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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSION
  8. Acknowledgments
  9. Conflicts of interest
  10. References

Objective  To evaluate the effectiveness of a new handheld vacuum delivery device.

Design  Randomised controlled trial.

Setting  Southmead Hospital, Bristol, UK.

Population  One hundred and ninety-four women requiring delivery with vacuum extraction.

Methods  Women were recruited into the study while still in early labour. If during delivery, vacuum extraction was indicated they were then randomly allocated to either a new handheld vacuum device, the Kiwi Omnicup, or to a ‘standard’ vacuum cup selected by the obstetrician. The ‘standard group’ consisted of the silastic (66/98, 67%) or the metal cups (32/98, 33%). Study data were recorded at completion of the delivery and at 24–48 hours. An additional brief symptom questionnaire was completed by the mother at 10 days postnatally. Statistical analyses were done on an ‘intention-to-treat’ basis.

Main outcome measures  The primary outcome was the successful completion of delivery with the allocated instrument. Secondary outcomes were substantial fetal scalp trauma and substantial maternal trauma.

Results  The Omnicup was associated with a significantly higher failure rate than the standard cup [34%vs 21%, odds ratio (OR) = 1.9 (1.01–3.6)]. This difference persisted after adjusting for confounding factors [adjusted OR = 2.25 (1.01–5.0)] and resulted in significantly more forceps deliveries in the Kiwi Omnicup group [22%vs 10%, unadjusted OR = 2.5 (1.1–5.5)]. The caesarean section rates were similar in both groups [9.4%vs 8.2%, OR = 1.2 (0.4–3.2)]. Rates of substantial scalp trauma and maternal trauma were similar in both groups. However, there were more cases of jaundice in the standard cup group [2.3%vs 12.0%, unadjusted OR = 0.18 (0.04–0.82) and adjusted OR = 0.10 (0.01–0.88)].

Conclusions  The Omnicup is not as successful at achieving a vaginal delivery as the combination of the currently used vacuum cups.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSION
  8. Acknowledgments
  9. Conflicts of interest
  10. References

Instrumental delivery is one of the most common obstetric interventions, with vacuum extraction being considered the instrument of first choice,1 as it causes fewer maternal injuries. However, this recommendation has been criticised2 because vacuum extraction is more likely to fail.3 There are many different types of vacuum extractors, although there are two main groups: soft cup and metal cup ventouses. A systematic meta-analysis demonstrated that soft vacuum cups had a lower scalp injury rate, but also a higher failure rate.4 The Royal College of Obstetricians and Gynaecologists have produced a guideline,5 which states that practitioners should use the most appropriate instrument for individual circumstances, and published data suggest that there has been an increase in the number of vacuum extractions relative to forceps deliveries in the last 10 years.6

Recently, a new vacuum cup has been introduced. The Kiwi Omnicup (Kiwi Omnicup, Clinical Innovations Inc, Murray, Utah, USA) has a design similar to the Bird posterior metal cup but is made of rigid plastic. A number of advantages have been proposed7: the design allows it to be used for occipito-anterior as well as occipito-transverse and occipito-posterior positions. It incorporates a handheld pump, which eliminates the need for a separate electric pump. It is also a single use instrument, which may reduce the potential transmission of infections, particularly prions.8 However, its efficacy is unclear.

Observational studies have found varying results in the efficacy and safety of this new vacuum device. In one retrospective study, the Omnicup was found to be successful in achieving a vaginal delivery in 98% of a series of vacuum extractions without any obvious increase in fetal scalp trauma.7 This study did not have a comparison group and can therefore only provide limited information. Another prospective observational study,9 comparing the Omnicup to the standard cups found that its success in achieving a vaginal delivery was comparable to that of the standard cups (87.3%vs 90.6%), but there was a significant increase in the incidence of superficial scalp abrasions in the Omnicup group. Because this study was not randomised, the results are subject to bias. Therefore, we conducted a randomised controlled study of the Omnicup versus the currently used vacuum cups.

The study hypothesis was that the Omnicup would achieve a higher proportion of successful instrumental deliveries with less fetal scalp trauma.

METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSION
  8. Acknowledgments
  9. Conflicts of interest
  10. References

Southmead Hospital in Bristol, UK, has an annual delivery rate of around 5000 deliveries per year. The annual instrumental delivery rate is about 12–13%, with vacuum deliveries accounting for about 8–10%.

