Arrhythmia Overview - Mechanism of Bradyarrhythmia and Tachyarrhythmia

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Case description

Arrhythmias mean loss of rhythm and is used to describe an abnormal rhythm of the heart.
Cardiac arrhythmias are accelerated, slowed, or irregular heart rates caused by abnormalities in the electrical impulses of the myocardium.

The heart is made up of four chambers the right and left atrium and right and left ventricles. Contraction of the ventricle ejects blood out of the heart to the rest of our body. All this is possible because of the amazing coordination of the heart's conduction system which is responsible for the creation and propagation of an impulse to the atrium and ventricular myocardium.

The SA node is the dominant pacemakers and initiates an impulse and causes contraction of the atrial myocardium. As the atrium is depolarised a P wave is transcribed on the surface ECG. After atrial contraction the impulse reaches the AV node. This structure generates a slow action potential and so there is a slight delay though this structure. Once the action potential traverses the AV node, it activates the proximal portion of the bundle of His, a specialised conducting tissue that generates a fast action potential. The period of time from the end of the p wave to the beginning of the QRS is termed the PR interval and is where the activation of both the AV node and bundle of His occurs. Abnormalities of conduction in the AV node and bundle of His are manifest on ECG as first, second or third degree heart block.

After impulse transmission through the bundle of His, the impulse is conducted to the right and left bundle branches which generate Fast action potential. The left bundle splits into the left anterior and posterior fascicles. The impulse travels from the bundles, fascicles and purkinje network, generating a fast action potential and resulting in rapid activation and depolarization of left and right ventricular myocardium. The depolarization and contraction of the ventricular myocardium is represented by the QRS complex on the ECG.
The contraction of the myocardium will eject blood of the ventricles into the aorta and pulmonary circulation. Following the QRS complex the T wave represents ventricular repolarisation, which is the time when the ventricular myocardium relaxes. Atrial repolarisation is hidden within the dominant QRS complex on the ECG.

tags: arrhythmia Bradyarrhythmia tachyarrhythmia


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