ANEXO 2 ALGUNAS EVIDENCIAS
8. NUESTROS CIMIENTOS REFERENTE TEÓRICO
8.1 APRENDIZAJE COOPERATIVO.
inhalation of 14% oxygen.
3.2.1 Background and aims
The results of the previous study showed that SpO2 declined with time over a five-minute
period of inhaling 14% oxygen but did not plateau. The aim of this experiment was to monitor the time course of changes to SpO2 when breathing 14% oxygen over 15 minutes
in healthy control participants. It was hypothesised that SpO2 readings will continue to
decline past the 5 minute period, but plateau within 15 minutes. Based on the literature, 15 minutes was believed to be sufficent to produce steady state hypoxia (Connolly et al. 2009b; Connolly et al. 2009a).
A second aim was to compare the timecourse in older and younger individuals. As neither group had respiratory problems, such as asthma, it was hypothesised that there would be no difference between the SpO2 readings of the two groups when breathing 14% oxygen.
The results of this study were required to determine the time required for a change in oxygenation level to have maximal effect on SpO2 .
3.2.2 Participants
Nine healthy control participants were recruited from the School of Optometry and Vision Sciences in Cardiff University, between the ages of 24-71. Inclusion and exclusion criteria were as described in section 3.1.2. Participants were divided into two groups; a younger
group age 24-36 years old and an older group age range 58-71yrs old. All procedures adhered to the tenets of the Declaration of Helsinki. The study was approved by the School of Optometry and Vision Sciences ethical committee.
3.2.3 Method
Participants were seated for the duration of the experiment and were advised to report any symptoms of hypoxia including dizziness. A finger pulse oximeter was used to measure the peripheral oxygen saturation data (see section 3.1.3). Baseline SpO2 data
were collected in normoxic conditions for 5 minutes, with the participant breathing room air. Then, using 8% oxygen delivered though a 60% venti mask (see section 3.1.3), the participant breathed 14% oxygen for 15 minutes. SpO2 data were collected at 1 minute
intervals in the hypoxic condition for a minimum of 15 minutes. The experiment was stopped if the SpO2 levels dropped below 85%, or if symptoms such as dizziness or
confusion were reported. After cessation of inhalation of 14% O2, SpO2 readings were
taken until the participant’s SpO2 readings had returned to a normal level.
3.2.4 Results
The mean age for group 1 was 27.8 +/- 4.50 years and the mean age for group 2 was 66.5 +/- 5 years. Full 15 minute datasets were collected for all participants.
97.8%. This dropped to 97.0% for group 1 when breathing 14% oxygen and 93.6% for group 2. The graph shows that mean SpO2 reached a minimum at around 8 minutes after
commencing breathing the hypoxic gas for group 1. For group 2 the mean SpO2 stabilised
after 8 minutes and reached a minimum after 11 minutes of breathing 14% oxygen. For both groups the data plateaued after this point. The SpO2 readings returned to within a
normal level within 1 minute for group 1 and 5 minutes for group 2. The confidence intervals show that the data were less variable for group 1. It also demonstrates that there was a significant difference between groups as the confidence intervals do not overlap. The SpO2 was significantly lower for group 2 at baseline, and was affected more
by hypoxia than for group 1, as evidenced by the greater drop in SpO2 value when
breathing 14% oxygen.
Figure 3-16. Group average data from younger (group 1) and older (group 2) participants, showing confidence interval bars.
84 86 88 90 92 94 96 98 100 1 2 3 4 5 6 7 8 9 101112131415161718192021222324252627 Sp O2 (%) group 1 means group 2 means
3.2.5 Conclusions
The results from this study show that a period of longer than 5 minutes is required for the inhalation of 14% gas to result in the maximum effect on SpO2 readings. The younger
group’s mean SpO2 fell at a faster rate, taking 8 minutes compared to 11 minutes for
group 2 to reach a minimum.
The results from this study appear to contradict earlier findings that suggested no difference in the SpO2 readings between older and younger populations. Feigl (2008)
found that when breathing 12% oxygen the SaO2 levels decreased significantly for both
groups, but that there was no difference between the two groups with the younger group dropping to 90% +/-2% and the older group dropping to 91% +/- 0.4%. However, the mean age of the older population used in their study was 55 years old, substantially younger than the older population used in this study (mean age 66.5 years old). It should also be noted that the sample size in our study was small and hence the comparisons between Feigl’s study and this one should be interpreted with caution.
The reduction in SpO2 measured in this study is less than that observed by other
investigators, where drops to between 85.7% to 90.6 were found when breathing 12- 14.1% oxygen for a 15 minute period (Connolly et al. 2006; Connolly 2011; Connolly et al. 2008b; Feigl et al. 2008; Feigl et al. 2007; Feigl et al. 2011; Connolly et al. 2008a).
Comparing the study designs, Connolly (2006) used a hypobaric chamber to simulate an altitude of 15,000 ft. This method of reducing oxygen may not be comparable with the
methods used in this study. Also, an altitude of 15,000 ft is equivalent to an oxygen concentration of less than 12%, therefore SpO2 would be expected to drop to a lower
level than in this experiment. Feigl (2008) and Feigl (2007) also used oxygenation concentrations lower than 14%, using 12% oxygen. Furthermore, different mask types were used in previous studies. For example, Connolly et al (2008) used a Royal Air Force Aircrew Respirator MK5, consisting of an oxygen mask and headpiece that covered the head, neck and shoulders. Connolly and Hosking (2008) and Connolly (2011) both used masks that were suspended from a cloth helmet that ensured an adequate facial seal. Feigl (2008), Feigl (2011) used non-rebreathing masks and Feigl (2007) used a mouthpiece and nose clip. However, although the gas delivery system used in this study was different, participants were checked for the correct fitting of the mask in this study and fitting was adjusted if required, hence the gas delivered should have been broadly as expected.
To summarise, the results of this study indicate that a period of 15 minutes adaption to a change in oxygen concentration is enough to allow SpO2 to reach a plateau and result in a
steady respiratory state. However an inhaled oxygen level of <14% may be necessary to achieve the SpO2 levels reported in the literature.