Previous audits at this unit found that the failure rate for vacuum deliveries was approximately 25%. These deliveries had been conducted using a variety of vacuum devices including the silastic, metal anterior and posterior cups, which we have classified as the ‘standard cups’ in this study. The Omnicup had not been in use at the time of these audits as it was not yet available at our unit. In order to get an estimate of the success rate likely to be achieved by using the Omnicup, we used information provided in a Cochrane review comparing the soft (silastic) and rigid (metal) vacuum cups,4 which found the failure rate for rigid cups to be about 10%. In the absence of reliable data evaluating the Omnicup, which is designed quite like a rigid metal cup, we assumed that it could have a similar success rate as the metal cup and could therefore reduce our failure rate from 25% to 10%. From this we estimated that a sample size of 200 patients, with 100 patients randomised to each group, would give our study an 80% statistical power to show this 15% difference, with a P value of <0.05 used to indicate statistical significance. Ethics committee approval was sought and obtained, and recruitment began on 18 February 2002 and completed 31 October 2002.

A woman was eligible to enter the study, if she had a singleton pregnancy with cephalic presentation and gestational age greater than 37 weeks. Women were approached to enter the study when they were admitted in early labour. If they agreed to take part, they signed a consent form and a note was made on the delivery suite board indicating that they would be randomised if they needed delivery by vacuum extraction. Randomisation was achieved by taking consecutively numbered sealed opaque envelopes containing the allocation to either arm of the trial (allocation was by a randomly generated computer sequence 1:1 ratio). The computer sequence was prepared prior to the commencement of the trial and was not known to the researchers. Both the mother and everyone involved with the trial and the delivery were blind to the allocation prior to opening the envelope. When an instrumental delivery was planned for a woman who had consented to take part in the study, an instrumental delivery pack with standard vacuum devices and forceps, as well as an Omnicup, were taken to the intended place of delivery (either the delivery room or theatre). The next consecutively numbered envelope was only collected and opened once a decision to proceed to a vacuum delivery had been made. If the woman was randomised to the ‘Omnicup’ arm of the trial, an Omnicup (Clinical Innovations Inc) was to be used. If she was randomised to the ‘standard cup’ arm, the exact type of cup to be used (i.e. whether silastic, metal, etc.), was at the discretion of the obstetrician performing the delivery, excluding the Omnicup.

For both arms of the trial, the vacuum employed was 0.8 kg/cm2. The pump used to generate the vacuum for the silastic and the metal cups was an Ameda Egnell Senator 30 (Ardo Medical, Switzerland). A ‘successful’ delivery was defined as delivery with the allocated vacuum instrument. If delivery with the selected instrument was not successful, the subsequent course of action (i.e. delivery with another instrument or caesarean section) was at the discretion of the obstetrician conducting the delivery.

The two main indications for delivery were fetal distress (suspected or actual) and delay in the second stage of labour. Fetal distress was diagnosed by suspicious or pathological findings on the cardiotocograph and/or fetal blood sampling. Delay in second stage was diagnosed, if the woman was actively pushing for more than 60 minutes and delivery was not imminent. It is the hospital policy to allow up to 1 hour of ‘passive’ descent of the presenting part for women with epidural analgesia. Episiotomy was performed at the discretion of the obstetrician. All instrumental deliveries were performed by medical staff.

Following delivery, maternal trauma was assessed and repaired by the obstetrician. Details of the delivery and maternal trauma were recorded both in the notes and on the study data collection sheets. Substantial maternal trauma was defined as third or fourth degree tears or extended perineal tears that needed repair in theatre. We also collected data on ease of cup application on a scale of 1 to 10, where 1 was very difficult application and 10 very easy application.

The obstetric senior house officer collected postnatal data 24–48 hours after delivery. Perineal pain/discomfort was assessed on a four-point scale (i.e. no pain, mild pain, moderate pain and severe pain, as reported by the mother). At a similar time interval, the babies were assessed by the neonatal senior house officer and data were recorded on a proforma. Neither the neonatal nor the obstetric senior house officers were blinded to the kind of cup used. The pre-specified categories of fetal scalp trauma were cephalhaematoma, bruising >5 cm, bruising <5 cm, lacerations and others, as previously used by Cohn et al.10 Substantial scalp trauma was defined as presence of one of either cephalhaematoma, bruising >5 cm or lacerations. Neonatal jaundice was assessed clinically with additional blood testing as necessary. We also recorded Neonatal Intensive Care Unit (NICU) admissions and their indications.

In addition, the mother was asked to complete a short questionnaire 10 days postnatally. This was mainly to assess perineal pain/discomfort at 10 days and also to assess whether there had been any re-admission to hospital.

