Image-Guided Percutaneous Technique for Direct Pleural Pressure Measurement in a Swine Model

Summary

This protocol describes a minimally invasive percutaneous technique for inserting anterior and posterior pleural catheters in swine, enabling direct measurement of regional pleural pressure without thoracotomy.

Abstract

Pleural pressure measurement enables advanced physiologic assessment, including partitioning respiratory system mechanics into lung and chest wall components, calculating transpulmonary pressure, and quantifying regional pleural pressure gradients. This information is essential for optimizing mechanical ventilation, assessing the impact of altered chest wall physiology, and advancing translational research in lung injury. However, current techniques for direct pleural pressure measurement often require open-chest surgical implantation of pleural pressure transducers via thoracotomy, which is time-consuming, high-risk, and may impair chest wall mechanics. To overcome these limitations, we developed a minimally invasive, percutaneous, image-guided technique for pleural catheter placement. The procedure was performed in anesthetized, mechanically ventilated swine. Under ultrasound guidance, a Tuohy needle was advanced into the pleural space, and a small artificial pneumothorax (50–100 mL of oxygen) was created to separate the pleural layers. Over a guidewire, a 9 Fr sheath introducer was placed, allowing advancement of a balloon-tipped catheter into the anterior pleura. The animal was then turned in a semiprone position, and a second catheter was inserted dorsally after increasing the same pneumothorax by an additional 50–100 mL of oxygen. Placement was confirmed by computed tomography (CT) and physiologic tracings. After evacuation of the pneumothorax and lung re-expansion, imaging verified the position. Successful anterior and posterior catheter placement was achieved in 16 of 20 animals (80%), supporting the procedural feasibility of this approach. Unsuccessful procedures were due to unintended pneumothorax during anterior pleural access or anterior catheter malposition outside the target ventral region. This method provides a less invasive and reproducible approach for direct regional pleural pressure monitoring in large-animal models. By avoiding surgical access, this percutaneous, image-guided technique is well-suited for experimental protocols investigating respiratory mechanics, lung and chest wall physiology, and mechanical ventilation in translational research.

Introduction

Pleural pressure has been widely used in experimental studies as a key indicator of chest wall mechanics, helping to describe the partitioned behavior of the respiratory system and its interaction with the lung during mechanical ventilation¹. This variable is essential for calculating transpulmonary pressure, which represents the primary determinant of lung stress². In lung injury, pleural pressure can also help describe end-expiratory mechanics and provide insight into the superimposed pressure gradient, a phenomenon linked to the load acting on dependent lung regions^3,4. This regional pressure distribution creates ventro-dorsal differences in pleural and transpulmonary pressure, which during controlled mechanical ventilation are mainly determined by the gravitational load and the severity of lung injury, whereas during spontaneous or assisted breathing, they may be further amplified by regional inspiratory muscle activity⁵.

Direct pleural pressure monitoring is not performed in clinical practice because the procedure is too invasive to be considered safe. For this reason, esophageal pressure (Pes) is commonly used as the best surrogate of pleural pressure⁶. However, Pes is influenced by several factors that can affect its accuracy, including catheter position, pressure transmission assessed by the Baydur ratio, and ventilation mode⁷. The esophageal balloon sits inside a compliant structure, and its signal is shaped by both the esophageal wall and adjacent mediastinal organs. Even minor shifts in catheter depth or alignment may change the measured pressure or the size of its tidal fluctuations⁸. Moreover, Pes primarily reflects mid-to-dorsal pleural pressure, making it less representative of ventral regions and the overall pleural environment in heterogeneous lung injury⁹.

For these reasons, direct pleural pressure measurement remains the gold-standard approach in translational and experimental physiology¹⁰. Several devices have been used for direct pleural pressure measurement. Wafer-like pleural sensors are frequently used in experimental models^11,12, and in some studies, esophageal balloon catheters have also been introduced into the pleural space to record pleural pressure directly^13,14. Despite the availability of different tools for direct pleural pressure measurement, their insertion has traditionally required either open thoracotomy or thoracoscopic access. These approaches are invasive, require advanced surgical expertise, prolong procedural preparation, and disrupt normal chest wall mechanics, significantly altering the physiological conditions under investigation.

