Simulation of embryo transport in a closed uterine cavity model.

OBJECTIVE: Uterine peristalsis is induced by synchronized contractions of the non-pregnant myometrium. The resulting periodic motions of the anterior and posterior walls of the uterus generate intrauterine motions which serve as transport vehicles for crucial processes of the early stages of mammalian reproduction. Among other duties, these flow currents are responsible for transporting the embryo to a successful site of implantation when the time window for implantation is open. In this study we investigated the transport characteristics of embryos with a new computational model. STUDY DESIGN: A computational model of idealized peristaltic flow within a two-dimensional channel with a closed end was implemented to study pre-implantation transport of embryos within the sagittal cross-section of the uterine cavity, which is closed at the fundal end and open towards the cervix. A commercial finite-element numerical package was implemented to solve the complex unsteady flow field. RESULTS: The numerical simulations revealed that the fluid flow field and the resulting transport characteristics of peristaltic flow in a closed channel are strongly affected by the closed end. The velocity profiles are also dependent on wall motility, level of asymmetry and frequency of the peristalsis. The trajectories of massless particles demonstrate periodic recirculation in small moving loops around their initial location along and across the channel. Particles which were initially separated by one wavelength along the channel length were transported in almost identical patterns. Particles which were initially located at full wavelengths experience very small velocities, and thus, their net axial displacement is negligible. CONCLUSIONS: The major outcome of the model simulation is that particles at almost any location, except those at full wavelengths of the peristalsis, are most likely to recirculate in small moving loops around their initial location. Accordingly, embryos are expected to be implanted in the uterine wall in the vicinity where they were initially placed, either naturally or artificially, within the uterine cavity.

Fetal blood flow in branching models of the chorionic arterial vasculature.

Fetal development depends on adequate exchange of materials between the fetus and maternal circulatory systems, which requires optimal distribution of blood vessels over the chorionic plate to ensure perfusion of the whole placental volume. Based on a previous investigation of the architecture of the chorionic vessels in the human placenta, we developed in this study typical models for the dichotomous and monopodial segments of the chorionic arteries of a mature placenta. Each model also included some intraplacental (IP) vessels that branch off into the cotyledons perpendicular to the chorionic arteries. Computational analysis of steady blood flow through these models was performed to explore the distribution of fetal blood over the chorionic plate. The results demonstrated that energy losses are small in the monopodial model, which explains their efficient delivery of fetal blood over the chorionic plate in cases of a marginal cord insertion. On the other hand, the dichotomous model is efficient in distributing a relatively large volume of blood over large areas near the bifurcation. Accordingly, the combination of dichotomous and monopodial bifurcation in a normal chorionic plate ensures a uniform blood perfusion of the placenta. Simulations with narrow daughter and IP vessels did not result in significant changes in the main mother tubes, supporting clinical observations in which umbilical blood flow remains normal although some peripheral vessels may be occluded.

Evaluation of the embryo transfer protocol by a laboratory model of the uterus.

OBJECTIVE: To investigate the dispersion of transferred matter within a uterine laboratory model during and after embryo transfer (ET) as a function of uterine position, catheter placement, and injection speed. DESIGN: Experimental setup. SETTING: Reproductive bioengineering laboratory. PATIENT(S): N/A INTERVENTION(S): N/A MAIN OUTCOME MEASURE(S): Laboratory simulations of ET into a rigid transparent uterine model. The catheter was loaded with sequences of liquid and air as in the clinical setting. Experiments were conducted for different inclinations of the uterine model, various transfer speeds of the catheter load, and placement of the catheter tip near and remote from the fundus. RESULT(S): Dispersion of the transferred matter depended on the position of sagittal cross-section of the uterine cavity with respect to the horizon. The air bubbles were pulled up by buoyant forces toward the highest point of the cavity (e.g., the fundus) and thereby dragged behind them the transferred media with the embryos. Placement of the catheter tip near the fundus appeared to transfer the embryos into the tube when transfer was performed at fast speeds, possibly leading to ectopic pregnancies. CONCLUSION(S): Fundal level of the uterine cavity, location of the catheter tip, and transfer speed determine the potential for embryo implantation. Adjustment of ET protocol to individual patient anatomy is recommended.

