The IOP change during the cardiac cycle, called PA, is not directly a measure of volume flow. It rather reflects the pressure change that is associated with the volume flow. The point where scleral stiffness comes in is the calculation of POBF. This procedure has been formulated by Silver and Farrell.
8 A key point in this calculation is the transformation of PA into the associated volume change. Because this volume change is not directly measurable, Silver calculated it from PA by assuming a standard ocular rigidity.
8 In the approach presented in the present paper, this procedure was not applied. Instead, we assumed that the ocular volume change can be estimated based on measurement of FPA. Assuming that FPA is proportional to the change in Δ
V during the cardiac cycle a factor E1 = (log IOP
1 − log IOP
2)/FPA was calculated as a measure of the mechanical properties of the eyeball based on the Friedenwald equation. IOP
2 and IOP
1 are the highest IOP during systole and the lowest IOP during diastole, respectively. This factor related to ocular rigidity was chosen as the main outcome variable and compared between the patients and control subjects. All outcome parameters are given in absolute values as the mean ± SD. The unpaired
t-test was used to compare the outcome parameters between the two groups. Linear correlation analysis was used to investigate the association between FPA and the outcome parameters of pneumotonometry. Finally, a multiple regression model was used to investigate whether E1 is dependent on factors such as age, sex, systemic blood pressure, pulse rate, and ocular perfusion pressure (OPP). This analysis was performed separately in the patients and healthy control subjects.