Atrial Depolarization Sequence in Normal Hearts
Direct Answer
In a healthy adult heart, the right atrium depolarizes first during atrial depolarization, as the sinus node is located in the upper anterior part of the right atrium and electrical activation spreads from there to both atria. 1
Normal Atrial Depolarization Sequence
Initiation and Spread
The cardiac impulse originates in the sinus node, which is anatomically positioned in the upper anterior part of the right atrium. 2
Sinus node depolarization spreads through the right atrium first, then propagates to the left atrial myocytes, with the entire process reaching the atrioventricular (AV) node within 200 milliseconds. 1
The electrical activation spreads from the right atrium to the left atrium via specialized interatrial conduction pathways, including Bachmann's bundle superiorly and other mid-atrial and inferior pathways. 1, 3
Clinical Significance
A properly timed atrial contraction, initiated by this sequential depolarization, can increase cardiac output by 25% to 30% through optimized ventricular filling. 1
The atrial contribution to ventricular filling can account for up to 30% of cardiac output by way of the atrial "kick" that occurs with coordinated atrial contraction. 2
Electrocardiographic Manifestations
On surface ECG, normal P waves reflect this right-to-left atrial activation sequence, with the initial portion representing right atrial depolarization and the terminal portion representing left atrial depolarization. 1
Abnormalities in interatrial conduction can manifest as prolonged P waves (≥110 ms) or biphasic P waves in inferior leads, indicating delayed left atrial activation. 4, 3
Common Pitfalls
Do not confuse ectopic atrial rhythms with normal sinus rhythm—ectopic atrial bradycardia originates from an atrial pacemaker other than the sinus node and may alter the normal right-to-left atrial activation sequence. 1
Premature atrial contractions (PACs) arise from abnormal and premature depolarization sites within the atria, producing P waves with different morphology from sinus P waves, indicating altered atrial activation sequences. 5, 6
In patients with interatrial conduction delays, the left atrium may activate significantly later than normal, which can be detected by prolonged interatrial conduction times (>115 ms) on electrophysiologic studies. 7