Fig. 11.1
Catheter-loading technique employed at Rotunda-The Center for Human Reproduction
After scoring and selecting the best embryos for transfer, prior to catheter loading, the identity of the patient is reconfirmed by the embryologist. The culture dish with the respective embryos is removed from the incubator and embryos loaded into the ET catheter as follows:
A 1 mL syringe is filled with 0.5 mL culture medium. (Caution: Ensure that the syringe does not contain any air bubbles.)
The nozzle of the filled syringe is firmly fixed to the inner soft catheter. (Caution: Ensure that the fitting is tight otherwise the assembly may detach at the critical point of embryo transfer.)
The entire medium is dispelled out rinsing the lining of the inner catheter. This results in the entire inner catheter being filled with media, without the existence of any dead space.
The catheter is loaded with about 20 mm column of air. (Tip: This will ensure enough air to push out the embryos.)
The catheter tip is brought near the embryos and the catheter loaded with the embryos in a continuous column of medium (~5–8 μL). The medium segment containing the embryos should be no more than 10 μL. This is followed by small column of air (~3 mm) and then ~2.5 μL of culture medium. (Caution: The position of the embryos within the catheter plays a very important role in retained embryos.)
During embryo transfer, once the syringe plunger is pushed, it is kept pressed until withdrawal of the catheter. (Caution: Releasing the pressure before complete withdrawal of the catheter can cause retained embryos due to negative pressure.)
The catheter is slowly withdrawn after injecting the embryos. (Caution: Rapid withdrawal may create a negative pressure and result in the withdrawal of the embryos following the catheter.)
Inspection of Retained Embryos
After the embryos have been transferred, the catheter is handed back to the embryologist. The catheter is flushed by in-and-out suction in media on the sterile dish cover. The catheter tip is gently rolled on the lid and inspected for retained embryos, blood, and mucus under the microscope.
Air-Fluid Versus Fluid-Only Method
When the formation of air bubbles within the small volume containing the embryos was carefully avoided, no differences in implantation and pregnancy rates were reported in a prospective, randomized study that compared two catheter-loading techniques: (i) Group 1 (n = 52), 200 μL of air in the syringe, 100–125 μL of air in the proximal part of the catheter, 20–25 μL of medium containing the embryos to be transferred, and 10 μL of air at the tip of the catheter; (ii) Group 2 (n = 50), the syringe and the entire catheter filled with medium and the embryo-containing medium (20–25 μL) without being bracketed by air spaces. The authors concluded that the air loaded into the transfer catheter to bracket the embryo-containing medium has no negative effect on ET success [26]. These results are in agreement with the study by Krampl et al. [15].
A meta-analysis of randomized trials that compared air bubbles to bracket the embryo-containing medium during embryo transfer versus fluid-only methods also showed no significant differences between the two methods with regard to live birth (OR = 1.34; 95 % CI = 0.59–3.07), ongoing pregnancy (OR = 1.34; 95 % CI = 0.59–3.07), and clinical pregnancy (OR = 1.13; 95 % CI = 0.70–1.83) rates or rates of implantation, miscarriage, multiple and ectopic pregnancies, and retained embryos. The authors concluded insufficient evidence to suggest the superiority of fluid-only method over the use of air brackets during embryo loading and suggested a need for well-designed and powered randomized trials to determine any possible benefit to either method [27].
Marek et al. [18] reported significantly higher implantation and clinical pregnancy rates in the air-column (n = 278) group compared to the group in which a full column of medium (n = 264) was used to expel the embryos despite no significant difference in the incidence of retained embryos (1.1 % in both groups) [18]. They opined that if an increase in reactive oxygen species occurs with the presence of air in the catheter, this does not translate into lower implantation and pregnancy rates [15, 18, 26].
