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The Physics, Clinical Measurement and Equipment of Anaesthetic Practice for the FRCA$
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Patrick Magee and Mark Tooley

Print publication date: 2011

Print ISBN-13: 9780199595150

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780199595150.001.0001

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PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 20 June 2021

Electrophysiology and Stimulation

Electrophysiology and Stimulation

Chapter:
Chapter 18 Electrophysiology and Stimulation
Source:
The Physics, Clinical Measurement and Equipment of Anaesthetic Practice for the FRCA
Author(s):

Patrick Magee

Mark Tooley

Publisher:
Oxford University Press
DOI:10.1093/oso/9780199595150.003.0022

This chapter covers the processing and application of electrical signals from the body, in particularly the electroencephalogram (EEG), the electrocardiogram (ECG), and the electromyogram (EMG). The EEG and ECG will be considered in their monitoring capacity. The EMG will be discussed along with simulation and neuromuscular blockade and monitoring. The electrocardiogram (ECG) is a surface reflection of the propagation of electrical depolarisation and repolarisation over the various contractile chambers of the heart. Depolarisation is the trigger for releasing the stored contractile energy in the cardiac muscle. Each chamber also produces electrical action and polarising recovery potentials associated with the mechanical contribution of the recovery. The ECG can be divided into two major components: one associated with the propagation of excitation and recovery of the atria; the other with these events occurring in the ventricles. Excitation of the atria gives rise to the P wave, after which the atrial contractions propel blood into the ventricles. An atrial recovery wave exists, but it is rarely seen, as it is obscured by ventricular excitation, which is signalled by the QRS wave. During the later part of the QRS wave, ventricular contraction commences. Recovery of the ventricles is preceded by the T wave. The ECG labels, i.e. PQRST, are shown in Figure 18.1. To localise the direction of excitation and recovery of the heart chambers (and also to estimate the extent of cardiac injury), a variety of electrode arrangements can be used. The electrodes (which are normally disposable silver–silver chloride as described in Chapter 5) are positioned on easily located anatomical landmarks such as the right arm (RA), the left arm (LA), and the left leg (LL), with the right leg usually providing the reference or common. The standard (1, II, III), augmented (aVR, aVL, aVF) and precordial (V) leads are routinely recorded by electrocardiographers. It is possible to locate the direction of excitation and recovery by considering that the direction of the event (excitation or recovery) is at right angles to the isoelectric lead (i.e. the lead with equal forces in the positive and negative). This can be demonstrated by forming an equilateral triangle (Einthoven’s triangle) such as in Figure 18.2(a).

Keywords:   auditory evoked responses (AER), cardiac vectors, coherent averaging, digital differentiation, electrocardiogram (ECG), electroencephalogram (EEG), evoked responses, nerve stimulators, neuromuscular function monitoring, postsynaptic potentials, somatosensory evoked responses, thalamic discharges, visual evoked responses

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