Atrial Systole

Atrial systole is the period of the cardiac cycle during which the atrial myocardium contracts. It occurs near the end of ventricular diastole and provides the final active contribution to ventricular filling before ventricular systole begins.

Atrial contraction follows atrial depolarization, which normally originates in the sinoatrial node and spreads through the right and left atrial myocardium. On the surface electrocardiogram, atrial depolarization is represented primarily by the P wave.

During atrial systole, the atrioventricular valves are normally open, allowing blood to move from the right atrium into the right ventricle and from the left atrium into the left ventricle. The pulmonary and aortic valves remain closed because ventricular pressures are below the pressures in the pulmonary trunk and aorta.

Position in the Cardiac Cycle

Atrial systole occurs during the final portion of ventricular diastole.

Most ventricular filling has already occurred passively before atrial contraction begins. Atrial systole then provides an additional active contribution to end-diastolic ventricular volume.

Sequence of Events

Event	Result
SA node depolarization	Initiates atrial electrical activation
Atrial depolarization	Produces the P wave on the ECG
Atrial contraction	Raises atrial pressure
Blood moves into ventricles	Completes ventricular filling
AV nodal delay	Allows atrial contraction to occur before ventricular contraction
Ventricular depolarization	Begins the transition toward ventricular systole

Electrical Initiation

Normal atrial systole begins with an electrical impulse generated by the sinoatrial node, the normal pacemaker of the heart.

The impulse spreads through atrial myocardium and specialized atrial conduction pathways, producing coordinated depolarization of both atria.

Sinoatrial Node

The sinoatrial node is located in the right atrium near the junction with the superior vena cava.

Pacemaker cells within the node spontaneously depolarize and normally establish the timing of each cardiac cycle.

Atrial Depolarization

Electrical activation spreads from the sinoatrial region across the right atrium and into the left atrium.

This depolarization precedes mechanical contraction because excitation must occur before the contractile apparatus develops force.

P Wave

The P wave on the electrocardiogram primarily represents atrial depolarization.

Mechanical atrial systole begins shortly after electrical activation and continues during the interval following the P wave.

Electrical Versus Mechanical Events

Electrical Event	Mechanical Event
P wave	Atrial contraction follows
PR segment	Atrial contraction and AV nodal conduction continue
QRS complex	Ventricular depolarization initiates ventricular systole shortly afterward

Atrioventricular Node

The impulse reaches the atrioventricular node after spreading through the atria.

Conduction through the AV node is relatively slow, producing an important delay between atrial and ventricular electrical activation.

Importance of AV Nodal Delay

The AV nodal delay helps provide time for atrial contraction and late ventricular filling before ventricular contraction begins.

This temporal separation contributes to coordinated sequential pumping by the atria and ventricles.

Mechanical Contraction

Atrial depolarization triggers calcium-dependent contraction of atrial cardiomyocytes.

Contraction decreases atrial chamber volume and raises atrial pressure, creating a pressure gradient that favors movement of blood into the ventricles.

Right Atrial Systole

During right atrial systole, contraction of the right atrium moves additional blood through the open tricuspid valve into the right ventricle.

The right ventricle is simultaneously near the end of its filling phase.

Left Atrial Systole

During left atrial systole, contraction of the left atrium moves additional blood through the open mitral valve into the left ventricle.

This completes the final active component of left ventricular filling before ventricular systole.

Atrioventricular Valves

The tricuspid and mitral valves are normally open during atrial systole because atrial pressure exceeds ventricular pressure.

This permits forward flow from the atria into the ventricles.

Semilunar Valves

The pulmonary and aortic valves remain closed during atrial systole.

Pressure within the pulmonary trunk and aorta remains greater than ventricular pressure during this late diastolic phase.

Valve Status During Atrial Systole

Valve	Status
Tricuspid valve	Open
Mitral valve	Open
Pulmonary valve	Closed
Aortic valve	Closed

Ventricular Filling

Ventricular filling begins earlier in diastole when ventricular pressure falls below atrial pressure and the atrioventricular valves open.

