Editor's note  

This contribution is reproduced by kind permission of the authors and editors from the textbook Colonoscopy: Principles and practice, by Waye JD, Rex DK and Williams CB, Blackwell Publishing 2003. Although targetted at colonoscopy, it is a very valuable review of a complex topic that deserves wider readership.

 Introduction   

Hemostasis and the ablation of pathologic tissues are the most important indications for thermal techniques in colonoscopy. However, because the colon wall is thin, it is not the ideal organ for the application of thermal techniques. The thickness of the three layers of the colon wall, comprising the mucosa, submucosa, and muscularis propria, varies from 1.5 to 3 mm (Fig. 1) throughout the length of the large intestine. Following insufflation, the wall can be even thinner. Since damage to the muscularis propria of the colon should be avoided during endoscopic interventions, thermal injury must not extend beyond the submucosa in order to avoid complications. As a consequence, only about half of the 1.5–3.0 mm constituting the thin wall of the colon is accessible to the endoscopist for thermal interventions. The necessity that endoscopically applied thermal techniques do not damage the muscularis propria of the colon makes their application within the colon difficult, especially when the lesion to be treated is large.

The application of thermal techniques in the colon requires knowledge of thermal effects in biologic tissues. In addition, the endoscopist must have sufficient training and master the available endoscopes, instruments, and peripheral equipment. This article deals with the theoretical principles concerning the application of thermal techniques, especially in the colon.

 Relevant thermal effects in biological tissues   

All thermal effects in and on biological tissues—whether intentional or unintentional—depend on the intensity and duration of temperature in the tissue (Fig. 2) almost regardless of the way in which this temperature is reached.

Thermal treatment is among the oldest of therapeutic techniques. Although high-frequency (HF) surgery was introduced about 100 years ago and laser surgery about 30 years ago, the terminology uses words that are centuries old and described various types of cautery. As an example, coagulation is the only term in current use to describe thermal hemostasis, even though different thermal techniques can be used for this purpose. The term 'coagulation' actually encompasses many different tissue effects such as devitalization, coagulation and desiccation.

Thermal devitalization   

Thermal devitalization is defined as irreversible death of tissue. More precisely, devitalization of a target tissue means irreversible as well as complete death of tissue. Biologic tissue becomes devitalized if its temperature reaches 41.5°C. The higher the temperature, the faster the devitalization. Unfortunately devitalization is not a visible phenomenon and hence can occur in an uncontrolled fashion, and thus it is not used for destruction of pathologic tissue. Even if thermal devitalization is not employed intentionally, some degree of tissue death occurs outside the border of the coagulation zone. The depth of the invisible thermal devitalization zone depends on many different parameters, and it should be assumed for the sake of safety that it occurs in direct proportion to the visible coagulation effect.

Thermal coagulation   

Thermal coagulation is defined as conversion of colloidal systems from sol to gel state. Biologic tissue becomes coagulated thermally if its temperature increases to approximately 60°C. When this temperature is exceeded, the structure of the cell changes causing the following effects:

• change of the color of the tissue;
  
• formation of derivatives of collagen, e.g. glucose;
  
• contraction of collagen.
  

The change in color of the tissue is the only way to visually control intended as well as unintended coagulation. Unfortunately, color changes can only be seen on the surface but not within the tissue.

Even if thermal devitalization could be used for destruction of pathologic tissue, it is not used for this purpose because it is not controllable. Therefore the coagulation effect is used as a means of controlled devitalization. It should be noted that an invisible thermal devitalization zone of variable depth is unavoidable outside the border of the coagulation zone.

The formation of derivatives of collagen, e.g. glucose, can become adherent after desiccation.

The contraction of collagen can result in narrowing of the lumen of blood vessels and hence cause hemostasis. Even though the term 'coagulation' is used as a synonym for thermal hemostasis, thermal coagulation alone is only efficient for hemostasis of small vessels. Larger vessels (> 0.5 mm) must be compressed mechanically during thermal coagulation to achieve hemostasis.

