Operative Vaginal Delivery

Operative Vaginal Delivery

Introduction

Unfortunately, obstructed labor still complicates thousands of deliveries worldwide, claiming hundreds of maternal and fetal/neonatal lives and causing significant morbidities in the survivors (such as is the case with obstetric fistulas) that could have been otherwise prevented with timely obstetrical interventions.

Obstetrical forceps and vacuums refer both to a heterogeneous group of instruments and the procedures employed to assist deliveries using these instruments. Though obvious differences between the mechanism and techniques of forceps and vacuums, the theoretical principles and clinical considerations related to their indications, contraindications, conditions for application, intrapartum and post-partum conduct and documentation apply to both types of instruments and in general will be dealt with as a common topic.

Since destructive obstetrical procedures will not be considered here the term assisted vaginal deliveries (AVDs) is preferred to the term operative vaginal deliveries, which is more inclusive. In addition, assisted vaginal deliveries will be restricted here to those assisted by obstetric forceps (FADs) and vacuums (VADs). Although some authors make a distinction between obstetrical forceps and spatulas (non-articulated forceps), for the purposes of this review they will be considered within the same category.

An algorithm to facilitate clinical decision making when considering an assisted vaginal delivery is provided. (Figure 1)

Figure 1.

Biomechanical model of delivery

Basic model of delivery

In a basic biomechanical model of delivery (Figure 2) the uterus provides an expelling force that is able to propel a rather passive (from the mechanical standpoint) fetus through a pelvic canal that offers an opposing resistance vector, usually of lower magnitude. When the expelling force and resistance are of a similar magnitude, delivery can be prolonged indefinitely unless an intervention is performed or, historically, until the mother and/or fetus succumb in the process.

Figure 2.

The biomechanical interventions able to favorably alter the arrested delivery can take the form of an increase in the expelling force (pulling and/or pushing), a decrease in the resistance vector or combination of these two.

Assisted delivery and actions of forceps and vacuum

Forceps and obstetric vacuums (see Figure 3) are employed to assist a vaginal delivery for their potential to do three things:

Figure 3.

• Increase the expelling force (adding or replacing the maternal expelling forces).

• Decrease the resistance force of the maternal birthing canal by modifying the perimeter of the fetal head (correction of malpositions, asynclitism, and deflection).

• Decrease the resistance of the birthing canal by increasing the perimeter of the soft pelvis (in the case of forceps).

Anatomy of instruments

Forceps

Most forceps are articulated clamps with two branches (left or right; according to which side of the mother they relate to), a lock and a handle (see Figure 4). The portion of the blade that relates to the fetal head is referred to as blade and has two curvatures: cephalic, which adapts to the fetal head; and pelvic, which follows the contours of the maternal pelvis. Since the forceps adapts to the pelvic curvature the concavity of the blade determines which edge is anterior and which blade is right or left.

Figure 4.n

Over 600 forceps have been described, with a dozen or so still in use worldwide. Some of them, e.g. the Piper and Kielland forceps, have been designed for a specific purpose, such as the delivery of the aftercoming head or the management of a transverse arrest of descent, respectively. In general, the selection of forceps or vacuum and the type of instrument in any given patient depends on their availability, location of delivery, obstetrician’s preference and clinical features of the case (see selection of instruments).

Vacuum

An obstetric vacuum is typically constituted of a cup, which provides the attachment to the fetal scalp; a stem (though some stem-less vacuums exist); a handle; and a vacuum system, which may attach to the vacuum handle, or more recently – as is the case of disposable instruments – incorporated in the vacuum itself (see Figure 5).

Figure 5.n

Operative vaginal delivery nomenclature

Maternal pelvis

In 1988 The American College of Obstetricians and Gynecologists ACOG) adopted the following classification to standardize assisted vaginal deliveries.

Outlet:

• Scalp is visible without labial separation.

• Fetal skull has reached pelvic floor.

• Fetal head is at or on perineum.

• Sagittal suture is in antero-posterior or right or left anterior or posterior position

Low:

• Leading point of the fetal skull is at +2 or lower but not on pelvic floor.

• Rotation of 45 degrees or less is exerted.

• Rotation of greater than 45 degrees is exerted.

Mid-forceps: Head is engaged but higher than +2.

High: Not included in the classification.

This classification also applies to the use of obstetric vacuum.

Practical Tips:

From a practical standpoint both the station of the procedure and any degree of rotation from the original instrument application need to be documented (see section on documentation).

In terms of fetal evaluation of descent, the bony structures of the skull (and not the caput) are the ones used to determine the fetal station.

Be careful with the evaluation of station of occipito-posterior varieties of presentation as they often seem lower in the birthing canal than they truly are.

Application of forceps:

The application of the forceps is defined as the relationship between the axis of the blades and the axis of the pelvis. It is the way the forceps is originally applied and articulated in the maternal pelvis.

There are 8 main applications of forceps (see Figure 6 for example):

Figure 6.n

• Direct: Anterior and posterior

• Lateral: Right and left

• Right: Anterior and posterior

• Left: anterior and posterior

Intermediate applications to the ones described here also exist. The main goal of the forceps application is to adapt to the fetal position in the pelvis in order to achieve a symmetric biparieto-bimalar grip (see below).

The application of vacuum will be discussed below as it pertains more to the anatomy of the fetal head.

Practical Tip

With all types of instruments, preserving or increasing fetal flexion will decrease the head diameters and associated head perimeters offered to the pelvis. This strategy decreases the resistance of the pelvis, facilitates the assisted delivery and decreases the potential for maternal and fetal trauma.

Fetal head

In the human fetus the head is the largest segment of dystocia (compared to the fetal shoulders and fetal pelvic girdle). For evolutionary reasons the human fetus is the only live born who undergoes an intrapelvic rotation of the fetal head during the birthing process.

i. Orientation in the space

The head is a tridimensional structure with 3 planes defining its precise spatial position in the birthing canal:

• Sagittal plane (or plane of flexion):Determines 4 distinct modalities of cephalic presentation: face; brow; deflexed vertex (or sinciput) and vertex. (Figure 7)

Figure 7.n

• Transverse plane (or plane of position): Determines the fetal position of the modality of presentation, which is the orientation of a point of reference with the right or left; anterior or posterior sides of the maternal pelvis. (Figure 8)

Figure 8.n

There are eight main fetal positions recognized: (see Figure 6 for example):

v Direct: Anterior and posterior

v Transverse: Right and left

v Right: Anterior and posterior

v Left: anterior and posterior

Coronal plane (or plane of synclitism): Determines the lateral obliquity of the presentation and position relative to the mid-axis of the pelvis. Only anterior (posterior deviation of the sagittal suture; results in more anterior parietal being palpable) and posterior asynclitism (anterior deviation of the sagittal suture; results in more posterior parietal being palpable) are recognized. (Figure 9)

Figure 9.n

ii. Concept of grip:

The relationship between the axis of the blades and the major axis of the fetal head is defined as fetal grip (also known as fetal application). It represents the way the forceps holds the fetal head within the pelvis.