Modeling myometrial smooth muscle contraction.

Existing models of uterine contractions assumed a top-down approach in which the function at the organ or tissue level was explained by the behavior of smaller basic units. A new model of the excitation-contraction process in a single myometrial myocyte was recently developed. This model may be used in a bottom-up approach for the description of the contribution of cellular phenomena to the overall performance of the tissue or organ. In this review, we briefly survey current knowledge of uterine electrophysiology and contractility as well as current modeling techniques, which were successfully used to study the function of various types of muscle cells. In the physiological part of the review, we relate to mechanisms of intracellular Ca(2+) control, Ca(2+) oscillations, and Ca(2+) waves and to the various membranal transport mechanisms regulating ion exchange between the intracellular and extracellular spaces. In addition, we describe the process leading from excitation to contraction. In the modeling part of the review, we present the Hodgkin-Huxley (HH) model of excitation in the squid axon as well as models of Ca(2+) control and the latch-bridge model of Hai and Murphy describing the kinetics of smooth muscle cell (SMC) contraction. We also present integrative models describing more than one of these phenomena. Finally, we suggest how these modeling techniques can be applied to modeling myometrial contraction and thus may significantly contribute to current efforts of research of uterine function.

Bioengineering studies of the embryo transfer procedure.

Embryo transfer (ET) is the final manual intervention in extracorporeal fertilization in which an embryo is transferred into the uterus by a transcervical catheter. The low rates of embryo implantation within the uterus are attributed, among other factors, to the ET technique, which depends on a multitude of anatomical, physiological, and mechanical aspects. We developed computational and experimental models to simulate ET to examine the contribution of mechanical features to the success of this procedure. The experimental model allowed laboratory simulations of the dispersion of the catheter load as a result of different injection speeds into a tilted uterine model. The mathematical model analyzed potential trajectories of the transferred embryos resulting from the interaction between the injection velocity and the intrauterine flows caused by uterine peristalsis. The simulations revealed the important contribution of mechanical parameters, such as the position of the uterus and the presence of air in the catheter load. The latter was found to increase the potential for the embryo to be near the fundal area during the time limit for implantation. Based on the results of our simulations, we recommended performing ET in a patient-specific position in which the fundus will be the highest point above the horizon and that the load be delivered slowly, that is, not less than 10 s. We also recommended placing the tip of the catheter at the mid cavity to avoid ectopic pregnancy.

Analysis of cervical dynamics by ultrasound imaging.

Preterm birth is generally due to cervical ripening during the second trimester of pregnancy. The diagnosis of cervical incompetence is mostly based on the measurement of the shortened cervical length from transvaginal ultrasound (TVUS) images. We investigated the cervical dynamic response to spontaneous or imposed variations of intrauterine pressure, which may induce cervical shortening. The TVUS images of the cervix sagittal cross-sections were recorded from six women in mid pregnancy. The cervical dynamics was observed while the subject was either in a supine position, kneeling, or had undergone transfundal pressure in a supine position. Each subject was tested in all three positions, but the dynamic response was observed in only one of them. The time-dependent analysis was performed on consecutive TVUS images at time intervals of 1 s to extract the dynamic response of the funneling geometry and the closed cervical length. The internal os was considered as being a point on the uterine wall and characterized by a sharp gradient of the inner wall of the uterine cavity. Dynamic evaluation of TVUS images revealed that shortening of the cervical length was greater than 30% and that the funneling percentage was greater than 40%. This study demonstrates the clinical potential for dynamic assessment of cervical response due to excessive uterine pressure, in addition to its application for the conventional measurement of cervical length.

Mathematical model of excitation-contraction in a uterine smooth muscle cell.

