Cardiac Conduction System and Atrial Flutter
Introduction
The heart is critical to the body as it pumps oxygen-rich blood that is necessary for normal functioning to other organs (Bugnitz & Bowman, 2016). The heart must keep pumping blood to prevent it from clotting as clots can cause severe conditions, including stroke. The cardiac cycle is controlled by the cardiac conduction system, which produces and directs electrical signals that enable this function (Laske et al., 2016). This system consists of the sinus or sinoatrial (SA) node, the atrioventricular (AV) node, the AV bundle, or bundle of HIS, and the bundle branches called Purkinje fibres.
Body
The upper wall of the right atrium contains the SA node, which is the heart’s anatomical pacemaker. The central nervous system activates the electrical signals that are sent from the sinus node throughout the atria until they reach the AV node found in the centre of the heart just above the right atrioventricular valve (Laske et al., 2016). The AV node regulates these impulses before they pass to the ventricles via the bundle of His to ensure that the atria fully contracts before the lower chambers are activated to pump blood out of the heart (Kennedy et al., 2016).
Understanding how a heartbeat is represented on an electrocardiogram (ECG) is crucial to deciphering the cardiac cycle. The elements of an ECG are the P wave, PQ segment, QRS complex, ST segment, and the T wave. In a normal ECG, during the p wave, the SA node releases electrical pulses causing the atria to contract (van Weerd & Christoffels, 2016). In the PQ segment, the AV node receives impulses from the SA node and begins transmission to the AV bundle (Kennedy et al., 2016). In the QRS complex, depolarization of the ventricles occurs in three stages. The Q wave depolarises the intraventicular septum, the R wave acts on the main ventricular mass, and the S wave represents the final depolarization phase at the base of the heart (van Weerd & Christoffels, 2016). In the ST segment, the ventricles contract and pump blood out of the heart, and during the T Wave, the ventricles repolarize and relax before the cycle repeats.
Tachycardia is any condition that causes a rapid heartbeat that cannot be explained by factors such as age, emotional state, or physical activity. Atrial flutter is a supraventricular tachycardia as it affects the upper chambers of the heart (Cosío, 2017). It is dangerous because it stops the atria from filling with enough blood before they contract, thus reducing blood flow to the rest of the body. Atrial flutter causes rapid contractions of up to 250-300 beats per minute (bpm) while the ventricles contract at a standard rate, thus creating an arrhythmia.
In case the sinus node fails at its pacemaking function, automaticity foci located in the atria send electrical signals as a backup (Cosio, 2017). During atrial flutter, an irritable automaticity focus beats at a rate of 250-300bpm, sending electrical impulses in circular patterns. When the signal is sent to the AV node, the ventricles regulate by reducing it by a half or fourth to between 75 – 150 bpm. Additionally, the AV node has a refractory period so that an amount of time must pass before it transmits another electrical impulse, which causes the ventricles to maintain a normal pace (Bugnitz & Bowman, 2016; Cosio, 2017). On an ECG, the heartbeat presents as regularly regular with equidistant intervals between R-waves and a series of sawtooth-like P-waves.
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