Considering the contact of the forceps blades with the fetal vault structures three different fetal grips are recognized:

1. Biparieto-bimalar (which can be symmetric or asymmetric): The forceps makes contact with the parietal bones and then with the malar regions of the face (constituted mostly by the zygomatic bone). The ideal grip is a symmetric biparieto-bimalar. (Figure 10)

Figure 10.n

2. Fronto-mastoid: Asymmetric grip where one of the blades makes contact with the frontal bone and the other one with the mastoid area of the temporal bone. To be avoided. (Figure 11)

Figure 11.n

3. Fronto-occipital: Asymmetric grip where one of the blades makes contact with one or both frontal bones and the other one with the occipital bone. To be avoided. (Figure 12)

Figure 12.n

The fetal grip of forceps depends mainly on the fetal position and the forceps application. Asynclitism may play also a smaller role. (See Figure 13).

Figure 13.

iii. Point of flexion

The fetal head has a wide range of three-dimensional motions owing to its attachment and physical characteristics of the cervical spine. With the head in indifferent flexion the hinge point or hypomoclion is projected on the vault as a point in the sagittal suture slightly posterior to the midpoint between the bregmatic and lambdoid sutures. From the physical standpoint, to promote flexion the supplemental force of vacuum or forceps needs to be applied slightly posterior to this projection point in a spot located approximately 2.5-3 cm anterior to the lambdoid fontanel. This point is widely known (albeit incorrectly) as the “flexion point.” Technically speaking, this point (located approximately 3 cm anterior to the angle of the lambdoid fontanel) is the superficial projection (on the skull) of the true flexion point which is located at the base of the skull in its articulation with the cervical spine. (Figure 14)

Figure 14.n

iv. Application of vacuum: (Figure 15)

Figure 15.n

Relative to the location of the vacuum cup to the lambdoid fontanel, the projection of the flexion point and the sagittal suture, the applications of vacuum can be median or paramedian; flexing or deflexing.

The ideal application of a vacuum is a median flexing (See Figure 15).

Practical tips

• During any vaginal exam an effort to describe the fetal head needs to be described in the 3 axis of the space, according to its modality of presentation, position and synclitism (or asynclitism).

• In addition to the indication and the station of the procedure, the location of the fetal head in the 3 axis of the birthing canal determines the choice of instruments (see below).

• Proper placement of vacuum and forceps relative to the projection on the vault of the head flexion point will promote flexion of the presentation and facilitate the assisted delivery.

• Palpation of both fontanels is helpful in the correct identification of the presentation.

• Documentation of an assisted vaginal delivery should include the forceps and vacuum application used and forceps grip achieved.

Indications for an assisted delivery

Different professional societies propose different indications for the use of VDs (Table I). From the biomechanical standpoint the reasons for an assisted delivery can be divided into prophylactic and therapeutic indications and furthermore, into maternal or fetal (See Table II and Table III). This classification allows for the appropriate planning and preparation of the particular procedure (e.g. prophylactic assisted delivery for maternal cardiac disease vs. a 90 degree rotation and delivery for a transverse arrest of descent). It also provides physicians with a framework to improve communication with expectant families eager for information regarding the need for assistance in their deliveries.

Table I.n

Table II.n

Table III.n

Practical Tips

• It is fundamental to the success of all AVDs for the obstetrician to diagnose, understand, and document the indication(s).

• It is important for the patient (and family) to understand the rationale for the use of instrument on her delivery.

• The preparations of the OVD should be consistent with the indications for the procedure.

Contraindications

A brief evaluation should also be made to determine whether the candidate for an assisted delivery (i.e. pregnant mother with an indication) has any contraindication for an assisted instrumental delivery. Some proposed contraindications are included in Figure 16.

Figure 16.n

Uncertainty regarding the fetal position is an absolute contraindication for assisted vaginal deliveries.

Recently the use of ultrasound has been proposed to evaluate the fetal position in prospective assisted vaginal deliveries. Though the use of ultrasound as an adjunctive tool in the evaluation and management of labor is encouraged, this practice needs further evaluation before it can be systematically implemented as an adjunct in operative vaginal deliveries. From the clinical standpoint there are two main concerns regarding its use for the diagnosis (not the confirmation) of the fetal position:

• The presence and magnitude of plastic phenomena of the fetal head (caput succedaneum, cephalic molding and modeling) increase with cephalo-pelvic disproportion to such an extent that traditionally an assisted delivery is not recommended when the magnitude of the caput precludes the clinical diagnosis of presentation and position. By allowing a diagnosis of fetal position in these circumstances, an ultrasound may lead to the attempt of a very difficult AVD.

• If the clinical diagnosis of the presentation, position and station cannot be clinically determined, even if the instrument is correctly applied in one axial plane, the correct application on the three axial planes and the progress of an assisted delivery cannot be properly monitored (without the use of an ultrasound). This practice could potentially lead to a misapplication of instruments and late abandonment of a procedure.

Conditions/Prerequisites

Conditions are a variety of details aimed at increasing the success of the procedure and decreasing, if not preventing, morbidities associated with the potential use of instruments. A list of proposed conditions is provided in Figure 17.

Figure 17.n

Preliminary evidence shows that minimal changes in policy toward the preparation for an OVD may decrease not only its rates but also the incidence of traumatic births, with a relative increase in the number of trials of instrumental deliveries in the operating room and a relative decrease in the number of cesarean sections performed for no progress of labor or during the second stage without a trial of forceps or vacuum. Leung and colleagues speculate that these changes were the result of operator improvement in performance, possibly resulting from the awareness of being evaluated, a phenomenon known as the “Hawthorne effect.”

