The pregnant ewe: an animal model for fetoscopic surgery

Advancements in medical research using fetal sheep: Implications for human health and treatment methods

Sheep are typically considered as industrial animals that provide wool and meals. However, they play a significant role in medical research in addition to their conventional use. Notably, sheep fetuses are resistant to surgical invasions and can endure numerous manipulations, such as needle puncture and cell transplantation, and surgical operations requiring exposure beyond the uterus. Based on these distinguishing characteristics, we established a chimeric sheep model capable of producing human/monkey pluripotent cell-derived blood cells via the fetal liver. Furthermore, sheep have become crucial as human fetal models, acting as platforms for developing and improving techniques for intrauterine surgery to address congenital disorders and clarifying the complex pharmacokinetic interactions between mothers and their fetuses. This study emphasizes the significant contributions of fetal sheep to advancing human disease understanding and treatment strategies, highlighting their unique characteristics that are not present in other animals.

The effects of partial amniotic carbon dioxide insufflation in an ovine model

OBJECTIVE

We aim to assess the effect of partial amniotic carbon dioxide insufflation (PACI) at increasing pressures on fetal acid-base, fetal-placental perfusion, and fetal membrane morphology in an ovine model.

METHOD

Pregnant ewes and fetuses were instrumented under isoflurane anesthesia at 105 days gestation (term 145 days) to monitor utero-placental blood flow, fetal and maternal blood pressure, heart rate, and blood gas status. One group (n = 6) was exposed to PACI (unheated dry CO₂ ), involving 10 mm Hg stepwise increases in insufflation pressure (5 to 25 mm Hg), for 80 minutes followed by 20 minutes of desufflation. Un-insufflated controls (n = 5) were monitored for 100 minutes. At postmortem, fetal membranes were collected for histological analysis.

RESULTS

PACI at 25 mm Hg caused severe fetal hypercapnia (PaCO₂  = 143 ± 5 vs 54 ± 5 mm Hg, P < 0.001), acidosis (pH = 6.85 ± 0.02 vs 7.25 ± 0.02, P < 0.001), hypoxia (SaO₂  = 31 ± 4% vs 57 ± 4%, P = 0.01), and reduced uterine artery flow (50 ± 15 vs 196 ± 13 mL/min/kg, P = 0.005) compared with controls. These effects were greater at higher PACI pressures. PACI resulted in leukocyte infiltration in the amnion (1.77 × 10^-5  ± 0.61 × 10^-5 vs 0.38 × 10^-5  ± 0.19 × 10^-5  cells/μm² , P = 0.04) and chorionic membranes (2.94 × 10^-5  ± 0.67 × 10^-5 vs 0.84 × 10^-5  ± 0.42 × 10^-5  cells/μm² , P = 0.01).

CONCLUSION

Higher PACI pressures results in larger disturbances in fetal acid-base, uterine blood flow, and fetal membrane inflammation in sheep. Differences between human and sheep utero-placental structure should be considered.

Partial amniotic carbon dioxide insufflation for fetal surgery

Partial amniotic carbon dioxide insufflation (PACI) involves insufflating the amniotic sac with carbon dioxide (CO₂ ) and, in some cases, draining some of the amniotic fluid. The creation of a gaseous intra-amniotic compartment improves visualization, even in the presence of limited bleeding, and creates the work space required for complex fetoscopic procedures. Clinically, PACI is mostly used to perform fetoscopic myelomeningocele (MMC) repair, enabling a minimally invasive alternative to open fetal surgery. However, evidence of the fetal safety of PACI is limited. Previous animal experiments in sheep demonstrate that PACI induces fetal hypercapnia and acidosis with largely unknown short and longer term implications. In this review, we examine the literature for the physiological effects of intrauterine insufflation pressure, duration, humidity, and the role of maternal hyperventilation on fetal physiology and well-being.

Domestic animal models for biomedical research

Experimental animals in biomedical research provide insights into disease mechanisms and models for determining the efficacy and safety of new therapies and for discovery of corresponding biomarkers. Although mouse and rat models are most widely used, observations in these species cannot always be faithfully extrapolated to human patients. Thus, a number of domestic species are additionally used in specific disease areas. This review summarizes the most important applications of domestic animal models and emphasizes the new possibilities genetic tailoring of disease models, specifically in pigs, provides.

The use of ruminant models in biomedical perinatal research

Animal models are of critical importance in biomedical research. Although rodents and lagomorphs are the most commonly used species, larger species are required, especially when surgical approaches or new medical devices have to be evaluated. In particular, in the field of perinatal medicine, they are critical for the evaluation of new pharmacologic treatments and the development of new invasive procedures in fetuses. In some areas, such as developmental genetics, reproductive biotechnologies and metabolic programming, the contribution of ruminants is essential. The current report focuses on some of the most outstanding examples of great biomedical advances carried out with ruminant models in the field of perinatal research. Experiments recently carried in our research unit using ruminants are also briefly described.

