What is Sinus Rhythm
Sinus rhythm is a cardiac rhythm that originates in the sinoatrial node, representing the normal electrical activation pattern of the heart. 1
Definition and Origin
Sinus rhythm is defined as an electrocardiographic finding of a cardiac rhythm that originates in the sinoatrial node. 1 The sinoatrial node is a specialized pacemaker structure located at the junction of the superior vena cava and right atrium, specifically in the intercaval region bordering the crista terminalis. 2
Electrocardiographic Characteristics
In normal sinus rhythm, specific ECG features must be present: 1
- P waves are positive in leads I, II, and aVF, and negative in lead aVR 1
- The P wave axis in the frontal plane lies between 0 and 90 degrees 1
- In the horizontal plane, P waves are directed anteriorly and slightly leftward, therefore may be negative in leads V1 and V2 but positive in leads V3 to V6 1
- P waves have a normal contour, though amplitude may vary 1
Heart Rate Parameters
The normally innervated sinoatrial node generates impulses at approximately 60 to 90 beats per minute and responds to autonomic influences. 1 When the rate exceeds 100 bpm, it is classified as sinus tachycardia rather than normal sinus rhythm. 1
Physiological Regulation
The sinoatrial node's spontaneous activity results from specialized pacemaker cells with unique electrophysiological properties that promote automaticity. 3 The rhythmic impulse initiation occurs through spontaneous diastolic depolarization involving multiple ionic currents including the hyperpolarization-activated "pacemaker" current (If), calcium currents (L-type and T-type), delayed potassium current, and sodium-calcium exchange current. 4
Autonomic regulation significantly modulates sinus rhythm: 4
- Vagal (parasympathetic) tone causes hyperpolarization and slows the spontaneous diastolic depolarization, decreasing heart rate 4
- Sympathetic stimulation via catecholamines increases diastolic depolarization speed, resulting in sinus tachycardia 4
- At rest, observed heart rate is typically lower than the intrinsic sinoatrial node frequency due to predominant vagal tone 4
Clinical Significance
Sinus rhythm represents optimal cardiac function for exercise capacity and quality of life compared to arrhythmias. 4 The sinoatrial node's complex three-dimensional intramural structure and conduction pathways are essential for maintaining synchronized cardiac pacemaking and proper source-sink electrical balance. 3, 2
Common pitfall: Surface multielectrode mapping during clinical procedures may misidentify intramural sinoatrial node pacemaker sites as epicardial or endocardial exit sites, as these exit sites can be distributed up to 50 mm along the crista terminalis beyond the approximately 20-mm-long anatomic sinoatrial node structure. 2