Consent/Assent

First and foremost the mother has to give consent to proceed with an assisted delivery. A detailed conversation with the patient or couple should, in the majority of circumstances precede the actual procedure. Our assessment of the progression of labor, fetal condition and the options available, along with the likelihood of success for an assisted delivery, are vital pieces of information for a family that is generally distressed by the lack of progress of the delivery process or by a non-reassuring electronic fetal heart rate tracing.

If time allows, prior to the assisted delivery, an entry in the medical record shall document the results of the maternal and fetal assessment, the mechanism of labor, the options given to the mother and the general plan for delivery. Alternative plans if the procedure is not successful or deemed potentially traumatic also should be included. The different elements to incorporate in such a preoperative note are included in Figure 18.

Figure 18.n

In the case of a prolonged second stage the patient may decide that waiting a few more hours may be the best course of action for her and her child. In the absence of other clinical indications to change clinical conduct, this request can be considered and granted, particularly in the presence of a reassuring fetal condition and when the indication is only a prolonged second stage. Such involvement in the decision making may provide some women with an improved sense of control over the birthing process and may reinforce the fact that the intervention proposed if needed, was indeed required.

Procedure

Any medical intervention during the delivery process has the potential for additional morbidity and even mortality. To substantiate the need of a medical procedure that may have maternal and neonatal implications, one needs to weigh the risks versus the benefits of the intervention with the patient or the couple. When an operative intervention is required in the second stage of labor, the options, risks, and benefits of vacuum, forceps, and CS must be considered and weighed. The individual and social success of the assistance of the birthing process is the outcome of the healthiest mother and child. In cases of AVDs it has been said that a true failure is not when a vaginal delivery is not accomplished but when a preventable injury is inflicted. It follows then that an instrumented vaginal delivery deserves as much consideration, thought, and preparation as its alternative: the abdominal delivery (i.e. cesarean delivery). The safety and success of these procedures hinge on a systematic approach to the issue (Figure 1). This guideline for clinical practice follows specific instructions to ensure not only that the procedure is performed under the right circumstances, but also sets its performance under conditions that would minimize the potential for maternal or fetal morbidity.

Details of the assisted vaginal delivery, including the preparation of the patient, subjective criteria for the selection of instruments, application steps and post-procedure conduct are included in its streamlined form in Figure 19, Figure 20, Figure 21, and Figure 22.

Figure 19.n

Figure 20.n

Figure 21.n

Figure 22.n

Selection of the instrument

Although certain generalities can be made about one group of instruments versus the other (see Figure 23), the information available is of insufficient quality to recommend forceps over vacuums in each specific case (see section of forceps versus vacuum below). The choice of intervention needs therefore to be individualized. Given an indication for an assisted vaginal delivery the choice of instruments depends on: the particular indication for the procedure, the anesthesia (in place or available), the availability of instruments, the training and experience of the obstetrician and the preference of the patient.

Adherence to clinical guidelines, conditions, and steps recommended could improve the outcome and decrease the potential for morbidity associated with the use of forceps and vacuum.

Practical Tips

• Verify the adequate function of the instruments prior to their application.

• A phantom application of forceps and vacuum based on the most recent exam is useful to recapitulate the interventions required in any given case and to determine the instrumental application.

• Do not insert the blades or rotate during uterine contractions.

• Do not force the insertion of forceps blades or their articulation (locking).

• Keep two fingers on the vacuum cup during traction.

• Keep continuous electronic fetal monitoring of the heart rate or frequent heart rate auscultation (every 5 minutes, including after the contraction).

• Provide feedback to the mother (or couple) and assisting personnel on the evolution of the procedure.

• Exert the pulling vector on the forceps or vacuum with the dominant hand in a longitudinal axis parallel to the mother.

• During vacuum and forceps deliveries increase and decrease the traction force in a similar manner to the increase and decrease in intensity of the uterine contraction.

• Do not use greater pulling force than the one allowed by the upper extremities. If the perception of the force required to exert traction is high, perform the traction from a sitting position.

• Try to remove the forceps blades once the fetal presentation reaches the pelvic floor and is distending the vulva, ideally between contractions.

• Many lacerations occur at the time of removal. Use the fetal head as a sliding surface for their careful removal in the opposite order to their insertion.

• Avoid if at all possible the use of combined instruments (vacuum and forceps).

• Anticipate the possibility of a postpartum hemorrhage and/or genital tract lacerations.

Episiotomy and assisted deliveries

Episiotomy has not been proven to be an essential part of an assisted vaginal birth, as it does not reduce and may increase the incidence of maternal trauma. The routine use of episiotomies has not been shown to decrease the duration of the second stage or the rates of shoulder dystocia associated with AVD.

The available evidence suggests that the routine use of episiotomy is harmful and can increase the rates of perineal lacerations in patients undergoing instrumented deliveries (both vacuum and forceps). In the uncommon case in which the perineum is preventing delivery, an episiotomy may expedite a vaginal birth (although no prospective evidence is currently available).

Median episiotomies are associated with an increased risk of severe perineal (third-degree or fourth-degree) lacerations, particularly when associated with an AVD (including VAD). Medio-lateral (and lateral) episiotomies, on the other hand, have been found to be protective in several retrospective studies.

In a case control study of 100 primigravidas, Eogan and colleagues demonstrated a 50% relative decrease in risk of having a third-degree tear for every six degrees of separation from the perineal midline that an episiotomy was cut.

Anal sphincter lacerations at the time of delivery may be complicated by future fecal incontinence in up to 57% of patients, despite immediate identification and repair, and are to be dutifully avoided.

Practical Tips

• Do not cut an episiotomy before the presentation is at the level of the pelvic floor (if one is deemed necessary) and after the forceps blades have been removed from the pelvis.

• If an episiotomy is needed in association with an AVD, a medio-lateral is preferable to a midline episiotomy.

• If an episiotomy is needed, begin the incision of the perineum at the vulvar fourchette at the time of vulvar distension during a uterine contraction with an angle of at least 30 degrees off the midline.

Instrumental rotations

At this time there are no randomized controlled trials comparing rotation with expectant management or instrumental rotations to cesarean sections.

