Is there any danger to not being in the recommended zone of 450-600mmHg?

The literature has made clear that lower levels of vacuum (e.g. 450mmHg or less) lead to a greater number of sudden detachments (pop-offs), which may increase the incidence of scalp trauma and serious complications such as subgaleal and intracranial hemorrhages. However, there have been no reported studies showing that a ‘higher level’ of vacuum (e.g. 600mmHg and higher) leads to greater injuries. Thus, it is most physicians’ practice to induce the vacuum to the highest level (top of the green or 600mmHg). In fact, a review of the literature revealed that greater than 90% of the reported trials on vacuum delivery devices, listing the vacuum level, reported using 600mmHg.

Furthermore, it should be remembered that atmospheric pressure is 760mmHg at sea level and this pressure decreases with increasing elevation. Thus, with a perfectly designed vacuum pump, the maximum amount of pressure that could be obtained is 760mmHg (at sea level). However, there is no product on the market that is ‘perfect’ and thus most devices cannot exceed 700mmHg no matter how long they are pumped. Regardless, it is important to stress that no evidence exists to show that higher levels of vacuum are associated with an increased rate of fetal scalp injury. It is the sudden detachments that occur—more often at the lower vacuum range—that lead to higher incidences of fetal injuries, not the vacuum level itself.

Are all gauges accurate?

Consensus opinion holds that that majority of fetal scalp or intracranial injuries are most likely due to incorrect placement of the cup on the fetal head. However, the FDA database of injuries asserts that gauge inaccuracy has lead to fetal injury and even death. A study reported at the 2004 Annual ACOG Meeting by Dr. Victor Vines (Medical City Dallas Hospital, Dallas, Texas) showed that almost half of all ‘reusable’ pumps were inaccurate. Dr. Vines showed that the Bourdon gauges, used on all vacuum devices except for Clinical Innovations Kiwi PalmPump, were easily made inaccurate when exposed to any impact or vibration—which often occurs during cleaning and storage of these pumps. In addition, the Bourdon gauges are subject to impact during shipping (even the single-use vacuums) which could make them less accurate. The single-use Kiwi PalmPump, with a tire-gauge approach, appears to not be as prone to the error introduced by the vibration and shock of storing and shipping.

Specific to the Kiwi PalmPump

Of note, when the vacuum is obtained with the PalmPump, an audible click can be appreciated with each pump. This click is a valve releasing the pressure from within the cup, after several pumps (approximately 22-25) the audible click will disappear, showing that no more vacuum can be obtained. Thus, a practitioner can pump the PalmPump as much as he/she wants, but will be hard pressed to obtain a vacuum greater than approximately 700mmHg—which appears to be a safe level. However, since it is the standard of care to obtain 450-600mmHg, this is the advice we continue to recommend.

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