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Review
. 2023 Mar;23(3):84-91.
doi: 10.1016/j.bjae.2022.11.006. Epub 2023 Jan 4.

Electrical safety, defibrillation and diathermy

M A Tooley  1   2
Affiliations
Review

Electrical safety, defibrillation and diathermy

M A Tooley. BJA Educ. 2023 Mar.
No abstract available

Keywords: defibrillation devices; diathermy; electrical safety.

PubMed Disclaimer

Conflict of interest statement

The author declares that he has no conflicts of interest.

Figures

Fig 1
Fig 1
Diagram of the domestic mains supply, supplying class 1 equipment. The equipment has a load of 230 Ω, and so the normal current would be 1 A. The case of the equipment is earthed. The leakage current by capacitance is shown by C. See text for details. (Reproduced with permission from Magee and Tooley, The Physics, Clinical Measurement and Equipment of Anaesthetic Practice.)
Fig 2
Fig 2
Diagram of a class 1 equipment which supplies a patient connected part via an isolation transformer. The mains transformer core and one side of the secondary is earthed for safety. (Reproduced with permission from Magee and Tooley, The Physics, Clinical Measurement and Equipment of Anaesthetic Practice.)
Fig 3
Fig 3
(A) A situation in which there is a dual fault. The connection from earth to the metal case is broken and there is a fault resistance to the case caused by an internal fault. There will be current going to the earth through the person. The magnitude of the current will be dependent on the person resistance and the person-earth resistance. R is the fault resistance. (B) A situation in which there is an isolation transformer present. In this case, the dual fault situation described in (A) does not give a complete current path owing to the transformer, and no current flows through the person. (Reproduced with permission from Magee and Tooley, The Physics, Clinical Measurement and Equipment of Anaesthetic Practice.)
Fig 4
Fig 4
An example of a sensitive circuit breaker. C in the diagram represents circuit components which give a voltage output proportional to the current flowing through them. These outputs, one from the live line, the other from the neutral line, are supplied to a comparator circuit. This circuit compares the two inputs and gives an output if there are different by a desired amount. This output will operate an electronic switch and break the circuit. (Reproduced with permission from Magee and Tooley, The Physics, Clinical Measurement and Equipment of Anaesthetic Practice.)
Fig 5
Fig 5
A typical monophasic defibrillator circuit. (A) The mains version. The mains voltage is transformed up to a very high voltage (e.g. 5000 V). This voltage is rectified by a diode, which only passes the positive part of the alternating cycle to the capacitor. When the switch is in the charge position, the capacitor is charged up to the supplied voltage. When the switch is passed over to discharge, it is discharged through the inductor and patient. (B) The battery version. This part replaces the 230 V AC supply shown in (A), but the transformer has a much higher step up ratio. (Reproduced with permission from Magee and Tooley, The Physics, Clinical Measurement and Equipment of Anaesthetic Practice.)
Fig 6
Fig 6
Critically damped waveform commonly used in the output circuits of monophasic defibrillators. The values shown are typical ones. (Reproduced with permission from Magee and Tooley, The Physics, Clinical Measurement and Equipment of Anaesthetic Practice.)
Fig 7
Fig 7
Underdamped biphasic waveform. (Reproduced with permission from Magee and Tooley, The Physics, Clinical Measurement and Equipment of Anaesthetic Practice.)
Fig 8
Fig 8
Diagram showing a biphasic waveform, the truncated exponential. (Reproduced with permission from Magee and Tooley, The Physics, Clinical Measurement and Equipment of Anaesthetic Practice.)
Fig 9
Fig 9
A typical truncated exponential biphasic defibrillator circuit. The capacitor is charged up as described in the monophasic system. When switch C is passed over to discharge, it is discharged through the patient. After a certain time, the timing circuits switch over the electronic switches A and B, so the polarity is reversed. (Reproduced with permission from Magee and Tooley, The Physics, Clinical Measurement and Equipment of Anaesthetic Practice.)
Fig 10
Fig 10
Set-up for an isolated monopolar surgical diathermy system, showing the electrodes. a and b denote active and passive, respectively (Figure adapted from Tooley.6).
Fig 11
Fig 11
Diathermy circuit demonstrating stray capacitance. a and b are outputs from the diathermy machine, and the patient has a poor contact with the passive electrode. An earthed object, such as a drip stand, is touching the patient. Stray capacitance is shown between the passive electrode and the theatre floor. (Figure adapted from Tooley.6).
See this image and copyright information in PMC

References

    1. Tooley MA. Electricity, magnetism and circuits. BJA Educ 2023; 23: 61–5 - PMC - PubMed
    1. Tooley MA. Electronics and biological signal processing. BJA Educ 2023; in press - PMC - PubMed
    1. Schuder J.C., Rahmoeller G.A., Stoeckle H. Transthoracic ventricular defibrillation with triangular and trapezoidal waveforms. Circ Res. 1966;19:689–694.
    1. Peleska B. Cardiac arrhythmias following condenser discharges and their dependence upon strength of current and phase of cardiac cycle. Circ Res. 1963;13:21–32. - PubMed
    1. Bardy G.H., Marchlinski F.E., Sharma A.D., et al. Multicenter comparison of truncated biphasic shocks and standard damped sine wave monophasic shocks for transthoracic ventricular defibrillation. Circulation. 1996;94:2507–2514. - PubMed

Further Reading

    1. Magee P., Tooley M. second edition. OUP; 2011. The physics, clinical measurement and equipment of anaesthetic Practice. For more in-depth information on this topic, please refer to chapters 6, 20 and 21 from.