The important points to be considered in this analysis are the relative difficulty of these procedures, and the potential morbidity associated with the natural history of fetal head malpositions or with instrumental rotations. One of the arguments in favor of rotations to occipito-anterior (OA) are the higher rates of perineal trauma encountered with deliveries in posterior varieties of presentation. In a retrospective study of 588 forceps procedures by Benavides and colleagues, anal sphincter injury occurred significantly more often in the OP group (51.5%) than in the OA group (32.9%) (OR 2.2; 95% CI 1.3–3.6). In a logistic regression model controlled for OP position, maternal body mass index, race, length of second stage, episiotomy, birth weight and rotational forceps, an OP head position was 3.1 times more likely (95% CI 1.6–6.2) to be associated with an anal sphincter injury than an OA head position. The neonatal morbidity appears also to be increased in OVDs in the OP position when compared with OVDs in the OA position.

The role of rotational forceps in fetal-neonatal morbidity is controversial. Hankins and colleagues did not find an increase in maternal or neonatal morbidity outcomes when forceps rotations of 90 degrees or more were compared with lesser degrees of rotation. In a study evaluating more than 140 rotations, Schiff and colleagues found similar maternal and neonatal outcomes in a comparison with a matched control group of VADs. Feldman and colleagues reported higher rates of admission to the neonatal intensive care unit that were unrelated to the mode of delivery. These investigators also noted lower rates of maternal morbidity associated with forceps rotations with the Leff forceps when compared with traditional low or outlet forceps, a difference likely reflecting the expertise of the operators with this particular instrument. Similarly, Al-Suhel and colleagues report that the use of Kielland forceps had maternal outcomes no different than non-rotational vacuums and lower rates of adverse neonatal outcomes than all other forms of instrumental delivery. In a report of VADs applied to 167 cases of OP presentation, Neri and colleagues reported a 79.6% success rate of the instrument in effecting an “autorotation” to OA.

Recommendations:

The available evidence suggests that perineal trauma could be diminished by rotating the fetus to occiput anterior instead of performing a direct occiput posterior delivery. Deciding on whether to proceed with a rotation greater than 90 degrees depends not only on the experience of the obstetrician but, also on the patient’s parity, size of the fetus, size and conformation of the pelvis, station of the presentation, presence of plastic phenomena (molding, caput), fetal well-being assessment and availability of instruments. Such procedures should be abandoned if the rotation is deemed difficult or potentially traumatic.

Practical tips

• A manual rotation might prevent an instrumental rotation.

• Consider rotations of the presentation in all cases of OP position with the exception of the occipito-sacral at the low to outlet stations.

• Malrotations of the fetal head are frequently associated of asynclitism and deflection, which need to be corrected before (asynclitism) and after (deflection) the rotation of the fetal head.

• Rotational forceps deliveries should not be attempted in hospitals not equipped to perform an immediate CS unless exceptional circumstances exist.

• Rotation of the fetal head should only be attempted between contractions.

• After a successful rotation, encourage the patient to push her fetus after disarticulation of the branches and removal of the forceps.

When to abandon the procedure

There are no randomized trials looking at this issue. Different operational criteria have been proposed for the abandonment of an AVD. The RCOG proposes to abandon a FAD when the forceps’ blades cannot be applied easily, the handles do not easily approximate, a rotation is not easily effected with gentle torque, or there is no evidence of progressive descent with each pull or when delivery is not imminent following three pulls of a correctly applied instrument by an experienced operator.

The Canadian guidelines propose to consider abandonment of the vacuum and/or forceps when it fails to achieve delivery of the fetus in a reasonable time, although no operational criteria for the time allowed is provided.

The threshold for abandoning an instrumental delivery and resorting to an alternative mode of delivery is likely to differ between clinicians and the clinical circumstances.

In a report by Murphy and colleagues, 82% of OVDs were completed within three pulls. More than three pulls at attempted OVD was associated with increased neonatal trauma for both completed and failed deliveries; the majority of these deliveries were accomplished by “less experienced trainees” with 1-5 years of obstetric experience.

In comparing the need for zero to two pulls with three or more pulls with a vacuum, nulliparous women required significantly more pulls than parous women (p=0.001). In this study more than three pulls were required by 4.9% of the nulliparous and 2.6% of the parous women. In the study of vacuum by Vacca, 84% of the patients required four pulls or fewer. In his report, five pulls or more were required in 40% of cases in which the cup was deflexing, 29% of the mid-pelvic extractions and 16% of the rotational procedures.

Recommendations

Most operative deliveries, including the application of a vacuum, should be considered a trial. Unless the practitioner is certain that an AVD is going to be successful, the possibility of failure needs to be anticipated. In these circumstances, an alternative plan that will result in a safe and expeditious birth must be in place and implemented promptly if the planned operative birth is unsuccessful.

Practical Tips

• An experienced operator or supervisor needs to be in attendance for all AVDs.

• The obstetrician needs to be prepared to abandon a forceps or vacuum procedure if the instrument cannot be applied properly, if there is no progress after proper application, or if further attempts are deemed potentially traumatic.

• The number of pulls should not be the only criterion used to abandon an AVD.

Post-procedure conduct

Debriefing

Couples and mothers expect some feedback to be provided by their physicians regarding the course of the procedure, the lack or presence of complications and the prognosis for recovery.

AVDs can be associated with fear of subsequent childbirth, and in a severe form may manifest as a post-traumatic stress type syndrome that has been termed “tokophobia.” There is no evidence to support the use of formal debriefing in reducing the risk of subsequent postnatal depression for women who have experienced an AVD. Nonetheless, women report the need for a review following delivery to discuss the indication for delivery, the management of any complications, and the implications for future deliveries. The optimal timing, setting and health care professional for post-delivery review require further evaluation. When compared with women who had cesarean sections, women who underwent an AVD were more likely to prefer a vaginal delivery a year after the event and to actually have a vaginal delivery in subsequent pregnancies. This fact may have an impact on the current epidemic of cesarean deliveries.

Documentation

Practical tips

• Afford the assisted vaginal delivery note the same importance and detail afforded to a cesarean delivery note.

• Be acquainted with the specific procedures of the hospital or system where the delivery occurs.

• Standardized operative vaginal delivery forms are becoming popular to ensure consistent documentation across practitioners.

Potential complications

The available information to evaluate the contribution of these procedures to maternal, fetal, or neonatal morbidity is of variable quality, has a wide range of reported outcomes, and for the most part does not take into account either the indications for its use or the comorbid conditions (prolonged second stage for example) associated with it. Yet when controlled by other clinical variables associated with difficult deliveries, some of the perceived associations with the use of instrument do not remain significant.

