Diathermy and its safe use

Diathermy Diathermy is frequently used to coagulate a bleeding vessel or to cut tissues. Unipolar diathermy is commonly used. Components 1. Diathermy active electrode. 2. Patient's plate. 3. Diathermy case (where the frequency of the current used can be adjusted.) An isolating capacitor is situated between the patient's plate and earth.

Mechanism of action Heat is generated when a current passes through a resistor. The amount of heat generated (H) is proportional to the square of current (I2) divided by the area (A) (H=I2/A). A large amount of heat is produced at the_tip of the diathermy forceps because of its small size (high current density). Whereas at the site of the patient's plate, because of its large surface area, no heat or burning is produced (low current density).

Mechanism of action A high frequency current 500 000 -1 000 000 Hz is used. This high frequency current behaves differently from the standard 50 Hz current. It passes directly across the preccordium without causing ventricular fibrillation. High frequency currents have a low tissue penetration without exciting the contractile cells.

Mechanism of action The isolating capacitor has low impedance to a high frequency current, i.e. diathermy current. The capacitor has a high impedance to 50 Hz current Thus protecting the patient against electrical shock.

Mechanism of action Earth-tree circuit diathermy can be used. The patient, the tip of the diathermy forceps and the patient plate are not connected to earth. This reduces the risk of burns to the patient. This type of circuit is known as a floating patient circuit

Mechanism of action Bipolar diathermy does not require a patient plate. The current flows through one side of the forceps, through the patient and then back through the other side of the forceps. Usually low power can be achieved from a bipolar diathermy. Bipolar diathermy is frequently used during neurosurgery or ophthalmic surgery.

Problems in practice and safety features If the area of contact between the plate and patient is reduced, the patient is at risk of being burned at the site of the plate. If the plate is completely detached, current might flow through any point of contact between patient and earth, for example earthed ECG electrodes or temperature probes. Modern diathermy machines do not function with any of the above.

Problems in practice and safety features If the area of contact between the plate and patient is reduced, the patient is at risk of being burned at the site of the plate. If the plate is completely detached, current might flow through any point of contact between patient and earth, for example earthed ECG electrodes or temperature probes. Modern diathermy machines do not function with any of the above.

Problems in practice and safety features Electrical interference with other electrical monitoring devices. The use of electrical filters can solve this problem. Interference with the function of cardiac pacemakers. Damage to the electrical circuits or changes in the programming can occur. This is more of a hazard with cutting diathermy than with coagulation diathermy. Modern pacemakers are protected against diathermy.

Pacemaker interference Electromagnetic interference (EMI) 0 and 109 Hz (e.g. AC power supplies and electrocautery) microwaves with frequencies between 109 and 1011 Hz (including ultra high frequency radio waves and radar) can cause device interference. X-rays, gamma rays and infrared and ultraviolet light do not cause interference. EMI may enter the pacemaker or defibrillator by direct contact between the patient and the source, exposure to an electromagnetic field Device leads act as antennae.

Pacemaker interference Devices are generally protected by circuit shielding using titanium casing noise protection algorithms that filter out unwanted signals. Possible responses to external interference include inappropriate inhibition or triggering of a paced output asynchronous pacing reprogramming (usually into a backup mode) damage to device circuitry and triggering a defibrillator discharge. Asynchronous pacing and mode resetting are the most common outcomes of EMI and should be considered if pacing modes appear to change suddenly or intermittently on ECG monitors

Pacemaker interference In some devices EMI may also cause a pacing mode change. This is usually the ‘backup’ or ‘reset’ mode (often the same mode as the ‘battery depletion’ mode). Assessed using telemetric interrogation. The backup or reset mode is usually VVI or VOO.

Pacemaker interference The EMI generated by electrocautery that may affect the device is related to the distance and orientation of the current to the patient’s device and leads. Radio frequency signals may be interpreted as cardiac impulses, leading to inappropriate inhibition. Prolonged application of cautery can repeatedly trigger the NSP, resulting in asynchronous pacing, with function returning to normal when electrocautery is stopped. If electrocautery interference results in mode resetting, for example from DDD to VVI or VOO, AV synchrony will be lost and may result in haemodynamic embarrassment. Such resetting will persist even after electrocautery is stopped.

Pacemaker interference Implanted defibrillators may interpret electrocautery interference as ventricular fibrillation, resulting in an inappropriate shock. pacemaker circuitry damage resulting in output failure, activation of maximum rate response pacing pacing lead overheating and myocardial damage transient threshold alteration.