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Pathophysiology

Normal Sinus Rhythm

The rhythm originates from the sinus node in normal sinus rhythm (NSR). The rhythm is often regular with constant P-P intervals. When the rhythm has some irregularity, it is known as sinus arrhythmia. Generally, adults’ normal heart rate ranges between 60 and 100 beats per minute. However, normal variations exist depending on the individual’s age and gender. A sinus rhythm with a rate above the normal range is called sinus tachycardia, and 1 below the normal range is called sinus bradycardia.

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In NSR, the P wave is less than 120 milliseconds long and less than 0.15 mV to 0.25 mV in height in lead II. The permissible maximum varies based on the lead. If a biphasic P wave exists in lead V1, the terminal component should be less than 40 milliseconds in duration and 0.10 mV in depth. The P wave should also have a normal axis (0 degrees to more than 90 degrees) and constant morphology. The normal axis is indicated by P waves that are:

There are some cases of NSR in which the P wave duration and morphology may be abnormal. This usually reflects atrial disease and/or an atrial electrical conduction defect. The normal PR interval ranges between 120 ms and 200 ms. It tends to be in the lower range as the heart rate increases due to rate-related shortening of action potentials. Conversely, slower heart rates tend to increase the PR interval towards the upper range. Nevertheless, the PR interval is independent of the presence or absence of sinus rhythm.

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Sinus Node Dysfunction

Sinus node dysfunction is often due to abnormal impulses produced by the pacemaker cells or abnormal conduction across the perinodal cells. It can be acquired or inherited; the acquired form is more common. Patients may or may not be symptomatic. There are several types and variations of sinus node dysfunction. Some of these include sinus pause, arrest, exit block, arrhythmia, and wandering atrial pacemaker (WAP). Because the mass of the sinus node is too small to create a significant electrical signal, it is not manifested directly on the ECG. Instead, SA nodal pacemaker activity must be inferred from the P waves of atrial depolarization. Hence, sinus node dysfunction is often noted with an inappropriate SA nodal response to the body’s metabolic demands and/or the absence of P waves.

Sinus Pause and Arrest

Sinus pause or arrest results when there is a problem with initiating electrical discharge from the SA node. As a result, the ECG shows a transient absence of sinus P waves. This can last for a few seconds or even several minutes. Because the sinus node stops firing and can start back up at any moment, there is often no relationship between previous P waves and those that follow (ie, non-compensatory). Also, the sinus pause or arrest tends to permit enough time for escape beats or rhythms to follow. A sinus pause of a few seconds is not always pathologic and may be seen in non-diseased hearts. However, if a sinus pause and arrest go on for longer, patients can become symptomatic, experiencing lightheadedness, dizziness, presyncope, syncope, and possibly death.[6][7]

SA Nodal Exit Block

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SA nodal exit block occurs when the sinus node fires, although the impulse cannot reach neighboring atrial tissue. It is believed to involve the perinodal (T) cells. Similar to sinus pause and arrest, the atria do not receive the proper signal to contract, and thus, the ECG shows an absence of P waves. There are 3 degrees of SA nodal exit block: first, second, and third degree. They follow the conventional atrioventricular (AV) nodal blocks. To conceptualize these, there are 3 components to keep in mind: 1) a relatively constant input from the SA node, 2) an area across which the block occurs, and 3) output (ie, the P waves). The SA nodal exit block type can be determined by evaluating the P waves.

Sinus Arrhythmia

Sinus arrhythmia represents small variations in the sinus cycle length. More precisely, it is defined as a variation in the P-P interval of 120 milliseconds or more in the presence of normal P waves or a change of at least 10% between the shortest and longest P-P intervals. P wave morphology remains relatively unchanged, but there can be small variations in the PR interval. Sinus arrhythmias are more commonly seen in young individuals and those exposed to morphine or digoxin. The 2 predominant types are a result of normal respiration and digoxin toxicity. Therefore, unless the patient has been receiving digoxin, patients are often asymptomatic and do not require treatment.

Wandering Atrial Pacemaker

WAP is not pathologic and is often seen in young, healthy individuals. It results from a change in the dominant pacemaker focus from the sinus node to ectopic atrial foci. At least 3 dominant ectopic atrial foci must meet the diagnostic criteria for WAP. This can be seen on ECG by a variation in P wave morphology and the PR interval. Each variation in P wave morphology represents a different ectopic focus. The closer the ectopic focus is to the AV node, the shorter the PR interval. Because WAP is not considered pathologic and is often asymptomatic, there is no indication for treatment.

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