Open Access Study protocol

IMOP: randomised placebo controlled trial of outpatient cervical ripening with isosorbide mononitrate (IMN) prior to induction of labour – clinical trial with analyses of efficacy, cost effectiveness and acceptability

Shrikant Bollapragada, Fiona Mackenzie, John Norrie, Stavros Petrou, Margaret Reid, Ian Greer, Inass Osman and Jane E Norman*

BMC Pregnancy and Childbirth 2006, 6:25  doi:10.1186/1471-2393-6-25

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Amendment to the protocol

Shrikant Bollapragada   (2006-12-08 11:55)  Division of Developmental Medicine, Glasgow Royal Infirmary, Glasgow, UK email

Dear Sir

We write to inform you of a change in the study protocol since our original submission. This relates to an increase in sample size from 300 to 350 women. We gained approval from MHRA and the ethics committee before increasing the sample size and we had submitted this request as a substantial amendment.

The original protocol stated that 300 women will be recruited to the study. The primary outcome on which this study is powered is the time interval from hospital admission to delivery. With 150 women in each of the IMN and placebo groups (300 in total) the study has 96% power at a 5% level of significance to detect a difference in mean time from admission to delivery of 4 hours, assuming a common standard deviation of 9 hours, using a two sample two sided t-test.

In a meeting of the Trial Management Committee after 200 women were recruited, it was decided to increase the sample size to 350 women. The reasons for doing this were that

1. Women scheduled for induction of labour at term are recruited and randomised up to seven days prior to scheduled admission for induction of labour. However, one quarter of women are not initiating on randomised treatment as they commenced spontaneous labour before they could start the outpatient treatment.

2. The times from admission to delivery are much longer than expected - average time in women who take the medication is 29 hours as compared to 20 hours which was stated in the initial trial application.

3. There was more variability in the primary outcome than we assumed - standard deviation of 17 hours as compared to 11 hours which was stated in the initial trial application.

This is expected to reduce the power of the study and therefore the sample size was increased to protect the original power. The current treatment effect was not looked at, when deciding to increase the sample size. The decision to increase the sample size to 350 was empirical, based on likely recruitment within the time available.

Shrikant S Bollapragada1

John Norrie2

Jane E Norman1

1Division of Developmental Medicine, Glasgow Royal Infirmary, Queen Elizabeth Building, 10 Alexandra Parade, Glasgow, G31 2ER, UK

2Centre for Healthcare Randomised Trials (CHaRT), Health Services Research Unit, Aberdeen University, Aberdeen AB25 2ZD, UK

Competing interests



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