The use of instruments in a delivery is in itself a good surrogate parameter of difficulty in the birthing process. Large epidemiologic studies have shown that when compared with spontaneous vaginal deliveries, the women who required a FAD are older, nulliparous, have heavier fetuses, are more frequently postdates, are induced more frequently, have dystocia of their labors (including longer labors, abnormal uterine contractility patterns and fetal malpositions), and have more diagnoses of fetal distress, pregnancy-induced hypertension, abruptio of the placenta, and chorioamnionitis.

The risks attributed to the use of the instruments are therefore difficult to differentiate from the effects of the indication that prompted their use. In the case of AVD literature, association and causation are frequently confused. Given that few studies have prospectively evaluated the elective use of forceps when seemingly no assistance was required, it is difficult to ascertain what proportion of the morbidity over the baseline the instruments are actually responsible for (their attributable risk).

It is not surprising that the outcomes of AVDs in different parts of the world, by different operators with variable experience, associated with the use of different instruments for different indications and in different circumstances, are as widely variable as the ones summarized in Table IV and Table V. These considerations also explain why, in expert hands and in the absence of a medical indication, the morbidity associated with the use of forceps and vacuums is negligible.

Table IV.n

Table V.n

Al Suhel and colleagues, reporting on 1067 instrumental deliveries, noted that the use of rotational Kjelland forceps was associated with lower odds of adverse maternal (defined as severe perineal lacerations, postpartum hemorrhage or fever) and adverse neonatal (defined as 5 min Apgar score of less than 7, cord pH under 7.15 or admission to the NICU) than vacuum deliveries (rotational or not), non-rotational forceps or the sequential use of instruments. Although the authors recognize that these instruments are used by experienced operators (and therefore the potential for operator bias), it also illustrates the low morbidity that can be achieved through high levels of proficiency, emphasizing the role of training in these procedures. With the exception of pelvic floor dysfunction, little attention has been given to date to long-term maternal and neonatal outcomes in relation to the mode of delivery. In 2004, Bahl and colleagues reported on the fertility and pregnancy outcomes at 3 years in relation to the mode of delivery in the cohort previously reported by Murphy et al.87 Women who had an AVD had significantly higher odds of achieving a planned pregnancy at 3 years (adjusted odds ratio of 2.09; 95% confidence interval of 1.1-4.28) than women who underwent a CD during the expulsive phase in the previous pregnancy. In addition, women who had an instrumented delivery were more likely to aim and succeed at having a vaginal delivery when compared with women who underwent a CD, despite a high rate of VBAC success. The authors concluded that AVDs offer advantages for future delivery outcomes over cesarean deliveries and urge further attention be paid to the psychological impact of operative deliveries in general.

Vaccum versus forceps

Experts often provide conflicting evidence for and against the use of these procedures. The literature discussing the use of vacuum and forceps includes prospective randomized trials comparing the outcomes after forceps-assisted and vacuum-assisted births. These trials do not use the same inclusion criteria or the same instruments, or look at the same outcomes, making comparison of the outcomes of these interventions difficult. Neonatal mortality and serious morbidity related to spontaneous vaginal birth, vacuum or forceps, or CS have also been reviewed in large retrospective studies. None of these prospective or retrospective studies discuss specifics of the technique used for vacuum or forceps procedures, the time required, or the criteria for abandonment of the procedure. In addition, the analysis of morbidities (particularly long-term) associated with the use of instruments is fraught with bias (selection/ascertainment/response/recall), lack of power to find true differences by route of delivery, lack of pre-pregnancy baseline data, lack of long-term follow-up, inconsistencies, unaddressed covariables and an incomplete understanding of physiologic processes being evaluated.

In a 10-year analysis of operative deliveries during the second stage in two hospitals in the United Kingdom, Loudon and colleagues noted a secular trend of increasing failure rates of AVDs, which was correlated with the increased failure rates of vacuums.

As far as morbidities, each type of instrument has a different profile of complications. The generalities of the instruments and the patterns of morbidity tend to be consistent, and are outlined in Figure 23.

Figure 23.n

Individualized considerations and care of these cases is indispensable for optimal outcomes.

Summary

Assisted vaginal deliveries with vacuum and forceps are meant to facilitate and expedite delivery for mother and infant, a process that has inherent morbidities for both.

An appropriate use of instruments calls for a systematic approach to its potential need, a keen understanding of what processes are called for their use, and when/how they are to be corrected with instruments.

As long as the vaginal route continues to be an option for human delivery, forceps and vacuums will continue to assist women and children to carry out the delivery process unharmed. Their safe and rational use continues to be taught and can have an important role in the prevention of cesarean deliveries in patients in need of assistance during the second stage.

What is the evidence for specific management and treatment recommendations

Rosenberg, K, Trevathan, W. “Birth, obstetrics and human evolution”. BJOG. vol. 109. 2002. pp. 1199-206.

Wittman, AB, Wall, LL. “The Evolutionary Origins of Obstructed Labor: Bipedalism, Encephalization, and the Human Obstetric Dilemma”. Obstetrical and Gynecological Survey. vol. 62. 2007. pp. 739-48.

Arrowsmith, S, Hamlin, EC, Wall, LL. “Obstructed Labor Injury Complex: Obstetric Fistula Formation and the Multifaceted Morbidity of Maternal Birth Trauma in the Developing World”. Obstetrical & Gynecological Survey. vol. 51. 1996. pp. 568-74.

Cook, RJ, Dickens, BM, Syed, S. “Obstetric fistula: the challenge to human rights”. Int J Gynaecol Obstet. vol. 87. 2004. pp. 72-7.

Khan, KS, Wojdyla, D, Say, L, Gülmezoglu, AM, Van Look, PFA. “WHO analysis of causes of maternal death: a systematic review”. Lancet. vol. 367. 2006. pp. 1066-74.

Hofmeyr, GJ. “Obstructed labor: using better technologies to reduce mortality”. Int J Gynaecol Obstet. vol. 85 Suppl 1. 2004. pp. S62-72.

Zupan, J. “Perinatal mortality in developing countries”. N Engl J Med. vol. 352. 2005. pp. 2047-8.

Gei, AF, Pacheco, LD. “Forceps: still an option?”. Curr Womens Health Rev. vol. 4. 2008. pp. 56-75.

Drife, J. “The start of life: a history of obstetrics”. Postgrad Med J. vol. 78. 2002. pp. 311-5.

Breen, J. “Observations on the management of tedious labour, with an account of the number of cases that occurred in the lying-in hospital, Dublin, from November 1800 to the 31st July 1816”. The Edinburgh Medical and Surgical Journal. vol. 15. 1819. pp. 161-74.