Hence, the use of small air spaces to bracket the embryo-containing medium in the catheter does not adversely affect the implantation and pregnancy rates provided that the total volume of transfer is small, <10 μL of air at the tip of the catheter is injected, and air bubbles within the volume of transfer media containing the embryos are avoided [9], both of which were observed in Moreno et al.’s [26] study. Results of early, uncontrolled studies, [22, 23] suggesting an improvement in pregnancy rates by the removal of air from the transfer catheter can be explained by a reduction in the total transfer volume (including both transfer medium and air) and other refinements in the IVF program rather than by a specific deleterious effect of the air [9]. In our study, the total volume of transfer was small and included only the 10–12.5 μL of medium containing the embryos and the 2.5–5.0 μL of air that almost unavoidably exists within the catheter when a full column of medium is not used [14].
A transient motion of the ET-associated air bubbles has been observed using our standard embryo transfer loading technique. Hence, our group conducted a prospective, randomized study in a private academically affiliated infertility center, using a new embryo transfer catheter-loading technique to test whether the embryos actually retain their position of deposition in the upper uterine cavity and do not migrate. Fifty-six patients undergoing donor egg IVF underwent ultrasound-guided ET by a single physician with a standardized technique, randomly using one of the two loading techniques: (i) the standard technique in which the syringe was filled with medium right up to the proximal part of the ET catheter, which was followed by 2.5 μL air, followed by 7.5 μL of embryo-containing medium, followed by 2.5 μL air, and eventually by 2.5 μL of culture medium at the tip of the catheter (Group A), and (ii) that in which the syringe and the entire catheter were filled with medium followed by 12.5 μL of embryo-containing culture medium followed by 2.5 μL of air at the tip (Study Group B). The 2.5-μL volume of air at the catheter tip, in the study group, was used to prevent spillage of the medium containing the embryos. The 2.5 μL gap of air between the embryo-containing medium and the next bolus of medium in the control group was used to prevent the transport of the embryos within the catheter [14]. The liquid that does not contain the embryos and extends into the fluid column in the syringe was used to push the catheter load during transfer to avoid embryos from sticking to the catheter wall [14]. The visualization of the ET-associated air bubble was better in Group A, but the air bubble movement was eliminated using the new loading technique in Group B. Though the implantation (22.5 % vs. 17.7 %) and clinical pregnancy rates (47.2 % vs. 45 %) were similar in both the groups, the observations led us to conclude that the new loading technique enables embryos to retain their place of deposition and we could consistently deposit embryos at the desired site in the uterine cavity [14].
Velocity of Embryo Expulsion
The embryo transport pattern within the uterine fluid is controlled by the uterine peristalsis factors, such as amplitude and frequency of the uterine wall’s motility, as well as the synchronization between the onset of catheter discharge and uterine peristalsis [4].
A gentle and atraumatic ET technique that avoids uterine contractions and placement of the embryo(s) at 1.5–2 cm below the top of the uterine cavity have been rated as among the most important parameters that may secure the highest chance of implantation. However, prospective, randomized studies on the velocity with which the embryos should be expelled into the uterine cavity have rarely been addressed in literature on ET techniques [6].
Theoretically, low-velocity embryo expulsion into the uterine cavity may increase the risk of the embryos being retained in the catheter, adhering to the tip of the catheter, thus resulting in embryo deposition in the cervical canal on retraction of the catheter. Hence, wrong placement of the embryos will not be discovered when the catheter is checked under the microscope by the embryologist after ET. While it would seem that these hazards could be diminished or avoided by delivering embryos with high velocity, this could thrust the embryos into the Fallopian tubes or by a forced retrograde flow, down into the cervical canal, decreasing the chance of implantation and increasing the risk of an ectopic pregnancy. In addition, expulsion of embryos with high velocity may provoke uterine contractions, which would tend to adversely affect the chances of intrauterine implantation [6]. It has been demonstrated that 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 [3].
Using a computational model to simulate ET within the uterine cavity with a fluid-filled catheter inserted into a two-dimensional channel with oscillating walls, Yaniv et al. [1] demonstrated 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 [1].
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