Thermal desiccation   

Thermal desiccation is defined as heat-induced dehydration of tissue. If the temperature of tissue is equal to the boiling temperature of intra or extracellular water (c. 100°C), the desiccation effect can dehydrate the tissue quickly, depending on the density of power applied to the target tissue. Thermal desiccation can cause:

• contraction and shrinkage of tissue, by dehydration;
  
• an adhesive effect of glucose;
  
• a dry layer that acts to insulate tissue electrically.
  

Thermal desiccation causes significant contraction by drying and shrinkage of vessels, resulting in hemostasis of small vessels. Larger vessels (> 0.5 mm) must be mechanically compressed during thermal hemostasis.

Desiccation of glucose as a derivative of collagen results in a glue effect, which in turn causes sticking of desiccated tissue to coagulation electrodes, heater probes, the distal end of laser fibers, and also to polypectomy snares.

Desiccated tissue has a relatively high specific electric resistance. A layer of desiccated tissue functions like an electric isolating layer. This can cause a problem during polypectomy if the tissue adjacent to the snare becomes desiccated. When this occurs, there is no cutting effect and the snare can get stuck within the desiccated tissue of the polyp and cannot be moved forward or backward. During use of the argon plasma coagulator (APC) the desiccated electrically isolating layer automatically limits the maximum penetration depth of the thermal effect, described in more detail below.

Thermal carbonization   

Thermal carbonization is defined as partial oxidation of tissue hydrocarbon compounds if the temperature exceeds 200°C. Because the temperature of tissue containing water does not exceed approx. 100°C, only desiccated and relatively dry tissue can become heated above 200°C and carbonized. Dry tissue will achieve temperatures above 100°C only by an electric spark or laser.

If the temperature of desiccated tissue increases above 200°C in the presence of oxygen (room air), it becomes carbonized after desiccation. However if the target tissue is bathed by a noble gas such as argon, the tissue does not become carbonized.

Even though carbonization of tissue is not a goal in therapeutic colonoscopy, it is relevant during tissue vaporization by laser, because the absorption of light increases when the tissue becomes carbonized to a black color.

Thermal vaporization   

Thermal vaporization is defined as combustion of desiccated and carbonized tissue. Tissue becomes vaporized during or after desiccation and carbonization when the temperature increases to approximately 500°C and it is bathed in oxygen-containing gas, e.g. air. If the target tissue is within inert gas (e.g. CO₂) or noble gas (e.g. argon), the tissue does not become vaporized.

Thermal vaporization can be used directly for the ablation of pathologic tissues as well as indirectly for tissue cutting. In colonoscopy only laser, especially Nd:YAG laser, is used for tissue ablation by vaporization, and only high-frequency surgery is used for thermal cutting of tissue.

 Generation of temperature in thermal tissue   

Various energy forms, and their respective sources, applicators and application techniques are available for thermal intervention in the colon (Fig. 3). A description of these properties and their relevance for endoscopic applications in the colon follow.

The temperature of tissue can be increased either exogenously, e.g. by means of a heater probe, or endogenously, e.g. by means of electric current or laser; it can also be increased by a combination of both, as in high-frequency surgical cutting, where endogenous heat is caused by electric current and exogenous heat is caused by electric arcs between the active electrode and tissue. For thermal interventions in the colon it is important that the temperature required for an intended purpose is only delivered at the target tissue.

Unintentional thermal damage to adjacent tissues must be avoided. This stipulation is difficult to achieve since it is not possible to heat part of a tissue to a desired temperature without at the same time heating adjacent tissue. Although it is not possible to avoid heat transfer, it may be possible to keep thermal damage of adjacent tissues to a minimum. Where possible, the distance between the target tissue and deeper surrounding tissue can be increased for the purpose of limiting thermal damage by submucosal injection with physiological NaCl solution (Fig. 4).