Das, K. “Obstetric Forceps: Its History and Evolution”. Calcutta, The Art Press. 1929.

Dumont, M. “Histoire et petite histoire du forceps”. J GynecolObstetBiolReprod. vol. 13. 1984. pp. 743-57.

Gei, AF, Pacheco, LD. “Operative vaginal deliveries: practical aspects”. Obstet Gynecol Clin North Am. vol. 38. 2011. pp. 323-49.

Simon-Toulza, C, Parant, O. “Spatules : description, mécanique, indications et contre-indications”. J Gynecol Obstet Biol Reprod (Paris). vol. 37. 2008. pp. S222-30.

Gei, AF, Belfort, MA. “Forceps-assisted vaginal delivery”. Obstet Gynecol Clin North Am. vol. 26. 1999. pp. 345-370.

Kielland, C. “Uber die anlegung der Zange am nichit notierten Kopf mit beschreibung eines neuen Zangermodelles, und einer neun Anlegungsmethode”. Monatschr f Geburtsh u Gynak. vol. 43. 1916. pp. 48

Piper, EB, Bachman, C. “The prevention of fetal injuries in breech delivery”. JAMA. vol. 92. 1929. pp. 217-21.

“Society of Obstetricians and Gynaecologists of Canada (SOGC) clinical practice guidelines: guidelines for operative vaginal birth”. Int J Gynaecol Obstet. vol. 88. 2005. pp. 229-36.

“Royal College of Obstetricians and Gynaecologists (RCOG). Operative vaginal delivery”. Clinical Guideline 26. RCOG. 2011.

Vayssiè, C, Beucher, G, Dupuis, O. “French College of Gynaecologists and Obstetricians. Instrumental delivery: clinical practice guidelines from the French College of Gynaecologists and Obstetricians”. Eur J Obstet Gynecol Reprod Biol. vol. 159. 2011. pp. 43-8.

“Royal Australian and New Zealand College of Obstetricians and Gynaecologists. Guidelines for use of rotational forceps”. 2004.

Vacca, A. “Handbook of vacuum extraction in obstetrical practice”. 1992.

Williams, MC. “Vacuum-assisted delivery”. Clin Perinatol. vol. 22. 1995. pp. 933-52.

O’Grady, JP, Pope, CS, Patel, SS. “Vacuum extraction in modern obstetric practice: a review and critique”. Current Opinion in Obstetrics and Gynecology. vol. 12. 2000. pp. 475-80.

Vacca, A. “Vacuum-assisted delivery”. Best Practice & Research Clinical Obstetrics & Gynaecology. vol. 16. 2002. pp. 17-30.

Riethmuller, D, Ramanah, R, Maillet, R. “Ventouses: description, mecanique, indications et contre-indications”. J Gynecol Obstet Biol Reprod. vol. 37. 2008. pp. S210-21.

Molina, FS, Nicolaides, KH. “Ultrasound in Labor and Delivery”. Fetal Diagn Ther. vol. 27. 2010. pp. 61-7.

Ghi, T, Youssef, A, Maroni, E. “Intrapartum transperineal ultrasound assessment of fetal head progression in active second stage of labor and mode of delivery”. Ultrasound Obstet Gynecol. vol. 41. 2013. pp. 430-5.

Sherer, DM. “Can sonographic depiction of fetal head position prior to or at the onset of labor predict mode of delivery?”. Ultrasound Obstet Gynecol. vol. 40. 2012. pp. 1-6.

Raia-Barjat, T, Fanget, C, Trombert, B. “Étude préliminaire du diagnostic d’engagement par une échographie transpérinéale avant une extraction instrumentale”. J Gynecol Obstet Biol Reprod (Paris). vol. 41. 2012. pp. 346-52.

Verhoeven, CJ, Rückert, ME, Opmeer, BC. “Ultrasonographic fetal head position to predict mode of delivery: a systematic review and bivariate meta-analysis”. Ultrasound Obstet Gynecol. vol. 40. 2012. pp. 9-13.

Sentilhes, L, Gillard, P, Descamps, P. “Indications et pre´ requis a` la re´alisation d’une extraction instrumentale: quand, comment et ou` ?”. J Gynecol Obstet Biol Reprod. vol. 37. 2008. pp. S188-201.

Leung, WC, Lam, HS, Lam, KW. “Unexpected reduction in the incidence of birth trauma and birth asphyxia related to instrumental deliveries during the study period: was this the Hawthorne effect?”. BJOG. vol. 110. 2003. pp. 319-22.

Nichols, CM, Pendlebury, LC, Jennell, J. “Chart documentation of informed consent for operative vaginal delivery: is it adequate?”. South Med J. vol. 99. 2006. pp. 1337-9.

Cohen, WR. “Influence of the duration of second stage labor on perinatal outcome and puerperal morbidity”. Obstet Gynecol. vol. 49. 1977. pp. 266-9.

Albers, LL, Schiff, M, Gorwoda, JG. “The length of active labor in normal pregnancies”. Obstet Gynecol. vol. 87. 1996. pp. 355-9.

Menticoglou, SM, Manning, F, Harman, C, Morrison, I. “Perinatal outcome in relation to second-stage duration”. Am J Obstet Gynecol. vol. 173. 1995. pp. 906-12.

Rouse, DJ, Owen, J, Savage, KG. “Active phase labor arrest: revisiting the 2-hour minimum”. Obstet Gynecol. vol. 98. 2001. pp. 550-4.

Towner, D, Castro, MA, Eby-Wilkens, E. “Effect of mode of delivery in nulliparous women on neonatal intracranial injury”. N Engl J Med. vol. 341. 1999. pp. 1709-14.

Gardella, C, Taylor, M, Benedetti, T. “The effect of sequential use of vacuum and forceps for assisted vaginal delivery on neonatal and maternal outcomes”. Am J Obstet Gynecol. vol. 85. 2001. pp. 896-902.

Wen, SW, Liu, S, Kramer, MS. “Comparison of maternal and infant outcomes between vacuum extraction and forceps deliveries”. Am J Epidemiol. vol. 153. 2001. pp. 103-7.

Demissie, K, Rhoads, GG, Smulian, JC. “Operative vaginal delivery and neonatal and infant adverse outcomes: population based retrospective analysis”. BMJ. vol. 329. 2004. pp. 1-6.

