.
17. Place a 16-French chest tube in the left pleural cavity through a separate incision, secure it to the skin, and then connect to a closed chest tube drainage unit at a pressure of -20 cm·H₂O. Place an untied U-stitch around the tube to facilitate closure after chest tube removal.
18. Administer an intercostal nerve block (0.5-1 mg/kg bupivacaine) for postoperative analgesia.
19. Close the thoracotomy with figure-of-eight, #2 polyglactin 910 sutures. Close the pectoralis muscle layer with running #0 polyglactin 910. Close the subcutaneous tissue in layers of running #2-0 polyglactin 910 sutures and staple the skin.
20. Reposition the animal to dorsal recumbency, remove the orogastric tube, and then discontinue isoflurane.
21. Continue mechanical ventilation and supportive care until arterial blood pH > 7.35 and pCO₂ < 55 mmHg.
22. Extubate once the animal is breathing spontaneously, lifting its head, and chewing on the endotracheal tube. Remove the chest tube prior to full anesthetic recovery. Tie the U-stitch to close the chest tube incision.
23. Transfer the animal to its cage while monitoring its anesthesia recovery. Ensure supplemental oxygen (3-5 L/min by facemask) is available at all times while the sheep remains immobile. Monitor vital signs every hour for the first 4 h, every 8 h for the next 24 h, and once daily after that.

5. Postoperative recovery

1. Monitor the thoracotomy and port implantation sites daily for signs of infection. Administer long-acting antibiotic (ceftiofur, 5 mg/kg intramuscularly) within 24 h after the procedure and every 3-4 days after that for 1 week.
2. Continue the fentanyl patch postoperatively for a total of 72 h. After that, provide additional analgesia (e.g., meloxicam, 1 mg/kg once daily intramuscularly) if the animal continues to show signs of pain (i.e., teeth grinding, elevated heart rate).
3. Remove the external sutures and skin staples 10-14 days after the surgery or as recommended by veterinary staff.
4. Ensure port site protection from the animal rubbing or scraping the port sites against surrounding structures using a tubular dressing (Figure 2D).

6. Chronic PA banding (9 - 10 weeks)

1. Transfer the sheep to a small enclosure. Shear off the excess wool around the implanted ports.
2. Clean the shaved areas with 70% isopropyl alcohol. Apply topical lidocaine spray for local anesthetic.
3. Prepare two pressure transducers for monitoring RV and occluder cuff pressures (Figure 3A).
   1. For both transducers: Connect the female luer end of pressure tubing (36 in or longer) to the male luer end of the transducer. Connect the male luer end of the pressure tubing to one of the female luer connections on a three-way stopcock. Finally, connect a 22 G Huber needle to the male luer end of that three-way stopcock.
   2. For RV pressure transducer: Hang a heparinized saline bag (2 IU/mL), puncture the bag with the IV administration set, and connect the IV administration set's male luer connection to the female luer connection of the RV pressure transducer. Then, pressurize the saline bag (e.g., pressure bag).
   3. For the occluder transducer: Prime the transducer and the pressure tubing fully. Put a male luer cap on the female luer end of the pressure transducer to prevent the cuff fluid from leaking out back to the transducer.
   4. Connect both the transducers to the data acquisition hardware using an appropriate cable or adapter.
4. Calibrate the transducers as specified in Supplementary File 1.
5. Click on Start on the top right of the software window to start recording the data acquisition software to capture RV and PA cuff pressure waveforms at 400 Hz.
6. Have an assistant provide mild restraint of the animal prior to port access. Insert Huber needle from the RV pressure transducer to the RV port. Attach a 10 mL syringe to the three-way stopcock and attempt to draw blood back into the syringe from the RV port (Figure 3B).
   1. If it is difficult to pull back on the syringe, first inject 5-10 mL saline into the RV port to dislodge the source of occlusion.
   2. If clogging persists, instill 2 mg of tissue plasminogen activator (tPA) into the port as fibrinolytic agent and leave it overnight. Check the following day to aspirate the tPA.
7. Once the RV pressure line is established, connect the Huber needle from the PA cuff transducer.
8. Capture the starting values of RV and PA cuff pressures (Figure 3C). Note any drastic changes from previous readings.
   1. If PA cuff and/or RV pressure dropped substantially from the previous reading, it may be a sign that the PA cuff is leaking.
   2. Observe another obvious sign of PA cuff leak by studying the PA cuff waveform. If the average PA cuff pressure drops at a discernible rate, then there is a high chance that the cuff is leaking.
      NOTE: Re-check that all the luer connections on the pressure transducer, tubing, and stopcock are tightened. The highly pressurized fluid content from the PA cuff can flow back and leak out of loose luer connections.
      1. If the PA cuff is leaking, determine the extent of leakage. If the leakage rate is slow, then a more frequent banding strategy can overcome the leakage to make the disease model still effective.
9. Slowly inject 3% hypertonic saline into the occluder port while paying attention to RV and cuff pressures.
   1. Adjust the amount of injection based on desired PH disease severity and RV phenotype. A weekly increase of cuff pressure by 100-150 mmHg is a reasonable target to develop an adaptive compensating RV phenotype.
   2. More rapid increases in cuff pressure (>250 mmHg per week) will likely produce a decompensating RV phenotype.
10. Once the PA cuff is inflated to the desired amount, remove the Huber needle from the cuff port.
11. Obtain a blood sample from the RV port.
    1. Aspirate 10 mL of blood out of the RV port in a sterile fashion and set aside.
    2. Place a new syringe in place of the aspiration syringe and aspirate as much blood as needed without going over the weekly blood draw limit of 7.5% of the total blood volume.
    3. Reconnect the original syringe with aspirated blood and return it through the RV port.
    4. Pull on the valve lever of the pressure transducer to flush heparinized saline from the saline bag into the RV port. Continue flushing until the entire line becomes clear and colorless.
12. Flush the RV port with 10 mL of saline. Then, further flush the port with 5 mL of 1000 U/mL of heparin sodium.
13. Repeat the steps 6.1-6.12 every 1-4 days for 9-10 weeks.

