Effect of altitude on retinal hemodynamics: A physiology-enhanced theoretical analysis

Abstract

Purpose : Glaucoma is a leading cause of blindness worldwide with elevated intraocular pressure (IOP) being the major risk factor and its reduction being the only approved treatment. In many patients, impaired ocular hemodynamics has also been shown to be involved in its pathogenesis. While it is known that IOP is affected by altitude, the relationship between elevation and ocular hemodynamics is unclear. Here we use mathematical modeling for a physiology-enhanced analysis of the Mont Blanc Study (MBS) (Bruttini et al 2020) to estimate the effects of altitude on retinal hemodynamics.

Methods : IOP, mean arterial pressure (MAP) and heart rate (HR) were measured in 33 healthy volunteers at 77m (Pavia, PV, Italy) and at 3,466m (Pointe Helbronner, PH, Mont Blanc Mountain, Italy) above sea level. A validated mathematical model for retinal hemodynamics (Guidoboni et al 2014) is utilized to analyze the MBS data. Pressures, flow rates and resistances in the central retinal artery and vein (CRA and CRV) and in the retinal microvasculature are predicted by the model, based on individually measured values of IOP, MAP and HR. The model accounts for IOP and MAP induced variables resistances in the retrobulbar segments of the CRA, CRV and in retinal venules (RV). Results between PV and PH are analyzed using t-test with p<0.05 statistically significant.

Results : With altitude, the results predict a statistically significant increase in all retinal pressures (p<0.001, which follows the increasing MAP with altitude observed in the MBS) and in the retinal blood flow across the full retinal circulation (p<0.001, from the CRA inflow to the CRV outflow via the retinal microvasculature), Figure 1. With altitude, the results predict a significant decrease in CRA (p<0.001) and RV (and p=0.003) resistances, and a non-significant increase in the CRV resistance (p=0.253), Figure 2. The resistance in the RV is markedly higher than in the CRA and CRV, suggesting that the venules are withstanding most of the IOP load on the vasculature.

Conclusions : Mathematical modeling can be used to enhance measured datasets with hemodynamic variables that cannot be measured directly, such us the venous resistance. This analysis suggests that venules bear the significant portion of IOP pressure load within the ocular vasculature. Advancement of this approach may include the study of the effect of altitude on oxygen saturation via autoregulation.

This is a 2021 ARVO Annual Meeting abstract.

 

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