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C. J. Roberts, A. M. Mahmoud, R. H. Small; The Response of Ocular Pulse Amplitude (OPA) to an Increase in Intraocular Pressure (IOP) Generated by Changing Subject Position, in Order to Investigate Ocular Blood Flow and Validate a Novel Electrical Analog Model. Invest. Ophthalmol. Vis. Sci. 2010;51(13):565.
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Ocular Pulse Amplitude (OPA) is affected by both ocular biomechanics and ocular blood flow, as recognized by Perkins. The purpose of this preliminary study was to evaluate the response of OPA to a change in subject position, and attribute that response to either biomechanical or vascular mechanisms, in order to validate a novel electrical analog model.
9 eyes of 9 subjects were included. Triplicate measurements were acquired in each of 3 positions using a rigid gas-permeable contact lens (Paragon Vision Sciences) with an embedded pressure sensor (Ziemer Ophthalmics) connected by wire to the PASCAL Dynamic Contour Tonometer (DCT) base. Positions included sitting and supine in randomly selected order, followed by Trendelenberg, with 10 minutes of adjustment in each position prior to data acquisition. Systemic blood pressure (BP) and heart rate (HR) were also acquired using an automated cuff. One-sided, paired t-tests were performed to compare both intraocular pressure (IOP) and OPA between each of the 3 positions. A simulated carotid waveform was generated from measured BP and HR, and used as input to an electrical analog model of the interdependence between aqueous circulation and ocular blood flow. Model parameters were adjusted systematically so that the output matched measured IOP and OPA for each subject.
The mean values of IOP in the sitting, supine, and Trendelenberg positions were 15.7±3.5 mmHg, 18.4±3.8 mmHg, and 18.8±3.5 mmHg, respectively. The mean values of OPA in the sitting, supine, and Trendelenberg positions were 2.1±0.74 mmHg, 2.4±0.6 mmHg , and 2.1±0.5 mmHg, respectively. Both IOP (p<0.002) and OPA (p<0.038) were significantly greater in the supine than the sitting position. OPA (p<0.028) was significantly lower in the Trendelenberg position than supine, with no difference in IOP (p=0.3). IOP (p<0.002) was significantly greater in Trendelenberg than sitting, without a difference in OPA (p=0.4). Increases in IOP without corresponding increases in OPA were shown to be a result of reduced ocular blood flow in the model.
The increase in IOP and corresponding increase in OPA in the sitting to supine transition represents a biomechanical response. However, in the transition from supine to Trendelenberg, OPA is reduced without a precipitating reduction in IOP. The failure of IOP and OPA to change in the same direction is a sign of reduced ocular blood flow, and is predicted by the model.
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