The primary outcome of the study was the successful completion of delivery with the allocated instrument. Secondary outcomes were substantial fetal scalp trauma and substantial maternal trauma.

Statistical analyses were performed on an ‘intention-to-treat’ basis using Stata 7.0 (Stata, Texas, USA). Proportions were compared using the χ2 test or Fisher's exact test as appropriate. Two-sample t tests and the Mann–Whitney test were used for continuous outcomes data as appropriate. Logistic regression was used to adjust for confounders. Proportions, odds ratios (OR), P values and 95% confidence intervals are reported.

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSION
  8. Acknowledgments
  9. Conflicts of interest
  10. References

During the trial period there were 3187 deliveries. Of these, 703 babies (22%) were delivered by caesarean section and 367 (12%) by instrumental delivery, with 261 (8%) of these being delivered using a vacuum device and 106 (3%) using forceps. A summary flowchart of trial randomisation is shown in Fig. 1.

image

Figure 1. Randomisation flowchart.

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Two hundred consented women were initially randomised into the study, but six had to be excluded from all analyses due to errors that occurred prior to the randomisation process (Fig. 1). In all six cases the envelopes were opened prior to any decisions to assist the deliveries. Of the remaining 194 women, 96 were randomised to the Kiwi Omnicup arm and 98 to the standard cup arm. The primary instrument in the standard cup arm was the silastic cup for 66 cases, the anterior metal cup for 18 cases and the posterior metal cup for the remaining 14 cases.

Baseline characteristics for the two groups were broadly similar (Table 1). In both groups, more than 80% of the deliveries were performed with regional analgesia. Almost one-third of the fetuses in the study population were presenting in the occipito-transverse or occipito-posterior position (30.9%).

Table 1.  Baseline and labour characteristics. Values are presented as n (%), median (range) and mean [SD] as appropriate.
 OmnicupStandard cup
Age30 [1.0]28 [1.1]
 
Parity
Nulliparae85 (89)86 (88)
Multiparae11 (12)12 (12)
 
Previous caesarean section2 (2)5 (5)
Gestation at delivery (weeks)40 [1.3]40 [1.3]
Birthweight (g)3547 [427]3530 [494]
 
Primary indication for delivery
Fetal distress39 (41)31 (32)
Failure to progress55 (58)65 (66)
Maternal condition1 (1)2 (2)
 
Regional analgesia
Spinal6 (6)2 (2)
Epidural78 (81)78 (80)
 
Delivery in theatre23/96 (24)20/98 (20)
 
Station of vertex at cup application
At spines11 (13)10 (11)
+154 (61)52 (55)
+222 (25)32 (34)
+31 (1)0
 
Position at application of cup (%)
Occipito-anterior59 (66)68 (72)
Occipito-transverse13 (15)10 (11)
Occipito-posterior17 (19)17 (18)
 
Operator status
Consultant6 (6)8 (8)
Registrar64 (67)55 (56)
Senior house officer26 (27)35 (36)

Delivery outcome, including maternal trauma, is summarised in Table 2. There were more failures to deliver with the allocated instrument using the Omnicup and the level of the difference reached statistical significance [34%vs 21%, unadjusted OR = 1.9 (1.01–3.6), P= 0.04]. This difference persisted after adjusting for grade of operator [senior house officer, specialist registrar (SpR) or consultant], gestation, fetal birthweight and sex, use of regional anaesthesia, maternal age, fetal head position and station [adjusted OR = 2.2 (1.01–5.0), P= 0.047].

Table 2.  Delivery outcome data. Values are presented as n (%), median (range) and mean [SD] as appropriate.
 Omnicup (n= 96)Standard cup (n= 98)Unadjusted OR (95% CI)Adjusted OR (95% CI)#
  • NS; P > 0.05. Statistical analysis for perineal trauma and pain was performed for both overall incidence and individual degree of severity.

  • #

    Adjusted for position, station, grade of operator (operator status), baby birthweight and sex, gestation, maternal age and regional anaesthesia.

