How to Identify Sinus Rhythm
Sinus rhythm refers to the normal rhythmic beating of the heart, typically ranging between 60 and 100 beats per minute (bpm). The heart is composed of countless cardiac muscle cells, which are categorized into regular muscle tissue and specialized conducting tissue. The specialized conduction system includes the sinoatrial (SA) node, internodal tracts, atrioventricular (AV) node, His bundle, left and right bundle branches, and Purkinje fibers. These structures are responsible for generating and transmitting electrical impulses that regulate the coordinated contraction of the atria and ventricles. Under normal conditions, the SA node acts as the heart's natural pacemaker, initiating each heartbeat and maintaining a regular rhythm known as sinus rhythm.
Criteria for Identifying Sinus Rhythm
On an electrocardiogram (ECG), sinus rhythm can be identified by several key characteristics:
- Upright P waves in leads I, II, and aVF, with an inverted P wave in lead aVR.
- A PR interval ranging between 0.12 and 0.20 seconds, indicating normal conduction from the atria to the ventricles.
- A heart rate between 60 and 100 bpm.
- Consistent P-P intervals within the same lead, with a variation of less than 0.16 seconds.
Variants of Sinus Rhythm
1. Sinus Tachycardia
Sinus tachycardia occurs when the heart rate exceeds 100 bpm while maintaining a sinus rhythm. This condition is often a physiological response to stress, exercise, fever, or emotional excitement, where the body requires increased blood flow. The SA node accelerates its firing rate to meet these demands, resulting in a faster but still regular heartbeat.
2. Sinus Bradycardia
Sinus bradycardia is defined as a heart rate below 60 bpm with a regular sinus rhythm. It is commonly seen in athletes or individuals with high cardiovascular fitness, where the heart can efficiently pump blood at a slower rate. As long as there are no symptoms such as dizziness, fatigue, or shortness of breath, sinus bradycardia is considered a normal variant and does not require treatment.