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Traction Force in Vacuum-Assisted Delivery

January 2007 

With the ever-increasing malpractice premiums and multi-million dollar lawsuits, it is surprising to see practitioners continue to document and describe operative deliveries with such non-descript terms as, a “difficult delivery”, an “easy pull”, a “moderately hard pull”.  Although medicine is rapidly advancing in areas, such as electronic patient records and extensive simulation models used for training, an accurate method of determining traction force during operative vaginal deliveries continues to be elusive. 

Sudden detachment of a vacuum device from the fetal head (a “pop-off”), is associated with an increase in the rate of injury to the fetal scalp¹.  It is therefore  important to determine the ‘appropriate’ amount of traction force that should be applied; keeping in mind this will vary according to a number of obstetrical factors (i.e., position, station, size of fetus, maternal pelvis, etc.).  Moreover, the propensity for detachments is also related to cup placement on the fetal head (i.e., a flexing median application will lead to fewer pop-offs—see Innovative Insights Archive September, 2002 for more information).

So what is the right amount of force?

Birth is a mechanical process in which maternal expulsive forces must overcome the soft tissue resistance of the birth canal.  Scientific measurements of the forces involved would seem imperative if optimum fetal results are to be expected.  Yet, until recently the literature on this

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subject has been conflicting and vague with regards to the appropriate amount of force that should be used to perform an operative delivery.  However, it is logical to approach this issue  through a combination of experimental investigations and past experience.  Lindgren²  showed that the maximum ‘normal’ amniotic pressure during the second stage of  labor (assuming an average fetus with a cranial cross-sectional area of 10cm) approximates 120 mm Hg of pressure, 

or thirty-three pounds of force (assuming an average fetus with a cranial cross-sectional area of 10cm).  Following on from that logic then, the maximum force that should be applied to the fetal head with a vacuum (or forceps) is 33 pounds.  However, several authors have reported that during instrumental deliveries (i.e., forceps and vacuum) greater forces still resulted in ‘normal’ infants in the majority of cases.^3,4,5  Furthermore, clinical experience has shown that the majority of vacuum deliveries can be completed with a traction force of 25 pounds or less⁶.

A recent landmark article in the Australian and New Zealand Journal of Obstetrics and Gynaecology confirmed this clinical experience⁷.  The study included 119 nulliparous women who required a vacuum-assisted delivery.  The Kiwi OmniCup vacuum device was attached to a strain gauge transducer that allowed accurate measurements of the traction force that was applied during each delivery.  The results showed that 80% of the deliveries were successfully completed with 25 lbs of traction force or less.  The remaining 20% required up to 30 lbs of traction force.  In addition, the study showed that the incidence of scalp abrasions and cephalohematomas increased with increasing traction force.  Thus, it is reasonable to conclude that for the majority of vaginal vacuum-assisted deliveries the maximum amount of traction force should be in the range of 25-30 lbs.

 How does one determine the force they use during an operative vaginal delivery?

Even if the appropriate amount of traction force has been determined, how does one accurately and consistently provide the ‘right’ amount of force during an operative delivery?  In 2005, physicians from the University of New Mexico performed an investigational study to answer that question⁸.  They utilized an isometric strength-testing unit to provide the physicians with a real-time computer printout of the force applied, when pulling. They concluded that untrained individuals may be unable to gauge how much or how little force they are actually applying during an operative delivery.   Thus, they recommended the use of repetitive motion training and intrinsic sensory feedback augmented with visual feedback to enhance the application of reproducible force within the acceptable range.  With direct visual feedback safe limits were retained and variability of traction forces were diminished. 

The Kiwi Vac-6000MT vacuum device (Clinical Innovations, Murray, UT) provides this important real-time visual feedback by implementing a unique traction force indicator situated between the traction bar and the vacuum cup.  This proprietary design can assist physicians to apply appropriate traction force during vacuum-assisted deliveries, by providing a visual confirmation of the force exerted.  

Now that the recommended range of traction force is better understood

and more clearly defined in the literature, it is important that the force of a vacuum delivery is accurately reflected in the medical record of the patient.  No longer is it acceptable to describe a pull as “easy” or “difficult”, the Kiwi Vac-6000MT allows the physician to clearly document “25 lbs of force was applied over three contractions”.  This will not only assist the physician in providing levels of traction force that should be safe for the fetus and adequate for the delivery, but may also offer protection in the case of a poor fetal outcome.

¹ Bird, J. “The use of the vacuum extractor” Clinics in Obstet & Gynaecol 1982;9:641-661.

² Lindgren L. “The causes of fetal head molding in labor”  Acta Obstet Gynaecol Scand 1960; 39:46.

³ Wylie B. “Traction in Forceps Deliveries” Am J Obstet Gynecol 1935; 29:425-433.

⁴  Moolgaoker, Arvind S., Ahamed, Syed O.S., Payne, Peter R.. “A Comparison of Different Methods of Instrumental Delivery Based on Electronic Measurements of Compression and Traction” Obstet & Gynecol 1979; 54(3): 299-309.

⁵ Pearse, Warren H. “Electronic Recording of Forceps Delivery” Am J Obstet Gynecol 1963; 86:43-51.

⁶ Vacca A. “The ‘sacral hand wedge’: a cause of arrest of descent of the fetal head during vacuum assisted delivery” Br J Obstet Gynaecol September 2002; 109:1063-1065.

⁷ Vacca A. “Vacuum-Assisted Delivery: An analysis of traction force and maternal and neonatal outcomes” Aust N Zealand J of Obstet & Gynaecol 2006; 46: 124-127.

⁸  Leslie K,  Dipasquale-Lehnerz P, Smith M. “Obstetric Forceps Training Using Visual Feedback and the Isometric Strength Testing Unit” Obstet Gynecol 2005 105: 377-382.