In-utero stenting: development of a low-cost high-fidelity task trainer

Authors

  • J. F. Nitsche,

    1. Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Mayo Clinic College of Medicine, Rochester, MN, USA
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  • D. T. McWeeney,

    1. Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Mayo Clinic College of Medicine, Rochester, MN, USA
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  • W. D. Schwendemann,

    1. Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Mayo Clinic College of Medicine, Rochester, MN, USA
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  • C. H. Rose,

    1. Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Mayo Clinic College of Medicine, Rochester, MN, USA
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  • N. P. Davies,

    1. Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Mayo Clinic College of Medicine, Rochester, MN, USA
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  • W. Watson,

    1. Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Mayo Clinic College of Medicine, Rochester, MN, USA
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  • B. C. Brost

    Corresponding author
    1. Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology, Mayo Clinic College of Medicine, Rochester, MN, USA
    • Department of Obstetrics and Gynecology, Mayo Clinic, 200 First Street, Rochester, MN 55905, USA
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Abstract

Objective

To develop an in-utero stent placement training model.

Methods

The in-utero stent task trainer was constructed using a formalin-preserved gravid pig uterus. Altering the size of the uterine segment, changing the fluid level in the uterus and addition of a large Ziploc freezer bag variably filled with differing amounts of ultrasound gel can vary the procedural skill required.

Results

Thoracoamniotic and vesicoamniotic shunts can be simulated using this life-like model. The cost of eight to 10 learning stations is approximately US $ 60. Fetal position, maternal size and amniotic fluid status can be altered rapidly to increase the complexity of the procedure.

Conclusions

This low-cost and realistic task trainer can provide the opportunity to practice in-utero shunt procedures in a non-clinical environment. This model should enhance learning and reinforce acquired skills. Copyright © 2009 ISUOG. Published by John Wiley & Sons, Ltd.

INTRODUCTION

Lower urinary tract obstruction (complicating approximately 2.2 per 10 000 births) and fetal hydrothorax (complicating approximately one in 15 000 pregnancies) are both rare congenital defects1–3, and are not always amenable to therapy. In-utero stenting has been proposed as a potential therapeutic option under the right conditions for the management of fetal hydrothorax or lower urinary tract obstruction1–3.

The rarity of these conditions makes learning this invasive skill and obtaining proficiency in it prior to patient contact difficult. In the case of amniocentesis, this problem has been circumvented by the creation of an inexpensive task trainer using gravid pig uteri and fetal pigs4 that allows proficiency with amniocentesis to be achieved before performing procedures on patients. No such model for more complex needle-guided procedures has been described in the literature. Here we report a modification of the gravid pig uterus amniocentesis model into an in-utero stenting task trainer model that can realistically reproduce the relevant clinical conditions, and allow procedural practice to improve procedural times and potentially patient safety in an educational setting.

METHODS

Preparation of the uterine model

A preserved porcine uterus with approximately 12-cm fetuses is obtained from a biological supply company. Segments from each uterine horn containing a fetal pig are identified. Each segment of formalin-fixed uterus is incised to yield a tube approximately 20–25 cm in length. The fetal pig is removed temporarily after cutting the umbilical cord and removing any placental/membrane tissue from within the uterine segment. One end of the uterine segment is then closed with 1-inch binder clips, making a watertight seal. The formalin-fixed fetal pig is replaced into the uterus and the segment filled with water. The other end of the uterine segment is then closed with 1-inch binder clips, again being careful to obtain a watertight seal (Figure 1).

Figure 1.

Photograph of the prepared uterine model, prepared using a segment of gravid pig uterus.

Preparation of the fetal model

The fetal pig can be prepared to simulate pleural effusions or bladder outlet obstruction. To make a pleural effusion model, an approximately 1-cm hole is incised in the lateral fetal thorax through the skin, muscle and ribs. The thoracic cavity contents including the heart and both lungs are removed bluntly. The fetal ribs maintain the original thorax contour so closure of the defect is not required. With this technique a 1.5–2-cm thoracic space is developed (Figure 2) to receive the pigtail catheter.

Figure 2.

Photograph of the prepared fetal model. An approximately 1-cm hole was incised in the lateral thorax of the fetal pig and the thoracic cavity contents were removed.

To make a bladder outlet obstruction model, an approximately 1-cm hole incision is made in the paraspinal lumbar region. The abdominal cavity contents including the fetal bowel are removed bluntly. The incision is then closed with suture and water is instilled into the abdominal cavity with a syringe before each use. This technique will result in an approximately 3–4-cm abdominal space to receive the pigtail catheter.

Preparation of the in-utero stent task trainer

The developed pleural effusion or bladder outlet obstruction cavity is filled with water and the fetus is introduced into the uterine segment without spillage. The uterine tube is then filled from the open end until the segment is completely filled with water. One-inch paper binder clamps are used to make the uterine segment watertight, with care taken to exclude air from the uterus. The uterine segment is then rotated so that the incised hole faces the bottom/back of the uterine segment. The binder clip handles are positioned so that they will not interfere with the procedure.

