Cardiac Cycle Overview

The cardiac cycle is a sequence of events that occur with every heartbeat, divided primarily into two main phases:

1. Systole
   • Ventricular Contraction
2. Diastole
   • Ventricular Relaxation

Basic Principles

• Blood Flow: Blood flows from higher to lower pressure.
• Pressure Changes:
  • Contraction increases chamber pressure.
  • Relaxation decreases chamber pressure.
• Valve Mechanics:
  • AV Valves: Open when atrial pressures > ventricular pressures; close when reversed.
  • Semilunar Valves: Open when ventricular pressures > aortic/pulmonary pressures; close when reversed.

Initiation of the Cardiac Cycle

• SA Node Firing: Initiates the cycle by stimulating atrial depolarization, noted as the P-wave on an ECG.
• Atrial Contraction: Begins after the P-wave, elevating atrial pressure and propelling blood into the ventricles.
  • Contributes a smaller fraction to ventricular filling as ventricles are mostly filled by passive flow.
• Reversal of Pressure Gradient: As atrial contraction ends, atrial pressure falls, causing AV valves to close, marked by the first heart sound (S1), signaling the start of systole.

Systole

• Ventricular Depolarization (QRS Complex): Leads to ventricular contraction, increasing pressures.
• Isovolumetric Contraction:
  • Semilunar valves remain closed temporarily; no blood ejection occurs, and ventricular volume remains constant.
• Ventricular Ejection:
  • Begins when ventricular pressures surpass aortic/pulmonary pressures, opening the aortic and pulmonic valves for rapid blood ejection.
• Ventricular Repolarization (T-wave):
  • Initiates pressure fall and reduced ejection force.
• Closure of Semilunar Valves:
  • Occurs when ventricular pressures fall below aortic/pulmonary pressures, ending systole and beginning diastole, marked by the second heart sound (S2).

Diastole

• Isovolumetric Relaxation:
  • Initial phase with all valves closed; ventricular pressures drop rapidly without volume change.
• Atrial Filling:
  • Atria fill with blood, slowly increasing atrial pressures.
• Ventricular Filling:
  • As ventricular pressures reduce below atrial pressures, AV valves open, allowing passive blood flow into ventricles.
• Atrial Contraction:
  • Completes ventricular filling, repeating the cardiac cycle.