1.9.8 Atrial Systole Definition
Atrial systole is the contraction phase of the atria, contributing to blood flow into the ventricles during the cardiac cycle.
Atrial Systole Definition is the contraction of the atrial myocardium that occurs at the very end of diastole, immediately before the onset of ventricular systole. During atrial systole, the atria actively contract to propel their remaining blood volume through the open atrioventricular (AV) valves into the ventricles, completing ventricular filling and establishing the final end-diastolic volume upon which ventricular contraction will act.
Timing Within the Cardiac Cycle
Atrial systole occupies a precise and brief position within the overall sequence of the cardiac cycle.
Preceding Events
Atrial systole follows the passive filling stages of diastole, namely rapid filling and diastasis, during which the ventricles have already received the majority of their diastolic volume through pressure-driven flow across the open AV valves.
Electrical Trigger
Atrial systole is initiated by atrial depolarization, which originates at the sinoatrial node and spreads through the atrial myocardium, corresponding to the P wave on the electrocardiogram. A short delay at the atrioventricular node separates atrial depolarization from the subsequent ventricular depolarization, allowing atrial contraction to be mechanically completed before ventricular systole begins.
Subsequent Events
Atrial systole ends as the AV valves close at the onset of ventricular systole, transitioning the cardiac cycle into isovolumetric contraction.
Contribution to Ventricular Filling
Atrial systole, often referred to as the "atrial kick," supplies the final increment of blood to the ventricles before contraction.
Volume Contribution
In a resting, healthy heart, atrial systole typically contributes a modest fraction of total ventricular filling, since the majority of filling occurs passively during rapid filling. However, this contribution becomes proportionally much greater when diastolic filling time is reduced.
Importance at Elevated Heart Rates
As heart rate increases and diastole shortens, passive filling time diminishes, making the active contribution of atrial systole increasingly important for maintaining adequate end-diastolic volume and stroke volume.
Hemodynamic Effects
Atrial systole produces measurable changes in atrial and ventricular pressure.
Atrial Pressure Wave
Atrial contraction produces a distinct rise in atrial pressure, recorded as the "a wave" in atrial and central venous pressure tracings.
Ventricular Pressure
The additional volume delivered by atrial systole causes a small, final rise in ventricular pressure and volume just before the onset of isovolumetric contraction, contributing to the end-diastolic pressure–volume relationship.
Clinical Relevance
Loss of coordinated atrial contraction, as occurs in atrial fibrillation, eliminates the atrial kick and can reduce end-diastolic volume and cardiac output, particularly in individuals with reduced ventricular compliance who depend more heavily on active filling. Restoration of normal atrial rhythm is therefore an important therapeutic goal in many patients with structural or electrical cardiac disease.