In this method article, we present a novel percutaneous technique for placing esophageal balloon catheters into the pleural space of swine for direct regional pleural pressure monitoring. This approach is minimally invasive and does not require thoracotomy or advanced surgical training. Anterior pleural access is performed using a standard ultrasound-guided puncture technique, while computed tomography (CT) imaging is used to confirm placement of the intrapleural guidewire and catheter and to assist in planning posterior pleural access. The method allows placement of catheters in different regions of the pleural cavity for regional assessment of partitioned respiratory mechanics. The technique aims to reduce procedural complexity and risk while preserving high-fidelity pleural pressure recordings for physiological and translational research.

Protocol

After Institutional Animal Care and Use Committee approval (2020I000073) at Massachusetts General Hospital, Boston, MA, experiments were performed from March to November 2025 in 20 healthy Yorkshire pigs weighing 30–40 kg.

CAUTION: This protocol intentionally creates a pneumothorax and uses 100% oxygen. Perform the procedure under continuous physiologic monitoring by personnel trained in large-animal anesthesia and pleural access and eliminate ignition sources during oxygen insufflation.

1. Animal preparation

1. Sedate the animal with intramuscular tiletamine–zolazepam (4.4 mg/kg), xylazine (2.2 mg/kg), and atropine (0.04 mg/kg).
2. Establish intravenous access and administer fentanyl (2 µg/kg) and propofol (2 mg/kg) intravenously to allow safe endotracheal intubation.
3. Intubate the animal and confirm correct airway placement by verifying bilateral chest expansion, bilateral breath sounds, and the presence of a consistent end-tidal CO₂ waveform.
4. Start volume-controlled ventilation (VCV) with a fraction of inspired oxygen (FiO₂) of 0.4, tidal volume of 10 mL/kg, positive end-expiratory pressure (PEEP) of 5 cmH₂O, and respiratory rate of 15–20 breaths/min. Adjust the respiratory rate to maintain end-tidal CO₂ between 35 and 45 mmHg.
5. Maintain continuous anesthesia and analgesia throughout the procedure with intravenous propofol (10 mg∙kg^-1∙h^-1) and fentanyl (5 µg∙kg^-1∙h^-1). Before administration of neuromuscular blockade, confirm adequate anesthetic depth by assessing jaw relaxation, absence of palpebral reflex, absence of withdrawal response to interdigital or hoof pinch, and absence of hemodynamic response to stimulation. After neuromuscular blockade, monitor anesthetic adequacy using physiological variables, including heart rate, arterial pressure, and absence of autonomic responses to procedural stimulation. Apply ophthalmic ointment after induction of anesthesia to prevent corneal drying.
6. Monitor heart rate, blood pressure, oxygen saturation, end-tidal CO₂, and airway pressures continuously. Maintain heart rate at 70–140 beats/min, mean arterial pressure above 60 mmHg, oxygen saturation above 92%, end-tidal CO₂ between 35 and 45 mmHg, and peak airway pressure below 35 cmH₂O whenever possible.
7. Administer a neuromuscular blocking agent (pancuronium 0.1 mg∙kg^-1∙h^-1) and confirm the absence of spontaneous breathing activity before beginning pleural access.

2. Equipment preparation

1. Prepare the sterile equipment required for ultrasound-guided pleural access and catheter placement, as listed in the Table of Materials.
2. Clip, clean, and drape the anterior and posterior thoracic surfaces under sterile conditions.

