Completion of lung function measurements

2.8.1 Measurement of infant length and weight

Following completion of respiratory ftmction tests, the baby’s crown-heel length was measured using a calibrated stadiometer (Harpenden Infantometer, CMS, London) and naked weight obtained using Seca electronic scales (Seca Ltd, Birmingham). If feasible, crown-rump length (with the nappy removed) was also obtained.

The crown-heel and/or crown-rump lengths were measured by two people. One person gently held the baby’s head in a mid-line central position at the top o f the stadiometer whilst the other gently depressed the baby’s knees to fully extend the legs. A moving footplate was then positioned firmly against the soles o f the baby’s feet. The length was read off a counter once the footplate had been locked in a fixed position. This procedure was repeated at least twice and results reported as the mean of 2 measurements within

0.5 cm o f each other. A similar process was carried out to obtain the crown-rump length. In this case, one person ensured that the baby’s head was in the mid-line central position at the top o f the stadiometer, and that his/her body was straight along the midline, and the back resting flat on the stadiometer. Both o f the baby’s legs were straightened and flexed toward the abdomen, and the footplate positioned against the tuberocities. Once the footplate was fixed, the length was read off the counter. The crown-rump length was repeated at least twice and results reported as a mean o f 2 measurements within 0.5 cm of each other.

Each infant was weighed at least twice. When 2 consistent readings were obtained, that value was reported as the test weight.

2.8.2 Follow-up arrangements

Infant and maternal details were documented using a project specific questionnaire (section 2.6.2 and Appendix A.3). Follow-up appointments were arranged with the

• parents. A week before the next appointment, a letter was sent out to the parents to remind them of the appointment and advise them that a telephone call would be made a day or two before the appointment to confirm the date and time, and to check that the infant was not suffering from any upper respiratory tract infection. If this was the case, the appointment would be deferred for at least 3 weeks.

Following respiratory ftmction tests, babies and mothers were escorted back to the wards or nursery if they were in-patients, or to the hospital reception if they had come from home.

2.8.3 Datastorage

Immediately after each study, known signals of flow, volume and Pao were recorded to check the validity of the pre-study calibration factors. Once the original data were backed-up and stored on the project specific optical disk, data would be compressed {if necessary) and stored onto a set of floppy disks. Each set o f individual data was identified and labelled with the unique study number allocated to the baby which indicated study, subject and measurement occasion. Later on, results o f the data analysis would be stored onto subject-specific floppy disks as well as being backed up onto a master analysis disk, using unique identifiers.

2.8.4 Data entry and Epi-Info database

A double entry system was used to enter and validate infant details and analysis data for each infant using Epi-Info, version 5.01a (Atlanta), on an IBM compatible computer. Statistical analyses of the data took into account the matching o f serial measurements between the infant groups and cross-sectional comparisons (see chapter 3).

2.8.5 Equipment cleaning

After each study, various components o f the apparatus were disconnected and thoroughly cleaned. Face masks and parts o f the dismantled Hans Rudolph PNT were

.soaked in Milton solution for at least 30 minutes. The metal wire mesh screens from this PNT were cleaned with 90% Ethanol (alcohol) solution. They were then left to dry in air. Since the Fleisch PNT could not be dismantled, it was rinsed and cleaned with 90% Ethanol (alcohol) solution and left to dry. The shutter and connectors were sent to the Hospital Sterilisation and Disinfection Unit (HSDU) for autoclaving at a low temperature (121°C). Used silicone therapeutic putty was also sterilised in this way. The cable release mechanism, oximeter probe and PNT heating circuit lead were wiped clean with alcohol swabs (Seton Prebbles Ltd, Oldham, England). The cot and mattress were cleaned with soap and water, and when dried, the cot was made up with clean sheets. The work surfaces, stadiometer and scales were cleaned with hard surface disinfectant wipes (Iso-Bact, Griffiths and Nielsen Ltd., Billingshurst, Sussex).

2.9 Summary

• The aims and objectives o f the current study have been described (section 2.1). The study design and protocol, the sample size and power calculations have also been summarised.

• A brief summary o f the theoretical background to infant respiratory function testing has been discussed, followed by detailed description o f the equipment used in this study with regards to their function and calibration. Further description o f the equipment assessment and outcome are presented in Appendix A4.

• Data collection and calculation of both HBIR activity and passive respiratory compliance have been described, with details o f methodology, data acquisition and quality control.

In chapter 3, all cross-sectional and longitudinal results matched for GA, PNA and PCA are presented with a summary o f infant characteristics. The possible influence of sedation, ethnicity and maternal smoking during pregnancy on HBIR activity are also discussed.

3.1 Introduction

The influence of gestational and postnatal age on the vagal control o f breathing in healthy preterm and fullterm infants was investigated by making serial measurements o f HBIR activity and respiratory system compliance during the first 6 months of life. Although

recordings were performed during natural sleep in infants less than 44 weeks PCA, light sedation was needed in many infants for their final measurement. The study design, techniques and equipment used in this study have been described in chapter 2. This chapter will present the results of serial measurements o f HBIR activity and their relationship with other respiratory parameters, and describe the influence o f postnatal and postconceptional age on HBIR during early life.