The maximum accuracy that T
1 can be measured, in addition to the temperature dependence of this technique, introduces a certain amount of error into the absolute pO
2 measurements. Although these errors are reflected in the difference in SDs between the MRI-derived vitreous pO
2 and the intraoperative vitreous pO
2, the mean MRI value obtained is comparable to several other studies.
50,53 The combined pre-PPV mean pO
2, from both the experimental and control eyes, was 13.4 ± 6.7 mm Hg, which is higher, although not markedly, than the measurements taken in the patients at the time of surgery, 7.2 ± 0.6 mm Hg. One of the first studies that measured vitreous pO
2 with a polarographic electrode oxygen probe, reported a mid vitreous pO
2 of 15.9 ± 2.8 mm Hg.
53 Shui et al. measured oxygenation at various points in the vitreous cavity of rabbits and reported a similar mid cavity pO
2 of 13 ± 2 mm Hg.
50 Park et al. recently published a study comparing probe designs for determining intraocular oxygen distribution both in vivo and in vitro, and raised issues relating to the accuracy of polarographic electrodes, as used in our study and several mentioned beforehand, to measure pO
2 in small areas such as the vitreous cavity.
54 However, our intra-operative pO
2 measurements were highly consistent, and closely matched other human data in which the potentially more accurate optical fluorescence fiber optic oxygen probes were used. For example, Holekamp et al. found a mean mid vitreous pO
2 of 7.1 ± 0.5 mm Hg in 69 eyes
28 , and 8.5 ± 0.6 mm Hg in a further 29 eyes.
27 In addition, we have performed preliminary work comparing the pO
2 readings from polarographic and optical fluorescence fiber optic probes in BSS, and found no difference (Dowell NG, unpublished data, 2011). The previously described study by Muir et al. reported a mid vitreous pO
2 of 18.3 mm Hg, which is comparable to our result (13.4 ± 6.7 mm Hg).
51 Apart from the variability inherent to the technique's limitations, the reasons for the small difference between the results are likely to be the same that accounted for the differences in both studies phantom experiments, namely temperature and magnetic field strength. The fact that our result is closer to our own intraocular measurements, and to the large number of eyes measured by Holekamp et al.,
27,28 would suggest that the mid vitreous temperature is lower than Muir's assumed temperature of 37°C temperature assumption used by Muir et al.
51 However, interestingly Muir et al. investigated T
1 changes with eyes open and closed (in our study the participants had their eyes open) and found a difference that suggested vitreous temperature increased a small amount with closed eyes. Although it is not explicitly stated in their article, the 18.3 mm Hg mid vitreous pO
2 reported could be for closed eyes, or an average of open and closed, thereby, increasing the temperature closer to their assumed value, which could explain why the difference between pO
2 in both our studies was only 4.9 mm Hg, far less than would be expected given the 2°C difference in our temperature assumptions.