The Effect of Hypercarbia and Hyperoxia on the Total Blood Flow to the Retina as Assessed by Magnetic Resonance Imaging

Abstract

Purpose: The feasibility of measuring total blood flow to the retina with Arterial Spin Labeling Magnetic Resonance Imaging (ASL-MRI) has been described previously. In the present study, the hypothesis was that the reactivity that the ASL-MRI detects at the human retina is dominated by the choroidal blood flow, and thus it may serve as a useful tool for quantitative assessment of the choroidal vascular reactivity.

Methods: Before imaging, the intraocular pressure (IOP) was measured in the study eye of nine clinically healthy subjects (four males) while the subject performed the ventilatory protocol subsequently imaged by the scanner. End-tidal CO₂ partial pressure (P(ET)CO₂) was increased to target 45 mm Hg, (baseline P(ET)CO₂ = 40 mm Hg and baseline P(ET)O₂ = 100 mm Hg). P(ET)O₂ was then increased to target 300 and 500 mm Hg while keeping P(ET)CO₂ constant at 45 mm Hg. A background-suppressed, pulsed-continuous ASL sequence was used for blood flow imaging.

Results: The measured total blood flow increased significantly from 1.55 ± 0.17 μL/mm²/min at the baseline to 1.96 ± 0.18 μL/mm²/min during hypercarbia. With increasing P(ET)O₂, the measured blood flow did not change significantly relative to the hypercarbia condition but remained significantly elevated relative to the baseline. There were no significant changes in systolic, diastolic, or mean blood pressure, heart rate, or IOP during all four breathing conditions.

Conclusions: The lack of change in the ASL signal under hyperoxic conditions is consistent with the hypothesis that this noninvasive assessment technique is predominantly weighted by choroidal blood flow. The results indicate that a CO₂ provocation challenge, in combination with ASL-MRI, is a promising noninvasive approach for investigating choroidal vascular reactivity under normal and disease states.