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pressure was used as a covariate to rescale the saturation values based on the regression of mean arterial pressure with saturation levels over the applied levels of IOP. Least squares mean values for the site by pressure interaction means were tested for statistically significant differences using protected t-tests and an alpha level correction method based on a simulation method.71

Analysis of the compliance test images was conducted to determine whether any statistically significant alteration in the MPD (mean position of the disc) occurred due to hypercompliance or increased movement of the ONH during the repeated testing sessions applied to each eye. The statistical design for the evaluation of compliance was a two-way factorial ANOVA with time (0, 10, or 30 minutes within a testing session) and examination date as the main effects and MPD as the outcome. Individual mean values of the time by examination date interaction were separated using the same protected t-tests and alpha correction method applied above.71

All data management and statistical methods were carried out using programs and procedures of the statistical analysis system (SAS Institute, Cary NC).72

4.4.2. Results

4.4.2.1. Compliance testing

Overall, the change in MPD with increased IOP did not exceed 25 µm for any of the eyes in any of the compliance testing sessions. A comparison of the compliance test results for the first eye of each monkey before and after the HSI sessions showed no damage caused by acute pressure elevations to the ONH.

4.4.2.2. Blood spectra from ONH structures

Differences in average spectral amplitudes in Figs. 4.19 and 4.20 were the result of changes in the position of the retina and the pupil alignment caused by elevation of IOP.

4.4.2.3. Oxygen saturation of ONH structures

Fifteen measurements (three measurements at 1 min intervals at a given IOP per session × five sessions over a five-week period for each eye of each

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Fig. 4.19. Reflectance spectral curves in artery and vein in monkey 3. Data are from images obtained 30 minutes after IOP was set to 10, 30, 45, and 55 mmHg. Reprinted with permission from Khoobehi, B., Kawano, H., Ning, J., Burgoyne, C.F., Rice, D.A., Khan, F., Thompson, H.W., and Beach, J.M. Oxygen saturation changes in the optic nerve head during acute intraocular pressure elevation in monkeys. In: Manns, F., Soderberg, P.G., and Ho, A., (eds.), Ophthalmic Technologies XIX, Proc of SPIE. 7163, 716320. ©2009 SPIE.

monkey) produced standard deviations of mean for the percent O2 saturation of 1.3–1.6 for each IOP, and a correlation between different measurements of 0.999 (P < 0.0001). At the three lowest IOPs in Fig. 4.21, the artery and vein saturations gradually decreased with IOP, the vein dropping at a greater rate. At 55 mmHg, the artery showed a large and significant (P < 0.0001) drop in saturation. The rate of decrease for the vein was unchanged, with significantly reduced saturation at 45 and 55 mmHg. In the ONH, saturation of all structures was significantly reduced at 45 mmHg, with the exception of the temporal cup (see Table 4.5) The temporal cup showed a relatively stable saturation versus IOP, similar to that in the retinal vein. In the nasal cup and the rim, saturation was reduced more rapidly with increased IOP, with a range of relative stability observed in the nasal cup between 10 and 45 mmHg that was not present in the rim structures. Table 4.6 shows average values for mean arterial pressure of each monkey over the 10 sessions (five

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Fig. 4.20. Reflectance spectra from five regions of the ONH in monkey 3. Data are from images obtained 30 minutes after IOP was set to 10, 30, 45, and 55 mmHg. Reprinted with permission from Khoobehi, B., Kawano, H., Ning, J., Burgoyne, C.F., Rice, D.A., Khan, F., Thompson, H.W., and Beach, J.M. Oxygen saturation changes in the optic nerve head during acute intraocular pressure elevation in monkeys. In: Manns, F., Soderberg, P.G., and Ho, A., (eds.), Ophthalmic Technologies XIX, Proc of SPIE. 7163, 716320. ©2009 SPIE.

for each eye). At IOP = 55 mmHg, perfusion pressure values approached the single digits.

4.4.2.4. Oxygen saturation maps

Figure 4.22 shows the reproducibility of the saturation maps of structures within the optic disc border from three-image sets taken at different times

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Fig. 4.21. Percentage O2 saturation (±SE) vs. IOP from retinal vessels overlying the ONH (top) and five regions of the ONH (bottom). n = 150 readings for each data point (three readings per eye at each session × five sessions for each eye × two eyes per monkey × five monkeys). Data from Table 4.5. Reprinted with permission from Khoobehi, B., Kawano, H., Ning, J., Burgoyne, C.F., Rice, D.A., Khan, F., Thompson, H.W., and Beach, J.M. Oxygen saturation changes in the optic nerve head during acute intraocular pressure elevation in monkeys. In: Manns, F., Soderberg, P.G., and Ho, A. (eds.) Ophthalmic Technologies XIX, Proc of SPIE. 7163, 716320. ©2009 SPIE.

during one-recording session at IOP of 10 mmHg. Color codes represent low to high saturations in the order blue, cyan, green, yellow, and red. High saturation is seen in arteries within the border of the disc and in the ONH tissues surrounding the vessels. Low saturation is visible in veins within the disc border. The presence of choroidal pigments outside the border of the disc alters the relationship between color codes and saturation, hence the color codes in images we present apply only to structures inside the border.73

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