5. Surgical Diathermy Flashcards
Use
Diathermy is used widely in surgical practice, both for coagulation and for cutting,
and relies on the heat generated as an electric current passes through a resistance that
is concentrated in the probe itself
Heat generation
Heat generation is proportional to the power that is developed, typically 50–400 W.
Heat is proportional to I2 (current)/A (area).
Cutting
high-frequency sine waveform is used for cutting, typically 0.5 MHz
coagulation
damped waveform is used for coagulation, typically 1.0–1.5 MHz.
High frequency
High frequency is necessary because muscle is very sensitive to DC and to AC at low
frequencies.
Mains frequency is low, at 50 Hz, which is a frequency that is particularly
efficient at precipitating VF.
Very high-frequency current has minimal tissue
penetration and passes across the myocardium without ill effect.
There are two types of diathermy
two connections to the patient; the neutral (or indifferent)
patient plate, and the active coagulation or cutting electrode
Current passes
through both, but the current density at the active electrode is very high and
generates high temperatures.
current density is dispersed
over a wide area and heating does not occur
patient plate, and hence the
patient, is kept at earth potential, which reduces the risks of capacitor linkage
incorporate isolating capacitors to minimize the problem. An
alternative is to use an earth-free or floating circuit
Bipolar:
The current is localized to the instrument;
it passes only from one blade of the forceps to the other.
Bipolar diathermy uses low power, and this limits its efficacy in the coagulation of all but small vessels. The circuit is not earthed.
Risks of Diathermy
- thermal injury
a - Electrode
b - metal contact site
c - prothesis
thermal injury
at the site of the indifferent electrode (the diathermy
plate) which must be in close and even contact with a large area of skin, ideally an
area that is well perfused and so which will dissipate heat. Adhesive and conductive
gels are useful. If the area of contact is small, the current density increases to the
point at which a burn is probable
may occur if the plate is detached or malpositioned.
The diathermy current may flow to earth through any point at which the
patient is touching metal (such as the operating table, lithotomy poles or ECG
electrodes) and cause a burn.
The plate should not be placed over an area where there is a metal prosthesis in place
(usually the hip). Metal has a low resistance in comparison with tissue, and so the
current will flow preferentially through the prosthesis, generating a potentially
dangerous current density.
Other risks
Alcoholic skin preparation solutions have ignited after diathermy activation
completed via a route that does not include the indifferent
electrode; this may also result in a burn
interfere with monitoring devices. This problem can be minimized
by the use of electrical filters
Diathermy may lead to ischaemia and infarction of structures supplied by fine endarteries.
Classic examples include the penis (hence unipolar diathermy must be
avoided in circumcision) and the testis, which has a vulnerable vascular pedicle
Diathermy may interfere with cardiac pacemaker function.
The indifferent electrode should be sited as far distant as
possible from the pacemaker,
and bipolar diathermy should be used wherever possible.
If the use of unipolar diathermy is unavoidable,
it should be deployed in short bursts.
Cutting diathermy causes more of a problem than coagulation.
Implantable cardiac defibrillators (ICDs) are increasingly common.
In emergency surgery these can be protected from the effects of surgical diathermy by
the application of a magnet which will deactivate the ICD but not the pacing function.
