The ECG shows leads I, II and III recorded from an asymptomatic 30 year old man.
Rhythm: The ECO shows sinus rhythm. Normal P waves are present in front of every QRS complex and every P wave is conducted. The PR interval is constant and of normal duration (between .12 and .21 seconds). P wave and QRS complex morphology is constant in any one lead. However, the P wave rate varies in a cyclical fashion giving rise to an identical cyclical variation in QRS rate.
Sinus arrhythmia is present. This arrhythmia is usually a normal finding, especially common in children and young adults. The cause is a cyclical variation in the vagal tone occurring during respiration. Vagal tone is reduced during inspiration (with acceleration of SA node discharge). The arrhythmia may be abolished at faster heart rates, with exercise for example. During the slow phase of the rhythm a junctional escape rhythm may sometime occur and trigger ventricular depolarization until sinus rhythm returns as the SA node accelerates.