Novel methodology to comprehensively assess retinal arteriolar vascular reactivity to hypercapnia

Abstract

Purpose

(1) Describe a new methodology that permits the comprehensive assessment of retinal arteriolar vascular reactivity in response to a sustained and stable hypercapnic stimulus. (2) Determine the magnitude of the vascular reactivity response of the retinal arterioles to hypercapnic provocation in healthy, young subjects.

Methodology

Eleven healthy subjects of mean age 27 years (SD 3.43) participated in the study and one eye was randomly selected. A mask attached to a sequential rebreathing circuit, and connected to a gas delivery system, was fitted to the face. To establish baseline values, subjects breathed bottled air for 15 min and at least 6 blood flow measurements of the supero-temporal arteriole were acquired using the Canon Laser Blood Flowmeter (CLBF). Air flow was then decreased until a stable increase in fractional end-tidal CO₂ concentration (F_ETCO₂) of 10–15% was achieved. CLBF measurements were acquired every minute (minimum of 6 measurements) during the 20-minute period of elevated F_ETCO₂. F_etCO₂ was then reduced to baseline levels, and 6 further CLBF measurements were acquired. Respiratory rate, blood pressure, pulse rate and oxygen saturation were monitored continuously.

Results

Retinal arteriolar diameter, blood velocity and blood flow increased during hypercapnia relative to baseline (p = 0.0045, p < 0.0001 and p < 0.0001, respectively). Group mean F_ETCO₂ showed an increase of 12.0% (SD 3.6) relative to baseline (p < 0.0001).

Conclusions

This study describes a new methodology that permits the comprehensive assessment of retinal arteriolar vascular reactivity in response to a sustained and stable hypercapnic stimulus. Retinal arteriolar diameter, blood velocity and blood flow increased significantly in response to a hypercapnic provocation in young, healthy subjects.

Introduction

Vascular reactivity is the magnitude of change of hemodynamic parameters to a provocative stimulus, for example, an increase in partial pressure of oxygen or carbon dioxide in the blood. It has been shown that both the retinal and the cerebral vessels react similarly by constricting to oxygen (O₂) and by dilating to carbon dioxide (CO₂) (Dorner et al., 2002, Kety and Schmidt, 1948, Meadows et al., 2003, Raper et al., 1971). Hypercapnia, that is an increased partial pressure of CO₂ (PaCO₂) in systemic arterial blood, is known to be a potent vasodilatory stimulus. For the purpose of non-invasive studies in healthy subjects at rest, the end-tidal partial pressure of CO₂ (P_etCO₂) is considered to be within 2 mm Hg of the arterial PCO₂ (PaCO₂) and tracks it closely when P_etCO₂ is varied (Robbins et al., 1990). In this paper, we will also refer to F_etCO₂ which is the fractional end-tidal concentration of CO₂; P_etCO₂ = F_etCO₂ × barometric pressure.

The vascular reactivity response of the retina to a 10–15% increase in F_ETCO₂, achieved by the use of various methods of modulating inspired CO₂, has been quantified using laser-Doppler-based techniques (Chung et al., 1999, Harris et al., 1996, Roff et al., 1999, Venkataraman et al., 2005). The response of the ocular vasculature to breathing a mixture of 95% air and 5% CO₂, or 95% O₂ and 5% CO₂ (i.e. carbogen), has also been assessed using the Oculix and Retinal Vessel Analyser (RVA) (Dorner et al., 2002) (Luksch et al., 2002), blue field entoptic technique (Sponsel et al., 1992) and the Pulsatile Ocular Blood Flowmeter (POBF) (Kergoat and Faucher, 1999). In addition, Harino et al. (1995) determined change in macular blood velocity by rebreathing into a bag to elevate F_ETCO₂, using the blue field entoptic technique. Recently, our group has determined the effect of hypercapnia, induced using a sequential rebreathing circuit, on retinal capillary blood flow (Venkataraman et al., 2005). Unlike previous studies, the use of a sequential rebreathing circuit permitted a sustained and a stable increase in F_ETCO₂ with minimal concomitant change in end-tidal PO₂ concentrations. In addition, all of the hemodynamic assessment techniques detailed above assess only one aspect of hemodynamics (i.e. vessel diameter or effectively blood velocity).

The determination of quantitative retinal blood flow necessitates the simultaneous measurement of vessel diameter and blood velocity. The Canon Laser Blood Flowmeter (CLBF), model 100 (Canon, Tokyo, Japan), is the only retinal hemodynamic instrument that can simultaneously measure vessel diameter and centerline blood velocity and therefore for the first time quantify volumetric blood flow in absolute units (Harris et al., 2003). It utilizes bidirectional photodetectors to quantify centerline blood velocity, densitometry to measure vessel diameter and an eye tracker system to minimize the impact of eye movement (Feke et al., 1998, Yoshida et al., 2003). Using the CLBF and a sequential rebreathing circuit, our group has recently defined the timeline response of the retinal arterioles to isocapnic hyperoxia (Gilmore et al., 2005) and have demonstrated that retinal blood flow varies directly with the arterial PCO₂, as reflected in the P_etCO₂ (Gilmore et al., 2004). However, the magnitude of the vascular reactivity response to hypercapnia has not been systematically addressed in previous work.

The aims of the study were to: (1) describe a new methodology that permits the comprehensive assessment of retinal arteriolar vascular reactivity in response to a standardized, sustained and stable hypercapnic stimulus with minimal concomitant change in end-tidal PO₂. The concomitant change in end-tidal pO₂ during hypercapnia is a problem that previous studies have typically ignored. (2) Determine the magnitude of change in vessel diameter, blood velocity and flow of the retinal arterioles in response to sustained and stable hypercapnic provocation in young, healthy subjects. The magnitude of retinal arteriolar vascular reactivity will be used as a reference for future studies that investigate the impact of disease upon this hypercapnic provocation. A sequential rebreathing circuit was used to induce a stable, sustainable change in F_ETCO₂ (Gilmore et al., 2004, Sommer et al., 1998).