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Surgical diathermy
EQUIPMENT AND CLINICAL PHYSICS
Current density – for a certain supplied voltage, the average current throughout the circuit is the same. The current density is the current per unit area, therefore if the material in which the current passes is smaller, the heating effect increases. Another way of looking at it is that the resistance of the material is proportional to its size, therefore as the material becomes smaller its resistance gets larger. The heating power is the product of the current squared and the resistance (Power=I2 x R). Physiotherapy – if both of the electrodes are large (> 5 cm2), the current density will be low and evenly distributed throughout the body with maximum heating effects about 1 cm from both of the electrodes. No tissue damage is caused in this mode. Surgical diathermy (or electrosurgery) – one or both of the electrodes is very small, and is used to cut and coagulate tissue. The small electrode can be made into a pointed surgical tool and localized heating occurs at the tip of the instrument. The smaller and more pointed the instrument, the greater the current density at the tip. This electrode is classified as the active or live one. The current densities around this electrode can be as much as 10 A/cm2, and the total heating power about 200 W. The other electrode in the circuit is much larger and situated on another part of the body (Figure 1). This is the passive electrode (also called dispersive, neutral, plate, patient or indifferent) and because of its large surface area, the current density is low, and no heating occurs. The current density is high only around the active electrode and quickly becomes low in the rest of the body (Figure 1).
Surgical diathermy Mark Tooley
When a voltage source (such as a battery) is supplied across a conductor, a current flows. If the voltage is supplied across the body via suitable electrodes the body becomes part of the circuit (Figure 1) and a current flows, the magnitude depending on the properties of the tissues in its path, particularly the resistance. This current can cause heating or other physiological effects, depending on the frequency of the driving voltage. Constant or direct current (DC) (normally obtained from batteries) and low frequency alternating current (AC), such as the UK domestic mains supply at 50 Hz (cycles a second), cause predominantly physiological effects proportional to the size of the current. Low currents (mA) cause tingling and pain, and higher currents (tens of mA) cause muscle stimulation, convulsions, tetanic muscle stimulation and ventricular fibrillation. Diathermy As the frequency of the driving voltage is increased, the heating increases and the stimulation decreases. At frequencies above 100 kHz (i.e. radio frequencies) the effect is entirely heating. This heating effect in the body by electric current is called diathermy, but the location, concentration and how this heat is used depends on the electrode design and the current concentration or current density at any point in the circuit.
Surgical effects and associated waveforms Cutting: a fine-intensity arc from the pointed active electrode produces very rapid heating of tissue over a very small area. This causes microexplosive boiling of intracellular fluid, the energy developed by the creation of steam. The tissue is cut without dissipation of heat in the surrounding tissue. The waveform for the most efficient cutting is a continuous sine wave (Figure 2a), at a voltage of 250–3000 V, the voltage depending on the application, electrode and power needed. Smaller voltages can be used if the tip of the electrode is very pointed, sharp and clean (the sharper the tip, the higher the current density and heating).
Mark Tooley is Consultant Senior Lecturer in Medical Physics at the University of Bristol and the United Bristol Health Care NHS Trust, and a Chartered Engineer. He took his first degree at Bath University, then further degrees at London University (St Bartholomew’s Hospital). He is a Fellow of the Institute of Physics and Engineering in Medicine (IPEM) and the Institution of Electrical Engineers (IEE). He is an affiliate of both the Royal College of Physicians and the Royal College of Anaesthetists.
Coagulation: although the continuous waveform produces excellent cutting, the edges bleed freely and so the coagulation effect
Set-up for an isolated surgical diathermy system in the monopolar mode Surgical diathermy machine Alarms
Active electrode connection
Active electrode Current density high Low
A Live Radiofrequency generator
Modulator
Monopolar
B Live Isolated voltage source
Passive electrode
Passive electrode connection
Theatre floor Earth
1
ANAESTHESIA AND INTENSIVE CARE MEDICINE 5:11
369
© 2004 The Medicine Publishing Company Ltd
EQUIPMENT AND CLINICAL PHYSICS
as steam and the tissue is dried out. The waveform shape is not critical, and the power (and voltage) are lower.