Representative Results

A representative group of 12 sheep is used to show the efficacy of this model for developing varying degrees of PH-RVF. Among these sheep, the mean PA cuff pressure increased from 32 ± 20 mmHg at week 1 to 1002 ± 429 mmHg at week 9. This resulted in increasing the RV mean and systolic pressures from 28 ± 5 and 57 ± 7 mmHg at week 1, respectively, to 44 ± 7 and 93 ± 18 mmHg by week 9. Furthermore, PA cuff pressure profile was superimposed onto mixed venous oxygen saturation (SvO₂) to demonstrate the efficacy of the model at fine-tuning disease phenotype (Figure 4). Specifically, faster PA banding led to a more rapid decline in SvO₂. In comparison, those that experienced a more gradual PA banding strategy maintained a physiologic range of SvO₂ between 70% and 80%. A representative transthoracic echocardiogram acquired after 9 weeks of progressive PA banding shows RV dilation and septal bowing due to pressure overload (Supplementary Video 1). In a previously published case report¹⁰, the model can also be used to induce end-stage RV failure, which leads to pleural effusions and abdominal ascites.

Figure 1: Overview and timeline for the overall experiment. (A) Experimental timeline for the chronic pulmonary hypertension (PH) right ventricular failure (RVF) model and the suggested data acquisition strategy. (B) The schematic diagram for the first survival surgery to establish the foundation for the chronic pulmonary hypertension (PH) right ventricular failure (RVF) model. The main pulmonary artery (PA) occluder is implanted, the left pulmonary artery (LPA) is ligated, and a pressure tubing is placed in the right ventricular outflow tract (RVOT). Finally, both RVOT and PA cuff pressure lines are connected to their respective ports, both of which are implanted subcutaneously for recurrent access and monitoring. (C) Photograph of the PA cuff, the subcutaneous port, and the plastic fitting to protect their barbed connection. Please click here to view a larger version of this figure.

Figure 2: Photographs of key surgical steps to establish the ovine pulmonary hypertension (PH) model. (A) Isolation of the main pulmonary artery (PA) and implantation of the PA cuff (circle). (B) Implanted PA cuff (circle), Penrose tubing (star), and right ventricular outflow tract (RVOT) pressure tubing (white triangle). (C) Subcutaneous implantation of ports for RVOT and PA cuff. (D) Tubular dressing and foam padding fitted around the sheep's body to protect the implanted ports. Please click here to view a larger version of this figure.