A comparative biomechanical analysis of term fetal membranes in human and domestic species

OBJECTIVE

The purpose of this study was to biomechanically characterize and compare human, porcine, equine, and ovine fetal membranes.

STUDY DESIGN

Noncontact metrology was used for topographic analyses. Uniaxial tensile testing was performed to resolve specific biomechanical values. Puncture force and radial stresses were determined with biaxial puncture testing. Microstructure and surface tortuosity were analyzed histologically.

RESULTS

Equine and human membranes sustained larger magnitude loading, but ovine and porcine membranes exhibited stronger material properties. Biaxial puncture validated uniaxial results; human and equine groups accommodated the largest loads but lowest stresses. Equine membranes were mostly vascularized; tortuosity was highest in porcine membranes. Species' gestation length was correlated positively with membrane thickness.

CONCLUSION

The anatomy of placentation and length of species gestation show distinct relationships to membrane biomechanics. Unlike other species, human fetal membranes do not compensate for structural weakness with a thicker membrane. This finding may explain the high incidence of preterm premature rupture of membranes in humans.

Gasless fetoscopy: a new approach to endoscopic closure of a lumbar skin defect in fetal sheep

OBJECTIVE

To develop a new endoscopic approach to the correction of a myelomeningocele-like defect in fetal sheep.

METHODS

The fetuses of 9 pregnant ewes, with an average gestational age of 115 days, were subjected to a 3.0 x 2.0 cm removal of the skin over the lumbar spine, performed through hysterotomy. The uterus was closed, and three 5-mm endoscopic cannulas, without valve mechanisms, were inserted. In the pilot phase (2 animals), we initially worked exclusively in the amniotic fluid space. In the study phase, we partially withdrew the fetus from the amniotic fluid to completely expose its back. By simply allowing air to enter the amniotic cavity (without gas injection), a working space was created using a uterine lift device. The skin around the defect was dissected, and a biosynthetic cellulose material was applied to cover the area. A continuous suture of the skin was performed to completely hide the material.

RESULTS

The combined air/fluid space allowed the skin to be successfully closed in 6 out of 7 cases in the study phase. All fetuses were alive at the end of the procedures. Time to complete the endoscopic part of the procedure fell from 3 to 1 h by the end of this series. Premature birth occurred in 2 of the 4 cases allowed to continue with the pregnancy.

CONCLUSION

A new gasless fetoscopic surgery technique was developed as an alternative to current techniques used for fetal endoscopic surgery.

Radiofrequency driven cord occlusion for selective termination of pregnancy: evaluation in the fetal sheep

OBJECTIVE

This study was designed to assess the ability of an ultrasound-guided radiofrequency (RF)-driven procedure to induce complete and irreversible cord occlusion using a 90 days fetal sheep model.

STUDY DESIGN

Twenty 90 days gestation sheep underwent general anesthesia. The first ten fetuses were exposed under hysterotomy, and RF electrode was inserted visually in the middle of the umbilical cord and deployed. Fetuses were then replaced into the amniotic fluid and RF procedure (average target temperature of 100 degrees C during 10 minutes) was applied. For the next ten fetuses, RF electrode was inserted into the cords under trans-parietal ultrasound guidance and the same RF procedure was applied. Cord occlusion was assessed by Doppler examination (absence of cordonal flows at the end of the procedure and until fetal heart failure occurred) and by subsequent histopathological analysis.

RESULTS

Cord occlusion was always complete at Doppler examination at the end of RF procedure for the ten experiments realized under hysterotomy. No cordonal reperfusion was observed until fetal heart failure. Histopathological analysis confirmed cordonal occlusion at the site of impact. Neither cordonal rupture nor cordonal bleeding was observed for any of the ten experiments. When RF electrode was inserted under ultrasound guidance, complete occlusion could be obtained only for 6 of the ten experiments.

CONCLUSION

Our results suggest that RF might be an appropriate method for selective termination of pregnancy. Yet, optimal insertion of the electrode is required to engender a complete and irreversible cord occlusion, and ultrasound-guidance training seems necessary before current human application.

Minimal access fetal surgery

The development of fetal surgery has led to promising therapeutic options for a number of congenital malformations. However, preterm labor (PTL) and premature rupture of membranes continue to be ubiquitous risks for both mother and fetus. To reduce maternal morbidity and the risk of prematurity, minimal access surgical techniques were developed and are increasingly employed. Congenital diaphragmatic hernia (CDH), obstructive uropathy, twin-to-twin transfusion syndrome (TTTS), and sacrococcygeal teratoma have already been successfully treated using minimal access fetal surgical procedures. Other life-threatening diseases as well as severely disabling but not life-threatening conditions are potentially amenable to treatment. The wider application of minimal access fetal surgery depends on a continued improvement in technology and a better understanding of complications associated with fetal intervention.