Different waveforms for different types of surgery a 250 V
c 350 V
Blended modes (cutting and coagulation): modes can be blended, so that a mixture of reasonable cutting and coagulation is provided. The waves can be 50% active/50% off (Figure 2c), or 25% active/75% off (Figure 2d), and any combination in between. The less the waveform is active, the more useful the coagulation, and vice versa (Figure 2c, d). These ratios are controlled by the modulator in the equipment (Figure 1).
50% off Time
Time 50% active
2 µs Excellent cutting b 8 kV
d 350 V
Good cutting Reasonable coagulation
Electrode systems The electrode systems used in surgical diathermy can be monopolar or bipolar. Each has different characteristics and uses. Monopolar systems (Figure 1): the power available in this mode is high, and can produce very efficient cutting. The active electrode is available in an assortment of tips: loops to cut and fulgurate, needles to cut and coagulate, ball electrodes for desiccation and fulguration, and blades to cut and coagulate. However, as the patient forms a major part of the electrical circuit, care must be taken. This mode should not be used in areas where the target tissue is connected to adjoining tissue via small delicate structures because these small structures could have high current densities and be damaged by excessive heat. If a metal prosthesis, or a pacemaker, forms part of the circuit, it could become a major part of the circuit and most of the current could flow through it. The device will become warm, and any tissues joining sharp extremities of the device could become areas of high current density with associated damage. Bipolar systems: in the bipolar arrangement both electrodes are small and produce high current densities. The intense heating effects are the same at each electrode. The body does not form part of the circuit. The electrodes can be incorporated, for example, into a pair of forceps. Bipolar surgical diathermy has a localized, precise effect on the tissue and suits delicate surgery (e.g. ophthalmic). The technique has good cogulation effect but less cutting ability because of the low power available.
75% off Time 3 µs 47 µs Excellent coagulation
Time 25% active Reasonable cutting Good coagulation
a Continuous wave at 500 kHz at 250 V peak, suitable for cutting. b A waveform suitable for coagulation (the time scales are expanded for clarity). c A 50% active waveform. d A 25% active waveform.
2
is poor. Coagulation can be provided by fulguration (literally lightning) and desiccation. Fulguration (also called spray coagulation because of multiple affected sites around the electrode position) is the destructive charring of the tissue by arcing, and normally a flat spatula or ballshaped electrode is used. Blunted instruments need higher voltages (up to 9 kV) than cutting instruments to provide the current density needed for arcing. The instrument is positioned 2–4 mm above the surgical site. The waveforms used are not continuous, and the wave can be active for as little as 6% of the complete cycle. For example, the 500 kHz wave could be active for 3 µs and then off for 47 µs (Figure 2b). Desiccation – coagulation by desiccation (pin-point or contact coagulation) is the heating of tissue resulting from contact between the active electrode and tissue. The tissue temperature is increased at the point of contact and intracellular water is slowly driven off
Safety of surgical diathermy equipment Older diathermy machines used to have the passive electrode earthed to keep the patient at zero voltage, but directly earthing
Diathermy circuit demonstrating stray capacitance
diathermy machine, and the patient has a poor contact with the passive electrode. An earthed object (e.g. a drip stand) is touching the patient. Stray capacitance is shown between the passive electrode and the theatre floor.
Active electrode Current path Earthed object
A Burns Live
Passive electrode
B
Isolated Earth
Theatre floor Poor connection to patient Stray capacitance
3
ANAESTHESIA AND INTENSIVE CARE MEDICINE 5:11
370
© 2004 The Medicine Publishing Company Ltd
EQUIPMENT AND CLINICAL PHYSICS
the patient is not advisable for safety reasons (see page 366). Modern machines have outputs completely isolated from earth (Figure 1) to minimize spurious diathermy circuits to earth. To make a complete electrical circuit for the maximum designed diathermy current to flow, the circuit must be completed from the active electrode connection to the active electrode (Figure 1), via the patient resistance, to the passive electrode, back to the passive electrode connection. In theory, if the passive electrode became disconnected, no current would flow. If an earthed object (e.g. a drip stand) touches the patient, it should have no effect, because it is not part of the return current path. However, there is the problem of stray capacitance.
