Decalogo de la Calidad

Experiments were begun immediately after all preparation steps had been finalised to prevent unwanted gas mixing by diffusion. Data acquisition from the gas sensor and flow meter were initiated at a sampling frequency of 100 Hz followed by starting of the pump. The system was allowed to run until the CO2 concentration decay had plateaued and a level of less than 600 ppm (0.06 %V/V) was

achieved. The concentration of CO2 in the earth’s atmosphere is approximately 400 ppm (Wayman et

al., 2013) however this level can be much higher in inhabited buildings, thus the lower limit of 600 ppm was chosen as it was the lowest level which was consistently attainable in the experimental area.

9.1.3

Data Processing

Initially it was thought that the measured data from different experimental runs could be directly compared to one another. However it was soon obvious that due to the very low level of CO2

introduced into the system, as restricted by the sensor range, there was significant variation in the concentration of CO2 at the commencement of each individual experiment. The poor repeatability of

the initial CO2 concentration prompted a change in the method of data processing.

All concentration data was converted from units of ppm to %V/V for consistency with the later Experimental Method Version Two experiments. A reference level of initial CO2 was then set at 0.08

%V/V at which point analysis of data would begin. The time taken, in seconds, for the CO2

concentration to reach the second reference level of 0.06 %V/V was extracted from each data set. A theoretical time to attain the same reduction in concentration was also found assuming perfect mixing of inspired air with the rest of the system volume. This was calculated iteratively using equation [ 7 ] with variables Vs, the total system volume; Vt, the tidal volume; Ci, the average CO2 concentration in

the system at the beginning of breath cycle i, and Ca, the concentration of CO2 in the room air. The

equation states that during each breath cycle a portion of the total system volume, the tidal volume, is expelled via expiration and replaced with room air on inspiration. By assuming perfect mixing, the instantaneous CO2 concentration within the system at the end of a breath cycle is therefore a weighted

average of the initial system concentration and room air concentration based on their relative volumes. An example of the results of the iterative calculation is graphed in Figure 64.

s a t i t s i V C V C V V C_1((  )*  * ) [ 7 ]

Figure 64: Example of the outputs from the iterative calculation using Equation [ 7 ] to find the number of theoretical breath cycles for a 0.08 to 0.06 %V/V CO2 decay when assuming perfect mixing. This calculation was carried out for all four experiment scenarios performed using Set-up One and Set-Up Two. When calculating the theoretical time for the two experiments in which NHF therapy was used at a flow rate of 30 LPM, the occurrence of dead space washout was ignored. Thus the only influencing factors providing difference to the four scenarios were the variation in pump volume between Set-ups One and Two, and the period of the breath cycle between natural breathing and breathing with the 30 LPM NHF therapy.

9.1.4

Results and Discussions

From the theoretical calculations it was found that only 8 breath cycles would be required to attain the reduction from 0.08 to 0.06 %V/V CO2 in Set-up One when assuming perfect mixing. This was the

case for both scenarios of natural breathing and breathing with 30 LPM NHF therapy, as the difference in the tidal volume between the two was small at only 15 ml. Despite requiring the same

number of cycles, the decay time is slightly higher for the 30 LPM NHF therapy scenario as the breath period is longer than that of natural breathing. For Set-up Two the theoretical decay times were reduced to a mere 5 breath cycles for both breathing with 30 LPM NHF and natural breathing due to the reduction in the system volume. These results and the times are listed in Table 5 alongside the results extracted from the experimental data.

It was immediately apparent from the results of these experiments that metering a preset volume of CO2 into the pump volume could not be repeated accurately. Consequently it was necessary to set an

upper limit attained by all experiments from which the time analysis could be started. Figure 65 shows the full time series of the four decay experiments, with all reaching levels above 0.08 %V/V upper limit.

