We describe the causes and speed of transient blanching during copper vapour laser treatment of port-wine stains. Five watts of yellow (578 nm) light from a copper vapour laser was scanned over the lesion using a computer controlled scanning system.

The clinical response of the lesion to treatment is transient blanching, followed immediately by erythema. The clinical response of sclerosed vessels is different in that an intravascular coagulum is observed.

We measure the time taken for the lesion to blanch using two methods. First, high-speed photography is used to photograph the treatment process. Second, a photodiode measures the light re-emitted from the skin. Using illumination times of 3 to 5 ms and fluences of approximately , blanching times varied between 0 and 33 ms.

We conclude that the cause of the transient blanching is not thermal denaturation of either collagen or epidermal melanin. Rather it is the rapid expulsion of red blood cells from the treated vessels.

Our results have caused us to commence clinical trials using a new treatment protocol aimed at further improving the response of port-wine stains to copper vapour laser treatment.