Biomedical Equipments electro surgical unit

Electro Surgical Unit

1. UNIT-3 ELECTRO-SURGICAL UNIT Or SURGICAL DIATHERMY MACHINE

2. Introduction The term diathermy is derived from the Greek words “ Therma ”, meaning heat , and “ Dia ”, meaning through . Electrosurgery is a simple method of making surgical incisions, controlling bleeding and destroying unwanted tissue cells by the use of a high frequency “ electrosurgical current ”. Its benefits include the ability to make precise cuts with limited blood loss. Electrosurgical devices (Surgical Diathermy Machine) are frequently used during surgical operations helping to prevent blood loss in hospital operating rooms or in outpatient procedures.

3. Why high frequency current are used? Faraday proposed: Electric current through body tissues Intense neuromuscular stimulation, electrocution hazards & alteration of cardiac rhythm @ normal frequency of 50Hz (power supply feq . In India) will occur Neuromuscular stimulation gradually disappears at 50 KHz Power levels for machine are below the threshold of neural stimulation provided that the diathermy frequency is in the radio-frequency range.

4. Advantages of using high frequency currents The separation of tissues by electric current always takes place immediately in front of the cutting edge and is not caused by it. Electric cutting therefore, does not require any application of force. Instead it facilitates elegant and effortless surgery. The electrode virtually melts through the tissue instantaneously and seals capillary and other vessels, thus preventing contamination by bacteria. A simplified method of coagulation saves valuable time since bleeding can be arrested immediately by touching the spot briefly with the coagulating electrode.

5. GENERAL PRINCIPLES OF SURGICAL DIATHERMY

6. BASIC BLOCK DIAGRAM OF ELECTRO SURGICAL UNIT

7. PRINCIPLES OF ELECTRO SURGICAL CUTTING High frequency currents (in the range of 1-3 MHz) can be used in operating rooms for surgical purposes involving cutting and coagulation. Surgical diathermy machines depend on the heating effect of electric current. When high frequency current flows through the sharp edge of a wire loop or band loop or the point of a needle into the tissue (Fig. above ), there is a high concentration of current at this point. The tissue is heated to such an extent that the cells which are immediately under the electrode, are torn apart by the boiling of the cell fluid. The indifferent electrode establishes a large area contact with the patient and the RF current is therefore, dispersed so that very little heat is developed at this electrode. This type of tissue separation forms the basis of electro-surgical cutting.

8. Important considerations for voltage Biological tissue can only be cut when the voltage between the cutting electrode and the tissue to be cut is high enough to produce electric arcs between the cutting electrode and the tissue. The temperatures produced at the points at which the electric arcs contact the tissue like microscopic flashes of lightning are so high that the tissue is immediately evaporated or burned away . A voltage of 200 is required in order to produce the electric arc between a metal electrode and biological tissue. If the voltage is less than 200 , the electric arcs cannot be triggered and the tissue cannot be cut. Voltage suitable for cutting biological tissue ranges between 200 and 500 . If the voltage rises above 500 , the electric arcs become so intense that the tissue is increasingly carbonized and the cutting electrode may be damaged. A visible arc forms when the electric field strength exceeds 1 kV/mm in the gap and disappears when the field strength drops below a certain threshold level. Biological tissues are coagulated by thermal means if the requisite temperature is maintained at around 70°C.  

9. ELECTRIC ARCS

10. TYPES OF ELECTROSURGERY TECHNIQUES Depending on the amount of heat generated and type of electrodes used, surgical diathermy can be used to merely seal or to destroy tissue. Which is broadly classified as follows:- Cutting ( Electrotomy ) Coagulation Fulguration Desiccation Haemostasis

11. Cutting ( Electrotomy ) A combination of fine wire electrodes, high RF voltage and high cutting speeds are necessary for confinement of tissue destruction in electro-surgery. These parameters are of great value in microsurgery since localization of electrosurgical effects would be accompanied by coagulation and hemostasis. Evolving steam bubbles in the tissues at the surgical tip continuously rupture the tissue and are responsible for the cutting mechanism

12. Coagulation Electro-surgical coagulation of tissue is caused by the high frequency current flowing through the tissue and heating it locally so that it coagulates from inside . The coagulation process is accompanied by a grayish-white discoloration of the tissue at the edge of the electrode. In contrast to a thermocautery (A metal heated by electrical current is applied to the target tissue, to burn or coagulate the specific area of tissue used by dermatologists, ophthalmologists, plastic surgeons, urologists etc.) , better coagulation can be achieved by high frequency currents because it does not cause superficial burning. THERMOCAUTERY

13. Fulguration The term 'fulguration' refers to superficial tissue destruction without affecting deep-seated tissues. This is undertaken by passing sparks from a needle or a ball electrode of small diameter to the tissue. When the electrode is held near the tissue without touching it, an electric arc is produced , whose heat dries out the tissue. Fulguration permits fistulas and residual cysts to be cauterized and minor hemorrhages to be stopped.

14. Desiccation In desiccation, needle-point electrodes are stuck into the tissue and then kept steady . Depending upon the intensity and duration of the current , a high local increase in heat will be obtained. The tissue changes due to drying and limited coagulation .

