Tuesday, October 31, 2006
Why permanent facial redness can be hard to treat - lessons from the paper on IPL modelling
Re-iterates what I have read previously, that haemoglobin has absorption peaks around 560nm and again at around 590nm wavelengths, and drops strongly after that. This implies in my mind that using IPL cutoff filters over 590/595 is not useful for targetting blood vessels.
Temperatures higher than 70C in the centre of the blood vessel are needed for proper coagulation. Any higher and the vessel may vapourise (bruise), any lower and coagulation will not occur.
Physical properties of smaller vessels, such as those responsible for causing the erythema in rosacea, make them MUCH harder to succesfully heat to coagulation temperatures than larger vessels (e.g. visible telangiectasia). I have seen many discussions on the rosacea forums previously speculating whether hard to treat persistant redness may be "deep" redness... This paper is leading me to conclude that in many cases, the persistant redness is caused by very small vessels close to the surface of the skin. There are physical properties of these small vessels that make them hard to treat, as the paper explains:
"... it seems very unlikely to coagulate small vessels (60 um) using the given IPL spectra at radiant exposures less than 30J/cm2"
The following reasons were stated
"probably due to the limited amount of chromophores (hemoglobin) in such small vessels"
"Effect such as hypobaric pressure or dermal blood volume fraction play a significant role."
"Additionally, the cooling of a vessel by heat diffusion into the dermis has a major impact on the temperature in the vessel and the thermal damage. The cooling of a vessel during and after laser irradiation depends on the volume-surface ratio of each vessel and therefore on the vessel size. The smaller the vessel diameter, the faster the cooling and the smaller is the thermal damage inside the vessel".
The paper goes on to correlate this with previous clinical studies:
"Moreover, our findings support the results of clinical studies. Using IPL, a better clearance was observed in patients treated for visible teleangiectatic vessels compared to patients with diffuse erythema composed of much smaller vessels".
Because of the physical properties of small vessels, they may respond better to shorter pulse durations - long durations (over 10ms in this model) resulted in minimal increases in temperature because heat is being lost so quickly to the surrounding tissues.
Small vessels are best targetted by shoter wavelengths (from the graphs in the paper, around 530nm looks to be the peak). The paper states "There is a major contribution of the shorter wavelengths for effective heating of smaller vessels (60um and 150 um) [...] For larger vessels (300um and 500um), there is not much difference between individual wavelengths, although the photothermal effect is slightly more pronounced for longer wavelengths". This is what we have long suspected, however it does appear to indicate that for those with persistant, difficult to treat permanent redness, if you are only being treated with the 595nm+ cutoff filters, you may not successfully target that redness.
Theoretical take home points for your own IPL treatment, to discuss with your IPL doctor
* Erythema (permanent, diffuse redness) is hard to treat and this paper now gives sound theoretical reasons why that is the case.
* Visible vessels are much easier to successfully heat and treat.
* If you have permanent redness, shallow filters (less than 590) seem likely to be more successful
* Short pulse durations would appear effective for treating small vessels because maximum temperature is reached around 10ms into the pulse.
* Energy levels higher than 30J/cm2 at 10ms duration appear to be needed to successfully treat the small vessels which cause permanent erythema.
* In my opinion, based on this paper, long, multi-pulse shots (e.g. triple shots with long durations and pulse durations) are unlikely to be of much benefit in treating permanent redness - because the small vessels do not retain sufficient heat.
For anyone interested, I recommend obtaining the full paper. I look forward to seeing if clinical studies can be performed that attempt to back up the theory in this paper, in practice, and improve outcomes of IPL treatments!