Figure 3: Experimental approach for chronic pulmonary artery (PA) banding. (A) Schematic for setting up pressure transducers to measure and adjust right ventricular (RV) and PA cuff pressure values. (B) Photograph depiction of accessing RV outflow tract (RVOT) and PA cuff ports. (C) Representative pressure tracing of RV and PA cuff pressures. Please click here to view a larger version of this figure.

Figure 4: Pulmonary artery (PA) cuff pressure and corresponding mixed venous oxygen saturation (SvO₂). Longitudinal trends between pulmonary artery (PA) cuff pressure and corresponding mixed venous oxygen saturation (SvO₂) show differentiation in right ventricular phenotype based on the PA banding strategy. The color profile varies considerably between subjects that experienced a more rapid PA banding strategy in comparison to subjects that underwent a more gradual banding strategy. Please click here to view a larger version of this figure.

Supplementary Video 1: Representative transthoracic echocardiograms between healthy baseline state and after the pulmonary hypertension right ventricular failure (PH-RVF) disease model. The PH-RVF model recapitulates key features of the disease, including RV dilation and hypertrophy, and septal bowing. Please click here to download this Video.

Supplementary File 1: Data acquisition setup and calibration steps. Please click here to download this File.

Source	Noble Life Sciences, Woodbine, MD
Sex	Castrated male or female
Strain	Dorset cross
Weight	55-70kg at receipt
Diet	3 lb of pellets each day. Timothy hay given in the provided feed bag, filled up to twice a day
Light Cycle	Light cycle 12/12 hour light/dark periods; Lights on at 6:00am, off at 6:00pm unless otherwise indicated
Housing Condition	Sheep are housed individually or in pairs. Housing enclosures measure 6.3’w X 5.7’d (35.4 sq ft.) unless otherwise specified by the facility manager. Multiple enclosures can be connected for additional floor space as needed. Rubber mats are provided to all sheep upon receipt by the animal care technician. Mat(s) is/are sanitized weekly.

Supplementary Table 1: Relevant information on animal subject for this platform. 

Cases/Events	N (%)
Total	28 (100)
No complications	22 (78)
Infection, early termination	1 (4)
Compromise of implanted port	2 (7)
Compromise of implanted pulmonary artery cuff	2 (7)
RV decompensation at end of the model	1 (4)

Supplementary Table 2: Complications during the sheep pulmonary hypertension model.

Discussion

The presented PH-RVF model can reliably induce varying levels of disease severity to match the goals of the investigation. Two different approaches are used in combination to induce this disease model. First, the LPA ligation serves to increase pulmonary vascular resistance and decrease PA capacitance^11,12, thereby establishing the starting point of the chronic model at an already increased RV afterload state. Then, the implantation of the PA cuff and its progressive inflation serves to develop a targeted phenotype of PH-RVF. Controlling PA cuff pressure and its rate of change can differentially create compensating or decompensating RVs, demonstrated by either maintenance or decline of SvO₂ (Figure 4). By increasing cuff pressure by 250-300 mmHg per week, the sheep will start to display early signs of decompensation around 5-6 weeks. Increasing the cuff pressure by 100-150 mmHg per week, on the other hand, allows for a more adaptive profile over the entire 9-week duration.

Few large animal models of chronic PH and RVF exist in the literature. Pulmonary artery embolization in sheep has been most extensively reported and discussed^4,5. However, this approach has a high mortality rate, upward of 86%⁴ depending on dosage frequency and bead sizes, yet it yields only a marginal change in RV hemodynamics and function. On the other hand, the presented model can induce a much greater range of RV pressure overload with minimal procedurally related deaths. One animal that died due to this PH-RVF model developed several liters of pleural effusion and ascites¹⁰, correlating with the clinical and research findings of right heart failure in humans^13,14,15 and large animals¹⁶. These signs were observed without any evidence of left heart failure. This model can therefore serve as a clinically translatable large animal platform with the ability to produce titratable pathophysiology.

