The most common rhythm disturbance (which may not be abnormal), sinus tachycardia, is an acceleration of the sinus heart rate above 100 bpm [4]. In most cases, sinus tachycardia does not exceed 180 bpm. Sinus tachycardia is best diagnosed by identifying P waves, determining that they are of normal morphology, subsequently establishing that the PR interval is normal and constant, and determining that each QRS complex is preceded by the P wave and each P wave is followed by a normal QRS. In the course of normal daily activity, the heart rate rises in a gradual fashion and subsides in a gradual fashion [5]. Sinus tachycardia can occur during rapid eye movement (REM) sleep. Yet, patients suffering from REM sleep behavior disorder can have violent body movements without an increase in the heart rate due to the absence of autonomic arousal.

The opposite boundary of normal heart rate is sinus bradycardia. Sinus bradycardia is defined as a rate slower than 60 bpm [6]. Again, it is manifested by a normal P-wave appearance, a normal and constant PR interval, and a normal relationship of the P wave to the QRS complex, with a 1:1 sequence similar to that of sinus tachycardia. One observational pitfall in the patient with sinus bradycardia is the fact that, at times, U waves become very prominent and can easily be confused with P waves. As a result, blocked premature atrial contractions can be misdiagnosed. Use of β blockers can slow the heart rate as well as cause nightmares and sleep disruption.

Sinus arrhythmia is especially easy to notice with slowing of the heart rate during sleep. The P-wave morphology usually does not change. If it does change, the changes are phasic and the P-waves do not appear retrograde. There should be a 10 % difference between the maximum and minimum cardiac cycle length. Atrioventricular conduction is normal. This is manifested as a PR interval greater than 120 ms. A shorter PR interval with an abnormal P wave would indicate that the beats are not of sinus origin. The variations in sinus cycle length may be phasic, with respiration becoming shorter with inspiration due to reflex inhibition of vagal tone. This form of sinus arrhythmia disappears with apnea.

Premature atrial contractions are observed frequently in normal subjects and patients with a variety of diseases. They are manifested as an interruption in the heart rhythm with a premature beat having a narrow QRS complex. Because the origin of the atrial impulse is ectopic, the appearance of the P wave is abnormal, denoting its abnormal early origin. There is quite a wide spectrum in the incidence and frequency of premature atrial contractions. Their nature is classified as follows: If the premature atrial contractions occur singly, they are classified according to their incidence per period of time. Therefore, an ambulatory ECG report commonly describes how many premature atrial contractions were observed in a given time, such as an hour, a minute, or 24 h, according to how common they are. When premature atrial contractions are frequent, it is customary to further describe their nature (cyclic or noncyclic) and rate. For example, when premature atrial contractions occur cyclically, they may show a bigeminal pattern.

A variant of frequent premature atrial contractions is tachycardia, which is called multiform atrial tachycardia or chaotic atrial tachycardia [7]. This is a rhythm disturbance with definite clinical significance. It is identified by an irregular heart rhythm with narrow QRS complexes and rates in excess of 100 bpm. As the name implies, it is multifocal: The atrial beats originate in multiple sites in the atria. Consequently, the appearance of the P waves varies with the point of origin. There is variability in both the P-wave morphology and the PR interval. Multifocal atrial tachycardia is an arrhythmia that may have significant consequences. It is particularly common in patients with significant lung disease. These same patients often suffer sleep disorders. When analyzing ECG recordings, multifocal atrial tachycardia should not be confused with atrial fibrillation.

Atrial fibrillation is a very common rhythm disturbance that is important to diagnose, as the initial heart rate can be quite fast and drug therapy may be required to slow it down. Patients with chronic atrial fibrillation are at increased risk for thromboembolic phenomena and are therefore often admitted to the hospital for further management when this rhythm is diagnosed [8]. Management may be either rate control or attempted cardio version. Both approaches are typically accompanied by chronic antithrombotic therapy. The ECG hallmark of atrial fibrillation is a completely random and irregular heart rhythm with no reproducible R–R interval. Because the atria are fibrillating at a rate of 500 bpm, there are no P waves. The ECG baseline may appear irregular and erratic. This should not be confused with the variable P waves of chaotic atrial tachycardia or with U waves, as mentioned earlier. The ventricular rate in patients with atrial fibrillation tends to be fast when it first occurs. The rate may range around 150 bpm. A clue to underlying conduction system disease is a slow ventricular rate. In this case, caution needs to be exercised with therapeutic modalities, because of possible undesirable AV conduction problems [9, 10].

A variant of atrial fibrillation is a rhythm disturbance known as atrial flutter [11]. Atrial flutter differs in that atrial activity can be diagnosed as occurring 300 times per minute. At this rate, the ECG hallmark is a characteristic sawtooth pattern at a rate of 300 bpm. The usual presentation of atrial flutter is an atrial rate of 300 bpm with some degree of block between the atria and ventricles (the usual block is 2:1). Therefore, it is quite typical to recognize atrial flutter by the presence of a sawtooth baseline with a ventricular response of 150 bpm. The therapeutic goal in atrial flutter (similar to atrial fibrillation) is to slow down the ventricular response when it is fast. Again, caution is exercised when the initial ventricular response (with no medication) is an unduly slow rate with a conduction block of 4:1 or greater. Patients may benefit from radiofrequency catheter ablation.

The rhythm disturbances referred to earlier are classified as automatic rhythm disturbances. If properly diagnosed, they can be classified as disorders of cardiac automaticity. The warm-up phenomenon (gradual, nonabrupt increase in heart rate) is a hallmark of automatic tachycardia. Usually, an automatic tachycardia requires a search for its cause, which is then treated. For example, multifocal atrial tachycardia is typically seen in patients with lung disease, and improvement of hypoxemia often results in the return of the cardiac rhythm to normal. Sinus tachycardia frequently indicates a metabolic disturbance such as fever, thyrotoxicosis, or hypovolemia. Again, therapy of the cause is the proper approach rather than addressing the mechanism of the rhythm disturbance itself [12, 13]. Conversely, a group of tachycardias referred to as re–entrant are treated by addressing the mechanism of reentry. When this is corrected, the rhythm is restored to normal.