Much of the filling occurs passively before atrial systole. Atrial contraction provides the final active increment.

Rapid Ventricular Filling

Early in ventricular diastole, opening of the atrioventricular valves allows relatively rapid passive movement of blood from the atria into the ventricles.

This phase generally accounts for a substantial proportion of total ventricular filling.

Diastasis

Diastasis is the slower filling period that follows early rapid ventricular filling.

Atrial and ventricular pressures become more closely matched, reducing the rate of passive inflow before atrial systole occurs.

Atrial Kick

The additional ventricular filling produced by atrial contraction is commonly called the atrial kick.

Its relative contribution to ventricular filling varies with heart rate, ventricular compliance, loading conditions, and cardiovascular health.

End-Diastolic Volume

At the completion of atrial systole, the ventricles contain their end-diastolic volume.

This is the volume present immediately before ventricular systole and is an important determinant of ventricular preload.

Preload

Preload describes the degree of myocardial fiber stretch associated with ventricular filling before contraction.

Atrial systole can increase end-diastolic volume and therefore contribute to ventricular preload.

Frank-Starling Mechanism

Within physiological limits, increased ventricular filling increases myocardial fiber stretch and can enhance the force of subsequent ventricular contraction.

This relationship forms part of the Frank-Starling mechanism of the heart.

Atrial Pressure

Atrial contraction produces a transient increase in atrial pressure.

This pressure rise can be identified in recordings of atrial and venous pressure.

A Wave

The pressure increase caused by atrial contraction produces the a wave of the atrial pressure tracing.

On the right side of the circulation, the corresponding pressure change contributes to the a wave of the jugular venous pulse.

Jugular Venous Pulse

The jugular venous pulse reflects pressure changes transmitted from the right atrium into the large central veins.

The a wave occurs as a consequence of right atrial contraction near the end of ventricular diastole.

Atrial Pressure Waves

Wave	Major Cause
a wave	Atrial contraction
c wave	Early ventricular systolic events, including bulging of the closed tricuspid valve and transmitted pulsation
v wave	Atrial filling while the AV valve is closed

Venous Inflow During Atrial Systole

The atria receive blood from veins throughout much of the cardiac cycle.

Atrial contraction can transiently influence venous flow patterns because the major systemic and pulmonary venous entrances do not possess valves that completely isolate the atria during contraction.

Right Atrial Pressure and Systemic Veins

Right atrial contraction produces a pressure wave that can be transmitted backward into the superior vena cava and jugular veins.

This relationship allows the jugular venous pulse to provide information about right atrial mechanical activity.

Atrial Myocardium

The atrial walls contain cardiac muscle arranged to contract around the atrial chambers and venous inflow regions.

Atrial myocardium is thinner than ventricular myocardium because the atria generate much lower pressures than the ventricles.

Auricles

The right and left auricles are muscular appendages of the atria.

Their internal surfaces contain prominent pectinate muscles, which contribute to the muscular architecture of the atrial walls.

Pectinate Muscles

Pectinate muscles are parallel muscular ridges found prominently in the right atrial appendage and anterior right atrial wall and within the left atrial appendage.

They are part of the contractile atrial myocardium involved in atrial systole.

Atrial Contribution at Rest

At normal resting heart rates in healthy individuals, passive ventricular filling accounts for most end-diastolic filling.

Atrial systole supplies a smaller but physiologically meaningful final contribution.

Effect of Increased Heart Rate

As heart rate increases, the duration of diastole shortens disproportionately.

The time available for passive ventricular filling decreases, which can increase the functional importance of coordinated atrial contraction in appropriate circumstances.

Ventricular Compliance

Ventricular compliance describes how readily a ventricle expands as it fills.

When ventricular compliance is reduced, atrial contraction may become more important for achieving adequate end-diastolic filling, although higher filling pressures may also result.

Aging

Age-related changes can reduce ventricular relaxation and compliance.

Under these conditions, effective atrial contraction may contribute more importantly to ventricular filling than it does in a highly compliant young ventricle.