Figure 65: Full time series of CO2 concentration decay results from experimental Set-ups One and Two. Figure 66 shows all experiments with results beginning at the upper limit of 0.08 %V/V. In this plot the most significant observable difference is between the two setups used, with the Set-up Two results decaying to the lower limit much faster than the two datasets obtained using Set-up One. For Set-up One alone the use of NHF therapy at 30 LPM accelerated the rate of decay to a small extent when compared to the natural breathing case. Conversely, in Set-up Two the use of NHF therapy decreased the rate of CO2 decay when compared to the natural breathing result.

Figure 66: CO2 decay curves of both Set-up One and Two beginning at upper limit of 0.08 %V/V. The times taken for each experiment to decay from 0.08 to 0.06 %V/V CO2 were extracted and are

given in Table 5. Comparisons between the theoretical and experimental decay times show extreme differences with experimental times an order of magnitude greater than those calculated. It was not expected that perfect mixing would occur within the system and these large disparities show definitively that this assumption is inadequate.

Table 5: Theoretical and experimental CO2 concentration decay results.

Set-up One Set-up Two

0 LPM NHF 30 LPM NHF 0 LPM NHF 30 LPM NHF Breath Cycle Period 5.08 s 6.63 s 5.08 s 6.63 s

Tidal Volume 0.799 l 0.814 l 0.799 l 0.814 l

Theoretical Number of Cycles for Decay 8 8 5 5

Actual Number of Cycles for Decay 76.43 53.57 42.22 39.84

Theoretical Decay Time 40.64 s 53.04 s 25.40 s 33.15 s

Actual Decay Time 388.26 s 355.17 s 214.46 s 264.17 s

Comparison of the results from Set-up One show that the introduction of NHF therapy did reduce the time for CO2 decay from 388.26 seconds for natural breathing to 355.17 seconds. The total volume

exchanged to achieve this decay was also reduced by 15.25 litres. Set-up Two results also show a reduction in the total volume exchanged with the introduction of NHF; however, the decay time was faster for natural breathing. In spite of a lesser number of cycles and thus the lower volume exchanged with the introduction of 30 LPM NHF, the much shorter breath period of natural breathing produced a faster decay in this setup. As other contributing variables such as tidal volume differences were present in these experiments, the difference in the decay rates are most likely cumulative and not directly attributable to the introduction of NHF flow alone.

By comparing the results from the analogous experiments of Set-ups One and Two it is obvious that the system volume has a large influence on the rate of the CO2 decay. The reduced system volume of

Set-up Two resulted in much faster decay times of 214.46 seconds as opposed to 388.26 seconds and 264.17 seconds as opposed to 355.17 seconds for the natural breathing and 30 LPM NHF scenarios respectively. However there is no consistent reduction attributable to the use of NHF therapy in these four experiments.

Six repetitions of the Set-up Two natural breathing experiment were carried out to determine experimental repeatability, the results of which are plotted in Figure 67 with data all beginning at 0.08 %V/V. The figure shows that there are significant differences in the decay curves of the six experiments resulting in a wide variation of decay times. The decay times from the analysed curves are laid out in Table 6.

Figure 67: Repeatability tests of CO2 decay during natural breathing using Set-up Two.

Table 6: Results of CO2 decay repeatability experiments.

These repeatability results give an average decay time of 184.87 seconds with a standard deviation of 23.53 seconds for natural breathing with experimental Set-up Two. With the standard deviation this high, equal to 12.7% of the average value, the repeatability of these experiments is poor. It can also be seen in Figure 65 to Figure 67 that there are noticeable step changes in the CO2 concentration curves.

These step changes result from the low response rate of the sensor which was unable to accurately capture the decay. As a consequence of these issues, the non-physiological experimental system volume and the non-physiological introduction of CO2 as a single fixed volume as opposed to a

continuous flow, these results were concluded to be of little practical use. Subsequently Experimental Method Version One was not pursued further in favour of developing and using the physiologically representative Experimental Method Version Two.

One Two Three Four Five Six Actual Number of Cycles 42.22 42.16 32.62 34.36 31.93 35.06

Actual Decay Time 214.46 s 214.18 s 165.69 s 174.53 s 162.21 s 178.13 s