During photocoagulation, the change in optical properties of the tissue
(i.e., whitening) corresponds to a denaturation of tissue proteins,
usually thought to occur at approximately 55°C, 18°C above basal
temperature.
17 However, protein denaturation begins under
the photocoagulation threshold with a change in the folding of proteins
starting at 46°C to either 43°C or 49°C.
18 19 20 21 For
this reason we started our study with a laser power that induced a
faint whitening on the retina after a few minutes (150 mW). Because
there is a latency of tissue denaturation after the laser is stopped,
we assumed that this laser power of 150 mW corresponded to the
photocoagulation threshold. During the experiments, a range of power
from 92 to 127 mW was used for TTT (with fluences ranging from 400 to
550 mJ/cm
2). With these parameters, whitening was
not observed, except for some pulses made with the laser set at 127 mW.
This is consistent with TTT as proposed by Reichel et
al.,
5 who used a variable spot size of 1.2, 2.0, or 3.0 mm
and a duration of 60 seconds (maximum fluence, 680
mJ/cm
2). The temperature increase corresponding
to TTT has been recently calculated by Mainster and
Reichel
7 to be approximately 10°C.
7 The
basal body temperature of rabbit is 39.4°C, and some correlation
between the onset temperature for protein denaturation and body basal
temperature has been reported.
1 In our experiments, after
this correlation, the maximal temperature reached after 60 seconds’
irradiation can be estimated at approximately 50°C. However, for very
long exposures, such as 60 seconds, because of choroidal blood flow,
heat convection moderates the temperature increase and impedes a
reliable modeling of the temperature increase throughout the duration
of the laser pulse.
22