There are several notable challenges to executing this model. First, while using a left mini-thoracotomy facilitates expedient postoperative recovery, simultaneous surgical exposure of both the main PA and the LPA is technically challenging via this minimally invasive incision. Selecting the optimal intercostal space is essential and ultrasonography can be a helpful guide. The PA bifurcation is more distal and posterior compared to human anatomy, making ligation of the LPA the most challenging step of this procedure. While the ligation serves as a critical step to increase pulmonary vascular resistance and decrease PA capacitance, it is feasible that the main PA banding alone might achieve sufficiently high RV pressure.

Infection of indwelling ports and port-site wound dehiscence can be difficult to address and lead to devastating complications. In this pulmonary hypertension model, infections could be the acute metabolic insult that triggers cardiopulmonary compromise, collapse, and early mortality. High standards for sterile technique, meticulous skin closure, and port site protection significantly limit the incidence and impact of these occurrences.

Cuff rupture is a specific issue with the model that could lead to decreased RV pressure. Though uncommon, this problem has been observed previously. There are a few preventative and remedial steps for this issue. First, care should be taken to avoid puncturing the cuff while securing it around the PA with suture. Testing the cuff prior to closing the chest ensures its integrity at the conclusion of the initial operation. Next, the PA cuff size should be chosen based on the main PA diameter size. If the cuff leaks, then it will be important to assess the magnitude of leakage. If more frequent inflation of the PA band can overcome the rate of leakage, then the model can still achieve moderate PH-RVF, although it may no longer induce the desired severity of PH-RVF.

In our experience, this model has an overall success rate of 78% (Supplementary Table 2), but most of the complications have been in the earlier half of these trials. The more recent cohort of 13 subject has had a success rate of 100%, which suggests that this model can be reproducible and free of complications with enough experience. 

Finally, a key scientific limitation of the presented animal model is that it does not convey a key feature of pulmonary arterial hypertension, namely, pulmonary vascular remodeling. Hence, this model is not the ideal platform to develop and test therapeutics that are focused solely on the pulmonary vasculature. Instead, it is an effective platform to study RV dysfunction and failure from abnormal RV afterload. Patient outcomes in PH are largely driven by RV function, and favorable outcomes are associated with the preservation of this RV function¹⁷. Although this model does not capture all aspects of PH, it is a valuable model for understanding the molecular pathways leading to RVF and developing RV-targeted therapeutics to ameliorate RVF.

The LPA ligation and main incremental PA banding model can successfully recapitulate the complex pathophysiology of RVF secondary to PH. This model will provide investigators an experimental platform to develop new diagnostic biomarkers that differentiate between adaptive and maladaptive responses to PH on the RV, elucidate critical response pathways in RVF, and enable therapeutic innovations to treat RVF.

