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The heart is a complex myogenic organ composed of four chambers and a thick myocardium which has the ability to contract on its own without the intervention of the nervous system. This intrinsic rhythm is generated and controlled by the firing of action potentials of myocardial autorhythmic cells in the sinoatrial node (SAN) known as the pacemaker.

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The electrical conduction of the heart can be represented by an electrocardiogram (ECG), where each wave corresponds to different stages of the cardiac cycle, with reference to the heart anatomy in figure 3. 

To understand how an ECG is produced, The Einthoven’s Triangle provides us with different perspectives of the heart which is particularly helpful in distinguishing between depolarisation and repolarisation of the heart as similar waves of the ECG can represent either. Each of the three leads are composed of a negative and positive electrode; an upward deflection is seen when the current flows towards the positive electrode and a downward deflection is seen when the current flows towards the negative electrode. When the current is perpendicular to the axis of the electrode, no deflection is seen (Silverthron.D.U, 2016). The relationship between the three leads are shown in figure 1. Nowadays, a 12 lead ECG is used  (Philip R. Liebson, 2013).

In a normal ECG, there are a total of 5 clear waves in the order of P, Q, R, S, T which are separated by a P-R segment and a S-T segment. An ECG can also be divided into two sections: PR interval and QT interval. An ECG of one heartbeat is summarised in figure 2. 

Firstly, the cardiac cycle begins with atrial and ventricular diastole. This is where most of the blood from the atria flow into the ventricles with the force of gravity.

The first signal produced by the heart occurs during atrial depolarisation denoted by the P wave. The SAN located in the right atrium spreads a wave of electrical excitation across the Bachmann’s bundle to the walls of the atria causing it to contract, inducing atrial systole. The pressure in both atria increases greatly causing the tricuspid valves in the right atrium and bicuspid valves in the left atrium to open. Although the majority of the blood enters the ventricles during atrial diastole, the contraction of the atria forces the remaining 20% of blood volume (Shrestha, 2011) to fill the ventricles. Contraction of the right atrium fills the right ventricles with deoxygenated blood from the systematic circulatory system, while the left atrium receives oxygenated blood from the lungs via the pulmonary vein and fills the left ventricle.

During the P-R segment, the electrical impulse passes down to the atrioventricular node (AVN) via the internodal pathway. However, there is a slight delay of approximately 0.12s (Klabunde, 2008) in order to ensure that the remaining 20% of blood fills the ventricles during atrial systole.

Following the P wave is the QRS complex, composed of 3 separate waves. The Q wave signifies the depolarisation of the ventricular septum (Ashley, E.A., Niebauer, J. 2004) which is observed as a small downward deflection.

The ventricles enter isovolumic ventricular contraction which is the first stage of ventricular systole. The pressure in the atria drop and pressure builds up in the ventricles which causes the atrioventricular valves (tricuspid and biscupid valves) to shut preventing back-flow. This corresponds to the first heart beat sound S1. At the same time, the pressure in the ventricles isn’t great enough to open the semilunar valves, so they remain closed. During this phase, the blood volume in the ventricles stay constant. 

Ventricular depolarisation is illustrated by the significant positive deflection known as the R wave. The electrical impulse from the AVN reaches the Bundle of His down to the apex which bifurcates to the purkinje fibres along the walls of the ventricles. As the pressure in the ventricles exceed the  pressure in the arteries, the semi lunar valves (aortic and pulmonary valves) open. The ventricles enter systole which continues through to the S wave. It is also observed that the left ventricle has a thicker myocardium to produce larger forces of contraction to deliver blood around the body via the aorta. The spiral arrangement of the myocardium (Silverthron, 2016) and the wave of excitation propagating down the apex facilitates the ventricular contraction from the base upwards maximising ventricular ejection.

Simultaneously, the atria enters repolarisation during the QRS complex where blood starts to fill the atria, but the signal is masked by the ventricles depolarising. The AV valves are still shut during this phase.

While the ventricles continue to contract during the S-T segment, any stimulation to the myocardiocytes will not generate an action potential. This is because the cells are in their absolute refractory period which progresses into the relative refractory period during the T wave. The absolute refractory period is essential in preventing ventricular fibrillation, a type of cardiac arrhythmia.

The Q-T interval is a measurement of how long it takes for ventricular depolarisation and repolarisation to occur. This is an important marker for identifying patients with LQTS (Long QT Syndrome) which could be a result of a malfunctioning Na+ or K+ channels or side effects of medication (Silverthron, 2016). 

Lastly, the T wave represents ventricular repolarisation inducing ventricular diastole after the S2 is heard. As the blood leaves the ventricles, the pressure in the aorta and pulmonary artery surpasses the ventricular pressure. Consequently, the semi lunar valves shut and the chordae tendineae attached to the valves prevent them from inverting, thus preventing back-flow. The shutting of these valves can be heard as the second heart beat (S2). At this point, the ventricles are in isovolumic ventricular relaxation as both sets of AV valves and semilunar valves are closed. Both the T wave and P wave are similarly shaped but they represent different phases of the cardiac cycle, repolarisation and depolarisation respectively. 

In addition, an extra U wave is observed after the T wave. The U wave shows the repolarisation of purkinje fibres (Cadogan, 2017) although this is not usually noticed in an ECG trace due to its small amplitude.

The voltage on an ECG are usually displayed in millivolts (mV) on the y-axis and time on the x-axis in seconds. ECGs are often used to analyse a patient’s heart rhythm and identify heart conditions, for example: fibrillation, tachycardia, bradycardia, LQTS or a third-degree-block. 

The cycle repeats itself when the pressure in the atria rises and exceeds the pressure in the ventricles forcing the AV valves to open. The duration of one heartbeat is normally 0.80 seconds long (Shrestha, 2011) assuming a heart rate within the normal range.