What is the cardiac cycle and how does it work, including the role of diagrams in understanding its phases?

The Cardiac Cycle: Phases and Physiological Events

Overview of the Cardiac Cycle

The cardiac cycle represents the complete sequence of mechanical and electrical events that occur during one heartbeat, consisting of coordinated atrial and ventricular contraction (systole) and relaxation (diastole) phases that pump blood through the circulatory system 1, 2.

Electrical Foundation

The cardiac cycle is initiated and coordinated by electrical depolarization waves that precede and drive each mechanical phase of cardiac contraction and relaxation 2. The ECG provides the temporal framework for understanding these events:

• P wave: Represents atrial depolarization, triggering atrial contraction 3
• QRS complex: Represents ventricular depolarization, initiating ventricular contraction 3
• T wave: Represents ventricular repolarization, corresponding to ventricular relaxation 4, 5

The PR interval (120-200 ms) reflects the time for electrical conduction from atria through the AV node to ventricles 3.

Sequential Phases of the Cardiac Cycle

Phase 1: Atrial Systole (Late Ventricular Diastole)

• Atrial state: Contracting
• Ventricular state: Relaxed and filling
• AV valves (mitral/tricuspid): Open
• Semilunar valves (aortic/pulmonic): Closed
• Atrial contraction completes ventricular filling, contributing approximately 20-30% of total ventricular volume 1, 2

Phase 2: Isovolumetric Ventricular Contraction

• Atrial state: Relaxed
• Ventricular state: Contracting with all valves closed
• AV valves: Closed (following QRS complex)
• Semilunar valves: Closed
• Ventricular pressure rises rapidly without volume change until it exceeds aortic/pulmonary artery pressure 1, 2

Phase 3: Ventricular Ejection

• Atrial state: Relaxed and filling passively
• Ventricular state: Contracting and ejecting blood
• AV valves: Closed
• Semilunar valves: Open
• Blood is ejected into the aorta and pulmonary artery as ventricular pressure exceeds arterial pressure 1, 2

Phase 4: Isovolumetric Ventricular Relaxation

• Atrial state: Relaxed and continuing to fill
• Ventricular state: Relaxing with all valves closed
• AV valves: Closed
• Semilunar valves: Closed (after T wave)
• Ventricular pressure drops rapidly without volume change until it falls below atrial pressure 1, 2

Phase 5: Rapid Ventricular Filling

• Atrial state: Relaxed, passively emptying into ventricles
• Ventricular state: Relaxed and filling rapidly
• AV valves: Open
• Semilunar valves: Closed
• Approximately 70-80% of ventricular filling occurs passively during this phase 1, 2

Electromechanical Coupling During the Cycle

The ST segment on ECG corresponds to the plateau phase of the ventricular action potential, where transmembrane voltage remains at approximately +10 to -10 mV across all ventricular myocardial cells 4, 5. This electrical plateau maintains ventricular contraction during ejection 5.

During rapid repolarization (phase 3), the membrane potential returns from the plateau voltage to the resting level of approximately -85 mV, corresponding to the T wave and initiating ventricular relaxation 4, 5.

Pressure-Volume Relationships

The cardiac cycle can be visualized as a pump that first fills the ventricles from low-pressure venous compartments, then empties them into high-pressure arterial compartments 2. The Wright table provides an integrated view showing how each cardiac compartment's pressures and flows change over time, complementing traditional Wiggers diagram representations 2.

Clinical Relevance for Diagnosis

Understanding the precise timing and sequence of cardiac cycle events is essential for:

• Interpreting heart sounds: S1 occurs with AV valve closure at the onset of ventricular systole; S2 occurs with semilunar valve closure at the end of ventricular systole 2
• Analyzing valve disorders: Abnormal valve function alters the normal pressure-flow relationships and timing of cardiac cycle phases 2
• Assessing conduction abnormalities: Bundle branch blocks and other conduction defects alter the normal temporal sequence of ventricular activation and mechanical events 2

Common Pitfalls in Understanding

The cardiac cycle is often misunderstood when students focus solely on individual chamber events rather than the integrated sequence of pressure changes that determine valve opening and closing 1, 2. Valve function is entirely passive, determined by pressure gradients across the valves, not by active valve control 1.

The relationship between electrical events (ECG) and mechanical events (pressure/volume changes) must be understood sequentially: electrical depolarization always precedes and drives the subsequent mechanical contraction 2.