Principles of pressure transducers, resonance, damping and frequency response Mark R Stoker Methods of direct pressure measurement Accurate recording of dynamic physiological pressures involves the conversion of the pressure waveform into an electrical signal by a transducer so that it can be amplified, processed and displayed by electronic equipment. Two alternative transducer arrangements are used clinically (Figure 1). • A miniaturized sensor can be placed within the patient on the tip of a catheter directly at the point of measurement. • A fluid-filled pathway or manometer can be used to link a catheter or cannula to a transducer placed some distance away from the site of measurement. Although catheter-tip sensor systems are the most accurate, they tend to be more expensive because of their greater complexity, are prone to fibrin deposition if used within the circulation, and cannot be recalibrated within the patient. Manometric transducer systems are therefore in more widespread clinical use. Various physical methods are used to convert pressure changes into an electrical signal; the most common is the strain gauge. A strain gauge transducer consists of: • a force-gathering element, usually a flexible diaphragm that is deformed by incident pressure waves, creating a measurable strain • elements that transform the strain into a proportional electrical signal • a housing that protects the first two elements and incorporates the electrical connections. The marked change in electrical resistance of a semiconductor in response to physical deformation forms the basis of the piezoresistive transducer. This comprises a thin silicon wafer forming the diaphragm, onto the peripheries of which are etched semiconductor resistors. Deformation of the diaphragm alters the electrical resistance of the resistors, which are placed in two pairs, such that one pair is subjected to radial stress as the other pair is stressed tangentially (Figure 2). Circuitry required for amplification, temperature compensation and calibration may also be included on the same silicon wafer using integrated circuit technology. Piezoresistive strain gauges are in widespread clinical use because they are small, cheap to manufacture, resistant to mechanical vibration, and possess high sensitivity (a large change in resistance in response to a small diaphragmatic distortion). Despite electronic compensation, piezoresistive strain gauges are susceptible to the effects of temperature, and are accurate only over a comparatively narrow temperature range.
Stray capacitance: capacitors are electrical components consisting of two parallel plates separated by an insulator (which can be air). They store charge, block the flow of DC, but allow AC to pass. They can be considered as frequency-dependent resistances, in which the resistance deceases as the frequency increases. Capacitors can be ‘natural’ in that the two ‘plates’ can be, for example, the passive electrode and the ground of the theatre (which can be indirectly connected to earth). This ‘capacitor’ will have a low capacitance value and pass minimal currents at domestic mains frequencies. However, it will present a much lower resistance at the higher diathermy frequencies and so other stray circuits can be formed, which can result in burns. For example, if there is a poor connection between the passive electrode and the patient, and a sharp earthed object touches the patient (Figure 3), then an alternate current path could be formed for the return current. The current in this case flows from A in Figure 3 to the active electrode, into the patient, then some of current leaves by the earthed object and flows to earth. To complete the circuit back to B, the current passes from earth to the passive electrode via the stray capacitance. Burns can arise at the connection between the sharp object and the patient, but can also arise at parts of higher current density where only certain parts of the passive electrode are touching the patient. The adverse burns can be minimized by ensuring that the passive electrode is securely and completely attached to the patient (so that the correct circuit is the most ‘attractive’ to the current). Also ensure that no other objects are touching the patient when the diathermy is on. Alarms are installed in the equipment (Figure 1) to warn users that the device is active, and to indicate faulty electrode connections, and stray leakage currents. The stray currents get worse with increasing frequency and, to minimize this effect, modern isolated systems are designed to operate at the lower frequency range of diathermy machines (500 kHz). Older machines used to operate at frequencies up to 2 MHz. Interference with other equipment Surgical diathermy uses radiowaves, with frequencies around the top end of the medium wave AM band. The equipment can therefore act as a short-range transmitter, and sensitive monitoring equipment, such as ECG monitors, can be a receiver for these signals. Ideally, the ECG electrodes and leads should be placed as far from the diathermy electrodes as possible, and care must be taken with interpretation when diathermy is in use. Pacemakers can be interfered with by a similar method, and sometimes the interference can result in the pacemaker wrongly detecting a fast heart rate. In these cases, the pacemaker should be put into a fixed-rate mode.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 5:11
Mark R Stokerr is Consultant Anaesthetist at Peterborough Hospitals NHS Trust. He qualified from Oxford University and trained in anaesthesia in the Oxford and Anglia Regions. His interests include teaching and training, obstetric anaesthesia and public education.