Uterine contractility is generated by contractions of myometrial smooth muscle cells (SMCs) that compose most of the myometrial layer of the uterine wall. Calcium ion (Ca(2+)) entry into the cell can be initiated by depolarization of the cell membrane. The increase in the free Ca(2+) concentration within the cell initiates a chain of reactions, which lead to formation of cross bridges between actin and myosin filaments, and thereby the cell contracts. During contraction the SMC shortens while it exerts forces on neighboring cells. A mathematical model of myometrial SMC contraction has been developed to study this process of excitation and contraction. The model can be used to describe the intracellular Ca(2+) concentration and stress produced by the cell in response to depolarization of the cell membrane. The model accounts for the operation of three Ca(2+) control mechanisms: voltage-operated Ca(2+) channels, Ca(2+) pumps, and Na(+)/Ca(2+) exchangers. The processes of myosin light chain (MLC) phosphorylation and stress production are accounted for using the cross-bridge model of Hai and Murphy (Am J Physiol Cell Physiol 254: C99-C106, 1988) and are coupled to the Ca(2+) concentration through the rate constant of myosin phosphorylation. Measurements of Ca(2+), MLC phosphorylation, and force in contracting cells were used to set the model parameters and test its ability to predict the cell response to stimulation. The model has been used to reproduce results of voltage-clamp experiments performed in myometrial cells of pregnant rats as well as the results of simultaneous measurements of MLC phosphorylation and force production in human nonpregnant myometrial cells.

In vitro simulations of embryo transfer in a laboratory model of the uterus.

Embryo transfer (ET) is the final manual intervention during which the newly formed embryo is placed within the uterus by a transcervical catheter. The loading of the syringe-catheter complex with the transferred volume consists of the transfer media (which contains the embryos) separated by air spaces on both sides. The dynamics involved in injecting the syringe-catheter complex is not well understood nor has it been investigated to date. We developed an in vitro experimental setup for simulations of ET into a rigid transparent uterine model. The catheter was loaded in sequences of liquid and air as it is in the clinical setting. The transferred liquid was colored with a dye and its dispersion within the uterine cavity was recorded by a video camera. The results demonstrated, for the first time, the importance of having a gas phase in the catheter load. The resulting air bubbles within the uterus were carried upward towards the fundus by buoyant forces, thereby dragging behind them the transferred liquid which contained the embryos. This could be expected to substantially increase the probability for the embryos to be present near the fundal wall at the time window for implantation. There was also evidence of a dependency of the rate of injection upon the catheter load into the uterus: a low speed generated several air bubbles which led to more of the transferred liquid being carried towards the fundal end, thus possibly enhancing the potential for implantation.

Hemodynamic analysis of Hyrtl anastomosis in human placenta.

The Hyrtl anastomosis is a common connection between the umbilical arteries near the cord insertion in most human placentas. It has been speculated that it equalizes the blood pressure between the territories supplied by the umbilical arteries. However, its functional role in the regulation and distribution of fetal blood flow to the placenta has not yet been explored. A computational model has been developed for quantitative analysis of hemodynamic characteristic of the Hyrtl anastomosis in cases of discordant blood flow in the umbilical arteries. Simulations were performed for cases of either increased placental resistance at the downstream end or reduced arterial blood flow due to some pathologies upstream of one of the arteries. The results indicate that when placental territories of one artery impose increased resistance to fetal blood flow, the Hyrtl anastomosis redistributes the blood flow into the second artery to reduce the large pressure gradients that are developed in the affected artery. When one of the arteries conducts a smaller blood flow into the placenta and a relatively smaller pressure gradient is developed, the Hyrtl anastomosis rebuilds the pressure gradients in the affected artery and redistributes blood flow from the unaffected artery to the affected one to improve placental perfusion. In conclusion, the Hyrtl anastomosis plays the role of either a safety valve or a pressure stabilizer between the umbilical arteries at the placental insertion.

Renal blood flow alteration after paracentesis in women with ovarian hyperstimulation.