Clement, R, Bresson, C, Marcorelles, P. “Cerebellar-pulmonary embolism, cause of death in the newborn”. Journal of Clinical Forensic Medicine. vol. 13. 2006. pp. 361-5.

De Leeuw, JW, Struijk, PC, Vierhout, ME. “Risk factors for third degree perineal ruptures during delivery”. BJOG. vol. 108. 2001. pp. 383-7.

Handa, VL, Danielsen, BH, Gilbert, WM. “Obstetric anal sphincter lacerations”. Obstet Gynecol. vol. 98. 2001. pp. 225-30.

O’Mahony, F, Settatree, R, Platt, C. “Review of singleton fetal and neonatal deaths associated with cranial trauma and cephalic delivery during a national intrapartum-related confidential enquiry”. BJOG. vol. 112. 2005. pp. 619-26.

Lowe, B. “Fear of failure: a place for the trial of instrumental delivery”. Br J Obstet Gynaecol. vol. 94. 1987. pp. 60-6.

Edozien, LC. “Towards safe practice in instrumental vaginal delivery”. Best Pract Res Clin Obstet Gynaecol. vol. 21. 2007. pp. 639-55.

Schaal, JP, Equy, V, Hoffman, P. “Comparaison ventouse forceps”. J Gynecol Obstet Biol Reprod. vol. 37. 2008. pp. S231-43.

Lurie, S, Glezerman, M, Baider, C. “Decision-to-delivery interval for instrumental vaginal deliveries: vacuum extraction versus forceps”. Arch Gynecol Obstet. vol. 274. 2006. pp. 34-6.

Chalmers, JA, Chalmers, I. “The obstetric vacuum extractor is the instrument of first choice for operative vaginal delivery”. Br J Obstet Gynaecol. vol. 96. 1989. pp. 505-6.

Werner, EF, Janevic, TM, Illuzzi, J. “Mode of Delivery in Nulliparous Women and neonatal Intracranial Injury”. Obstet Gynecol. vol. 118. 2011. pp. 1239-46.

Myers-Helfgott, MG, Helfgott, AW. “Routine episiotomy in modern obstetrics: should it be performed?”. Obstet Gynecol Clin North Am. vol. 26. 1999. pp. 305-25.

Hartmann, K, Viswanathan, M, Palmieri, R. “Outcomes of routine episiotomy: a systematic review”. JAMA. vol. 293. 2005. pp. 2141-8.

Carroli, G, Mignini, L. “Episiotomy for vaginal birth”. Cochrane Database Syst Rev. vol. 1. 2009. pp. CD000081

Youssef, R, Ramalingam, U, Macleod, M. “Cohort study of maternal and neonatal morbidity in relation to use of episiotomy at instrumental vaginal delivery”. BJOG. vol. 112. 2005. pp. 941-5.

Helwig, JT, Thorp, JM , Bowes , WA. “Does midline episiotomy increase the risk of third- and fourth-degree lacerations in operative vaginal deliveries?”. Am J Obstet Gynecol. vol. 82. 1993. pp. 276-9.

Baskett, TF, Fanning, CA, Young, DC. “A prospective observational study of 1000 vacuum assisted deliveries with the omnicup device”. J Obstet Gynaecol Can. vol. 30. 2008. pp. 573-80.

Räisänen, S, Vehviläinen-Julkunen, K, Cartwright, R. “Vacuum-assisted deliveries and the risk of obstetric anal sphincter injuries-a retrospective register-based study in Finland”. BJOG. vol. 119. 2012. pp. 1370-8.

Rognant, S, Benoist, G, Creveuil, C, Dreyfus, M. “Obstetrical situations with a high risk of anal sphincter laceration in vacuum-assisted deliveries”. Acta Obstet Gynecol Scand. vol. 91. 2012. pp. 862-8.

Wheeler, TL, Richter, HE. “Delivery method, anal sphincter tears and fecal incontinence: new information on a persistent problem”. Curr Opin Obstet Gynecol. vol. 19. 2007. pp. 474-9.

Dahl, C, Preben, K. “Obstetric anal sphincter rupture in older primiparous women: a case–control study”. Acta Obstet Gynecol Scand. vol. 85. 2006. pp. 1252-8.

Aukee, P, Sundstrom, H, Kairaluoma, MV. “The role of mediolateral episiotomy during labour. Analysis of risk factors for obstetric anal sphincter tears”. Acta Obstet Gynecol Scand. vol. 85. 2006. pp. 856-60.

deVogel, J, van der Leeuw-van Beek, A, Gietelink, D. “The effect of a mediolateral episiotomy during operative vaginal delivery on the risk of developing obstetrical anal sphincter injuries”. Am J Obstet Gynecol. vol. 206. 2012. pp. 404.e1-5.

De Leeuw, JW, de Wit, C, Kuijken, JP. “Mediolateral episiotomy reduces the risk for anal sphincter injury during operative vaginal delivery”. BJOG. vol. 115. 2008. pp. 104-8.

Eogan, M, Daly, L, O’Connell, P. “Does the angle of episiotomy affect the incidence of anal sphincter injury?”. BJOG. vol. 113. 2006. pp. 190-4.

Majoko, F, Gardener, G. “Trial of instrumental delivery in theatre versus immediate caesarean section for anticipated difficult assisted births”. Cochrane Database Syst Rev. vol. 4. 2008. pp. CD005545

Benavides, L, Wu, JM, Hundley, AF. “The impact of occiput posterior fetal head position on the risk of anal sphincter injury in forceps-assisted vaginal deliveries”. Am J Obstet Gynecol. vol. 192. 2005. pp. 1702-6.

Bradley, MS, Kaminski, RJ, Streitman, DC. “Effect of rotation on perineal lacerations in forceps-assisted vaginal deliveries”. Obstet Gynecol. vol. 122. 2013. pp. 132-7.

Pearl, ML, Roberts, JM, Laros, RK. “Vaginal delivery from the persistent occiput posterior position”. Influence on maternal and neonatal morbidity. J Reprod Med. vol. 38. 1993. pp. 955-61.

Hankins, GD, Leicht, T, Van Hook, J. “The role of forceps rotation in maternal and neonatal injury”. Am J Obstet Gynecol. vol. 180. 1999. pp. 213-4.

Feldman, DM, Borgida, AF, Sauer, F. “Rotational versus non rotational forceps: maternal and neonatal outcomes”. Am J Obstet Gynecol. vol. 181. 1999. pp. 1185-7.

