Chapter 9: Arrhythmias and Antiarrhythmic Drugs

Learning Objectives

Introduction

The electrical activity of the heart can be affected by disrupting the conduction system, which can lead to rhythm disorders. The cardiac arrhythmias are classified into either bradyarrhythmias or tachyarrhythmias. Bradycardia is defined as a heart rate less than 60 beats per minute (bpm), whereas tachycardia is defined as a heart rate greater than 100 bpm. The former includes sinus bradycardia and various degrees of atrioventricular (AV) blocks. Tachyarrhythmias include both atrial and ventricular tachycardias.

There are many ways in which tachyarrhythmias can be classified. These can be based on the regularity of the rhythm, the site of origin of the rhythm, the duration of the QRS complex, and the mechanisms involved in the arrhythmia. Depending on the site of origin, the tachyarrhythmias can be classified into:

1. Atrial tachyarrhythmias

2. Ventricular tachyarrhythmias

Depending on the duration of the QRS complex, the tachyarrhythmias can be classified into:

1. Narrow QRS complex tachycardia (QRS ≤120 ms)

2. Wide QRS complex tachycardia (QRS >120 ms)

Bradyarrhythmias are treated by reversing the cause of the bradycardia or implanting a pacemaker. Tachyarrhythmias are also treated by reversing the cause if possible, or by using medications such as antiarrhythmics or ablations.

Normal Heart Rate

The normal heartbeat originates from the sinus node, also called the natural pacemaker of the heart. The normal heartbeat rate ranges from 60 to 100 bpm. The initial impulse is generated from the sinus node, and is conducted down to the AV node, which is effectively a “bridge” between the signals from the atrium to the ventricles. The impulse then travels from the AV node to the His-Purkinje system and separates into the left bundle (which activates the left ventricle) and the right bundle (which activates the right ventricle).

Normal Sinus Rhythm

The normal heart generates an impulse from the sinus node, which is regular, that is, the time interval between 2 beats (the R-R interval) is constant. Sinus arrhythmia is also a normal impulse from the sinus node but is slightly variable due to the vagal tone. The electrocardiography (ECG) findings in sinus rhythms or sinus arrhythmias consist of P wave morphology, in general, it is upright in leads 2/3 and aVF and biphasic in lead V₁.

Sinus Arrhythmia

The physiological variation in sinus rhythm in response to phases of the respiratory cycle can be observed in sinus ...