OBJECTIVE: To evaluate renal arterial resistance to flow by Doppler indices concurrently with ascites drainage in women with severe ovarian hyperstimulation syndrome. METHODS: We conducted an interventional clinical study of 19 women with severe ovarian hyperstimulation syndrome, manifested by free peritoneal fluid. The subjects were evaluated before and after therapeutic paracentesis by measuring urine output, blood urea nitrogen, intra-abdominal pressure, and renal artery flow measures by Doppler ultrasonography (systolic/diastolic ratio [S/D] and resistance index). RESULTS: An average of 3,340 mL of ascitic fluid was drained, and the intra-abdominal pressure decreased from 17.5 +/- 1.24 cm H2O to 10 +/- 1.22 cm H2O. Urine output was increased (by 65%, from 925 +/- 248 mL/d before paracentesis to 1,523 +/- 526 mL/d on the day after paracentesis, P <.001). The mean renal arterial S/D decreased from 3 +/- 0.15 to 2.29 +/- 0.13 (P =.001). Most of the decrease in intra-abdominal pressure as well as in renal vasculature resistance was apparent after an initial drainage of 2,000 mL. Additional fluid drainage had only negligible effect on intra-abdominal pressure and renal flow. CONCLUSION: Paracentesis lowered intra-abdominal pressure and decreased renal arterial resistance (lowered S/D and resistance index), ultimately resulting in increased urine production. It is plausible that the beneficial effects of paracentesis on urine output in ovarian hyperstimulation syndrome are due to improved renal blood flow from a direct decompression effect.

A glance into the uterus during in vitro simulation of embryo transfer.

BACKGROUND: The currently low implantation rate after embryo transfer (ET) is partially attributed to technical aspects, such as catheter type, catheter load, placement of catheter tip and physician skills. METHODS: Mock ET simulations were conducted with a transparent laboratory model of the uterine cavity. The catheter was loaded with alternating air and coloured liquid media. The transfer procedure was recorded by a digital video camcorder for later analysis. Different sequences of air and liquid volumes, as well as liquids of different viscosity were simulated. RESULTS: Injection of liquid with air into the uterus formed an air bubble which blocked the transport of the transferred liquid towards the fundus. The distribution of the transferred matter within the uterine cavity was determined by the composition of the liquid-air sequence and the viscosity ratio between the transferred liquid and the uterine fluid. CONCLUSIONS: It is suggested that the catheter load should contain minimal volumes of air in order to enhance the embryos' chances of reaching the site of implantation. The viscosity of the transferred liquid should be as close as possible to that of the uterine fluid in order to avoid transport of embryos towards the cervix.

A new power Doppler ultrasound guiding technique for improved testicular sperm extraction.

OBJECTIVE: To develop a noninvasive procedure that employs image processing of power Doppler ultrasound (PDUS) images of several orthogonal cross-sections of the testis to construct a three-dimensional (3D) mapping of preferential testicular regions in which spermatozoa are most likely to be found in nonobstructive azoospermic testes. DESIGN: Clinical study. SETTING: Ultrasound and andrology units in a large university-affiliated municipal hospital. PATIENT(S): Twenty-four nonobstructive azoospermic men. INTERVENTION(S): Before testicular sperm extraction was performed, PDUS images were acquired at seven cross-sections to reconstruct a 3D testicular vascularity index (TVI) matrix for spatial mapping of testicular regions in which spermatozoa are most likely to be found. The predictions based on TVI values of 107 regions were compared with the biopsy results. MAIN OUTCOME MEASURE(S): Prediction of presence or absence of spermatozoa by TVI values. RESULT(S): The prediction rate of the TVI matrix for the presence or absence of spermatozoa was 74.8%. The positive predicted value was 72%, negative predicted value was 75.6%, and specificity was 89.8%, but sensitivity was 47.3%. CONCLUSION(S): Our technique may obviate the need for arbitrary multiple biopsies that inflict some degree of damage upon testicular tissue and may increase the success rate of identifying viable spermatozoa in testicular tissue.

Biofluid aspects of embryo transfer.

Embryo transfer (ET) is the last stage of extracorporal fertilization during which the embryo is placed in the uterine cavity with a medium-filled catheter 2-3 days after in vitro fertilization. While fertilization in the laboratory occurs at very high rates (> 90%), the overall success of the procedure (i.e., take home baby) is still very low (< 25%) and assumed to be mainly due to implantation failure. A computational model was developed to simulate ET within the uterine cavity by a fluid-filled catheter inserted into a two-dimensional channel with oscillating walls. The results showed that the speed at which the embryos are injected from the catheter dominates the procedure and controls the velocity of their transport within the uterine cavity. ET at excessively high injection speeds may lead to ectopic pregnancies, while uterine peristalsis affects transverse dispersion only during injection at low injection speeds. The presence of the catheter within the uterus does not affect flow patterns downstream of its tip. The potential risks to implantation failure due to mechanical factors involved in the ET processes are discussed.