The uterine segment is then coated with a generous layer of sonographic gel. A gallon-sized Ziploc freezer bag (Dow, Indianapolis, IN, USA) containing sonographic gel is placed on top of the uterine segment. On ultrasound imaging this bag of ultrasound gel has an appearance similar to that of subcutaneous adipose tissue and is used to simulate maternal subcutaneous tissues in the model (Figure 3a and b). It can be variably filled to achieve the desired abdominal wall thickness.

Figure 3.

Ultrasound images of the hydrothorax stent task trainer: sagittal view of fetal hydrothorax model (a), transverse view of fetal hydrothorax model (b), trocar (arrow) placement into the thorax (c), and stent (arrow) placement demonstrated after deployment (d). a, simulated amniotic fluid; f, fetal pig; s, simulated subcutaneous tissue.

RESULTS

The in-utero stent procedure can proceed in the typical fashion under ultrasound guidance using this life-like model. The cost of eight to 10 learning stations is approximately US $ 60. Fetal position, maternal size and amniotic fluid status can be altered rapidly to increase the complexity of the procedure. Sonographic examples of the hydrothorax task trainer are shown in Figure 3. At the completion of the procedure, the stent can be noted to be appropriately placed both sonographically (Figure 3d) and on direct inspection upon disassembling the task trainer model (Figure 4).

Figure 4.

Photograph showing correct thoracic shunt placement after ultrasound-guided stent placement. The model can be disassembled to directly verify correct stent placement in the fetal pig.

DISCUSSION

The model described here provides a low cost and realistic task trainer upon which clinicians can practice a rare and technically challenging procedure. The ultrasound gel-filled bag realistically reproduces the appearance of the subcutaneous tissue on ultrasound imaging and allows the learner to ‘redirect’ the needle within this space as is possible in the human patient. Although the gravid pig uterus and fetal pig are fixed tissues, they are not overly rigid and approximate the consistency of human uterus and fetus encountered in the clinical setting. Thus, the force needed to penetrate these tissues is similar to that required during stent insertion in the clinical setting. The model itself is small enough to be placed on a table-top or other convenient surface, but can be placed in a commercially available torso model to better simulate performing the procedure on a patient at the bedside.

A learning curve exists for all medical procedures and is especially evident in the invasive diagnostic needle procedures performed in pregnancy. The variability in maternal size, gestational age, type of procedure, placental location and target tissue or fluid all make mastery more difficult. Nizard et al. showed that learners can reach a similar performance level for amniocentesis on a training model using a free-hand technique after approximately 100 procedures, with or without the use of an electronic guidance system5. Most providers consider chorionic villus sampling to be a more difficult procedure to master and it is suggested that the learning curve may not plateau until after 175–200 procedures6. Importantly, Pittini et al. showed that the use of a curriculum based on a high-fidelity simulator could effectively be used to shift the steep portion of the learning curve away from patients through use of a simulation laboratory7. The learning curve for in-utero shunt procedures has not been established.

The number of births in the USA is approximately 4 million per year, so we can estimate that there are approximately 270 cases of fetal hydrothorax and approximately 530 cases of posterior urethral valves annually. A large number of these spontaneously resolve or are not considered candidates for in-utero therapy on initial testing. This means that a small number of cases undergo shunt placement, making it difficult for trainee specialists in maternal–fetal medicine to gain the necessary experience to safely and effectively perform fetoamniotic shunting and for more experienced clinicians to maintain procedure competence. In some parts of the country this has resulted in centralization of the procedure to a very small number of specialized centers. This can place a significant burden, physically, emotionally and financially, on the couple already dealing with a difficult pregnancy. In many cases patients are unable or unwilling to travel to these regional centers to have the procedure performed. The task trainer described here will allow trainee specialists in maternal–fetal medicine to gain proficiency with in-utero stenting techniques before performing them in a clinical setting. By rehearsing the procedural steps and practicing the stent placement skills, it would be expected that the actual clinical procedural time would be reduced and successful stent placement increased, allowing the procedure to be performed more safely when it is not possible to perform it at a regional center.

This low-cost, high-fidelity model can be reused more than 10–20 times for each prepared fetal pig, making it an inexpensive tool that can enhance the training experience of specialists in maternal–fetal medicine by allowing focused repetition of a rare in-utero procedure. The benefits of such a preclinical trainer have been described for amniocentesis and formal evaluation of that model has shown that approximately 100 attempts are required to achieve proficiency5. Although in-utero stenting is a more technically demanding procedure than amniocentesis, nearly all clinicians with an interest in learning to perform in-utero stenting procedures would have extensive experience in ultrasound-guided needle placement. Thus it is likely that fewer than 100 procedures will be required to obtain proficiency in in-utero stenting. Now that a high-fidelity trainer has been developed it will be possible to introduce it into maternal–fetal medicine curricula to determine the optimum number of practice procedures necessary to achieve proficiency before performing the procedure in the clinical setting.

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