Materials

Name	Company	Catalog Number	Comments
0.9% Sodium Chloride Irrigation Pour Bottle by Baxter Healthcare, 1000 mL	Medline	BHL2F7124	Surgical Disposable
0.25% Bupivacaine	Hospira Inc	0409-1160-18	Medication, Intra-Operative
0.9% Normal Saline, 1000 mL	Baxter Healthcare Corp	0338-0049-04	Medication, Intra-Operative
0.9% Normal Saline, 500 mL	Baxter Healthcare Corp.,	0338-0049-03	Medication, Chronic PH
16 mm Heavy Duty Occluder with actuating tubing	Access Technologies	OC-16HD	Surgical Disposable
3-mL Skin Prep Applicator	Medline	MDF260400	Surgical Disposable
70% isopropyl alcohol prep pads	Medline	MDS090670	Disposable, Chronic PH
Adhesive bandage tape	Patterson Veterinary	07-835-7776	Disposable, Chronic PH
Adson forceps	V. Mueller	NL1400	Surgical Instrument
Allis tissue forceps	V. Mueller	CH1560	Surgical Instrument
Aortic clamp, straight (bainbridge forceps)	V. Mueller	SU6001	Surgical Instrument
Backhaus towel forceps	V. Mueller	SU2900	Surgical Instrument
Bags, Infusion: Nonsterile Novaplus Infusion Bag, 500 mL	Medline	TCV4005H	Disposable, Chronic PH
Berry sternal needle holder	V. Mueller	CH2540	Surgical Instrument
Blades, Electrode: Electrode Blade, 6.5", with 0.24 cm Shaft	Medline	VALE15516	Surgical Disposable
Blades: Stainless-Steel Sterile Surgical Blade, Size #10	Medline	B-D371210	Surgical Disposable
Blades: Stainless-Steel Sterile Surgical Blade, Size #11	Medline	B-D371211	Surgical Disposable
Blades: Stainless-Steel Sterile Surgical Blade, Size #15	Medline	B-D371215	Surgical Disposable
BNC Male to BNC Male Cable	Digi-Key	415-0198-036	Equipment
Castroviejo needle holder	V. Mueller	CH8589	Surgical Instrument
Cefazolin	Apotex Corp	60505-6142-0	Medication, Intra-Operative
Ceftiofur Crystalline Free Acid	Zoetis Inc	54771-5223-1	Medication, Post-Operative
Chest Drain, with Dry Suction, Adult-Pediatric	Medline	DEKA6000LFH	Surgical Disposable
Chest tube passer	V. Mueller	CH04189	Surgical Instrument
COnfidence Flowprobes for Research (PAU-Series)	Transonic	24PAU	Equipment, Perivascular Flow Probe
Cooley tangential occlusion clamp	V. Mueller	CH6572	Surgical Instrument
Data Acquisition Hardware	ADInstruments	PowerLab 16/30	Equipment
DeBakey Aorta clamp	V. Mueller	CH7247	Surgical Instrument
DeBakey multi-purpose clamp	V. Mueller	CH7276	Surgical Instrument
Debakey tissue forceps, 12’’	V. Mueller	CH5906	Surgical Instrument
Debakey vascular tissue forceps 7 3/4’’	V. Mueller	CH5902	Surgical Instrument
Debakey vascular tissue forceps, 9’’	V. Mueller	CH5904	Surgical Instrument
Electrosurgical Generator	Covidien	Force FX-C	Equipment
Endotracheal Tube, 10mm	Patterson Veterinary	07-882-9008	Surgical Disposable
Enrofloxacin	Norbrook Laboratories Limited	55529-152-05	Medication, Intra-Operative
Fentanyl Transdermal Patch	Apotex Corp	60505-7007-2	Medication, Pre-Operative
Ferris smith tissue forceps	V. Mueller	SU2510	Surgical Instrument
Finochietto rib spreaders, large	V. Mueller	CH1220-1	Surgical Instrument
Finochietto rib spreaders, medium	V. Mueller	CH1215-1	Surgical Instrument
Flexsteel ribbon retractor, 1” x 13”	V. Mueller	SU3340	Surgical Instrument
Flexsteel ribbon retractor, 2” x 13”	V. Mueller	SU3346	Surgical Instrument
Foerster sponge forceps, curved	V. Mueller	GL660	Surgical Instrument
Gauze Sponges: Sterile X-ray Compatible Gauze Sponges, 16-Ply, 4" x 4"	Medline	PRM21430LFH	Surgical Disposable
Gerald-DeBakey forceps	V. Mueller	CH04242	Surgical Instrument
Glassman Allis	V. Mueller	SU6152	Surgical Instrument
Halsted mosquito forceps	V. Mueller	SU2702	Surgical Instrument