Schiff, E, Friedman, SA, Zolti, M. “A matched controlled study of Kielland’s forceps for transverse arrest of the fetal vertex”. J Obstet Gynaecol. vol. 21. 2001. pp. 576-9.

Al-Suhel, R, Gill, S, Robson, S. “Kielland forceps in the new millennium. Maternal and neonatal outcomes of attempted rotational forceps delivery”. Aust NZ J Obstet Gynaecol. vol. 49. 2009. pp. 510-4.

Neri, A, Kaplan, B, Rabinerson, D. “The management of persistent occipito posterior position”. Clin Exp Obstet Gynecol. vol. 22. 1995. pp. 126-31.

Bahl, R, Van de Venne, M, Macleod, M. “Maternal and neonatal morbidity in relation to the instrument used for mid-cavity rotational operative vaginal delivery: a prospective cohort study”. BJOG. vol. 120. 2013. pp. 1526-32.

Traub, AI. “A continuing use for Kielland’s forceps?”. Br J Obstet Gynaecol. vol. 91. 1984. pp. 894-8.

Yeomans, ER. “Operative vaginal delivery”. Obstet Gynecol. vol. 115. 2010. pp. 645-53.

Lowe, B. “Fear of failure: a place for the trial of instrumental delivery”. Br J Obstet Gynaecol. vol. 94. 1987. pp. 60-6.

Murphy, DJ, Pope, C, Frost, J. “Women’s views on the impact of operative delivery in the second stage of labour: qualitative interview study”. BMJ. vol. 327. 2003. pp. 1132

Menage, J. “Post-traumatic stress disorder after childbirth: the phenomenon of traumatic birth”. CMAJ. vol. 156. 1997. pp. 831-5.

Fisher, J, Astbury, J, Smith, A. “Adverse psychological impact of operative obstetric interventions: a prospective longitudinal study”. Aust NZ J Psychiatry. vol. 31. 1997. pp. 728-38.

Creedy, DK, Shochnet, IM, Horsfall, J. “Childbirth and the development of acute trauma symptoms: incidence and contributing factors”. Birth. vol. 27. 2000. pp. 104-11.

Jolly, J, Walker, J, Bhabra, K. “Subsequent obstetric performance related to primary mode of delivery”. Br J Obstet Gynaecol. vol. 106. 1999. pp. 227-32.

Bahl, B, Strachan, B, Murphy, DJ. “Outcome of subsequent pregnancy three years after previous operative delivery in the second stage of labour: cohort study”. BMJ. vol. 328. 2004. pp. 311

Murphy, DJ, Pope, C, Frost, J. “Women’s views on the impact of operative delivery in the second stage of labour: qualitative interview study”. BMJ. vol. 327. 2003. pp. 1132

Murphy, DJ, Liebling, RE, Patel, R. “Cohort study of operative delivery in the second stage of labour and standard of obstetric care”. BJOG. vol. 110. 2003. pp. 610-5.

Vacca, A. “Operative vaginal delivery: clinical appraisal of a new vacuum extraction device”. Aust NZ J Obstet Gynaecol. vol. 41. 2001. pp. 156-60.

Gei, AF. “The prevention of the first cesarean section. The role of operative vaginal delivery”. Seminars in Perinatology. 2013.

Gillean, JR, Coonrod, DV, Russ, R. “Big infants in the neonatal intensive care unit”. Am J Obstet Gynecol. vol. 192. 2005. pp. 1948-55.

Gopalani, S, Bennett, K, Critchlow, C. “Factors predictive of failed operative vaginal delivery”. Am J Obstet Gynecol. vol. 191. 2004. pp. 896-902.

Johnson, JH, Figueroa, R, Garry, D. “Immediate maternal and neonatal effects of forceps and vacuum-assisted deliveries”. Obstet Gynecol. vol. 103. 2004. pp. 513-8.

Lucas, RM, McMichael, AJ. “Association or causation: evaluating links between “environment and disease””. Bull World Health Organ. vol. 83. 2005. pp. 792-5.

Carmona, F, Martinez-Roman, S, Manau, D. “Immediate maternal and neonatal effects of low-forceps delivery according to the new criteria of the American College of Obstetricians and Gynecologists compared with spontaneous vaginal delivery in term pregnancies”. Am J Obstet Gynecol. vol. 173. 1995. pp. 55-9.

Nichols, CM, Pendlebury, LC. “Chart documentation of informed consent for operative vaginal delivery: is it adequate?”. South Med J. vol. 99. 2006. pp. 1337-9.

Yancey, MK, Herpolsheimer, A, Jordan, GD. “Maternal and neonatal effects of outlet forceps delivery compared with spontaneous vaginal delivery in term pregnancies”. Obstet Gynecol. vol. 78. 1991. pp. 646-50.

“American College of Obstetricians and Gynecologists. Delivery by vacuum extraction”. ACOG Committee Opinion No. 208. ACOG; 1998.

Johanson, R, Menon, V. “Vacuum extraction versus forceps for assisted vaginal delivery”. Cochrane Database of Systematic Reviews. vol. 2. 1999. pp. CD000224-28.

O’Mahony, F, Hofmeyr, GJ, Menon, V. “Choice of instruments for assisted vaginal delivery”. Cochrane Database of Systematic Reviews. vol. 11. 2010. pp. CD005455-100.

Johanson, RB, Heycock, E, Carter, J. “Maternal and child health after assisted vaginal delivery: five-year follow up of a randomized controlled study comparing forceps and ventouse”. Br J Obstet Gynaecol. vol. 106. 1999. pp. 544-9.

Johanson, RB, Rice, C, Doyle, M. “A randomised prospective study comparing the new vacuum extractor policy with forceps delivery”. Br J Obstet Gynaecol. vol. 100. 1993. pp. 524-30.

Wax, JR, Cartin, A, Pinette, MG. “Patient choice cesarean: an evidence based review”. Obstet Gynecol Surv. vol. 59. 2004. pp. 601-16.

Ali, UA, Norwitz, ER. “Vacuum-assisted vaginal delivery”. Rev Obstet Gynecol. vol. 2. 2009. pp. 5-17.

Loudon, JA, Groom, KM, Hinkson, L. “Changing trends in operative delivery performed at full dilatation over a 10-year period”. J Obstet Gynaecol. vol. 30. 2010. pp. 370-5.

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