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Metal Exposure and Toxic Responses
Published in Stephen K. Hall, Joana Chakraborty, Randall J. Ruch, Chemical Exposure and Toxic Responses, 2020
Thallium is not an essential element in the body. Whether acute or chronic, the most characteristic features in thallotoxicosis are those involving the nervous system, skin, and cardiovascular tract. The usual patterns of damage in the peripheral nervous system are those of the “dying-back” type, with some involvement of the central nervous system. Hair loss generally occurs during thallium poisoning. Cutaneous effects may also include dry, scaly skin and impairment of nail growth often resulting in the appearance of Mee’s lines. Thallium poisoning may also produce a complex pattern of cardiovascular responses. Small doses of thallium have been reported to produce increases in heart rate and blood pressure but large doses may produce hypotension and bradycardia. Cardiovascular symptoms are often accompanied by retrosternal pain or electrocardiogram abnormalities such as flattening or inversion of the T-wave.
Measurement of Electrical Potentials and Magnetic Fields from the Body Surface
Published in Robert B. Northrop, Non-Invasive Instrumentation and Measurement in Medical Diagnosis, 2017
The duration of the S-T segment, SST, is from the end of the QRS complex to the onset of the T wave. The Q-T interval, IQT, is the time it takes for the ventricles to depolarize, contract, relax, and repolarize. There are several formulas to predict its normal range from other intervals in the ECG cycle, but in general, it should be shorter than 425 ms. The T wave occurs during ventricular muscle repolarization, and is normally positive, having a duration of from 100 to 250 ms. Its normal, mean, peak amplitude is 0.267 mV, with a minimum of 0 and a maximum of 0.8 mV. In some individuals, a small U wave is seen following the T wave. Its origin is uncertain (Rawlings 1991). Figure 4.29 illustrates one, normal, typical, ECG cycle of leads I, II, and III, V1, V3, and aVR. Note the inverted QRS spike in leads V1 and aVR (also seen in V2; not shown).
Electrophysiological Amplifier
Published in Mesut Sahin, Howard Fidel, Raquel Perez-Castillejos, Instrumentation Handbook for Biomedical Engineers, 2020
Mesut Sahin, Howard Fidel, Raquel Perez-Castillejos
The contraction of the heart starts with the firing of the SA node, which produces an action potential that spreads across the atria (also known as auricles) and into the AV node. The atria contract upon receiving the trigger signal from the SA node and push the blood into the ventricles through the cardiac valves. Then the AV node fires a pulse into the common bundle, the bundle branches, and ultimately, the Purkinje fibers that cause the contraction of the ventricles and the delivery of the blood outside of the heart into the bodily organs [9]. Cells in each one of the components of the cardiac conduction system fire at a different time and produce an action potential with a distinctive shape or signature (Figure 2.2). The waveform that we are used to seeing in ECGs is the combination of the action potentials produced by each one of the cells in the heart. The cardiac conduction system, despite its key role in the function of the heart, contributes only a small portion of the volume and the cells of the entire heart. By comparison, the muscle cells in the atria and the ventricles are much more numerous than the cells in the conduction system and as a result, the main factor for the recorded ECG signal is the firing of the muscle cells in atria and ventricles. The P wave in the ECG corresponds to the depolarization of the atria. The QRS complex, a component of the ECG waveform, arises from the depolarization of the ventricles; the repolarization of the atria also occurs during the time of the QRS complex, but the atrial signal is much weaker than that produced by the ventricles. The T wave is the result of the repolarization of the ventricles. Finally, the U waves are related to the repolarization of the Purkinje fibers although these waves are frequently not visible in ECGs. The ECG feature resulting from combining waves P through U is commonly referred to as electrical systole whereas the rest of the ECG between one U wave and the following P wave is known as electrical diastole.
Computational imaging of the cardiac activities using magnetocardiography
Published in Journal of Medical Engineering & Technology, 2019
The cardiac conduction starts from the sino atrial node, where the impulses spread across the muscles of the atrium causing the depolarisation of the atrial muscles and reach the atrioventricular node (AVN). A small pause near the AVN for the blood flow, the impulses run to the bundle of His, purkinje fibres, creating the depolarisation of the ventricles. After the contraction of the ventricular mass, the cells start repolarizing to its resting potentials. The sum of these activation waves reach the thorax as a functional waves characterising P, QRS and T [2]. P wave is due to the atrial depolarisation whereas QRS complex is activated due to contraction of ventricles. Finally, the repolarization of the ventricular muscles result in T-wave.