Diseño acero de refuerzo

The calculations for measurement of FRC were not incorporated into RASP at the time of the study, but the computer was used as a chart recorder, in order to measure the volumes and helium concentrations with maximum accuracy and resolution.

4.3.3 Calibration

The spirometer was filled with approximately 450 ml of distilled water. The bell was flushed with air and the bell was put in its lowest position before closing the shutter and the stopcock. A known volume of helium was added to the system using a

calibrated syringe. When a constant plateau of helium concentration was obtained, 100 ml of air was introduced into the system, using a calibrated syringe. The new helium concentration was recorded. Further 100 ml alliquots of air were added to the spirometer, recording the helium concentration each time. A 60 g weight was briefly applied to the spirometer bell, to ensure that there was no leak in the system. A graph of volume added against the reciprocal of the helium concentration was plotted, and the intercept on the volume axis was equal to the dead space of the system up to the shutter (figure 4.15). The dead space volume up to the face mask was measured by filling it with water. During the measurement of the dead space of the system, the helium concentration and volume signals were calibrated on the computer and the chart recorder.

4.3.4 Technique

(a) Preparation o f the infant

The infant, when asleep, was laid supine, and the oximeter and transcutaneous monitors were applied. A rim of silicone putty (Carters, Bridgend, Mid-Glamorgan) was applied around the mouth and nose, and to the face mask, which was applied to enclose the infant's mouth and nose, with the silicone putty providing an air-tight seal. The shutter was kept closed to the spirometer, so that the infant breathed room air. Having half filled the bell with air, by simultaneously closing the shutter to room air and opening it to the spirometer the infant was allowed to breathe from the spirometer circuit. The oxygen supply was switched on at the low flow meter and adjusted so that the volume trace end expiratory level of the spirometer remained constant, in order to determine the infant's oxygen consumption for the subsequent FRC

measurement (Merth 1991). The shutter was then simultaneously closed to the spirometer and opened to room air, thus removing or "switching" the infant from the spirometer circuit.

(b) Measurement o f FRC

The spirometer was flushed with fresh air, and helium was added into the circuit, until the concentration of helium in the circuit was 10% to 12%. Oxygen was then added until the concentration of oxygen in the circuit was approximately 22%. Once the helium concentration had stabilized, the computer and chart recordings were commenced, and the infant was "switched" into the circuit, simultaneously turning on the oxygen supply at the predetermined flow rate, such that the end expiratory level on the chart recorder remained stable. Equilibration was ensured by waiting until the helium concentration was stable (up to six minutes), or there had been minimal change over at least two minutes. The 60 g weight was briefly applied to the bell to ensure no leak and to verify equilibration. The temperature of the spirometer was recorded for BTPS corrections. Three measurements of FRC were made, allowing a washout time at least as long as the equilibration period between measurements. Results were expressed as the mean of technically satisfactory measurements (see section 4.3.4d).

(c) Calculation o f FRC

The final helium concentration (HeJ was obtained by extrapolating the final linear portion of the plot of helium concentration against time, back to the point in time at which a decline in helium concentration was first detected (figure 4.16). This compensates for any slow decline in helium concentration due to helium dissolving

in water and blood and tissues (Merth 1991).

The calculation for FRC is as follows:-

FRC (ml) = {(lYdspiRo + Vp] X [HCj - HeJ - V^mask) x CorrF} - He. where V,dSPIRO Vp

He,

He,

V,

= dead space of spirometer (540 to 600 ml, dependent on volume of water added)

= filling volume of spirometer bell (ml) = initial helium concentration (%) = final helium concentration (%

= infant's tidal volume above FRC when switched into the circuit (ml)

= additional dead space from shutter to mask (10 to 17 ml, dependent on mask size)

= correction factor for converting measurements from ATPS to BTPS

(d) Technical quality o f studies

The major problem with determining lung volumes by gas dilution, is ensuring a gas tight system. This was checked in two ways; firstly an oxygen analyser was included in the circuit, so that leaks were not inadvertently compensated for by changing the

secondly a weight was briefly applied to the spirometer bell, and when removed, provided that there was no leak, the volume tracing returned to the original end expiratory level (figure 4.17). Any measurements where leaks were detected were excluded from the analysis (figure 4.18), but most leaks, except around the face mask, were detected during calibration, so the system was checked and any leaks made air and helium tight before commencing lung volume measurements. Any measurements where the end expiratory level was unstable were excluded (figure 4.19). These were usually associated with the infant waking, or being in active sleep, so measurements were repeated once the infant was in quiet sleep again.