HF surgery

Use of an electrocautery for lipoma removal

In high-frequency surgery (hereinafter referred to as HF surgery ), high-frequency alternating current is passed through the human body in order to damage or cut tissue in a targeted manner through the heating caused by it. The diathermy or electrocautery (from Greek. Kaustos for burned ') here is a surgical method for the severing of tissue structures or complete removal of body tissue ( cautery ) with electrocautery . A major advantage over conventional cutting techniques with the scalpel is that the bleeding can be stopped at the same time as the cut by closing the affected vessels . The devices used are also known as electric scalpel.

When resecting malignant tumors , the use of the electric knife close to the tumor should not be used, as the pathologist cannot assess the burned cut surfaces and cannot make a statement as to whether the tumor has been completely removed ( in sano ). However, there is nothing to prevent the resection surfaces from being cauterized in order to destroy the tumor seed (of course not on the preparation side).

application

In contrast to the mechanical severing of tissue (e.g. with a scalpel ), the electrocautery uses a short, intense electrical current that severes or vaporizes the tissue depending on the duration of use.

Electro-caustic technology in operations is widespread today and is used in practically all routine operations, in particular as a practicable, rapid and essentially harmless possibility of sclerosing small and medium-sized blood vessels for the purpose of intraoperative hemostasis. It is especially important in dermatology , but also in areas where access is difficult and sensitive adjacent tissue could be injured. Therefore, electrocautery is a preferred method of brain surgery , especially in stereotactic brain operations such as B. the cingulotomy .

Physical principle

HF electrosurgery is based on Joule's law . When electrical current flows through the human body, three different effects occur:

Nerve stimulation
electrolysis
Joule electricity heat

The warming is used in high-frequency surgery. With high-frequency alternating current, electrolysis and nerve stimulation only occur to a very small extent.

The per tissue volume resulting heat capacity / is directly proportional to the Joule law according to specific resistance and the square of the surface current density . ${\ displaystyle \ Delta V}$ ${\ displaystyle \ Delta Q}$ ${\ displaystyle \ Delta t}$ ${\ displaystyle \ rho}$ ${\ displaystyle J}$

{\ displaystyle \ Delta Q = \ rho \ cdot J ^ {2} \ cdot \ Delta V \ cdot \ Delta t}

Values of 1 A / cm² to 6 A / cm² for the surface current density J are common. The conditions on which the type or appearance and effect of the cut depend are:

Current density
Duration of action or (cutting) speed of the moving electrode
Electrode shape
Current shape
Tissue condition

Application techniques

Monopolar application technology

The monopolar technique is used most frequently. One pole of the HF voltage source is connected to the patient via a counter electrode with the largest possible area. This electrode is often called a neutral electrode . The other pole is connected to the surgical instrument and this forms the so-called active electrode . The current flows from the active electrode to the neutral electrode via the path with the least resistance. The current density is highest in the immediate vicinity of the active electrode; this is where the thermal effect takes place most strongly. The current density decreases with the square of the distance.

The neutral electrode should be as large as possible so that the current density in the body is kept low and no burns occur. The skin on the neutral electrode is not noticeably heated by the large surface. Strict safety measures apply when attaching the neutral electrode. In order not to cause burns, the correct position and good contact of the neutral electrode (depending on the operating area) are decisive.

In the case of monoterminal application technology, a special form of monopolar technology, the neutral electrode is omitted. The generator is connected to earth on one side and the capacitive resistance ( impedance ) of a few hundred ohms of the human body to earth closes the circuit. The disadvantage is that the voltage across this capacitive resistor remains small enough only with small powers and this resistance and thus also the working current can also change if someone or z. B. a metal part touches the body. This is why this method is only used for interventions with small currents (e.g. dentistry and dermatology).

Bipolar application technology

With the bipolar technique, in contrast to the monopolar technique, the current only flows through a small part of the body - the one in which the surgical effect (incision or coagulation) is desired. Two mutually insulated electrodes, between which the HF voltage is applied, are led directly to the surgical site. The circuit is closed via the tissue in between. The thermal effect takes place in the tissue between the electrodes (in the picture the tips of the tweezers).

Compared to monopolar technology, 20–30% less power is required. The surrounding tissue is not damaged because there is no current flowing here and measuring devices on the patient (e.g. EKG) are not disturbed. This method is well suited for critical and precise applications such as microsurgery, neurosurgery and ENT surgery.

Frequency range

The frequency range used is generally between 300 kHz and 4 MHz. Below 300 kHz, amplified below 100 kHz, irritation of nerves causes disruptive muscle twitching, the so-called faradization or faradaysation.

The upper limit of the frequency range is determined by the capacitive leakage currents of the electrodes and cables. As the frequency rises, energy is also amplified and undesirably radiated, and control of the effect becomes more problematic. The risk of injuring the patient through burns elsewhere would increase. Therefore, in practice, the upper limit is 4 MHz. However, there are individual devices that use far higher frequencies.

