- WHY WE SHOULD KNOW? 1.Anesthesiologist may be blamed if burns occurs due to malposition of the plate 2. It can interfere with monitors e.g. ECG and pulseoximeters 3. It can disrupt pacemaker function in a patient, having it.
- Diathermy depends on the heat generated when a current pass through a tissue and is used to coagulate blood vessels and cut through tissues
- A high frequency current is necessary for this, as myocardium is sensitive to DC and low frequency AC [the usual mains frequency of 50 Hz] will precipitate VF. Very high frequencies have minimal tissue penetration and pass without harming the myocardium
- A 0.5 MHz alternating sinewave is used for cutting and a 1.0-1.5 MHz pulsed/ damped sinewave pattern is used for coagulation
- UNIPOLAR DIATHERMY & PROBLEMS: Here the forceps represent one electrode (small area, high current density and significant heat generation) and the diathermy plate ( indifferent electrode) over the patient represent the other electrode (large area, less heat). If the the plate is malpositioned, the current may pass through any point of metal contact *like ECG electrodes, metal poles of lithotomy, operation table etc, and may result in passage of high current density as the area of contact is small, resulting in a burn. So we should ensure that the plate is in close and proper contact with a large, highly perfused (will dissipate heat) area of skin (adhesive gels are useful). If we place it near to metal prosthesis (e.g. Hip), which has a low resistance than tissue, it will generate a high current density, resulting in burns. A unipolar diathermy can generate 150-400 Watts of energy.
- BIPOLAR DIATHERMY: Current passes between the two blades of the forceps; so requires no plate; safer in patients with pacemaker. But can generate only 40 Watts of energy. So efficacy is less and may be used for coagulation of small blood vessels
- OTHER PROBLEMS: Sometimes diathermies may cause ignition of skin preparation spirit. Newer diathermies dont have earthing; but if your machine is having earthing, an inappropriate earthing will result in current passing through other routes mentioned above*, resulting in burns.
- Cautious use of diathermy is required in patients with pacemakers:
Category: Anesthesia equipments
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VIVA SCENE: DIATHERMY AND THE ANESTHESIOLOGIST
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Damping
- Damping describes the resistance of a system to oscillation resulting from a change in the input. Damping is the result of frictional forces working in that system. So following a change in input there are several possible outcomes for the system:
- Perfect Response: any change in input would be instantly and accurately reflected in the output
- Under-damped – the output changes quickly in response to the step up in input, but it overshoots and then oscillates around the true value, before coming to rest at it. It will take some time before the true value is displayed and the peaks and troughs will over and underrepresent the true value. In a dynamic system, e.g. intra-arterial BP, the constantly changing input may result in wild fluctuations, rendering an under-damped system very inaccurate (although the MAP is still correct).
- Critically damped – the response and rise time of the system are longer than an under-damped response, but there is no significant overshoot and oscillations are minimal. ‘D’ is the damping factor and, by convention, in a critically damped system D = 1.
- Over-damped – defined as damping greater than critical. The output here could potentially change so slowly that it never reaches the true value. In a dynamic system, the response time may be too slow for the system to be useful.
- Optimally damped – in reality in clinical measurement systems, critical damping is not ideal and we are prepared to accept a few oscillations and some overshoot to achieve a faster response time. Hence, our systems are ‘optimally damped’ where 64% of the energy is removed from the system and D = 0.64. There is a 7% overshoot in this case.
- N.B: The ‘response time’ is the time taken for the output to reach 90% of its final reading. The ‘rise time’ is the time taken for the output to rise from 10 to 90% of its final reading.
- All instruments will possess damping that affects their dynamic response. This includes mechanical, hydraulic, pneumatic and electrical devices. In an electromechanical device such as a galvanometer there are mechanical moving parts such as the meter needle and bearings. Damping in these components arises from frictional effects on their movement. This may arise unintentionally or may be applied as part of the instrument design to control oscillation of the needle when it records a measurement. In a fluid (gas or liquid) operated device, damping occurs due to viscous forces that oppose the motion of the fluid. In an electrical system, damping is provided electronically by electrical resistance that opposes the passage of electrical currents.
- Damping is an important factor in the design of any system. In a measurement system it can lead to inaccuracy of the readings or display:
- Under-damping can result in oscillation and overestimation of the measurement.
- Over-damping can result in underestimation of the measurement.
- Critical damping is usually an optimum compromise resulting in the fastest steady-state reading for a particular system, with no overshoot or oscillation.
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#SCAVENGING IN #ANESTHESIA
Scavenging refers to the method of extracting waste gases from the breathing system and venting them to an area where they will not be directly inhaled by staff or other patients.
- Scavenging systems can be classified as open or closed.
- Open refers to the basic system of extracting the gas from its point of entry into the theatre
- Closed systems are more common and can be further subdivided into active and passive
- No conservation of volatile agent is possible with either the active or the passive systems; conservation must occur within the anaesthetic breathing system itself, by the use of a circle system and low-flow anaesthesia.
- The active and the passive both pass any waste gas to the atmosphere, polluting it to the same extent.
- In the passive scavenging system an exhaust port collects the waste gases from the expiratory valve of the breathing system or from the ventilator and the gases pass through the transfer system (which consists of 30 mm low-resistance tubing) to the outside of the building, preferably above roof level.
- If the theatre air is not recirculated, the waste gases can be piped to the exit port of the theatre ventilation system.
- In the passive system, the gases are pushed to the atmosphere solely by the expiratory power of the patient
- If the pathway to the atmosphere involves a vertical passage of gas, then the patient must overcome the atmospheric pressure required to push the gas over this distance; may be several floors of hospital! Means significant forces has to be overcome.
- The use of gases with higher density, like nitrous oxide, adverse atmospheric conditions like high winds etc, will further increase the forces required to expel waste gases; this can even affect the cardiopulmonary status of the patient.