Mode of delivery
First vacuum device used
 Successful63 (66)77 (79)  
 Failed33 (34)21 (21)1.9 (1.01–3.6)2.3 (1.01–5.0)
Delivery by different vacuum device3 (3)3 (3)  
Forceps delivery21 (22)10 (10)2.46 (1.1–5.5)2.7 (1.1–6.4)
Caesarean section after failed instrumental9 (9)8 (8)1.2 (0.43–3.15)0.80 (0.42–2.66)
 
Ease of cup application (scale 1–10)9 (1–10)9 (1–10)NS 
Estimated blood loss (mL)372 [17]381 [18]NS 
Episiotomy45 (47)51 (52)NS 
 
Perineal trauma  NS 
None11 (12)11 (11)  
First degree/second degree78 (81)79 (81)  
Third degree/fourth degree7 (7)8 (8)  
 
Perineal pain at 24–48 hoursn= 91n= 94NS 
None17 (19)15 (16)  
Mild33 (36)35 (37)  
Moderate38 (42)41 (44)  
Severe3 (3)3 (3)  
 
Perineal pain present at 10 days22/7526/70NS 

The higher proportion of failure to deliver with the allocated instrument resulted in more forceps deliveries in the Omnicup group [22%vs 10%, unadjusted OR = 2.5 (1.1–5.5), P= 0.03 and adjusted OR = 2.7 (1.1–6.4), P= 0.03]. There was no significant difference between the groups in the proportions of caesarean sections performed [9.4%vs 8.2%, adjusted OR = 0.80 (0.42–2.7), P= 0.77].

There were no significant differences in the ease of cup application, the estimated blood loss and the proportion of episiotomies or perineal trauma. There were also no significant differences in the pain reported by women at 24–48 hours post-delivery (data were available for 185/194 women).

The 10-day postnatal questionnaire was completed by 143/194 (74%) patients and there were no significant differences in the pain reported by the women at 10 days post-delivery.

Neonatal outcome is summarised in Table 3. Data on scalp trauma were available for 180/194 neonates and again there were no significant differences between the two groups. Following previous meta-analyses,4 we divided the scalp trauma into substantial (cephalhaematoma, bruising >5 cm and lacerations) and non-substantial (bruising <5 cm, oedema and petechiae). There were no significant differences between the two groups in the incidence of substantial scalp trauma [23%vs 24%, adjusted OR = 0.74 (0.34–1.60)]. Subgroup analysis showed a higher incidence of substantial scalp trauma after the use of metal cups than with either silastic cups or the Omnicup (Table 4). There were more cases of jaundice in the standard cup group (12%) than in the Omnicup group (2.3%), with the difference reaching statistical significance [adjusted OR = 0.11 (0.01–0.88), P= 0.04]. There was also a trend towards more NICU admissions in the standard cup group [5%vs 10%, adjusted OR = 0.19 (0.03–1.1), P= 0.19], although most of these admissions (8/15) were for suspected sepsis.

Table 3.  Neonatal outcome data. Values are presented as n (%) or n/N as appropriate.
 OmnicupStandard cupUnadjusted OR (95% CI)Adjusted OR (95% CI)#
  • NICU = Neonatal Intensive Care Unit.

  • #

    Adjusted for baby's gestational age, sex, birthweight, maternal age, fetal head position and station at delivery, grade of operator, sequential use of instruments (single vs two or more devices) and maternal use of regional anaesthesia.

Scalp trauman= 88n= 92  
None or not substantial68 (77)70 (76)  
Substantial scalp trauma20 (23)22 (24)0.94 (0.44–1.98)0.74 (0.34–1.60)
 
Apgar score <7 at 5 minutes1 (1)1 (1)  
Jaundice2 (2)11 (12)0.18 (0.04–0.82)0.11 (0.01–0.88)
Phototherapy1/23/11  
NICU admissions5/96 (5)10/98 (10)0.48 (0.12–1.63)0.19 (0.03–1.11)
Table 4.  Substantial scalp trauma rates with different vacuum cups.
Vacuum cup typeSubstantial scalp trauma rate
Silastic10/60 (17%)
Metal (anterior or posterior)12/32 (38%)
Omnicup20/88 (23%)

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSION
  8. Acknowledgments
  9. Conflicts of interest
  10. References

This is the first randomised study of the Kiwi Omnicup. It showed that the Omnicup was less successful in achieving delivery and resulted in a higher use of forceps in the Omnicup arm, with no difference in the caesarean section rate. There were no significant differences in the maternal or neonatal trauma rate. There were also no significant differences in either the ease of cup application, as judged by the operator, or the degree of perineal pain at 24–48 hours and 10 days.

In contrast to previous studies of the Omnicup, our study was randomised and therefore selection bias was minimised. Randomisation was successful, as the baseline characteristics were similar between the two groups. The postnatal questionnaire return rate was satisfactory at 74%. One possible weakness of the study was that a large proportion of deliveries (93%) were performed by trainees. However, this has been previously reported nationally in most UK hospitals2 and is therefore a more accurate representation of the ‘real-life’ situation. Furthermore, subgroup analysis showed that senior house officers had the lowest failure rate, suggesting that they were undertaking ‘easier’ deliveries under appropriate supervision. Indeed, the majority of deliveries by senior house officers were performed under direct supervision.

