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J. C. Downs, C. F. Burgoyne, Y. Liang, V. L. Sallee; Telemetric IOP Monitoring in the Adult Monkey Shows Ocular Pulse Varies With IOP. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2842.
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To test the hypothesis that the ocular pulse amplitude is higher at high IOP. Collagen-based tissue such as cornea and sclera stiffen significantly as they stretch, and also stiffen with age. We hypothesized that in conditions when the corneoscleral shell is stiffer (e.g. at higher baseline IOPs and/or in older eyes) the eye is less able to absorb transient IOP fluctuations and therefore the ocular pulse amplitude is higher.
We have adapted a proven implantable telemetric pressure transducer system (T30F, ITS, Dexter, MI) to monitor IOP by integrating the transducer into a custom, Baerveldt-style baseplate that is connected to the anterior chamber via a silicone tube. An implanted transmitter sends 500 IOP, ECG and body temperature measurements per second to an external antenna. Data is acquired with a commercially available system equipped with real-time barometric pressure compensation (ITS). We tested transducer accuracy from 5-60 mmHg by cannulating the anterior chamber with a 27-G needle connected to an adjustable saline reservoir equipped with an in-line digital manometer. At six weeks post op, we recorded continuous telemetric IOP and ECG during an IOP calibration test while the animal was under pentobarbital anesthesia, with steady heart rate of 110 beats/min and blood pressure of ~110/65. We calculated ocular pulse amplitudes at IOPs of 10, 20, 30, 40, and 50 mmHg.
The IOP transducer was accurate to within 1 mmHg from 5-60 mmHg, with a base noise level ≤ 0.4 mmHg. During the calibration test, average ocular pulse amplitude measured 0.7, 1.0, 1.2, and 1.5 mmHg at IOPs of 20, 30, 40, and 50 mmHg, respectively (Figure). There was no detectable ocular pulse at IOPs below approximately 15 mmHg. Peak ocular pulse lagged left ventricle contraction by approximately 50% of the cardiac cycle duration.
Ocular pulse amplitude in this adult monkey was ≤ 0.4 mmHg at IOPs below 15 mmHg, but increased with IOP to 1.5 mmHg at 50 mmHg. Such fluctuations may expose the optic nerve heads in eyes with stiffer corneoscleral shells and/or higher baseline IOP to cumulative IOP fluctuations of higher magnitude over time.
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