Some coagulation effect to adjacent (deeper or surrounding) tissue can also be desired in some cases, especially during cutting of vascularized biologic tissue, such as during polypectomy. During polypectomy, the tissue becomes vaporized in front of a cutting electrode and heat spreads to the adjacent tissue (the cut edges) to promote hemostasis.

These aspects should be taken into account when choosing the primary energy form, its source, applicators, and application techniques.

As mentioned previously, in the colon the distances between the tissues which are the desired subject of thermal heating and those tissues which are not intended to be thermally damaged are very small; as a consequence, the diffusion of heat within the surrounding tissue also has to be taken into account. Heat flows from tissues with a higher temperature into tissues with a lower temperature (Fig. 5). This diffusion effect is not used for therapeutic purposes in colonoscopy, and is limited by heating the target tissue to the temperature required only for the short amount of time necessary for the intended purpose.

In order to avoid unintentional damage to the tissue adjacent to the target tissue, it is necessary to know the maximum depth of the tissue injury and how to control the effect produced by the various thermal techniques.

Heater probe   

Heater probes belong to the family of cautery instruments, which have a very long history. In principle, cautery instruments consist of a handle with a distal tip, which can be heated to a temperature appropriate to cause one of the specific thermal effects in biologic tissue. The heater probe consists of a catheter with a special heat-generating device built into the tip, which converts electric energy to heat energy [1,2]. The heat generated outside the tissue (exogenously) can be applied to a target tissue by touching it with the hot tip.

The temperature of modern heat probes for flexible endoscopy is adjustable and automatically controlled. Modern heat probes are provided with irrigation from a nozzle on the tip, which can be used to clear blood from the site to facilitate a clear view and accurate positioning. A special coating on the tip prevents it from sticking to desiccated tissue.

Because heat probes can be pressed against the target tissue during heat application, even bleeding from medium size vessels can be treated by simultaneously compressing and coagulating the vessel (Fig. 6 a). However, this should be done very carefully to avoid thermal damage to the muscularis propria (Fig. 6 b).

High-frequency surgery   

General principles of high-frequency electric devices   

High-frequency surgery (HF surgery) is a thermal technique where the required temperature is reached by conversion of electric energy into heat energy within the target tissue, i.e. endogenously.

High-frequency alternating current (HF current) with frequencies greater than 300 kHz (ICE 6001-2-2) is well suited for the heating of biologic tissues because it does not stimulate either nerves or muscles. The electric energy (E) in tissue caused by the HF current becomes converted (®) endogenously into heat energy (Q). The amount of heat energy (Q) measured in watt-seconds (Ws) which is produced in the tissue is a function (f) of the electric resistance (R) and the square of the averaged value (I²) and the effective duration (Δt) of the HF current (Iav).

E®Q=f(R, Iav², Δt) (Ws)

The temperature of a biologic material rises proportionally to the amount of heat and inversely proportionally to the specific heat capacity of the tissue in question.

As mentioned above, a requisite for the application of thermal techniques in the colon is that the temperature required for an intentioned purpose is reached and becomes effective only at the target tissue, and unintentional thermal damage to adjacent or lateral tissues must be avoided. In HF surgery, this objective is achieved via the current density (j) and the current flow duration (Δt) in the target tissue. The current density (j) is a function (f) of the amount of current (i) measured in amperes (Amp) which flows through a defined area (A) measured in square centimeters (cm²) at a certain point in time (t) or averaged over a defined time interval (Δt).

j = f(i/A) (A/cm²)

The partial amount of heat (q) generated endogenously through electric current either partially or at an arbitrary point within the tissue is proportional to the specific electric resistance (ρ), the square of the current density (j²), and the effective current flow duration (Δt) at this point of the tissue.

q = f(ρ, j², Δt) (Ws)

Conduction of an electric current through any material requires that both poles of the electric source be connected to the tissue (through the patient) in an electrically conductive manner. Two electrodes are necessary for this purpose. The electrodes at the target tissue are called active electrodes. The electrodes through which the electric current is conducted away from the tissue (the patient), back to the energy source, without any thermal damage at this electrode, are called neutral electrodes. Applications which use an active and a neutral electrode are called monopolar applications, and the instruments used for these applications are called monopolar instruments (Fig. 7 a). Applications which use both electrodes simultaneously as active electrodes are called bipolar applications, and the instruments used for these applications are called bipolar instruments. As a rule, both active electrodes of bipolar instruments are located close by on the same instrument (Fig. 7 b).