Evolution of a percutaneous fetoscopic access system for single-port tracheal occlusion

BACKGROUND/PURPOSE

Prenatal tracheal occlusion currently is being assessed as a treatment modality for congenital diaphragmatic hernia (CDH). The development of a totally percutaneous fetoscopic access system would help avoid the need for maternal laparotomy and reduce the morbidity rate of fetal surgical procedures for the mother. Laparoscopic radial expansion sheaths and Seldinger technique-based vascular catheters both have been advocated as means of achieving amniotic cavity access. The authors have investigated these 2 systems in an attempt to develop a reliable method for achieving safe percutaneous fetoscopic access and present the first successful attempt to deploy an intratracheal balloon using an entirely percutaneous approach through a single port in an ovine model.

METHODS

A number of prototype systems were evaluated sequentially over a 3-year period in an ovine model: (1) the radially expanding InnerDyne step port system, (2) a new rigid cannula with a bulbous/sharp end preloaded onto the radially expanding InnerDyne port, (3) a conical removable addition to the rigid cannula in 2, (4) a modified bulbous/sharp ended cannula incorporating a circumferential protective insert, (5) a rigid split sheath with the radially expanding port placed through the lumen of the split sheath, (6) a flexible introducer and dilator with the split sheath (used in the Seldinger placement of central lines), and (7) a 2-needle approach using a superelastic shape-memory alloy Nickel-Titanium wire with the flexible dilator and sheath, incorporating a side perfusion port. For balloon tracheal occlusion, live anaesthetized time-mated pregnant ewes were used at 110 days' gestation. Tracheobronchoscopy was achieved using a 3-mm 0 degrees telescope, and the cutaneotracheal tract was secured by a 3.3-mm sheath incorporating a side-perfusion port. The rigid telescope was replaced by a flexible choledochoscope preloaded with a silicone balloon. The balloon was deployed 2 cm above the carina proximal to the right upper lobe bronchus.

RESULTS

The many problems encountered in the evolution of the preferred system related mainly to separation and tenting of the chorioamniotic membranes in the ovine uterus and inconsistent access to the fetal parts of interest. Each resulted in significant modifications to our approach. Furthermore, the use of rigid access devices commonly caused fetal injury. Successful access to the intrauterine cavity and cannulation of the trachea was achieved consistently with minimal trauma, irrespective of fetal position by method 7. Multiple port placement allowed visualization of the entry of all components of the system confirming minimal chorioamniotic membrane separation and tenting. Single port tracheal occlusion was undertaken first on 6 cadavers before being performed successfully on 3 live anaesthetized ewes. Fetoscopic access and cannulation of the trachea was achieved consistently in all live animals irrespective of fetal position.

CONCLUSIONS

This modified Seldinger technique using the unique properties of the memory-shaped alloy wire for initial uterine access offers a safe method for the percutaneous placement of fetoscopic ports in the ovine model for prenatal intervention. Successful placement of a tracheal balloon entirely through a single percutaneously placed port represents a further advance in prenatal therapy for CDH.

The combined method: a novel access technique for fetal endoscopic surgery

BACKGROUND/PURPOSE

To develop practical and less invasive techniques for fetal endoscopic surgery, new methods of lifting the uterine wall to allow fetal surgery without maternal laparotomy were developed and assessed.

METHODS

Fetal endoscopic surgical procedures, including tracheostomy and umbilical vascular cannulation, were performed using one of the three methods to enter the uterus without maternal laparotomy in pregnant goats (n = 6; 105 to 115 days' gestation): (1) direct uterine lifting with an air-cushion device; (2) indirect uterine lifting, in which the uterine wall was fixed to the maternal abdominal wall using balloon tip ports inserted percutaneously by Seldinger's method, then the maternal abdomen was lifted mechanically; and (3) combined method, in which low pressure CO2 (5 mm Hg for initial inflation and 2 mm Hg for maintenance) was insufflated into the uterus in addition to the indirect uterine lifting cited above.

RESULTS

The direct uterine lifting caused massive injury of myometrium and uterine membranes. The creation of intrauterine space and the protection of the membranes were not accomplished effectively by the indirect uterine lifting only. The combined method provided the adequate intrauterine space and excellent endoscopic visibility for completion of the endoscopic procedures with minimal uterine injury.

CONCLUSION

The fetal endoscopic surgery may be accomplished simply and safely by the combined method, a novel technique of uterine lifting to allow fetal surgery without maternal laparotomy.