ECG tutorial: Rhythms and arrhythmias of the sinus node
- Jordan M Prutkin, MD, MHS, FHRS
Jordan M Prutkin, MD, MHS, FHRS
- Associate Professor of Medicine, Division of Cardiology, Electrophysiology Section
- University of Washington
Sinus rhythm is present when the dominant pacemaker controlling impulse generation is the sinus node (waveform 1 and waveform 2). In this setting, activation of the atria is from right to left, superior to inferior, and anterior to posterior. (See "Normal sinus rhythm and sinus arrhythmia".)
Consequent to this activation pattern, the normal P wave in sinus rhythm may appear slightly notched since activation of the right atrium precedes that of the left atrium. The normal P wave is always positive (upright) in lead II and negative in aVR, usually positive in lead I, and it may be positive, negative, or biphasic in lead III. It is of variable polarity in lead aVL. In the precordial (chest) leads, V1 and V2, there is often a terminal negative component of the P wave, reflecting the posterior location (with respect to the right atrium) and later activation of the left atrium. The P wave is typically positive in the remaining precordial leads.
In normal sinus rhythm with 1:1 atrioventricular conduction, a P wave with a uniform morphology precedes each QRS complex. The rate is between 60 and 100 beats per minute and the cycle length is fairly uniform between sequential P waves and QRS complexes. In addition, the P wave morphology and PR intervals appear identical from beat to beat. It is possible to have normal sinus rhythm with abnormal AV conduction also, such as with second degree or third degree AV block. In these situations, the P wave is regular with a constant morphology, but there is either a pattern to the PR interval with intermittent dropped beats (second degree AV block) or no relationship at all between P waves and QRS complexes (third degree AV block).
Influence of the autonomic nervous system — Although the sinus node has an intrinsic automaticity and always produces an impulse, the rate of impulse generation is controlled by other factors, particularly the autonomic nervous system (figure 1). (See "Sick sinus syndrome: Epidemiology, etiology, and natural history", section on 'Autonomic nervous system and the SA node'.) With augmented parasympathetic (vagal) influence or reduced sympathetic stimulation, the sinus rate slows, and the PR interval prolongs due to a vagally mediated slowing of conduction through the atrioventricular node. By comparison, increased sympathetic activity and decreased vagal effects increase the sinus nodal rate and enhance atrioventricular nodal conduction, resulting in a shortened PR interval.