Harken clamp	V. Mueller	CH6462	Surgical Instrument
Heat Therapy Pump	Gaymar/Stryker	TP-400	Equipment
Heparin	Fresenius Kabi,	63323-540-31	Medication, Chronic PH
Hospira Primary IV Sets, 80"	Patterson Veterinary	07-835-0123	Surgical Disposable
Hypertonic saline 3%	Baxter Healthcare Corp.,	0338-0054-03	Medication, Chronic PH
Hypodermic Needle with Bevel and Regular Wall, 20 G x 1"	Medline	B-D305175Z	Disposable, Chronic PH
Interface Cable, Edwards LifeScience Transducer to ADInstruments  Bridge Amplifier	Fogg System	0395-2434	Equipment
Intravenous Infusion Pump	Heska	Vet/IV 2.2 Infusion Pump	Equipment
Isoflurane	Patterson Veterinary	14043-704-06	Medication, Pre-Operative
Kantrowitz thoracic clamp, 9-1/2”	V. Mueller	CH1722	Surgical Instrument
Kelly hemostats	V. Mueller	88-0314	Surgical Instrument
Lidocaine HCl, 2.46%	PRN Pharmacal,	49427-434-04	Medication, Chronic PH
Ligaclip Multiple-Clip Appliers by Ethicon	Medline	ETHMCS20	Surgical Disposable
Loop, Vessel, Mini, Red, 2/pk, Sterile	Medline	DYNJVL12	Surgical Disposable
Lorna non-perforating towel forceps	V. Mueller	SU2937	Surgical Instrument
Mayo dissecting scissors, curved	V. Mueller	SU1826	Surgical Instrument
Mayo dissecting scissors, straight	V. Mueller	SU1821	Surgical Instrument
Medipore Dress-It Pre-Cut Dressing Covers by 3M	Medline	MMM2955Z	Surgical Disposable
Meloxicam	Patterson Veterinary	14043-909-10	Medication, Post-Operative
Mixter thoracic forceps, 9”	V. Mueller	CH1730-003	Surgical Instrument
Mosquito hemostats	V. Mueller	88-0301	Surgical Instrument
Multi-Channel Research Consoles	Transonic	T402/T403	Equipment, Perivascular Flow Meter
Multi-Lumen Central Venous Catheterization Kits	Medline	ARW45703XP1AH	Surgical Disposable
Multi-Parameter Vital Signs Monitor	Smiths Medical	SurgiVet Advisor 3	Equipment
Needles: Hypodermic Needle with Regular Bevel, Sterile, 18 G x 1.5"	Medline	B-D305185Z	Surgical Disposable
No. 3 knife handle	V. Mueller	SU1403-001	Surgical Instrument
No. 7 knife handle	V. Mueller	SU1407	Surgical Instrument
Non-Vented Male Luer Cap	Qosina	13614	Disposable, Chronic PH
Octal Bridge Amplifier	ADInstruments	FE228	Equipment
Ophthalmic Ointment	Akorn Animal Health	59399-162-35	Medication, Pre-Operative
Penrose Tubing, 6 mm x 46 cm, 11 mm Flat	Medline	SWD514604H	Surgical Disposable
Perma-Hand Black Braided Silk:  2-0 SH Taperpoint Needle, Control Release, 30"	Medline	ETHD8552	Surgical Disposable
Perma-Hand Suture, Black Braided, Size 0, 6 x 30”	Medline	ETHA306H	Surgical Disposable
Perma-Hand Suture, Black Braided, Size 4-0, 12 x 30"	Medline	ETHA303H	Surgical Disposable
Phenylephrine	West-Ward	0641-6142-25	Medication, Intra-Operative
Polyhesive Cordless Patient Return Electrodes, Adult	Medline	SWDE7509	Surgical Disposable
Port-A-Cath Huber Needle, Straight, 22 G x 1-1/2"	Medline	AAKM21200724	Disposable, Chronic PH
PROLENE Monofilament Suture, Blue, Size 4-0, 36", Double Arm, RB-1 Needle	Medline	ETHD7143	Surgical Disposable
PROLENE Polypropylene Monofilament Suture, Blue, Double-Armed, RB-1 Needle, Size 5-0, 24"	Medline	ETH8555H	Surgical Disposable
Regional Block Needles, 22-gauge	Medline	B-D408348Z	Surgical Disposable
Schnidt tonsil artery forceps	V. Mueller	M01700	Surgical Instrument
Skin staple extractor	Medline	CND3031	Disposable, Chronic PH
Skin stapler 35 wide, with counter	Medline	STAPLER35W	Surgical Disposable
Sphygmomanometer	Patterson Veterinary	07-815-0464	Equipment
Sponge bowl	V. Mueller	GE-75	Surgical Instrument
Sponge, Lap: X-Ray Detectable Sterile Lap Sponge, 18" x 18", 5/Pack	Medline	MDS241518HH	Surgical Disposable
