Conduction system of the heart.

The heart pumps blood round the body thanks to its finely tuned system of electrical conduction. This system begins with the sinoatrial node (SA node) which is often referred to as the ‘pacemaker’ of the heart. It consists of a small cluster of specialised cells, located in the right atrium, which are able to depolarise without input from external sources. When the SA node fires it causes atrial contraction. Following on from this, there is a slight delay (about 0.1 seconds) before the atrioventricular node (AV node) then fires, causing ventricular contraction. This delay is very important, allowing blood to empty from the atria into the ventricles first. Next, the electrical impulses travel through the bundle of His, which then divides into the right and left bundle branches, and then ending in the Purkinje fibres.

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The Electrocardiogram (ECG) is a commonly used test for measurement of how well the conduction system of the heart is working. A number of stickers are placed over the chest, arms and legs to pick up the electrical changes in the heart. The basic ECG waveform consists of five waves (labelled as P, Q, R, S and T). The P wave represents electrical impulses firing in the SA node and moving across the atria, the QRS complex shows the rapid spread of the impulse from AV node to bundle of His and to the Purkinje fibres (the electrical activation of the ventricles), and lastly the T wave is where there is relaxation of the ventricular muscle.

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The ECG can be used by health professionals to determine a lot of information about the structure and function of the heart. Some of the measurements of interest on an ECG include the time interval between waves, mainly the P-R interval, The QRS width and the Q-T interval.

The information obtained from an ECG can also be crucial in the context of clinical trials. Some drugs possess the adverse side effect of prolongation of the Q-T interval and it is necessary to test this in the development of new pharmaceutical agents. The FDA guidance suggests that it is an important safety check to carry out to ensure a new drug does not delay repolarisation in the heart, potentially leading to cardiac arrthymias such as Torsades de pointes. For more information about Thorough QT studies (TQT) you could visit some drugs have been removed from use due to the risk of Torsades de pointes which can cause symptoms such as palpitations, dizziness, fainting or even sudden cardiac death. Clearly TQT studies are a vital ingredient in the regulation of drug development!

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