A schematic diagram of the methodology for measurements of retinal vascular PO
2 is shown in
Figure 1. Retinal arterial and venous PO
2 in the rat from the sham group was higher than the rat from the ligation group (
Figs. 1A,
1C). Retinal vessel boundaries, as automatically detected, are shown outlined on red-free retinal images in rats from the sham and ligation groups (
Figs. 1B,
1D). The position of one intravenous microsphere at two time points is displayed in a yellow box overlaid on the red-free retinal images. Reduced blood velocity can be visualized in the rat from the BCCAO group (
Fig. 1D) compared to the rat from the sham group (
Fig. 1B) by the shorter distance the microsphere traveled during the same time interval.
The
Table lists mean and standard deviation of retinal vascular O
2 contents, V
v, and TRBF in the sham, clamp and ligation groups. Compared to the sham group, O
2A was lower in both clamp (β = −2.9 mL O
2/dL) and ligation (β = −3.8 mL O
2/dL) groups (
P ≤ 0.01). Similarly, O
2V was lower in both clamp (β = −2.7 mL O
2/dL) and ligation (β = −5.3 mL O
2/dL) groups (
P < 0.04). O
2V was also lower in the ligation group compared to the clamp group (β = −2.6 mL O
2/dL;
P < 0.04). O
2AV was not significantly different among groups (
P > 0.1). There were no statistically significant differences in D
v and D
A (
P > 0.2). However, V
v was lower in the ligation group compared to the sham (β = −9.0 mm/minute) and clamp (β = −4.2 mm/minute) groups (
P ≤ 0.006). V
v was also lower in the clamp group compared to the sham group (β = −4.9 mm/minute;
P = 0.003). Likewise, TRBF was lower in the ligation group compared to the sham (β = −6.2 μL/minute) and clamp (β = −3.8 μL/minute) groups (
P ≤ 0.02). Occlusion of carotid arteries had a general effect of reducing retinal O
2A, O
2V, V
v, and TRBF, while D
v and D
A were not affected significantly. Additionally, O
2AV was not significantly changed with BCCAO due to comparable reductions in O
2A and O
2V.
Figure 2 displays mean and standard deviation of DO
2 and MO
2 in the sham, clamp and ligation groups. DO
2 was 948 ± 255 nLO
2/minute, 532 ± 356 nLO
2/minute, and 175 ± 137 nLO
2/minute in the sham, clamp, and ligation groups, respectively. DO
2 in the ligation group was lower compared to that in the sham (β = −772 nLO
2/minute) and clamp (β = −357 nLO
2/minute) groups (
P ≤ 0.04). DO
2 in the clamp group was lower compared to that in the sham group (β = −415 nLO
2/minute) (
P = 0.03). MO
2 was 466 ± 103 nLO
2/minute, 306 ± 135 nLO
2/minute, and 171 ± 135 nLO
2/minute in the sham, clamp, and ligation groups, respectively. MO
2 in the ligation group was lower compared to that in the sham (β = −295 nLO
2/minute) and clamp (β = −135 nLO
2/minute) groups (
P ≤ 0.05). MO
2 in the clamp group was lower compared to that in the sham group (β = −160 nLO
2/minute;
P = 0.03). Immediately after ligation of carotid arteries, DO
2 and MO
2 were decreased to 18% and 37% of the mean values in the sham group, respectively.
Figure 3 shows the mean and standard deviation of OEF in the sham, clamp, and ligation groups. OEF was 0.51 ± 0.11, 0.71 ± 0.29, and 0.98 ± 0.02 in the sham, clamp, and ligation groups, respectively. OEF was higher in the ligation group compared to that in the sham (β = 0.47) and clamp (β = 0.20) groups (
P ≤ 0.03). However, OEF in the clamp group was not significantly different than that in the sham group (
P = 0.1). The increase in OEF during carotid artery ligation indicates that up to 98% of the delivered oxygen was metabolized by the retinal tissue.
Figure 4 depicts the relationship between MO
2 and DO
2 based on compiled data from all 3 groups. The data were described by an exponential function: MO
2 = 520* (1 − e
–0.002*DO2;
r2 = 0.81). According to the mathematical model, MO
2 reached an asymptotic maximum value of 520 nLO
2/minute at high levels of DO
2. As DO
2 decreased, MO
2 initially decreased minimally, but eventually decreased more severely at low levels of DO
2. Using the equation derived from the mathematical model, at 50% of the mean MO
2 of the sham group (233 nLO
2/minute), DO
2 was calculated to be 297 nLO
2/minute. Based on this relationship, it may be possible to estimate MO
2 based on measurements of DO
2 and derive threshold values for maintaining MO
2.
Based on compiled data from all three groups, the relationship between OEF and DO
2 is depicted in
Figure 5. The data were described by an exponential function: OEF = 0.998* (1 − e
–649/DO2); (
R2 = 0.77). According to the mathematical model, OEF initially increased as DO
2 decreased, but eventually reached an asymptotic maximum value of 1 at low levels of DO
2. Using the DO
2 value of 297 nLO
2/minute, the value at which MO
2 was reduced to 50% of the mean in the sham group, OEF was calculated to be 0.89, reflecting a 75% increase with respect to the mean OEF value of 0.51 in the sham group. From this relationship, OEF threshold values for maintaining MO
2 may be estimated.