Faraday irritations can also occur in the operating frequencies due to low-frequency interference pulses. They are formed by the rectifying effect of the radio transfer from the electrode to the patient due to different work function and inhomogeneous field configuration. In order to suppress these low frequency components, a capacitor with less than 2.5 nF is installed in series with the electrode connections. However, muscle twitching in certain places (e.g. in the urinary bladder) cannot be completely avoided. This effect has not been clearly clarified and is probably based on thermal irritation effects.

Body tissue as an electrical conductor

At the frequencies used for HF surgery, the body tissue behaves like an ohmic resistance . The specific resistance depends strongly on the type of tissue. According to the above formula, the power input into the tissue at constant current is proportional to its specific resistance. Loss of performance is caused by heat dissipation, blood circulation and specific heat of vaporization.

The specific resistance of muscle tissue and tissue with a strong blood supply is relatively low. That of fat is around a factor of 15 higher and that of bones by a factor of 1000. The shape and level of the current must therefore be tailored to the type of tissue being operated on. In principle, the lowest possible frequency for the tissue is used.

The following guide values can serve as an illustration:

Tissue type (frequency = 1 MHz)	Specific resistance [ ] ${\ displaystyle \ rho}$ ${\ displaystyle \ mathrm {k \ Omega \ cdot cm}}$
blood	0.16
Muscle , kidney , heart	0.20
Liver , spleen	0.30
brain	0.70
lung	1, 00
fat	3.30

The effective resistance also depends on the type and shape of the electrode and the degree of tissue destruction. After scab formation, it increases to about ten times the value.

Device technology

Basic circuit diagram of a generator for HF surgery

Generators with a maximum power of 400 W are usually used for HF electrosurgery . The output voltage can be a high voltage of up to 4 kV when idling . In dentistry or ophthalmology, weaker devices with an output of max. 50 W with lower voltages common.

The illustration shows the circuit principle of a generator for HF surgery. The oscillator generated in this case, the operating frequency of about 700 kHz and controlled via a driver stage, the final stage . In the case of superficial coagulation, the oscillator is modulated at around 20 kHz in a ratio of 1: 5. The operating voltage of the driver stage and the power for cutting or coagulating are set with two separate potentiometers . In the output stage, several power transistors are connected in parallel, which work in switching mode in order to reduce the individual load and thus increase operational reliability.

The secondary circuit of the output stage transformer leads via high-voltage-resistant filter capacitors to suppress the disruptive low-frequency components to the connections of the active and the counter electrode. The connection of the counter electrode can be earthed directly or via a capacitance or the counter electrode is operated symmetrically to the active electrode. The patient protection circuit means that the generator can only be operated if the neutral electrode A and the counter electrode B are connected to the patient with good conductivity. Then a weak current (about 100 µA) flows between A and B and relay G is activated. In the event of poor contact with even just one electrode, the current falls below the minimum value and the relay interrupts energy generation by switching off the driver stage. A buzzer then often generates an acoustic alarm. This ensures that the high-frequency current always flows via the counter electrode to the HF generator and does not cause unintentional burns in other parts of the body.

If the counter electrode is connected to the device but not applied to the patient, the generator must not be operated; because even with symmetrical operation ("floating"), combustion cannot be completely ruled out. With capacitive or direct earthing of the counter electrode connection, the current flows to earth via the capacitance of the patient or via capacitively earthed transducers if the counter electrode is not in contact. It can reach the operating current and cause severe burns. Even if the counter electrode is properly applied, voltages occur in symmetrical operating mode ("floating") on this electrode, which lead to currents from this to earth if they cannot compensate with the currents of the active electrode (cause is the internal capacity of the Device, e.g. the transformer winding, to earth). This is always the case when the contact of the counter electrode is better than that of the active electrode. To reduce these currents, it is possible to operate the HF generator asymmetrically by capacitive or direct earthing of the counter electrode connection.

If bipolar instruments are used, e.g. forceps with two electrodes, the symmetrical operating mode should be selected in order to avoid body currents.

Monitoring of contacting

In order to prevent unwanted burns or electric shocks , the return flow of high-frequency currents via the counter electrode must be ensured when using monopolar tools. The counter electrode must therefore have good contact with the patient and the device. Otherwise the electricity could flow away in other ways. This is why powerful electrosurgical devices have a safety circuit based on the principle of a residual current circuit breaker , which checks the sum of the currents to the active electrode and to the counter electrode. If the currents cancel each other out insufficiently, the device is switched off.

Another method is to run a test voltage over the RF circuit, with which the tissue contact can be checked. Otherwise, the current is prevented from being switched on or the HF generator is forcibly switched off.

Application types

Coagulation

This fast and efficient hemostasis is used when there is no spontaneous coagulation and in most cases replaces the expensive fibrin glue or the complex ligature in small vessels .

The term coagulation encompasses two different surgical techniques: deep coagulation and (electrical) hemostasis.

With deep coagulation, the tissue is heated to 50–80 ° C over a large area. This is done with ball, plate or roller electrodes and is used to remove the tissue later. A large current density and current without pulse modulation are used. The depth of coagulation can be influenced by the magnitude of the current strength.

