Force, failure and trauma with vacuum extractor devices

Authors


Sir,

In your November issue, Attilakos et al.1 showed less efficacy and equivalent neonatal scalp trauma with the Kiwi Omni-cup compared with a tailored selection of vacuum extractor cups. Their results are interesting, particularly from a North American perspective. Rigid vacuum cups lost favour in North America in the 1960s after reports of increased neonatal trauma, often associated with what would now be considered as improper use. They were replaced by disposable soft cups such as the Mighty-Vac, which fail more often. The introduction of the increasingly popular Kiwi cup has renewed interest in the mechanics of vacuum extraction. As suggested by results of Attilakos et al., a tailored approach to vacuum delivery based on these mechanics may allow us to match the appropriate instrument to the task and minimise trauma.

The tailored approach to vacuum delivery apparently used in the study's control arm was first suggested by Johanson et al. Using a silicone cup for low and outlet deliveries, a 6-cm Bird Anterior cup for midcavity deliveries without rotation, and a 5-cm Bird Posterior cup for occiput transverse and occiput posterior deliveries, Johanson et al. demonstrated equivalent effectiveness to forceps, with less maternal trauma.2 Their rationale was based on cup mechanics. Successful vacuum delivery depends on a flexing application to the fetal scalp, just anterior to the posterior fontanelle. To achieve this, midcavity and rotational deliveries require a relatively flat vacuum cup, with an axis of traction that is independent of the plane of the cup. Only the Bird and Kiwi cups are so constructed. For low and outlet deliveries, a flexing application can be achieved with either Bird cups or larger cups having a handle perpendicular to the plane of the cup.

Similar to the metal 5-cm Bird Posterior cup, except for being made of rigid plastic, the Kiwi cup would be expected to perform similarly in OT and OP deliveries but less effectively for deliveries without rotation. For such deliveries, the larger diameters of the 6-cm Anterior Bird and 6.5-cm silicone cups generate 50 and 70% more traction, respectively, than the 5-cm cup because the traction force is proportional to the surface area of contact. This would very likely influence the instrument's effectiveness, particularly in this series where 25% of deliveries were ‘trials of vacuum’ performed in the operating theatre. Could the authors tell us which type of operative deliveries failed more often in the Kiwi group?

The increased force generated by the 6-cm anterior cup combined with its rigid metal construction may, however, potentially cause more scalp trauma and rarer intracranial complications. The trial by Attilakos et al. showed increased scalp trauma with its use: 38 versus 17% (silicone) and 23% (Kiwi). Perhaps the authors could also tell us if neonatal intensive care unit admissions (ten in the control group versus five in the Kiwi group) occurred more often following Bird Anterior cup deliveries.

Having only the Kiwi cup available for vacuum delivery makes as little sense as having only Kiellands available for forceps delivery. However, in North America, where the Bird cup is not marketed, the Kiwi cup is a significant improvement over disposable soft cup devices. The best ‘tailored’ approach in settings without access to Bird cups may be a silicone cup for low/outlet deliveries and a Kiwi cup for malposition.

Ancillary