The density of current within the target tissue can be varied in proportion to the size and shape of the contact surfaces of the active electrodes of HF instruments. Most active electrodes used in flexible endoscopy are in the shape of a needle, loop, or ball electrode (Fig. 7 c).

Apart from the shape, the size of the contact surface plays an important role as regards the current density and its distribution both in the target tissue and in adjacent tissue. A smaller contact surface results in a steep reduction in the current density and in the temperature profiles in the tissue independent of the distance from the contact surface (Fig. 8).

HF current can flow through biological tissue only when the tissue contains water and electrolytes. As a consequence, the temperature of tissue containing water cannot rise above the boiling point of water (approx. 100°C). Tissues that contain less water and are drier, have a lower electric conductivity and less HF current can flow through this tissue. Completely dry biologic tissue is an electric insulator, hence no electric current can flow through it, and the temperature cannot rise (Fig. 2a). This fact is of importance during use of argon plasma coagulation.

Electric arcs   

Electric arcs are ignited between an active electrode and tissues when the peak value of the HF voltage is equal to or greater than 200 V, which is typical if the active electrode consists of metal and the tissue contains water (Fig. 2b). Since these electric arcs reach temperatures far above 300°C, they generate exogenous heat, which raises the temperature of tissue above 100°C, thus causing carbonization and vaporization of dry tissue as described above.

In colonoscopy, carbonization and vaporization of tissue caused by an electric arc is not only unnecessary, but also annoying, since it generates a certain amount of smoke which impedes visibility. Because the depth of heat penetration cannot be controlled during electric arcing, it is not used as a therapeutic tool in endoscopy.

Even if the vaporization effect caused by electric arcs is not directly used in colonoscopy, it is useful indirectly for HF surgical tissue resection.

 Principles of high-frequency surgical coagulation   

In general, the term 'coagulation' includes the effects of devitalization, coagulation, and desiccation. In colonoscopy HF surgical coagulation can be used for thermal devitalization of pathologic tissue and for hemostasis. Thermal devitalization of pathologic tissue is performed by argon plasma coagulation (APC) or laser and is described in more detail below. Thermal hemostasis can be used to stop spontaneous bleeding as well as to prevent iatrogenic bleeding, for example during resection of polyps.

The spectrum of indications for thermal hemostasis is very wide. Equally wide is the spectrum of the techniques and instruments available for hemostasis, some of which have been developed or designed especially for application in flexible endoscopy. Because the wall of the colon is relatively thin, thermal hemostasis applied directly on the colon wall is a compromise between efficiency and thermal wall damage.

The method and instrument of thermal hemostasis is dependent on the size of the vessels causing bleeding. In small vessels, hemostasis can be achieved by thermal coagulation or desiccation alone. Control of bleeding from larger vessels requires mechanical compression during heat application. This principle is also applicable for hemostasis during polypectomy.

Monopolar coagulation instruments   

In their most simple form, monopolar coagulation instruments for flexible endoscopy consist of a catheter at the distal end of which is an electrode, often ball-shaped. Because this electrode can be pushed against the target tissue, this instrument is useful for hemostasis not only of small but also of larger vessels. In the colon the risk of deep thermal wall damage has to be taken into consideration. During hemostasis, coagulated or desiccated tissue can stick to the electrode, so that the source of bleeding can be reopened when the electrode is pulled off the site. This problem was addressed by the development of the electro-hydro-thermo probe and by addition of an antisticking coating.