If the power is too high, a scab forms (carbonization), which inhibits the further spread of heat into the depths. If you remove the electrode later, you also remove the burned tissue because it sticks to the electrode. If, on the other hand, you choose too little power and a long exposure time, the tissue around the electrode and a little deeper than the diameter of the electrode is cooked.

For hemostasis, tweezers are used as electrodes. The blood vessels are grasped with the tips of the tool and narrowed by the dehydration until they are completely closed. The work is carried out in bipolar mode, and monopolar forceps are rarely used. Large-area electrodes are operated with pulse-modulated current to stop oozing bleeding.

Special forms of coagulation are: fulguration and desiccation. During fulguration, superficial coagulation is carried out. The intra- and extracellular fluid evaporates through the sparkover from the tip of the electrode (usually needle electrode), which is passed over the tissue at a distance of a few millimeters. During fulguration, clusters of sparks can occur. Desiccation is coagulation using an inserted needle electrode.

Furthermore, the coagulation can be divided as follows:

Soft coagulation

It works with low voltage below 190 V. There are no arcs and no unwanted cutting, and carbonization is prevented.

Forced coagulation

Forced coagulation, also called forced coagulation, works with peak voltages of up to 2.65 kV. Smaller arcs are generated here in order to achieve a higher coagulation depth. Unfortunately, carbonization cannot be avoided. Small-area spherical electrodes are usually used for this.

Spray coagulation

With spray coagulation at operating voltages of up to 4 kV, long and stronger arcs occur which heat the tissue exogenously and endogenously. This can lead to the following complications during coagulation:

Adhesive effect with soft and forced coagulation
With dry tissue, there is no current flow and coagulation cannot occur

Electrotomy

The cutting of the tissue (instead of cutting with a scalpel) in HF surgery is called electrotomy. When cutting, the HF surgical device is operated with a needle or a narrow blade in monopolar mode. Recently, bipolar scissors have also been used very successfully for cutting.
As mentioned above, this is a cell explosion directly on the active electrode. The current density increases quadratically towards the active electrode. The heat output per volume element therefore increases with the fourth power to the reciprocal of the distance. This explains why one can achieve a locally very limited effect with a monopolar electrode.

The tissue is superficially coagulated on both sides of the incision. The depth of the coagulation seam depends on the tissue and the cutting speed. A distinction is made between the smooth and the scabbed cut. For the smooth cut, unmodulated or 100 Hz modulated current is used. For the scabbed cut, pulse-modulated current with a significantly higher modulation frequency is used. High instantaneous values mean a high output relative to the mean value. This results in greater superficial coagulation and closure of the wound edges. The advantage is low-bleeding cutting.

Safety measures

General safety precautions for monopolar electrosurgery:

To avoid burns in places other than intended or electric shock, the following safety measures must be taken:

The patient must be kept isolated on the operating table (dry cloths, plastic pads, etc.). It must also be stored isolated from all metal parts and conductive (antistatic) hoses.
Dry cellulose liners are required in folds of skin, breast folds and between extremities.
The counter electrode should be placed as close as possible to the surgical field. The only limiting factor is the sterile operating theater. It has to take up the current with as low a resistance as possible (i.e. through good contact) and return it to the generator.
Make sure that the counter electrode is in contact with a large area and firmly adheres to it.
Liquids must not get under the neutral electrode, as they lead to a high point current density.
So-called two-part neutral electrodes should always be used. These monitor the correct position of the neutral electrode. An additional measuring current is generated and monitored between electrode pairs. If this current is too low, one of the electrodes does not make good contact and the device switches off.
The electrode cables are as short as possible, the dosage of the HF power is to be selected as low as possible.
Only ECG cables with high-impedance inputs or HF chokes may be used for preoperative monitoring.
When using explosive anesthetic gases, the use of shielding gas is required (similar to shielding gas welding in metal construction).
Before the procedure, make sure that the current from the active electrode to the neutral electrode is not, or as little as possible, passed through the area of the heart.
DIN EN 60601-1 and the HF surgery standard DIN EN 60601-2-2 are used for the safety assessment

literature

Johannes Petres: Current treatment methods . Springer-Verlag, Berlin, Heidelberg 2013, p. 22 ff. ( Online )
Ingrid Moll: Dermatology . Georg Thieme Verlag, 6th edition Stuttgart 2005, p. 58 ff. ( Online )
Engelbert Mach: Introduction to Medical Technology for Health Professions . Facultas, Vienna 2009, pp. 76–90 ( online )
Rüdiger Kramme: Medical technology: procedures, systems, information processing . Springer-Verlag, Berlin Heidelberg 2013, pp. 396–412 ( online )
Hans-Dieter Reidenbach: High-frequency and laser technology in medicine: Basics and applications of high-frequency electromagnetic energy for therapeutic warmth . Springer-Verlag, Berlin Heidelberg 2013, pp. 8–192 ( online )

Web links

Explanation of high frequency (HF) surgery on pflege-im-op.de
Explanation of the electro caustic on healthline.com

Electro caustic (high frequency caustic ): definition and implementation on medhelp.at

High-frequency surgery: mode of action, risks and risk minimization on sempermed.com (PDF)

Electrosurgery: Manual on klsmartin.com (PDF)