McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 4 5 Antiarrhythmic agents

an ectopic focus, such as premature atrial contractions (PACs) or premature ventricular contractions (PVCs) , atrial flutter, atrial fibrillation (Box 45.1) or ventricular fibrillation; or by alterations in conduction through the muscle, such as heart blocks and bundle-branch blocks. Figure 45.2 displays an electrocardiogram (ECG) strip showing normal sinus rhythm; Figures 45.3 to 45.5 depict various arrhythmias.

cardiac cells generate an action potential internally to stimulate the cardiac muscle without other stimulation is known as automaticity. ■■ Electrolyte disturbances, decreases in the oxygen delivered to the cells, structural damage in the conduction pathway, drug effects, acidosis or the accumulation of waste products can trigger arrhythmias. ■■ Changes in the heart rate, uncoordinated heart muscle contractions or blocks that alter the movement of impulses through the system can disrupt heart rhythm. ■■ Arrhythmias change the mechanics of blood circulation (haemodynamics), which can interrupt delivery of blood to the brain, other tissues and the heart.

KEY POINTS

■■ Arrhythmias (also called dysrhythmias) are disruptions in the normal rate or rhythm of the heart. ■■ The cardiac conduction system determines the heart’s rate and rhythm. The property by which the

FIGURE 45.2  Normal sinus rhythm. Rhythm: regular. Rate: 60 to 100 beats/minute. P–R interval: 0.12 to 0.20 seconds. QRS: 0.06 to 0.10 seconds.

FIGURE 45.3  Atrial arrhythmias: premature atrial contractions (PACs). Rhythm: irregular due to the origination of a beat outside the normal conduction system (ectopic). Rate: normal sinus rate, except for PACs. P–R interval: P wave is abnormal, and interval may be slightly shortened in ectopic beat. QRS: normal. Atrial fibrillation. Rhythm: irregularly irregular. Rate: variable; usually rapid on initiation of rhythm; decreases when controlled by medication. P–R interval: no P waves are seen, replaced by an irregular wavy baseline. The atria are fibrillating because impulses are arising at a rate greater than 350 per minute. The ventricles respond when the atrioventricular (AV) node is stimulated to threshold and can receive the impulse. QRS: normal.