Electro-hydro-thermo probes   

Electro-hydro-thermo (EHT) probes for flexible endoscopy (Fig. 9) consist of a catheter with an electrode at the distal end (usually ball-shaped). On this electrode is a hole through which water or physiological NaCl solution can be instilled between the electrode and target tissue. When the electric current is applied the contact surface between electrode and tissue does not become dry and the electrode does not stick to the coagulated tissue [3,4]. The instillation of fluid can also be applied for the irrigation of bleeding sources. When applying EHT, the depth of the thermal effect cannot be well controlled. This problem has been addressed with the development of bipolar coagulation probes for flexible endoscopy.

Bipolar coagulation instruments   

In their most simple form bipolar coagulation instruments for flexible endoscopy consist of a catheter, at the distal end of which are at least two closely placed electrodes (Fig. 10). The HF current flows through the tissue only between these two electrodes. They can be applied either axially or laterally. The depth of the thermal effects which can be reached is relatively small, decreasing the risk of penetration; however, the efficacy is also limited, i.e. the instruments are useful only for small lesions. Bipolar instruments often have irrigation capacity and some have integrated injection needles [5,6].

 Principles of high-frequency surgical cutting with particular regard to polypectomy   

Biologic tissue can be incised electrosurgically when the HF voltage between an electrode and tissue is sufficiently high to produce electric arcs between the cutting electrode and the tissue; this concentrates the HF current at specific points of the tissue (Fig. 11a). The temperature produced at the interface where the electric arcs contact the tissue (like microscopic flashes of lightning) is so high that the tissue is immediately evaporated or burned away. As the active cutting electrode passes through the tissue, electric arcs are produced wherever the distance between the cutting electrode and the tissue is sufficiently small, producing an incision (Fig. 11b). As mentioned previously, a minimum peak voltage (Up) of 200 Vp is required in order to produce electric arcs between a metal electrode and biological tissue containing water. The intensity of the electric arcs increase in proportion to the peak voltage. Experience has shown that the depth of thermal coagulation along the cut edges increases with increasing peak voltage (Fig. 12).

In the system of HF surgical cutting, an increase of the voltage increases the electric power (P) by the square of the voltage (P = f(U²)), so it is necessary to modulate the amplitude of the voltage (turn it down) to compensate for the strong influence provided by the mathematical power of the square multiplier.

The higher the peak voltage (Up) and the degree of amplitude modulation, the deeper the thermal coagulation of the cut edges. If the voltage is not modulated and its peak value is low, the coagulation depth at the cut edges is minor or nil, it is called 'cut mode', and the HF current caused by this voltage is called 'cutting current.' If the voltage is strongly modulated and its peak value is high resulting in deep coagulation of the cut edges, it is called 'coagulation mode', and the HF current caused by this voltage is called 'coagulation current.' One reason for this confusing terminology is the fact that conventional HF surgical generators do not have the capacity for setting the output voltage, only the output power. Setting of the output power of HF generators is not the best option for polypectomy, but it is the standard at the present time.

In colonoscopy the depth of thermal coagulation and also the possibility of thermal devitalization outside the coagulation zone must be considered. It can be dangerous if the coagulation and/or devitalization occurs outside the desired zone of thermal devitalization. If deep thermal damage occurs, tissue histology may be interfered with. A useful aspect is that coagulation of the cut edge of the colon wall can cause hemostasis, which can be used advantageously. Hence, coagulation of the cut edges always is a compromise between these three aspects in colonoscopy.

Another problem with regard to the adjustability, reproducibility and constancy of the depth of coagulation common to all conventional HF surgical generators is the greater or lesser generator impedance Ri, making the HF output voltage Ua dependent on the HF output current Ia. The greater the generator impedance Ri, the more the HF output voltage Ua depends on the HF output current Ia. Conventional HF surgical generators have a generator impedance of between 200 and 1000 ohms.

The output voltage Ua, and hence also the intensity of the electric arcs and ultimately the depth of coagulation, vary considerably, since the load resistance Ra and current Ia vary from one cut to the next and also during each cutting process. During polypectomy for example the load resistance Ra, which is the electric resistance between a polypectomy snare and a polyp, depends among other things on the size of the polyp and increases during closing the snare because the contact between the snare and tissue becomes smaller and smaller.

