With unipolar diathermy, electric current flows through the patient and out via a large conductive pad placed on a nearby limb. The large surface area ensures that current density remains low, preventing burns at this site.
Chronaxie is the minimum time required for a constant electric current, which is double the threshold strength needed to stimulate a specific muscle or neuron. It is directly related to the density of voltage-gated sodium channels within the tissue, as these channels are responsible for phase 0 of the action potential and therefore the excitability of the cell. The myocardium has a chronaxie of three milliseconds. The high frequency of diathermy ensures that it does not exert a prolonged constant current, making it unlikely to trigger ventricular fibrillation.
Bipolar diathermy uses two closely positioned electrodes, such as the two tips of forceps. Current flows from one electrode through nearby tissues to the other electrode, completing the circuit. Bipolar diathermy is used in neurosurgery or eye surgery where delicate control is needed, or where current flow through the body may be harmful, for example when pacemakers are in situ.
Applied science
What risks are associated with diathermy use?
Diathermy uses an electrical current; all dangers associated with electrical devices are applicable. To prevent mains electricity from causing shock, diathermy uses an isolating capacitor, which permits high-frequency currents, but resists low-frequency AC.
Specific risks include burns, and monitoring and pacemaker interference.
Improperly positioned conducting pads or inadvertent patient contact with an earthed object may result in high current density at the contact site, causing burns. Small cross-sectional areas such as digits or metallic implants may cause channelling of the current, which can result in heating and burns. Operator error can also unintentionally damage surrounding structures, causing injury.
Arcing causes fires and explosions of flammable or combustible material. This occurs when air between the electrode and the skin becomes ionised by the strong charge at the electrode, and is able to carry current across the gap.
Monitoring, such as the electrocardiogram (ECG), detects and analyses subtle changes in skin potentials. Diathermy creates significant electrical noise, interfering with this equipment. Insulated wires, bandwidth filters and post-processing software can reduce the amount of interference.
Implantable pacemakers may interpret diathermy noise as cardiac electrical activity and be falsely inhibited. Implantable cardioverter defibrillators (ICD) may interpret noise as ventricular fibrillation, triggering an unnecessary cardioversion. Current flow through pacemaker/ICD electrodes inserted into the myocardium can cause burns at the contact point. This not only scars the myocardium, but can also render the device ineffective. Avoidance of diathermy is recommended in patients with these devices, although where this is not feasible bipolar diathermy is safer and preferred over unipolar.
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