3. Anterior percutaneous pleural access

1. Position the animal in a supine position with a slight rotation toward the side opposite the planned anterior pleural catheter insertion site.
2. Identify the sternal line and the diaphragmatic dome using ultrasound.
3. Identify the diaphragmatic dome using ultrasound, then move approximately 5 cm laterally and 5 cm cranially from the midline at the level of the diaphragm to select the anterior puncture site.
4. Visualize the anterior pleural space with the ultrasound probe.
5. Insert the ultrasound-enhanced Tuohy needle at the upper margin of the inferior rib of the selected intercostal space, with the curvature oriented upward.
6. Advance the needle toward the parietal pleura under real-time ultrasound guidance using an in-plane approach.
7. Apply gentle positive pressure to the syringe while advancing 1–2 mm further during an end-expiratory hold maneuver, until a sudden loss of resistance indicates entry into the pleural space.
8. Inject approximately 50–100 mL of 100% oxygen to create a small anterior artificial pneumothorax. Adjust the final oxygen volume based on computed tomography findings of pneumothorax expansion and pleural separation.
9. Insert the spring-wire guide through the needle and remove the needle.
10. Perform a CT scan to confirm correct guidewire placement within the pleural space, which is enlarged and clearly delineated by the artificial pneumothorax.
11. After CT confirmation, perform a small skin incision at the wire entry site.
12. Advance a 9 Fr introducer over the guidewire using the Seldinger technique.
13. Remove the guidewire and insert the esophageal balloon catheter into the anterior pleural cavity through the introducer.
14. Inflate the balloon with the optimal filling volume, determined from an in vivo balloon pressure-volume (PV) curve¹⁵, to ensure correct balloon expansion and reliable pressure transmission.
15. Connect the balloon catheter to an external differential pressure transducer using an air-filled pressure transmission system.
16. Zero the pressure transducer to atmospheric pressure before connecting the catheter to ensure accurate pressure measurements.
17. Connect the pressure transducer to a data acquisition system and acquire pressure signals using data acquisition software.
18. Acquire all pressure signals at a sampling frequency of 1,000 Hz without additional digital filtering. Begin recording once a stable pressure tracing is observed.

4. Posterior percutaneous pleural access

1. Reposition the animal in a semiprone position to promote dorsal migration of the intrapleural air.
2. Inject an additional 50–100 mL of 100% oxygen through the anterior catheter to enlarge the dorsal pneumothorax and facilitate posterior access.
3. Perform a CT scan to confirm dorsal migration of the artificial pneumothorax and identify the optimal posterior puncture area.
4. Mark the posterior puncture site on the skin halfway between the spinous processes and the posterior axillary line.
5. Insert the Tuohy needle perpendicularly at the upper margin of the inferior rib of the selected intercostal space, attaching an empty syringe for continuous aspiration.
6. Advance the needle during an end-expiratory hold maneuver through the muscular planes while aspirating continuously until air is obtained, confirming entry into the dorsal pneumothorax.
7. Insert the spring-wire guide and remove the needle.
8. Perform a CT scan to confirm correct guidewire placement within the pleural space, which is enlarged and clearly delineated by the artificial pneumothorax.
9. After CT confirmation, perform a small skin incision at the wire entry site.
10. Advance the 9 Fr introducer over the guidewire using the Seldinger technique.
11. Remove the guidewire and insert the second esophageal balloon into the dorsal pleural cavity through the introducer.
12. Inflate the balloon with the optimal filling volume, determined from an in vivo balloon pressure-volume (PV) curve, to ensure correct balloon expansion and reliable pressure transmission.
13. Connect the catheter to the differential pressure transducer and verify correct pleural waveform morphology on the monitor.

5. Final positioning and monitoring

1. Verify that ventilator parameters (tidal volume, inspiratory flow, and airway pressure) and hemodynamic values remain stable during and after catheter placement.
2. Drain the pneumothorax through the posterior and anterior introducers by gently aspirating intrapleural air with a 50 mL syringe until there is no resistance to drainage. Avoid creating excessive negative pressure within the pleural space.
3. Secure both esophageal balloon catheters to the skin with adhesive tape, and then carefully remove both 9 Fr introducers to prevent catheter displacement.
4. Perform a CT scan to assess residual pneumothorax. If residual pneumothorax is present, place the animal in the prone position and reduce FiO₂ to 0.25, if physiologically tolerated, to promote oxygen reabsorption.
5. Confirm the absence of an airway or pleural leak by comparing inspired and expired tidal volumes and performing an inspiratory hold. Verify a stable plateau pressure and return of flow to zero.
6. Reassess the anterior and posterior pleural pressure waveforms after introducer removal. Confirm that the expected waveform morphology remains stable; if it changes, do not begin recording until catheter position and waveform stability have been reassessed.
7. Begin simultaneous anterior and posterior pleural pressure recording only after confirming pneumothorax evacuation or reabsorption, stable ventilation and hemodynamics, absence of an ongoing leak, and stable pleural pressure waveforms.
8. Adjust ventilation keeping TV at 10 mL/kg and reducing the PEEP from 5 to 3 cmH₂O. The end-expiratory pleural pressure values were obtained during a stable end-expiratory hold maneuver as shown in Figure 1.
9. At the end of each experimental protocol, while the animals were maintained under general anesthesia, euthanasia was performed by intravenous administration of KCl (7% solution, 2 mL/kg).

