The article on vacuum extraction by Tracey Johnston and Tara Selman followed by the correspondence by Ajay Poddar and the authors' reply have raised a number of issues on the subject that invite considerable comment. Nevertheless, I would like to focus the readers' attention on just one, namely, misconceptions that all the authors seem to share about the purpose and use of the markings on the tube of the Kiwi™ OmniCup (Clinical Innovations LLC, Murray, Utah, USA).
As the designer of the omnicup I had the opportunity to incorporate a number of features intended to allow the operator to put important technical principles into practice. One of these, the markings on the tube of the device, has little significance per se but, when coupled with the technique of digitally calculating the distance of the flexion point from the posterior vaginal fourchette (the cup insertion distance), the marks can assist the operator to achieve a correct cup application to the fetal head more consistently.
The 6 cm and 11 cm tubal markings were never intended to indicate how far the cup should be inserted in OA or OP positions respectively. As Johnston and Selman point out the cup insertion distance will depend on the station of the vertex and, in particular, the attitude of the fetal head. Even at the same station in OP and OT positions, the distance from the flexion point to the fourchette can vary up to 4 cm depending on the degree of deflexion and/or asynclitism. Therefore, it is essential that the cup insertion distance is digitally measured in every case and the calculated distance extrapolated on to the omnicup tube using the tubal markings as reference points. This technique is easy to master and provides a more objective measurement to indicate how far the cup must be inserted into the vagina to achieve a correct application. Simply guessing the insertion distance or relying on generalisations is fraught with error.
Finally, as Johnston and Selman rightly conclude, successful vacuum-assisted delivery depends on a number of variables, the most important of which is operator competence. Competence in the use of a particular vacuum device involves familiarity with the special features of that device and the knowledge of how to apply them in clinical practice to achieve the best outcome.