Intravenously administered histamine increases choroidal but not retinal blood flow

To determine the effect of intravenously administered histamine on both retinal and choroidal blood flow in humans.

A randomized, double-masked, two-way crossover study was performed in 14 healthy volunteers. Placebo or histamine was administered intravenously in stepwise increasing doses (0.08 microg/kg/min, 0.16 microg/kg/min, and 0.32 microg/kg/min). Retinal vessel diameters were measured with a retinal vessel analyzer, and retinal venous blood speed was assessed by bi-directional laser Doppler velocimetry. Using these parameters retinal blood flow was calculated. Subfoveal and pulsatile choroidal blood flow were measured with laser Doppler flowmetry and laser interferometry, respectively.

After infusion of histamine pulsatile choroidal blood flow increased by 5 +/- 3%, 9 +/- 8%, and 14 +/- 7% (P = 0.001, ANOVA) and subfoveolar choroidal blood flow by 8 +/- 11%, 13 +/- 11%, and 13 +/- 12% (P = 0.003, ANOVA). Retinal arterial and venous vessel diameter significantly increased by 3 +/- 4%, 2 +/- 4%, and 3 +/- 5% (P = 0.047, ANOVA) and 1 +/- 2%, 3 +/- 2%, and 3 +/- 2% (P = 0.015, ANOVA), respectively. Red blood cell velocity in major retinal veins tended to decrease by -9 +/- 12%, -9 +/- 20%, and -13 +/- 12%, but this effect did not reach levels of significance. Calculated retinal blood flow was not changed by administration of histamine (-7 +/- 14%, -4 +/- 20%, and -8 +/- 12%, P = 0.28, ANOVA).

Intravenous histamine in the selected doses increased choroidal blood flow. Retinal vessels showed a small diameter increase, whereas red blood cell speed decreased, resulting in an unchanged total retinal blood flow. This may result from local differences in the receptor distribution in the posterior part of the eye.