Results

Pleural catheter placement using the percutaneous technique was attempted in 20 swine (Table 1). In 17 animals, a total of 34 pleural catheters (17 anterior and 17 posterior) were inserted. Of these, 33 catheters (97%) reached the intended pleural region, whereas one anterior catheter entered the pleural cavity but failed to reach the target ventral location. Correct regional placement of both anterior and posterior catheters was therefore achieved in 16 of the 17 animals in which catheter insertion was ...

Discussion

This percutaneous protocol provides a practical and reproducible method to obtain regional pleural pressure measurements without the need for thoracotomy or thoracoscopic access. Several aspects of the procedure are particularly important for ensuring successful catheter placement and high-fidelity pressure recordings.

A key element of the technique is the anterior puncture performed under direct thoracic ultrasound guidance. Ultrasound is used to clearly visualize the pleura and the chosen in...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Adhesive Tape	3M Tegaderm	1624W	Catheter fixation before introducer
Animal Clipper	3M Clipper	9661L	Thoracic hair removal
Atropine (Injectable)	Sparhawk Laboratories inc.	NDC:58005-354	Intramuscular premedication
Computed tomography scanner	NeuroLogica	Omnitom Elite PCD; 1-NL5100-060	Guidewire/catheter position and pneumothorax assessment;
Data acquisition software	ADInstruments	LabChart Pro 8.1.31	Acquisition Software at 1,000 Hz without digital filtering
Data acquisition system	ADInstruments	ML865	For pleural pressure acquisition
Differential pressure transducer	Harvard Apparatus / Hugo Sachs	73-3882	Pressure transducer for pleural pressure recording
Disposable OR towel	Cardinal Health	CAT 28700-004	Sterile field preparation (Step 2.1)
ECG & pulse oximetry monitor	Mindray	BeneView T5	Vital Parameters monitoring
Endotracheal tube, 7.0 mm	Teleflex Medical	REF 5-10314	Endotracheal Intubation
Esophageal balloon catheter	Cooper Surgical	REF 47-9005	Used as intrapleural pressure catheter (Step 2.1)
Fentanyl (injectable)	Hikma	NDC 0641-6030-01	IV analgesia
Gauze pads (sterile)	Medline	REF NON21430LF	Site preparation (Step 2.1)
Guidewire 0.025" × 33 cm	Teleflex Medical	REF AW-04025	For Seldinger technique (Step 2.1)
Infusion pump	Smiths Medical	Medfusion 4000	Continuous anesthetic delivery
Introducer sheath, 9 Fr	Cordis	REF 402-609X	For intrapleural catheter insertion (Step 2.1)
Mechanical Ventilator	Maquet – Getinge		Servo-I, Volume-controlled ventilation
Ophtalmic ointment	Lodi Veterinary Care	NDC 17033-211-38	Corneal protection after anesthesia induction
Oxygen 100% (medical grade)	Airgas	N/A	For controlled pneumothorax
Pancuronium (injectable)	Selleck. Chemicals	S2497	Neuromuscular blockade
Povidone–Iodine Solution	BICCA	CAT 3955-16	Skin antisepsis
Propofol (injectable)	NorthStar	NDC 16714-977-01	Induction and maintenance
Respiratory circuit (ventilator circuit)	Medline	REF NON026370	Standard circuit
Scalpel No. 11	Bard-Parker	REF 372611	For skin incision (Step 2.1)
Steril Drape	Medline	REF MDT2168286	Sterile field preparation
Sterile gloves (latex)	Biogel	REF 31280	Sterile protective gloves
Sterile gown	Halyard	REF 95121	Sterile surgical gown
Syringes 10 mL	Medline	REF SYR110010	For aspiration/injection (Step 2.1)
Syringes 50 mL	BD	REF 309680	For pleural air aspiration (Step 2.1)
Tiletamine–zolazepam (Injectable)	Virbac	NADA 71805-06	Telazol; intramuscular sedation
Tuohy needle, echogenic 18G × 100 mm	Pajunk	REF 521185-31C	For pleural puncture (Step 2.1)
Ultrasound probe (Butterfly iQ+)	Butterfly Network	950-20002-00	Used with Butterfly iQ App for iPad (Step 2.1)
Ultrasound software/application	Butterfly Network	Butterfly iQ 2.46; iPad 11 iPadOS 26	Butterfly iQ app for iPad
Xylazine (Injectable)	Cronus Pharma	LLC