During interpretation of our results, we acknowledge that the sample size of the study would not allow the detection of small differences in neonatal and maternal trauma between the two groups. Furthermore, the neonates were examined by one of a group of seven doctors who were not blinded to the cup used.

The overall delivery failure rate in our study was 28%, with the lower rate of 21% in the standard cup arm. In the Cochrane meta-analysis,4 the mean failure rate was 15% for the soft cups and 9.5% for the rigid cups (respective ranges are 9%–23% and 0%–19%). Other studies11 have also shown failure rates similar to ours, but a recent observational study9 reported success rates of 87% for the Omnicup and 91% for the standard vacuum equipment. We therefore sought possible explanations for our rather high failure rates. One possible explanation may be the relatively higher incidence of occipito-transverse and occipito-posterior position of the fetal head in our study (overall 30%). That compares to a rate of 21% in the previously referred to Omnicup study. In our study both trial arms were reasonably balanced with respect to fetal head position and this did not influence the comparisons between the groups. We also note that in our study, 23% of the study deliveries were performed in theatre, which means they were considered ‘difficult’ and caesarean section was likely if there was a failed instrumental delivery. A previous prospective observational study12 found that the caesarean section rate after attempted ventouse delivery in theatre is quite high at 52%. The same study also found that the occipito-posterior position was independently associated with an increased risk of delivery by caesarean section. The caesarean section rate in our study group was less than 10%, with an overall vaginal delivery rate over 90%, if forceps were used following failed vacuum delivery.

The high failure rate of the Omnicup could also be explained by the fact that it was introduced in our department only six months before the beginning of the trial and thus fewer obstetricians might have been comfortable using it. When the Omnicup was introduced in our unit, all obstetricians had the opportunity to watch a training video supplied by the company marketing the Omnicup. No further specific training was provided. The majority of deliveries (115/194) were performed by SpRs (total of eight in our unit) and senior house officers (13 senior house officers performed 61/194 deliveries) under direct supervision by SpRs. Two consultants and two staff grades performed 18/194 deliveries. The same SpRs were working in our unit for the whole duration of the study and all of them had at least three years experience in obstetrics and the use of the standard cups. No SpR or consultant had used the Omnicup in another hospital prior to the study. As so many different obstetricians performed deliveries in the study, it is possible they did not use the Omnicup on an adequate number of occasions to enable them to become familiar with the instrument. We examined the Omnicup and standard cup failure rates between the first and second half of the study and found there was no significant difference between the two. In fact there were more failures for the Omnicup in the second half of the study and the failure rate for the standard cups was stable throughout the study. This finding suggests there may not be a significant effect from a ‘learning curve’. The higher success rates demonstrated in the study by Hayman et al.9 may also suggest that when the obstetrician chooses the cup he/she feels is more appropriate, a successful delivery is more likely.

Our study demonstrates that the Omnicup may be less successful in achieving delivery than was initially believed. However, it does not appear to increase the incidence of scalp trauma and there is no difference in the caesarean section rate between the two groups. Clinicians and policy makers will have to decide whether the increased failure rate is an acceptable compromise when weighed against the benefits of having a single use instrument which would eliminate the need for purchasing different cups and vacuum pumps, and also sterilisation costs etc. In order to minimise the risk of transmission of vCJD, the British Department of Health has recommended the use of single use instruments for surgical procedures where it is practical and the clinical outcome is not compromised.13

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSION
  8. Acknowledgments
  9. Conflicts of interest
  10. References

The results of our randomised study suggest that the Kiwi Omnicup is not as successful at achieving a vaginal delivery as the combination of the currently used vacuum cups. However, there may be other benefits, particularly a reduction in transmissible infections. This means that its place in the obstetrician's armamentarium has yet to be decided and it could be influenced by other advantages such as being a single-use instrument.

Acknowledgments

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSION
  8. Acknowledgments
  9. Conflicts of interest
  10. References

The authors would like to thank the women who participated in the study, as well as the midwives and doctors at Southmead Hospital who helped with the data collection. We would also like to thank Dr Valentine Akande who offered statistical advice and helped with the generation of the randomisation sequence.

References

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSION
  8. Acknowledgments
  9. Conflicts of interest
  10. References

Accepted 19 May 2005