Laser energy and dye fluorescence transmission through blood in vitro

Because of the potential usefulness of evaluating and treating choroidal neovascularization obscured by blood, we designed this study to quantify the transmission of dye fluorescence and laser energy through blood.

Blood preparations anticoagulated with ethylenediaminetetraacetic acid with hematocrits of 0% (plasma), 46%, and 99% were placed in open cuvettes with path lengths of 100, 200, or 500 microns and were exposed for one minute to either 100% oxygen or 100% carbon dioxide. Each cuvette was then sealed. Photographs of the cuvettes of blood in front of a flask of fluorescein or indocyanine green solution were decoded and used to calculate the percent transmission of fluorescence through blood. Cuvettes of blood were also placed in the path of argon, krypton, and diode lasers for energy transmission measurements.

Plasma transmission of fluorescein and indocyanine green fluorescence and argon, krypton, and diode laser energy was 89% to 100% for all samples tested. Transmission of fluorescein fluorescence and argon laser energy through 99% hematocrit samples were both less than 5%. Transmission of indocyanine green fluorescence through 100-, 200-, and 500-microns-thick cuvettes filled with 99% hematocrit blood was 57%, 34%, and 4%. Transmission of krypton laser energy was 50%, 25%, and 6%; and transmission of diode laser energy was 60%, 35%, and 12% through 99% hematocrit blood. Intermediate transmission values were obtained for 46% hematocrit samples.

Krypton and, to a slightly greater degree, diode laser energy penetrate a thin film of blood. Indocyanine green fluorescence also penetrates a thin film of blood. If a layer of blood appears thinner than 500 microns, then indocyanine green angiography may be useful in imaging underlying pathologic features. If a lesion can be imaged with indocyanine green, then it can probably be treated with a krypton or diode laser.