371
© 2004 The Medicine Publishing Company Ltd
Current density – for a certain supplied voltage, the average current throughout the circuit is the same. The current density is the current per unit area, therefore if the material in which the current passes is smaller, the heating effect increases. Another way of looking at it is that the resistance of the material is proportional to its size, therefore as the material becomes smaller its resistance gets larger. The heating power is the product of the current squared and the resistance (Power=I2 x R). Physiotherapy – if both of the electrodes are large (> 5 cm2), the current density will be low and evenly distributed throughout the body with maximum heating effects about 1 cm from both of the electrodes. No tissue damage is caused in this mode. Surgical diathermy (or electrosurgery) – one or both of the electrodes is very small, and is used to cut and coagulate tissue. The small electrode can be made into a pointed surgical tool and localized heating occurs at the tip of the instrument. The smaller and more pointed the instrument, the greater the current density at the tip. This electrode is classified as the active or live one. The current densities around this electrode can be as much as 10 A/cm2, and the total heating power about 200 W. The other electrode in the circuit is much larger and situated on another part of the body (Figure 1). This is the passive electrode (also called dispersive, neutral, plate, patient or indifferent) and because of its large surface area, the current density is low, and no heating occurs. The current density is high only around the active electrode and quickly becomes low in the rest of the body (Figure 1).
Surgical diathermy Mark Tooley
When a voltage source (such as a battery) is supplied across a conductor, a current flows. If the voltage is supplied across the body via suitable electrodes the body becomes part of the circuit (Figure 1) and a current flows, the magnitude depending on the properties of the tissues in its path, particularly the resistance. This current can cause heating or other physiological effects, depending on the frequency of the driving voltage. Constant or direct current (DC) (normally obtained from batteries) and low frequency alternating current (AC), such as the UK domestic mains supply at 50 Hz (cycles a second), cause predominantly physiological effects proportional to the size of the current. Low currents (mA) cause tingling and pain, and higher currents (tens of mA) cause muscle stimulation, convulsions, tetanic muscle stimulation and ventricular fibrillation. Diathermy As the frequency of the driving voltage is increased, the heating increases and the stimulation decreases. At frequencies above 100 kHz (i.e. radio frequencies) the effect is entirely heating. This heating effect in the body by electric current is called diathermy, but the location, concentration and how this heat is used depends on the electrode design and the current concentration or current density at any point in the circuit.
Surgical effects and associated waveforms Cutting: a fine-intensity arc from the pointed active electrode produces very rapid heating of tissue over a very small area. This causes microexplosive boiling of intracellular fluid, the energy developed by the creation of steam. The tissue is cut without dissipation of heat in the surrounding tissue. The waveform for the most efficient cutting is a continuous sine wave (Figure 2a), at a voltage of 250–3000 V, the voltage depending on the application, electrode and power needed. Smaller voltages can be used if the tip of the electrode is very pointed, sharp and clean (the sharper the tip, the higher the current density and heating).
Mark Tooley is Consultant Senior Lecturer in Medical Physics at the University of Bristol and the United Bristol Health Care NHS Trust, and a Chartered Engineer. He took his first degree at Bath University, then further degrees at London University (St Bartholomew’s Hospital). He is a Fellow of the Institute of Physics and Engineering in Medicine (IPEM) and the Institution of Electrical Engineers (IEE). He is an affiliate of both the Royal College of Physicians and the Royal College of Anaesthetists.
Coagulation: although the continuous waveform produces excellent cutting, the edges bleed freely and so the coagulation effect
Set-up for an isolated surgical diathermy system in the monopolar mode Surgical diathermy machine Alarms
Active electrode connection
Active electrode Current density high Low
A Live Radiofrequency generator
Modulator
Monopolar
B Live Isolated voltage source
Passive electrode
Passive electrode connection
Theatre floor Earth
1
ANAESTHESIA AND INTENSIVE CARE MEDICINE 5:11
369
© 2004 The Medicine Publishing Company Ltd
EQUIPMENT AND CLINICAL PHYSICS
as steam and the tissue is dried out. The waveform shape is not critical, and the power (and voltage) are lower.