Another special problem of HF surgical resection of polyps is that HF surgical cutting can be done with minor mechanical force, as long as the HF voltage between the polypectomy snare and the tissue to be cut is above 200 Vp. Because the speed of the snare while cutting through the polyp has a major influence on the degree of hemostasis of the cut edges, the speed should be appropriate to the size of the polyp's attachment as well as controlled. Control of closure speed can be very difficult or really impossible if there is mechanical friction between the polypectomy snare and catheter or between the slider and the slider bar of the handle of the instrument (Fig. 13).

Mechanical friction can cause uncontrolled speed of the snare and hence uncontrolled or insufficient hemostasis, especially if the snare zips through the polyp. Most of the mechanical force on the polypectomy snare is caused by closing the snare intentionally.

 Technical aspects of polypectomy   

Polypectomy is one of the most important applications of HF surgery in the colon [7–11] and hemostasis is one of the main problems with polyp resection. If the problem of bleeding caused by resection did not exist, it would be possible to resect polyps or adenoma in a purely mechanical fashion with a thin wire snare in the absence of heat. This would have the advantage that neither the resected specimen (with regard to the histology) nor the wall of the colon (with regard to the risk of perforation) would be thermally damaged. This is possible for tiny polyps, but the endoscopist must tread the path between application of sufficient heat for hemostasis and yet avoid deep thermal damage. For safe polypectomy, the endoscopy team should be familiar with the equipment available for polypectomy [13–15].

Polypectomy snares   

The ideal polypectomy snares should cut perfectly, and should not coagulate the cut edge of the polyp to permit adequate histologic examination. In addition, the ideal snare should coagulate the cut edge on the colon wall to guarantee safe hemostasis, should not coagulate through the muscularis propria, and can be applied easily and safely. Unfortunately this ideal polypectomy snare is not available, as a number of problems must be addressed. For a perfect cut and minor thermal coagulation of the cut edge on the polyp margin the snare wire should be as thin as possible. For effective coagulation of the cut edge on the wall of the colon the snare wire should be as thick as possible. For easy and safe application on all different polyps the snare should be both flexible as well as stiff and should assume the optimal size for small as well as big polyps. In reality, the available polypectomy snares offer only a compromise of all these features.

A special problem can be caused by the nose at the distal end of polypectomy snares. If this nose is too long, because it is out of endoscopic view, it can touch the mucosa behind the polyp without the operator's knowledge and cause inadvertent damage when electrically activated.

The polypectomy snare handle   

Polypectomy snare handles should be designed ergonomically for both male and female hands, and should have minor friction between the slider bar and the slider. This is important to provide even loop closure allowing a consistent cut quality and even coagulation.

Polypectomy snare catheters   

Polypectomy snare catheters should be flexible enough for passing through working channels of twisted and looped endoscopes and have sufficient stiffness to prevent shortening when removing large polyps.

 Safety aspects of high-frequency surgery   

HF surgery can cause unintended thermal effects outside the target tissue during monopolar applications [12]. This can happen in tissue directly adjacent to the target tissue or remote from the target tissue when the HF current density is higher outside the target tissue.

To prevent thermal damage to the patient's skin, the neutral electrode must be firmly in contact with the skin as recommended in the instruction manual of the specific HF surgery generator.

HF surgery can cause interference in other electronic devices, such as a pacemaker where it can cause reversion of synchronous to asynchronous pacing or possibly pacemaker inhibition.

During polypectomy, the head of a big or stalked polyp must not touch the colon wall because HF current can flow through this contact resulting in uncontrolled thermal effects (Fig. 14 a).

If an endoloop is used for preventing bleeding and is placed on the stalk of a polyp between the colon wall and where the polypectomy snare is placed, the HF current density in the smaller diameter compressed by the endoloop can be much higher compared with the HF current density at the polypectomy snare; this will cause the narrowest part to become heated (within the endoloop) instead of the tissue within the polypectomy snare (Fig. 14 b,c).

