Electrical Coagulation
General
- The conversion of electric energy into heat creates tissue temperatures in the range of 60 to 100 °C during coagulation → Intracellular and extracellular fluid is evaporated, without the cell structures being destroyed.
- The heat generated is governed by Joule’s first law: Q (heat in joules) = I2 (current density) × R (resistance) × t (time)
- Tissue temperature at the active electrode tip is 1000°C, but 1 cm away from the tip it only reaches 38°C
- The effects of tissue coagulation:
- Protein molecules are denatured
- Tissue is shrunk
- Vessels are sealed
- The consequence of these processes: haemostasis
- Side effects
- Burns have also been reported from insulation failure at the active electrode or accidental contact between the active electrode and another conductor.
- Dispersive electrode contact monitoring pads have a dual foil design that allows measurement of skin impedance to ensure adequate skin contact. An audible alarm alerts staff to electrode failure.
- The isolated circuit reduces the risk of burns from contact with earthed objects, as the current must return to the generator to complete the circuit; however, burns can still occur due to capacitive coupling.
- Toxic smokes
- Diathermy smoke consists of 95% steam and 5% cellular debris, containing a variety of toxic mutagenic chemicals including hydrogen cyanide and benzene.
- Viruses and viable cancer cells can be transmitted in surgical smoke
- Diathermy machines now contain a surgical smoke evacuator with a 0.1 m filter, attached to the diathermy pencil < 2 cm from the site of smoke production.
- Interfere with both electroencephalography and electrocardiography monitoring electrodes.
- Interaction between EMI and cardiac pacemakers or implantable defibrillators.
- Bipolar is safer than monopolar, but can still cause EMI.
- Effects are unpredictable and include inappropriate pacing, damage to the device, inappropriate defibrillation, and myocardial heat damage.
- Modern pacemakers have a titanium shell and interference monitor to protect them from EMI.
- The use of a magnet to reset pacemakers to asynchronous continuous pacing is not predictable.
- Current advice suggests limiting the use of diathermy to short low power bursts and avoiding monopolar where possible.
- Placement of the dispersive electrode away from the device increases safety.
- Where appropriate the device should be checked and reprogrammed to monitoring mode prior to surgery.
Monopolar (Bovie) Coagulation
- Mechanism
- Electric current passes through the patient to a grounding pad.
- Possible transmission through electrically and thermally sensitive neural structures, this modality is not used directly on the brain or in proximity to named nerves (including cranial nerves) & nerve roots
- Indiscriminate mode of this cautery limits its use intradurally.
- Bovie loop electrodes can be effective with giant meningiomas, but they often char quickly, so their use is limited.
- The use of monopolar cautery must be avoided in patients with pacemakers because the electrical current of the cautery can interfere with pacemaker activity.
Bipolar Coagulation
- Mechanism
- Current passes only between the tips of the cautery device.
- Used for precise coagulation.
- When used directly on or near to the brain or nerves, the current setting is typically reduced from that employed for general use
- Usage
- Use the spring action of the bipolar forceps with sharp tips for gentle dissection of the arachnoid membranes.
- Gently avulse the thin arachnoid membranes among the frontotemporal opercula.
- Forceful blunt dissection of thick arachnoid membranes or adherent pial surfaces must be avoided and microscissors used instead.
- Can be used in place of multiple other tools, such as
- Microscissors
- Fine dissectors
- Fixed retractors.
- Avoid the unnecessary exchange of other instruments, therefore enhancing operative efficiency.
- “Closed” tip of forceps may also be used in place of fine dissectors to gently mobilize the Sylvian arteries.
- Fine tips of the forceps can grab the encasing arachnoid membranes of the cranial nerves and displace them away from the tumour capsule, without manipulating the sensitive nerves themselves.
- Avoid
- Charring at the tips of the forceps leads to ineffective coagulation. The forceps should not be completely approximated for smooth coagulation.
- Instead, a small amount of tissue should be left between the tips for the coagulation function to work effectively.
- High currents lead to charring and avulsion injury to the surrounding tissues with no hemostatic result.
- Irrigation at the tips can minimize charring and dissipation of heat.
- Do not carpet bomb
- Indiscriminate coagulation of the bleeding source along the cortex by the forceps. This method leads to further cortical injury and increased bleeding.
- Instead, controlled irrigation should be used to clearly pinpoint the exact source; correct application of the forceps along the axis of the source can provide haemostasis while protecting the neighbouring neurovascular structures from heat injury.
Laser
- Especially neodymium:yttrium-aluminum-garnet (Nd:YAG) laser
Thermal effects on biological tissue
Temperature | Effects |
37-40°C | None |
From ~40°C | Hyperthermia: initial tissue damage, edema formation, depending on the duration of application, the tissue can recover or die (devitalization) |
From ~60°C | Devitalization: destruction of the cells, shrinkage of the connective tissue through denaturation |
~100°C | Vaporization of the tissue fluid, depending on the speed of vaporization: 1. Tissue shrinkage through desiccation (drying out) or 2. Cutting due to mechanical tearing of the tissue |
From ~150°C | Carbonization |
From ~300°C | Vaporization: evaporation of the entire tissue |
Diathermy setting
Setting | Description |
Cutting | Using a pure continuous sine wave of low voltage rapidly produces high temperatures that vaporize tissue fluid causing cells to explode forming a gap in the tissues. The electrode need not be in contact with tissue, as an arc is formed producing a clean cut. With the coagulation setting on high power, cutting also occurs, but this produces a larger zone of greater thermal damage. |
Coagulation | Using an intermittent sine wave with a short ‘on’ time and a longer ‘off’ time (low duty cycle), and a higher voltage—the low rate of heat rise and produces a coagulation effect. |
Fulguration | Coagulation voltage is high enough to arc causing a zone of thermal damage around the vaporized tissue. |
Dessication | Requires the probe to be in contact with the tissue but using heat insufficient for cutting. This dries out the tissue and a coagulum is formed. |
Blend | It is also possible to use a blended waveform. A separate waveform with higher duty cycles giving more cut and less coagulation. |
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