Sponge, Peanut: X-Ray Detectable Sterile Peanut Sponge, Small, 3/8"	Medline	MDS72038	Surgical Disposable
Sterile Disposable Deluxe OR Towel, Blue, 17'' x 27'', 2/Pack	Medline	MDT2168202	Surgical Disposable
Sterile Luer-Lock Syringe, 3 mL	Medline	SYR103010Z	Disposable, Chronic PH
Sterile Luer-Lock Syringe, 5 mL	Medline	SYR105010Z	Disposable, Chronic PH
Sterile Surgical Equipment Probe Covers	Medline	DYNJE5930	Surgical Disposable
Stopcock: 3-Way Stopcock with Handle in OFF Position, Rotating Adaptor Male Collar Fitting, 45 PSI	Medline	DYNJSC301	Surgical Disposable
Stopcock: 3-Way Stopcock with Handle in OFF Position, Rotating Adaptor Male Collar Fitting, 45 PSI	Medline	DYNJSC301	Disposable, Chronic PH
Subcutaneous Port with 5-French Connector and Blue Boot	Access Technologies	CP2AC-5NC	Surgical Disposable
Super cut metzenbaum dissecting scissors	V. Mueller	CH2032-S	Surgical Instrument
Super cut nelson-metzenbaum dissecting scissors	V. Mueller	CH2025-S	Surgical Instrument
Syringes: Sterile Luer-Lock Syringe, 10 mL	Medline	SYR110010Z	Surgical Disposable
Thoracic Catheter, Straight, 28 Fr x 20"	Medline	SWD570549H	Surgical Disposable
Three-quarter surgical drape	Medline	DYNJP2414H	Surgical Disposable
Tiletamine + Zolazepam	Zoetis Inc	54771-9050-1	Medication, Pre-Operative
TourniKwik Tourniquet Set with Four 7.5" Bronze-Colored Tubes and 1 Snare, 12 French	Medline	CVR79013	Surgical Disposable
Transducer clip	Edwards LifeScience	TCLIP05	Equipment
Trigger Aneroid Gauge (Sphygmomanometer)	Patterson Veterinary	07-815-0464	Equipment
TruWave Disposable Pressure Transducer Kits by Edwards Lifesciences	Medline	VSYPX260	Surgical Disposable and Chronic PH
TS420 Perivascular Flow Module	Transonic	TS420	Equipment, Perivascular Flow Meter
Tubing, Suction: Sterile Universal Suction Tubing with Straight Ribbed Connectors, 1/4" x 12'	Medline	OR612	Surgical Disposable
Tubing: Pressure Monitoring Tubing with Fixed Male Luer Lock and Female Fitting, Low Pressure, 72" L	Medline	DYNJPMTBG72MF	Surgical Disposable
Tubing: Pressure Monitoring Tubing with Fixed Male Luer Lock and Female Fitting, Low Pressure, 72" L	Medline	DYNJPMTBG72MF	Disposable, Chronic PH
Tubular Elastic Dressing Retainer	Medline	DERGL711	Disposable, Chronic PH
Tuffier rib retractor	V. Mueller	CD1101	Surgical Instrument
Tygon E-3603 Flexible Tubings	Fisher Scientific	14-171-227	Surgical Disposable
U.S.A retractor	V. Mueller	SU3660	Surgical Instrument
Umbilical Tape, Cotton, 3-Strand, 1/8 x 36"	Medline	ETHU12TH	Surgical Disposable
Valleylab Button Switch Pencil	Medline	VALE2516H	Surgical Disposable
Vanderbilt deep vessel forceps	V. Mueller	CH1687	Surgical Instrument
Veterinary Anesthesia Machine	Midmark	Matrx VMC	Equipment
Veterinary Anesthesia Ventilator	Hallowell EMC	Model 2000	Equipment
Vicryl: Undyed Coated Vicryl 0 CT-1 36" Suture	Medline	ETHVCP946H	Surgical Disposable
Vicryl: Undyed Coated Vicryl 2 TP-1 Taper 54" Suture	Medline	ETHVCP880T	Surgical Disposable
Vicryl: Undyed Coated Vicryl 2-0 CT-1 18" Suture	Medline	ETHVCP739D	Surgical Disposable
Vital crile-wood needle holder, 10-3/8”	V. Mueller	CH2427	Surgical Instrument
Vital mayo-hegar needle holder, 7-1/4”	V. Mueller	CH2417	Surgical Instrument
Vital metzenbaum dissecting scissors, 14’’	V. Mueller	CH2009	Surgical Instrument
Vital metzenbaum dissecting scissors, 9”	V. Mueller	CH2006	Surgical Instrument
Vital ryder needle holder, 9”	V. Mueller	CH2510	Surgical Instrument
Yankauer, Bulb Tip: Sterile Rigid Yankauer with Bulb Tip, No Vent	Medline	DYND50130	Surgical Disposable