An important step in vacuum extraction is proper cup placement over the lexion point. his pivot point maximizes traction, minimizes cup detachment, lexes but averts twisting the fetal head, and delivers the smallest head diameter through the pelvic oudet. This improves success rates, lowers fetal scalp injury rates, and lessens perineal trauma because the smallest fetal head diameter distends the vulva (Baskett, 2008).
The lexion point is found along the sagittal suture, approximately 3 cm in front of the posterior fontanel and approximately 3em 6em /II -_ ...._-\ o\ I\ I\\ ........"------,///�
FIGURE 29-17 Drawing demonstrates correct cup placement at the flexion point. Along the sagittal suture, this spot lies 3 cm from the posterior fontanel and 6 cm from the anterior fontanel.
6 cm from the anterior fontanel. Because cup diameters range from 5 to 6 cm, when properly placed, the cup rim lies 3 cm from the anterior fontanel (Fig. 29-17). Placement of the cup more anteriorly on the fetal cranium-near the anterior fontanel-should be avoided as it leads to cervical spine extension during traction unless the fetus is small. Such placement also delivers a wider fetal head diameter through the vaginal opening. Last, asymmetrical placement relative to the sagittal suture may worsen asynclitism. For elective use, cup placement in OA positions is seldom diicult. In contrast, when the indication for delivery is failure to descend caused by occipital malpositionwith or without asynclitism or deflexion-cup positioning can be diicult.
During cup placement, maternal soft tissue entrapment predisposes the mother to lacerations and virtually ensures cup dislodgement, colloquially called a "pop of." Thus, the entire cup circumference should be palpated both before and after the vacuum has been created and again prior to traction to exclude such entrapment. Gradual vacuum creation is advocated by some and is generated by increasing the suction in increments of 0.2 kg/cm2 every 2 minutes until a total negative pressure of 0.8 kg/cm2 is reached 29-3). hat said, other studies have shown that negative pressure can be increased to 0.8 kg/cm2 in <2 minutes without a signiicant diference in eicacy or in maternal and fetal outcomes (Suwannachat, 2011, 2012) .
TABLE 29-3. Vacuum Pressure Conversions 100 10 3.9 1.9 0.l3 200 20 7.9 3.9 0.27 300 30 11.8 5.8 OA1 400 40 15.7 7.7 0.54 500 50 19.7 9.7 0.68 600 60 23.6 11s.6 0.82
Once suction is created, the instrument handle is grasped, and traction is initiated. Similar to forceps delivery, traction angles mirror that in Figure 29-10. Eforts are intermittent and coordinated with maternal expulsive eforts. Manual torque to the cup is avoided as it can cause cup displacement or cephalohematomas and, with metal cups, "cookie-cutter"-type scalp lacerations. hus, OA oblique positions are corrected not by rotation, but solely by downward outward traction. During pulls, the operator should place the nondominant hand within the vagina, with the thumb on the extractor cup and one or more ingers on the fetal scalp. So positioned, descent of the presenting part can be judged and the traction angle can be adjusted with head descent. In addition, the relationship of the cup edge to the scalp can be assessed to help detect cup separation.