![]() ![]() This cell architecture is referred to as a syncytium, which implies that the entire network of cells functions as one unit. As illustrated in Figure 2 all cardiac cells are connected, both electrically and mechanically, along their long axis. ![]() In contrast to skeletal muscle, cardiac cells display a branch-like morphology. The terms contractile myocardium, myocardium, or simply myocardial cell, refer to this cell type, and these terms are used interchangeably. Contractile myocardial cells carry out the actual contraction but are also capable of transmitting the action potential, albeit at a much lower speed than the conduction cells.These cells have virtually no contractile function. Conduction cells form the fiber networks that sprout into the myocardium and disseminate the action potential.Cell types in electrocardiologyįor the purpose of this discussion it is important to distinguish between two main types of cardiac cells: QRS complex), which has much stronger electrical potentials. Note that the ECG rarely shows atrial recovery (repolarization) since it coincides with ventricular depolarization (i.e. The T-wave reflects the recovery (repolarization) of the ventricles. Activation of the atria is reflected as the P-wave and activation of the ventricles results in the QRS complex. The action potential generates electrical currents that give rise to the classical ECG waveforms presented here. As the impulse spreads through the myocardium, it activates the cells which respond by contracting. The cardiac cycle starts when cells in the sinoatrial node discharge an action potential that spreads as an electrical impulse through the atria and – via the atrioventricular node – to the ventricles. Figure 1 illustrates the relevant components of the conduction system, the heart and the classical ECG waveforms. When the contractile myocardium receives the action potential, it is activated and contracts. These cells form bundles of fibers that act as electrical cords that spread the action potential rapidly and sequentially to contractile myocardium in the atria and the ventricles. To coordinate these two tasks, the heart has an electrical conduction system composed of specialized myocardial cells (henceforth referred to as conduction cells). Sequential activation implies that the atria are activated first and they fill the ventricles with adequate volumes of blood before ventricular contraction commences. Rapid activation is important in order to activate as much myocardium simultaneously as possible the more myocardium contracting at the same time the more efficient the pumping mechanism. To ensure effective cardiac pumping function, the atria and the ventricles must be activated rapidly and sequentially. When it does, treatment involves managing the underlying health condition, be it heart disease, high blood pressure, a congenital (present at birth) heart defect, or something else.Principles of cardiac electrophysiology and electrocardiography (ECG) A pacemaker is almost always required.Īnother type of conduction disorder, bundle branch block, occurs when a blockage in the right or left bundle branch causes one ventricle to contract slightly slower than the other.īundle branch block by itself often does not require treatment. If this happens, a person's pulse may dramatically slow, or there may be no pulse at all. Third degree heart block: The most severe degree of heart block, in which electrical signals completely fail to reach the ventricles.Depending on the underlying cause of this condition, a pacemaker may need to be considered. ![]() Second degree heart block: In second degree AV block, some of the electrical impulses from the atria reach the ventricles, but some are blocked and never reach the ventricles.First degree heart block: The least severe degree of heart block, in which the electrical signals are slowed but still reach your ventricles.There are three degrees of heart block severity: In most cases of heart block, the electrical signals are weakened but do not stop completely. This interference prevents the atria from telling the ventricles when to contract and pump blood. Heart block is a conduction disorder in which the heart's electrical signals are unable to move from the atria to the ventricles. ![]()
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