Different waveforms for different types of surgery a 250 V
c 350 V
Blended modes (cutting and coagulation): modes can be blended, so that a mixture of reasonable cutting and coagulation is provided. The waves can be 50% active/50% off (Figure 2c), or 25% active/75% off (Figure 2d), and any combination in between. The less the waveform is active, the more useful the coagulation, and vice versa (Figure 2c, d). These ratios are controlled by the modulator in the equipment (Figure 1).
50% off Time
Time 50% active
2 µs Excellent cutting b 8 kV
d 350 V
Good cutting Reasonable coagulation
Electrode systems The electrode systems used in surgical diathermy can be monopolar or bipolar. Each has different characteristics and uses. Monopolar systems (Figure 1): the power available in this mode is high, and can produce very efficient cutting. The active electrode is available in an assortment of tips: loops to cut and fulgurate, needles to cut and coagulate, ball electrodes for desiccation and fulguration, and blades to cut and coagulate. However, as the patient forms a major part of the electrical circuit, care must be taken. This mode should not be used in areas where the target tissue is connected to adjoining tissue via small delicate structures because these small structures could have high current densities and be damaged by excessive heat. If a metal prosthesis, or a pacemaker, forms part of the circuit, it could become a major part of the circuit and most of the current could flow through it. The device will become warm, and any tissues joining sharp extremities of the device could become areas of high current density with associated damage. Bipolar systems: in the bipolar arrangement both electrodes are small and produce high current densities. The intense heating effects are the same at each electrode. The body does not form part of the circuit. The electrodes can be incorporated, for example, into a pair of forceps. Bipolar surgical diathermy has a localized, precise effect on the tissue and suits delicate surgery (e.g. ophthalmic). The technique has good cogulation effect but less cutting ability because of the low power available.
75% off Time 3 µs 47 µs Excellent coagulation
Time 25% active Reasonable cutting Good coagulation
a Continuous wave at 500 kHz at 250 V peak, suitable for cutting. b A waveform suitable for coagulation (the time scales are expanded for clarity). c A 50% active waveform. d A 25% active waveform.
2
is poor. Coagulation can be provided by fulguration (literally lightning) and desiccation. Fulguration (also called spray coagulation because of multiple affected sites around the electrode position) is the destructive charring of the tissue by arcing, and normally a flat spatula or ballshaped electrode is used. Blunted instruments need higher voltages (up to 9 kV) than cutting instruments to provide the current density needed for arcing. The instrument is positioned 2–4 mm above the surgical site. The waveforms used are not continuous, and the wave can be active for as little as 6% of the complete cycle. For example, the 500 kHz wave could be active for 3 µs and then off for 47 µs (Figure 2b). Desiccation – coagulation by desiccation (pin-point or contact coagulation) is the heating of tissue resulting from contact between the active electrode and tissue. The tissue temperature is increased at the point of contact and intracellular water is slowly driven off
Safety of surgical diathermy equipment Older diathermy machines used to have the passive electrode earthed to keep the patient at zero voltage, but directly earthing
Diathermy circuit demonstrating stray capacitance
diathermy machine, and the patient has a poor contact with the passive electrode. An earthed object (e.g. a drip stand) is touching the patient. Stray capacitance is shown between the passive electrode and the theatre floor.
Active electrode Current path Earthed object
A Burns Live
Passive electrode
B
Isolated Earth
Theatre floor Poor connection to patient Stray capacitance
3
ANAESTHESIA AND INTENSIVE CARE MEDICINE 5:11
370
© 2004 The Medicine Publishing Company Ltd
EQUIPMENT AND CLINICAL PHYSICS
the patient is not advisable for safety reasons (see page 366). Modern machines have outputs completely isolated from earth (Figure 1) to minimize spurious diathermy circuits to earth. To make a complete electrical circuit for the maximum designed diathermy current to flow, the circuit must be completed from the active electrode connection to the active electrode (Figure 1), via the patient resistance, to the passive electrode, back to the passive electrode connection. In theory, if the passive electrode became disconnected, no current would flow. If an earthed object (e.g. a drip stand) touches the patient, it should have no effect, because it is not part of the return current path. However, there is the problem of stray capacitance.
