Tachyarrhythmias (tachycardias) and bradyarrhythmias (bradycardias) are abnormalities in the origin, timing or sequence of cardiac depolarization that result in a heart rate of >100 and <60 beats/min, respectively. The former are much more common and may be supraventricular, in which case they arise in either the atria or the atrioventricular node (AVN), or are ventricular in origin (see Chapter 50). Important bradyarrhythmias are described in Chapter 12. Where appropriate, ECG leads that best illustrate the abnormalities associated with each arrhythmia are shown here and in Chapter 50.

Most supraventricular tachycardias (SVTs) are troublesome rather than life-threatening, although rarely sudden death can occur. Common symptoms include lightheadedness, palpitations and shortness of breath.

Supraventricular premature beats (Figure 49a) are caused by ectopic (i.e. originating from a site other than the SAN) impulses arising in the atria or AVN earlier in the cardiac cycle than would be expected from the normal heart rate. They are typically conducted to the ventricles to cause a premature beat, which is generally followed by a pause as the normal rhythm is reasserted. With an atrial ectopic site the P wave is abnormally shaped because it is not generated in the SAN, and it may be inverted or missing entirely if the ectopic site is in or near the AVN.

Atrial tachycardia heart rate (120–240 beats/min) is frequently caused by an ectopic pacemaker, and can arise in either atrium (e.g. often close to the pulmonary veins in the left atrium). Other atrial tachycardias are re-entrant in nature, frequently following surgery that involves incision into the atrium. The tachycardia may start and stop suddenly or gradually. As with atrial ectopics, the P wave is abnormally shaped (Figure 49b).

Atrial flutter results from re-entry in an atrium (usually the right), often with an area of slowed conduction near the orifice of the inferior vena cava and a circuit involving the whole atrium. The atrial rate is typically ∼300 beats/min. As shown in Figure 49c, the AVN is often able to conduct only every other atrial impulse (2:1 AV block) to the ventricles because it is still refractory from the previous impulse, so that the ventricular rate is typically ∼150 beats/min. Less commonly, 3:1 or 4:1 block can occur, leading to correspondingly slower rates of ventricular contraction. The ECG has a ‘sawtooth’ appearance due to the presence of rapid regular F waves

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