Ultrasound-guided oocyte pick-up (OPU) is a well-established procedure during which ovarian follicles are aspirated with a needle connected to a suction pump. The suction pump creates negative pressure for aspirating the ovarian follicles.
Kitazato OPU needles are manufactured with different sizes (ranging from 16G to 21G), allowing to choose a smaller diameter to provoke less pain and discomfort to the patient.
The aspiration pressure required for an efficient oocyte pick-up varies according to different factors, such as the needle diameter, needle length and tubing length. Kitazato recommends using aspiration pressures between 100 and 225 Hgmm depending on the needle’s diameter. The thinner the needle, the higher aspiration pressure is required to maintain the same flow rate. An efficient aspiration pressure will maximize the oocyte recovery rate and reduce the risks of drastic temperature changes and mechanical stress to the oocyte. Therefore, it is important to notice that a higher aspiration pressure is not necessarily associated with any damage to the oocytes, especially if the the flow rate is stable and has been set according to the needle size and the manufacturer’s recommendations.
Continuous vacuum suction should be used just before penetrating the follicle. The positive pressure inside the follicle is dependent on the size, shape and the intraovarian position of the follicle. As the follicle matures it produces more follicular fluid. This increases the intrafollicular pressure causing the bulging of the external surface, especially when the follicle has reached maturity. The pressure of the fluid in the follicle at the moment of needle penetration might be significantly higher than the normal follicular pressure. As the needle tip is forced into the wall, the deformation of the follicular surface will cause the pressure to rise even further. The blunter the needle, the higher this pressure rise will become, increasing the risk of the follicular fluid spurting out when the follicle is punctured. Some of this fluid will flow up the needle, but a significant amount could be lost between the needle and the follicular wall. If continuous aspiration pressure has already been applied before the needle punctures the follicle, only little follicular fluid is expected to be lost.
A stable flow rate should be maintained during the entire procedure, because changes can induce turbulence. If turbulent flow is present, the oocyte may be tossed around, which could result in the cumulus cells being stripped off or the cracking of the zona pellucida. An intact cumulus layer is an important factor contributing to the resistance of oocytes to mechanical damage. Provided the aspiration pressure is maintained constant, the fluid velocity is the same throughout the needle and tubing line, if their diameters are also constant. Any diameter changes in the aspiration system (at the needle or in tubing line) will lead to sudden changes in flow rate, creating unwanted pressure changes and the presence of turbulent flow leading to mechanical stress of the oocyte.
As the follicular fluid volume is aspirated, the follicle collapses which should be continuously monitored by the ultrasound in order not to lose any oocytes. If complete follicle emptying cannot be verified, the oocyte can remain trapped inside the cavity). At this point, the needle should be gently withdrawn without applying any negative aspiration pressure to avoid a sudden forward flow of the follicular fluid towards the collection tube.
In conclusion, some of the most important practical considerations for an efficient and successful egg retrieval are:
- Setting the right aspiration pressure depending on the the thickness of the utilized needle (gauge).
- Applying continuous vacuum suction before penetrating the follicle.
- Maintaining a stable pressure throughout the entire procedure
- Avoiding applying negative aspiration pressure when withdrawing the needle from the follicle.
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