Biological tissue can only be coagulated by thermal means if the requisite
temperature of approximately 700 Celsius has been built up. It is difficult to obtain
what is known as the specific haemostasis, because it is impossible to provide
temperature required for coagulation as uniformly as possible without causing
thermal damage to the adjacent tissues.
The following critical temperatures must be noted:
1. Up to 400C : No significant cell damage.
2. Above 400C : Reversible cell damage, depending on the duration of exposure.
3. Above 490C : Irreversible cell damage = denaturation
4. Above 700C : Coagulation. Collagens are converted to glucose.
5. Above 1000C : Phase transition from liquid to vapour of the intra and extra
cellular water. The tissue rapidly dries out = desiccation (dehydration). Glucose
has an adhesive effect after dehydration.
6. Above 2000C : Carbonization = (pathological burns of the fourth degree).
If the temperature required for the denaturation, (500C) is not reached or the
temperature of approximately 700C required for the coagulation is exceeded,
additional problems may arise as the coagulum containing glucose dehydrates and
carbonizes.
When using the HF electric alternating current for endogenous heating of biological
tissue, the tissue temperature rises until it reaches 1000C or the boiling points of
these liquids.
The temperature rises at different rates at the various points on account of the
inhomogeneity of the electrical and thermal tissue properties, and, above all, of
the irregular current density distribution within the tissue. As a rule, the density
of the electric current is largely in the effective contact area between the electrode
and the tissue, decreasing with the distance from this contact area.