15. Haemostasis The concurrent use of continuous radio-frequency current for cutting and a burst wave radiofrequency for coagulation is called Haemostasis mode. The cutting current usually results in bleeding at the site of incision, whereas the surgeon would require bloodless cutting. The machines achieve this by combining the two waveforms The frequency of this blended waveform is generally the same as that used for cutting current. Cut waveform generated in a solid state diathermy machine Coagulate waveform generated in a solid state diathermy machine Blend waveform generated in a solid state diathermy machine

16. Operating modes of surgical diathermy Two modes:- Monopolar Mode Bipolar Mode

17. 1. Monopolar Mode Generator Active Electrode Patient Indifferent Electrode (Patient Plate) ESU – Base plate required to complete the circuit. – Minimum area of contact: 300 cm2 – Heat produced depends on: current density, tissue resistance and time – Current pass through patients body

18. 2. Bipolar Mode Generator one prong of active electrode intervening tissue Other prong of active electrode ESU – Current does not pass through patients body

19. Waveform used in surgical diathermy CUTTING For Cutting high amplitude low voltage continuous wave form current are used Cut waveform generated by electron tube circuit, showing RF modulated by 100 Hz Cut waveform generated in a solid state diathermy machine

20. COAGULATION For Coagulation high Voltage Low amplitude interrupted waveform are used. – Soft Coagulation (No Electric arcs generated) – Forced Coagulation (Electric arc generated), used in areas of high vascularity – Spray Coagulation (Intensely modulated high frequency voltage), used for haemostasis. Waveform used in surgical diathermy Coagulate waveform generated by spark gap generator Coagulate waveform generated in a solid state diathermy machine

21. Blended Mode (Amplitude and Voltage are equal)- both cutting & coagulation. Waveform used in surgical diathermy Blend waveform generated in a solid state diathermy machine

22. Electrosurgical (ESU) Electrodes Lancet electrodes are normally used for cutting applications • Needle electrodes are preferred for epilation and desiccation. • Loop electrodes are employed for exsecting (or opening up) channels and extirpating growths, etc.

23. Types of ESU Electrodes Active Electrode Passive Electrode

24. Active Electrodes Electrodes are connected to the RF power generator. Switching on the high frequency current can be done with a finger-tip switch in the electrode or a foot switch. Small size (thickness in mm (1mm) and wideness till 10. mm).

25. Active electrode for coagulation

26. Active Electrode Resistance The amount of power delivered to the tissue for a given ESU voltage depends on the active electrode-to-skin resistance R E. R I : Internal resistance of active electrode R E : active electrode-to-skin resistance R B : Body resistance R R : Return electrode resistance

27. Passive (Neutral, Return) Electrode/Indifferent Electrode The function of the patient return electrode is to remove current from the patient safely. A return electrode burn occurs when the heat produced, over time, is not safely dissipated by the size or conductivity of the patient return electrode. Two points are of practical importance: ( i ) Sufficiently large contact area. (ii) High electrical conductivity between the body and electrode.

28. Passive (Neutral, Return) Electrode/Indifferent Electrode

29. 1 st Approach -The low potential terminal of the radio frequency output leads is connected to the indifferent or dispersive electrode which is a lead plate (15 x 20 cm) wrapped in a cloth bag, soaked in saline solution and strapped onto the patient's thigh. 2 nd Approach -An alternative arrangement is to use a flexible non-crumpling stainless steel sheet plate without any covering. Good contact is established with the film of sweat rising between the plate and the patient's body. Quite often, a liberal amount of conductive paste like ECG paste is applied to the plat e. This gives excellent electrical contact and removes the need to keep a wet gauze pad. However, problems may arise if the paste is not cleaned from the plate after use as it may form a hard insulating layer . Passive (Neutral, Return) Electrode/Indifferent Electrode (Contd.)

30. 3 rd Approach -An alternative approach is to use capacitively coupled plates in which no direct contact is made between the metal of the indifferent electrode and the patient's skin. The electrode comprises a large sheet of thin metal sandwiched between two sheets of neoprene, which formed a capacitor with the patient's body . This capacitor allows an easy path for the passage of the high frequency diathermy currents. But there is a problem of introducing burn hazard when alternative current paths when other equipment with grounded patient connection is used. The common reason for faulty performance of an electro-surgical unit is improper placement of the indifferent electrode. This electrode must be placed in firm contact with a fleshy portion of the patient and as near as possible to the operating site. Poor contact or excessive distance from the operating site causes a loss of energy available for the actual surgical procedure. Passive (Neutral, Return) Electrode/Indifferent Electrode (Contd.)

31. Electro Surgical UNIT (ESU)/Surgical Diathermy Machine An ESU consists of basically a high frequency power oscillator: Spark-gap oscillator : Generates damped high frequency current which is specifically suitable for the coagulation of all kinds of tissues. Solid-state oscillator : Generates un-damped high frequency current which is suitable for making clean cutting.

32. Solid-State ESU The heart of solid-state ESU is the logic and control part which produce the basic signal and provides various timing signals for the cutting, coagulation and haemostatic modes of operation. In the cutting it delivers 400 W in 500 Ω load at 2000V. In the coagulation it delivers 150W. In coagulation, the burst duration is 10–15 s and repetition frequency of the burst is 15 kHz. The output circuit in ESU is generally isolated and carefully insulated from low frequency primary and secondary voltage by means of capacitors.

33. Block diagram of solid state electro-surgical unit

34. Solid state electro-surgical diathermy machine

35. Output circuit configuration in which the indifferent circuit is directly connected to the mains earth Use of capacitor in the output circuit. This permits RF currents to flow to earth through the diathermy machine. However, it effectively blocks the passage of low frequency currents (50 Hz) RF isolated output circuit configuration. Obviously, there is no direct connection of the indifferent circuit to ground. The RF leakage current is due to stray capacitance within the machine High Frequency Current Hazards