Measurement of Total Retinal Blood Flow and Oxygen Extraction in Patients With Diabetes and Healthy Subjects

To determine the total blood flow in the eye, OCT measurements were performed with a rectangular scanning pattern around the optical nerve head.

The DVA allows for the real time measurement of retinal vessel diameters in vivo.

The DVA is a commercially available system (IMEDOS, Jena, Germany) which comprises a fundus camera, a video camera, a real time monitor and a personal computer with an analyzing software for the accurate determination of retinal arterial and venous diameters. Every second a maximum of 25 readings of vessel diameter can be obtained. For this purpose the fundus is imaged onto the charge coupled device chip of the video camera. The consecutive fundus images are digitized using a frame grabber. In addition, the fundus image can be inspected on the real time monitor and, if necessary, stored on a video recorder. Evaluation of the retinal vessel diameters can either be done online or offline from the recorded video tapes

In particular, retinal oxygen saturation measurement is based on the image analysis by the DVA software of two monochromatic fundus images as recorded by a standard DVA. In an image, obtained by the camera and filter assembly, the operator has to mark the vessel of interest by a mouse click. The vessel is traced automatically applying the following procedure. The vessel walls are located as photometric edges in the vicinity of the mouse cursor in the green channel image. If edges are determined, the search is continued in their proximity.

We observe bi-directional blood flow and pulsatility of blood velocity in retinal vessels with a Doppler detection bandwidth of 12.5 kHz and a longitudinal velocity sensitivity in tissue of 200μm/s.

Peripapillary retinal nerve fiber thickness will be assessed using a commercially available device (Heidelberg Spectralis).

Central retinal thickness will be assessed using a commercially available device (Heidelberg Spectralis).

Pattern ERG will be performed according to the ISCEV standard for clinical pattern Electroretinography. Briefly described, the gold wire will be placed in a lower conjunctival sac, attached to the lateral nasal surface on one site and covered by the gold foil electrode on the temporal site. Reference and the ground electrodes will be placed in the outer canthus and the forehead, respectively. A black-and-white reverse checkerboard will be used with an aspect ratio of the width over the height of the stimulus field not exceeding 4:3.

Capillary blood glucose levels will be measured using a commercially available glucose meter (Accu- Check Go, Roche Diagnostics GmbH, Vienna). Capillary blood will be sampled from one fingertip with a single-use lancet.

The arterialized blood will be drawn into a thin glass capillary tube. Arterial pH, pCO2 and pO2 will be determined with an automatic blood gas analysis system (AVL 995-Hb, Graz, Austria).

Assesment of heart rate variability is a standardized, non invasive method for quantification of autonomic (sympathetic and parasympathetic) control and thus preferably used for evaluation of cardiovascular autonomic neuropathy (CAN) in diabetes. This measurement of CAN is based on analysis of heart rate variability either in time-domain (cardiovascular reflex tests to standardized stimuli including deep breathing, Valsalva maneuver and orthostatic load)or in frequency-domain, using a short-term modified orthostatic load. Short- term spectral analysis of HRV is obtained from recordings consisting of 256 seconds of artifact-free records each using a VariaCardio® system (Advanced Medical Diagnostics Group, UK).