Atrial Systole and the Cardiac Cycle

Parameter	State During Atrial Systole
Atria	Contracting
Ventricles	Relaxed and completing filling
AV valves	Open
Semilunar valves	Closed
Atrial pressure	Temporarily increases
Ventricular volume	Rises to end-diastolic volume

Transition to Ventricular Systole

After atrial systole, the electrical impulse travels through the His-Purkinje conduction system and activates the ventricular myocardium.

Ventricular pressure then begins to rise, leading to closure of the atrioventricular valves and the onset of ventricular systole.

QRS Complex

The QRS complex represents ventricular depolarization.

Atrial repolarization occurs at approximately the same time but is normally obscured by the much larger electrical signal generated by ventricular depolarization.

Closure of the AV Valves

When ventricular pressure rises above atrial pressure, the mitral and tricuspid valves close.

This marks the end of ventricular filling and contributes to the first heart sound.

First Heart Sound

The first heart sound (S1) is associated primarily with vibrations generated around closure of the atrioventricular valves and the onset of ventricular systole.

It therefore occurs after atrial systole rather than being produced directly by atrial contraction.

Fourth Heart Sound

A fourth heart sound (S4), when present, occurs during late diastole in association with atrial contraction against a relatively stiff or poorly compliant ventricle.

It is generally considered an abnormal finding in adults, although its significance depends on the clinical setting.

Clinical Significance

Atrial Fibrillation

In atrial fibrillation, coordinated atrial electrical and mechanical activity is lost.

The absence of effective atrial systole eliminates the normal atrial contribution to ventricular filling.

Loss of Atrial Kick

Loss of the atrial contribution may be tolerated reasonably well by some individuals with normal ventricular function.

It can be more consequential when ventricular compliance is reduced or when adequate filling depends more heavily on atrial contraction.

Mitral Stenosis

Mitral stenosis obstructs blood flow from the left atrium to the left ventricle.

Atrial contraction can produce a substantial rise in left atrial pressure when blood must pass through a narrowed mitral valve.

Tricuspid Stenosis

Tricuspid stenosis obstructs right atrial emptying into the right ventricle.

Right atrial systole against the narrowed valve can produce prominent systemic venous pressure changes.

AV Dissociation

When atrial and ventricular contractions occur independently, atrial systole may occasionally occur while an atrioventricular valve is closed.

This can produce marked backward transmission of atrial pressure into the venous circulation.

Cannon A Waves

Cannon a waves are prominent jugular venous pulsations that can occur when the right atrium contracts against a closed tricuspid valve.

They may be seen in rhythms characterized by atrioventricular dissociation.

Diastolic Dysfunction

When ventricular relaxation or compliance is impaired, late diastolic filling can become more dependent on atrial contraction.

Loss of coordinated atrial systole may therefore produce a greater reduction in filling in patients with significant diastolic dysfunction.

Key Features of Atrial Systole

Feature	Key Point
Timing	Late ventricular diastole
Electrical precursor	Atrial depolarization
ECG correlate	P wave precedes mechanical contraction
Pacemaker	Sinoatrial node under normal conditions
AV valves	Open
Semilunar valves	Closed
Main mechanical effect	Final active contribution to ventricular filling
Atrial pressure wave	a wave
Final ventricular volume	End-diastolic volume

Anatomical and Physiological Importance

Atrial systole links the electrical activity of the atrial conduction system with the mechanical completion of ventricular filling. The sinoatrial node initiates atrial depolarization, atrial myocardium contracts, and blood is driven through the open atrioventricular valves into the relaxed ventricles.

The timing of this event is coordinated by the cardiac conduction system. Delayed conduction through the atrioventricular node helps separate atrial contraction from ventricular contraction, allowing the atria to complete their contribution before ventricular pressure rises and closes the atrioventricular valves.

Although most ventricular filling can occur passively under normal resting conditions, atrial systole becomes particularly important when ventricular filling time is reduced or ventricular compliance is impaired. Loss of coordinated atrial contraction can therefore have markedly different hemodynamic consequences depending on the condition of the ventricles and the overall cardiovascular system.