Phases of the Cardiac Cycle

The cardiac cycle describes the sequence of mechanical and electrical events that occur from the beginning of one heartbeat to the beginning of the next. At a heart rate of 75 bpm, the cycle lasts approximately 800 ms.

Overview of the Cardiac Cycle

Two Main Phases

Division of the Cycle

The cardiac cycle is divided into seven phases:

1. Atrial systole (late diastole)
2. Isovolumetric ventricular contraction
3. Rapid ventricular ejection
4. Reduced ventricular ejection
5. Isovolumetric ventricular relaxation
6. Rapid ventricular filling
7. Reduced ventricular filling (diastasis)

Phase 1: Atrial Systole (Late Diastole)

Timing: Last 100 ms of diastole

Events:

• Atrial contraction (P wave on ECG)
• Pressure in atria rises (atrial kick)
• Additional blood (15-25% of ventricular filling) enters the ventricle
• AV valves are open
• Semilunar valves are closed

Pressure changes:

• Atrial pressure: 5-10 mmHg (a wave)
• Ventricular pressure: Increases slightly with filling
• Aortic pressure: Falls slowly (diastolic run-off)

Hemodynamics:

• Atrial contraction contributes 15-25% of stroke volume at rest
• Atrial contribution increases to 40% during exercise
• Loss of atrial kick (atrial fibrillation) reduces cardiac output

Phase 2: Isovolumetric Ventricular Contraction

Timing: First 50 ms of systole

Events:

• Ventricular contraction begins (QRS complex)
• Ventricular pressure rises rapidly
• AV valves close (S1 heart sound)
• All four valves are closed
• Ventricular volume remains constant
• Myocardial tension increases

Pressure changes:

• Ventricular pressure: Rises from ~10 to ~80 mmHg (LV)
• Atrial pressure: Rises (c wave from AV valve bulging)
• Aortic pressure: Continues to fall

Hemodynamics:

• No blood flow during this phase
• Muscle tension increases without shortening (isometric contraction)
• The ventricle changes shape (becomes more spherical)

Phase 3: Rapid Ventricular Ejection

Timing: First 100 ms of systole (after isovolumetric contraction)

Events:

• Ventricular pressure exceeds aortic pressure
• Semilunar valves open
• Blood rapidly ejects from the ventricle
• 70-80% of stroke volume ejected
• Ventricular volume decreases rapidly

Pressure changes:

• Ventricular pressure: Continues to rise (peaks at ~120 mmHg)
• Aortic pressure: Rises rapidly (anacrotic limb)
• Atrial pressure: Falls (atrial relaxation, x descent)

Hemodynamics:

• Peak aortic flow velocity: 1.0-1.7 m/s
• Peak ejection rate: 500-600 mL/s
• Ventricular volume drops by 60-70 mL

Phase 4: Reduced Ventricular Ejection

Timing: Last 100 ms of systole

Events:

• Ventricular pressure begins to fall
• Ejection slows
• Remaining 20-30% of stroke volume ejected
• Ventricular volume reaches minimum (end-systolic volume)

Pressure changes:

• Ventricular pressure: Falls below aortic pressure
• Aortic pressure: Peaks and then begins to fall (dicrotic notch marks aortic valve closure)
• Atrial pressure: Rises (v wave from atrial filling)

End-systolic volume (ESV): 40-60 mL

Hemodynamics:

• Ejection continues despite falling ventricular pressure
• Blood inertia maintains forward flow
• The dicrotic notch on the aortic pressure tracing marks aortic valve closure

Phase 5: Isovolumetric Ventricular Relaxation

Timing: First 100 ms of diastole

Events:

• Ventricular relaxation begins (T wave on ECG)
• Ventricular pressure falls rapidly
• Semilunar valves close (S2 heart sound)
• All four valves are closed again
• Ventricular volume constant (isovolumetric)

Pressure changes:

• Ventricular pressure: Falls rapidly from ~100 mmHg to ~5 mmHg
• Aortic pressure: Continues to fall (diastolic run-off)
• Atrial pressure: Continues to rise (v wave peaks)

Hemodynamics:

• No blood flow
• Ventricular relaxation is an active, energy-dependent process
• Impaired relaxation (diastolic dysfunction) elevates filling pressures

Phase 6: Rapid Ventricular Filling

Timing: First 100 ms of diastole (after isovolumetric relaxation)

Events:

• Ventricular pressure falls below atrial pressure
• AV valves open
• Blood rapidly flows from atria to ventricles
• 70-80% of ventricular filling occurs
• Ventricular volume increases rapidly

Pressure changes:

• Ventricular pressure: Rises slightly with filling
• Atrial pressure: Falls (y descent)
• Aortic pressure: Continues to fall

Hemodynamics:

• Early filling is determined by ventricular relaxation
• Suction effect: The relaxing ventricle generates a pressure gradient
• Impaired relaxation reduces early filling (E wave on transmitral Doppler)

Phase 7: Reduced Ventricular Filling (Diastasis)

Timing: Middle third of diastole (100-400 ms, rate-dependent)

Events:

• Atrial and ventricular pressures equalize
• Filling slows
• Flow continues from the pulmonary veins through the left atrium

Pressure changes:

• Atrial pressure: Stable
• Ventricular pressure: Slowly rising
• All pressures are relatively constant

Hemodynamics:

• Passive filling phase
• Duration is highly rate-dependent
• At fast heart rates, diastasis disappears entirely

Pressure-Volume Loop

The pressure-volume loop summarizes the cardiac cycle:

Key Points

Loop Components

• A to B: Isovolumetric contraction (pressure ↑, volume constant)
• B to C: Ejection (pressure ↑ then ↓, volume ↓)
• C to D: Isovolumetric relaxation (pressure ↓, volume constant)
• D to A: Filling (pressure ↑ slightly, volume ↑)

Stroke volume = EDV - ESV (typically 70-90 mL)

Ejection fraction = SV / EDV (typically 55-75%)

Factors Affecting the Cardiac Cycle

Right vs. Left Ventricular Cycle

Age-Related Changes