Purchase this article with an account.
Jeffrey W. Kiel, William J. Lavery; Effects of Head-Down and Head-Up Tilt on Episcleral Venous Pressure in a Rabbit Model. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1985.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
In humans, changing position from upright to supine elicits an approximately 10 mmHg increase in cranial venous pressure (CVP) caused by the hydrostatic column effect, but episcleral venous pressure (EVP) and IOP rise by only a few mmHg. This dissociation between CVP and EVP suggests a regulatory mechanism controlling EVP. The aim of the present study was to determine if the rabbit model is suitable to study the effects of postural changes on EVP despite its short hydrostatic column.
In anesthetized rabbits (n=50), we measured ear artery pressure (BP), IOP, and orbital venous pressure (OVP) by direct cannulation; carotid blood flow (BFcar) by transit time ultrasound, heart rate (HR) by a digital cardiotachometer, and EVP with a servo-null micropressure system. The protocol goal was to obtain measurement of supine EVP for approx. 15 minutes, followed by approx. 15 minutes of EVP measurement in either head-down tilt (n=43) or head-up tilt (n=7). The data (mean +/- standard error) were analyzed by paired t-tests.
Head-down tilt caused significant increases in IOP, OVP, and EVP. Head-up tilt caused IOP, OVP, and EVP to significantly decrease. From supine to head-down tilt, BP and BFcar were unchanged, IOP increased by 2.0 +/- 0.4 mmHg (p<0.01), EVP increased by 2.4 +/- 0.4 mmHg (p<0.01), OVP increased by 2.5 +/- 0.2 mmHg (p<0.01) and HR decreased by 9 +/- 3 bpm (p<0.01). For head-up tilt, BP decreased by 3.6 +/- 1.0 mmHg (p<0.05), IOP decreased by 3.0 +/- 1.0 mmHg (p<0.05), EVP decreased by 1.6 +/- 0.7 mmHg (p = 0.06), OVP decreased by 1.5 +/- 0.2 mmHg (p<0.01), BFcar decreased by 8.4 +/- 2.5 mL/min (p<0.05) and HR was unchanged.
Although the hydrostatic column in the rabbit is shorter than humans, the rabbit model permits sufficiently sensitive measurements of the pressures and systemic parameters likely involved in the EVP responses to posture change. The present results indicate directionally similar EVP and IOP responses to tilt as occur in humans and, as in humans, the responses are smaller than would be expected from the change in the hydrostatic column height. We conclude the rabbit model is appropriate for studying the mechanisms responsible for the relative immunity of EVP and IOP to posture change.
This PDF is available to Subscribers Only