Principles of pressure transducers, resonance, damping and frequency response Mark R Stoker Methods of direct pressure measurement Accurate recording of dynamic physiological pressures involves the conversion of the pressure waveform into an electrical signal by a transducer so that it can be amplified, processed and displayed by electronic equipment. Two alternative transducer arrangements are used clinically (Figure 1). • A miniaturized sensor can be placed within the patient on the tip of a catheter directly at the point of measurement. • A fluid-filled pathway or manometer can be used to link a catheter or cannula to a transducer placed some distance away from the site of measurement. Although catheter-tip sensor systems are the most accurate, they tend to be more expensive because of their greater complexity, are prone to fibrin deposition if used within the circulation, and cannot be recalibrated within the patient. Manometric transducer systems are therefore in more widespread clinical use. Various physical methods are used to convert pressure changes into an electrical signal; the most common is the strain gauge. A strain gauge transducer consists of: • a force-gathering element, usually a flexible diaphragm that is deformed by incident pressure waves, creating a measurable strain • elements that transform the strain into a proportional electrical signal • a housing that protects the first two elements and incorporates the electrical connections. The marked change in electrical resistance of a semiconductor in response to physical deformation forms the basis of the piezoresistive transducer. This comprises a thin silicon wafer forming the diaphragm, onto the peripheries of which are etched semiconductor resistors. Deformation of the diaphragm alters the electrical resistance of the resistors, which are placed in two pairs, such that one pair is subjected to radial stress as the other pair is stressed tangentially (Figure 2). Circuitry required for amplification, temperature compensation and calibration may also be included on the same silicon wafer using integrated circuit technology. Piezoresistive strain gauges are in widespread clinical use because they are small, cheap to manufacture, resistant to mechanical vibration, and possess high sensitivity (a large change in resistance in response to a small diaphragmatic distortion). Despite electronic compensation, piezoresistive strain gauges are susceptible to the effects of temperature, and are accurate only over a comparatively narrow temperature range.
Stray capacitance: capacitors are electrical components consisting of two parallel plates separated by an insulator (which can be air). They store charge, block the flow of DC, but allow AC to pass. They can be considered as frequency-dependent resistances, in which the resistance deceases as the frequency increases. Capacitors can be ‘natural’ in that the two ‘plates’ can be, for example, the passive electrode and the ground of the theatre (which can be indirectly connected to earth). This ‘capacitor’ will have a low capacitance value and pass minimal currents at domestic mains frequencies. However, it will present a much lower resistance at the higher diathermy frequencies and so other stray circuits can be formed, which can result in burns. For example, if there is a poor connection between the passive electrode and the patient, and a sharp earthed object touches the patient (Figure 3), then an alternate current path could be formed for the return current. The current in this case flows from A in Figure 3 to the active electrode, into the patient, then some of current leaves by the earthed object and flows to earth. To complete the circuit back to B, the current passes from earth to the passive electrode via the stray capacitance. Burns can arise at the connection between the sharp object and the patient, but can also arise at parts of higher current density where only certain parts of the passive electrode are touching the patient. The adverse burns can be minimized by ensuring that the passive electrode is securely and completely attached to the patient (so that the correct circuit is the most ‘attractive’ to the current). Also ensure that no other objects are touching the patient when the diathermy is on. Alarms are installed in the equipment (Figure 1) to warn users that the device is active, and to indicate faulty electrode connections, and stray leakage currents. The stray currents get worse with increasing frequency and, to minimize this effect, modern isolated systems are designed to operate at the lower frequency range of diathermy machines (500 kHz). Older machines used to operate at frequencies up to 2 MHz. Interference with other equipment Surgical diathermy uses radiowaves, with frequencies around the top end of the medium wave AM band. The equipment can therefore act as a short-range transmitter, and sensitive monitoring equipment, such as ECG monitors, can be a receiver for these signals. Ideally, the ECG electrodes and leads should be placed as far from the diathermy electrodes as possible, and care must be taken with interpretation when diathermy is in use. Pacemakers can be interfered with by a similar method, and sometimes the interference can result in the pacemaker wrongly detecting a fast heart rate. In these cases, the pacemaker should be put into a fixed-rate mode.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 5:11
Mark R Stokerr is Consultant Anaesthetist at Peterborough Hospitals NHS Trust. He qualified from Oxford University and trained in anaesthesia in the Oxford and Anglia Regions. His interests include teaching and training, obstetric anaesthesia and public education.
371
© 2004 The Medicine Publishing Company Ltd