If metallic hemoclips are used for hemostasis, the snare must not touch the clips as HF current will be conducted through it.

 Argon plasma coagulation   

The principle of argon plasma coagulation   

The principle of argon plasma coagulation (APC) is relatively simple [16]. When an electrode (E) is placed at a distance (d) from the surface of a tissue (G) and a HF voltage (U_HF) is applied between the electrode and the tissue, the gas between the electrode and the tissue becomes ionized and hence electrically conductive when the electric field strength (U_HF/d) exceeds a critical level. If the gas between the electrode and the tissue is a noble gas (argon, helium, etc.), an electric field strength of about 500 V/mm is needed for ionization. Argon is preferred because of its relatively low cost. The ionized argon forms argon plasma beams between the electrode and the tissue, which can be visualized as small sparks that conduct the HF current to the tissue. An important advantage of argon in comparison to air is its inert character, which neither carbonizes nor vaporizes biologic tissue so that the thermal effects of APC are limited to the devitalization (zone 1), coagulation (zone 2), desiccation (zone 3), and shrinking of tissue (zone 4) as a result of coagulation and desiccation (Fig. 15).

A special aspect of APC is that the direction of the argon plasma beams follows the direction of the electric field between the electrode and the tissue. The electrically active beams are directed from the electrode to electrically conductive tissue closest to the electrode, regardless of whether the tissue is in front of or lateral to the electrode. As soon as the target tissue becomes desiccated and hence loses its electric conductivity, the beams automatically move from desiccated to non-desiccated tissue until a large area of the target tissue is desiccated. As a result of the loss of electric conductivity at a treated site, the depth of desiccation, coagulation, and devitalization is limited.

Equipment for argon plasma coagulation   

The argon source is an argon cylinder with a pressure-reducing valve (Fig. 16). For safety reasons, the argon source must have automatically controlled flow rates and limitation of the pressure. The HF current source must provide both sufficiently high HF voltage for the ionization of argon as well as sufficiently high HF current to generate adequate heat within the target tissue.

APC probes for flexible endoscopy basically consist of a non-conductive flexible tube (Fig. 17) through which argon flows. An electrode within the distal end is connected to the HF generator by a wire through the lumen of this tube. For safety reasons, the electrode is recessed from the distal end of the tube so that it cannot come into contact with tissue.

As shown in Fig. 18, the depth of coagulation depends on power setting and on application time. In addition, the application technique has a significant influence on the depth. Movement of the activated probe tip will result in a shallower depth of thermal effect than is produced by directing the tip at one point.

When the probe is held at one site for between about 3 and 10 s, the depth of thermal coagulation is up to about 2 mm. Above 10 s the depth increases slowly to its maximum of about 3–4 mm.

Touching the foot pedal activates the flow of argon gas and simultaneously starts the flow of electric current. The time that the foot pedal is depressed may not be the same as the activation time, which refers to the interval when the argon plasma sparks actually touch the target tissue. There may be no or intermittent sparks if the distance between the probe and the tissue is too great.

Figure 19 shows that the shape of an argon gas beam consists of a zone of laminar argon flow, a zone of divergent argon flow and a zone where the flow becomes turbulent. Argon plasma beams can only reach the target tissue when there is argon gas between the distal end of the APC probe and the target tissue. This is the case when the argon gas beam is directed to the target tissue as shown in Fig. 20 (a) (axial APC probe) and Fig. 20 (b) (lateral APC probe). APC probes can be used laterally as well, but the lumen must be filled with argon. The ignited spark will direct itself to the nearest grounded tissue (Figs 20 c,d). If the target tissue is not within the argon gas beam or within an argon-filled lumen the argon plasma beams will not ignite, or plasma beams of air will ignite instead. Air plasma beams do not look very different from argon plasma beams; however, air plasma beams can cause carbonization as well as vaporization of tissue and hence can cause deep damage and perforation of organs. Endoscopists typically use air plasma beams when using the tip of the snare wire to 'spark' a polypectomy site to stop bleeding or destroy residual polyp fragments. Air plasma beams, consisting of ionized air, only travel over extremely short distances, and are uncontrollable.

