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Equipment was calibrated with respect to measurements o f F , Pao and Pj on a daily basis before each respiratory function test. Measurements were performed in the Research Room on the Special Care Baby Unit at Homerton Hospital.

3.4.1 Preparation of equipment

The computer unit was turned on and the RASP program loaded. The Hans Rudolph PNT, its heating source, and the amplifiers were switched on at least half an hour before calibration and measurement.

The two clear tubes attached to the Furness pressure transducer (± 0.2 kPa) were connected to the relevant pressure ports on the PNT to measure F . One tube from the (± 5kPa) pressure transducer was connected to the Pao port on the PNT, whereas the remaining tube was left unattached as a reference to atmospheric pressure. Similarly, jacket pressure was recorded by attaching one o f the tubes from the Pj transducer (± 1 0 kPa) to the pressure port situated on the proximal portion o f the rigid connector from the RTC air reservoir tank to the jacket. The second tube was left open as reference to atmospheric pressure. Care was taken to ensure that the tubes attached to either side o f each o f these differential transducers were o f equal proportions (Sly and Davis, 1996).

3.4.2 Flow calibration

Prior to V calibration, a draught-free environment was ensured by closing the door and window in the Research Room. To maintain accuracy over the range o f signals obtained, the stability and full-scale deflections were checked for any offset from zero in the RASP configuration menu, before known set points were applied to the transducer.

A Hans Rudolph PNT with a linear range, appropriate for the age and size o f the infant to be tested (Table 3.2) was connected to a source o f air supply via a calibrated

rotameter. A zero reference point was established when there was no flow through the PNT, A high reference point was obtained when a flow o f 100 mL.s'' (6 L.min'^) or 50 mL.s'^ (3 L.min ') was passed through the PNT with linear range o f 0-35 L.min'* or 0-10 L.min'% respectively.

To validate the calibration factors, a series of flow signals, which encompassed the entire range o f flow to be measured, was delivered and checked. This was achieved by recording the flow for approximately 5 seconds at 0, 50, 100 mL.s*' for the smaller PNT (0-10 L.min’’) and an additional recording o f 150 and 200 mL.s*’ for the larger PNT (0-35 L.min'^). Each section of recorded flow at these different rates was checked for accuracy and linearity against known values. If the signals were within ± 2% o f the delivered values, a note was made regarding the signals and then saved as part o f the study file.

3.4.3 Volume check

As F ' is the first time derivative of volume, the F ’ calibration was further validated by applying a known volume to the assembled PNT, using a Hans Rudolph calibrated syringe. Following a recording o f approximately 5 seconds of zero flow, a 40 mL volume was injected and then withdrawn through the calibrated PNT (0-35 L.min'*) at a frequency approximating that o f the respiratory rate o f the infant to be studied. A 10 mL calibrated syringe was used when checking the small PNT (0-10 L.min*'). After recording 6-8 o f these signals, the amplitude was checked by the placement o f the cursors in the peaks and troughs o f the ‘inspiratory’ and ‘expiratory’ phase o f the volume signals. Validation was accepted and saved if the measured signals were within ± 2% o f the known value used.

3.4.4 C alibration of pressure at airway opening (Pao)

The Pao signal was calibrated by delivering low and high set points to the Furness differential transducer (range ± 4.910 kPa or ± 50 cmH20). One o f the two transducer tubings was connected to a digital manometer, Digitron (Digitron Instrumentation Ltd., Herts, England) via a 3-way tap while the other tubing was opened to the atmosphere. A zero reference point was recorded when the 3-way tap

was opened to the atmosphere. To establish a high reference point, a 5-mL syringe was attached to the 3-way tap and a pressure o f 2 kPa was delivered to the differential transducer via the manometer.

This was subsequently validated and the accuracy o f the calibration assessed by delivery o f known pressure signals using a 5-mL syringe via the manometer. Recordings o f approximately 5 seconds duration at zero or 0 kPa, 1 kPa, 3 kPa and 4 kPa were made. The displayed signals were checked for accuracy by placing the cursors over the respective portions of the pressure signal. The calibration check was accepted and saved if the values were within ± 2% o f the known signals.

3.4.5 Calibration of jacket pressure (Pj)

Similarly, Rj was calibrated as for Pao- One o f the tubes attached to the Furness differential transducer (Pj) was connected to the Digitron via a 3-way tap while the other tube was opened to the atmosphere. A zero reference point was set when the 3- way tap was left opened to the atmosphere. To establish a high reference point, a 5- mL syringe was attached to the 3-way tap and a pressure o f 5 kPa was delivered via the manometer to the differential transducer and the value recorded.

The calibration was assessed by delivering known pressure signals i.e. 0 kPa, 3 kPa, 6 kPa and 10 kPa to the transducer via the manometer using a 5-mL syringe as described in section 3.4.4. The calibration check was accepted and saved if the values were within ± 2% of the known signals.

Once the F , Pao and Pj had been satisfactorily calibrated, the calibration factors were saved in a specific RASP profile, i.e. FEXP737.PRF. This profile was used to collect tidal breathing parameters and forced expiratory flows. A record o f the calibration factors was printed and kept as part o f each infant’s study record.