Safety aspects of argon plasma coagulation   

As in any monopolar electrosurgical procedure, the neutral electrode must be applied to the skin surface.

Because argon gas is insufflated into the colon during APC, extensive distension of the colon can occur. The distal end of the APC probe must never be pressed against the mucosa or perforation can occur. If the superficial mucosal layer is destroyed by the probe pressure against the colon wall, the flow of argon gas will create instantaneous submucosal emphysema.

 Laser   

Principle of Nd:YAG laser   

'Light Amplification by Stimulated Emission of Radiation (LASER)', first described theoretically by Albert Einstein in 1917, and put into practice by T.H. Maiman 5 years after Einstein's death, made possible the generation of electromagnetic radiation in the range of optical wavelengths (light) with an extremely high power density. For about 50 years, the high energy of the laser has been used in medicine. The endoscopic application of laser began in 1975 [54], and following the successful development of a flexible light conductor. Of all the different laser sources, only the argon laser (a = 488/515 nm, P_max= 20 W), and the Nd:YAG laser (a = 1064 nm, P_max= 100 W) can be used endoscopically because their wavelength can be conducted through thin flexible light guides without significant loss of energy.

Specific characteristics of Nd:YAG lasers in flexible endoscopy   

With an adjustable power of up to approx. 100 W, Nd: YAG laser sources are able to generate light with a wavelength of 1064 nm, which is located in the infrared range and is thus invisible to the human eye. With a light guide of 0.6 mm, this light can be guided through an instrumentation channel of a flexible endoscope. The laser light emanates from the light guide in an axial direction with a divergence of approx. 10 degrees.

The invisible Nd:YAG laser is combined with a 'pilot light' in the visible range in order to see where the beam is directed.

The thermal effects within the radiated tissue are primarily dependent on the density of absorption (W/mm²) in the tissue and the duration of effect (Δt). The density of absorption refers to the power of light (W) which is absorbed by the tissue per mm³. The density of absorption is dependent on several variables: the distance (x) of the distal end of the light guide from tissue (Fig. 21), the angle with which the light radiates onto the surface, and the absorption and reflection characteristics of the tissue. The parameters may change rapidly due to coagulation, desiccation, or even carbonization. The latter can cause a dramatic increase in absorption or a decrease in reflection, leading to intentional or unintentional vaporization of tissue or even a perforation of the colon.

For intentional vaporization of larger tumor masses, the distal end of the light guide has to be placed close to the target tissue and the power of the laser has to be set sufficiently high.

When using lasers for thermal hemostasis, the experience of the operator is most important; if the light guide is too far away the density of power could be too low for hemostasis, and if too close, the source of bleeding could be started by vaporization rather than stopped by coagulation (Fig. 22).

The introduction of APC into flexible endoscopy has reduced the need for lasers in endoscopy [57,60].

 Safety aspects of Nd:YAG laser in flexible endoscopy   

The laser can cause unintended thermal effects outside the target tissue during applications.

During Nd:YAG laser applications all persons including the patient must protect their eyes, even when the distal end of the laser fiber is within the colon, because the light guide can break outside the endoscope. Since Nd:YAG laser is invisible, a break of the laser fiber can damage the retina of unprotected eyes.

 Summary   

All the different thermal modalities described in this chapter have their special advantages and disadvantages. None of the methods or equipment can yet be regarded as ideal for all cases. A modern endoscopy facility should have adequate equipment to provide optimum treatment for each case as listed in Fig. 23. But endoscopists should not only have the equipment available, they should also be familiar with the physical background as well as with the advantages and disadvantages of all modalities which are available in the endoscopy suite. This, combined with practical skill and experience, and last but not least competent assistance, is the prerequisite for obtaining successful results.

 References  

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