Introduction
The cardiac cycle refers to the sequence of electrical,
mechanical, pressure, volume, and flow changes that occur in the heart during
one complete heartbeat. It includes the coordinated contraction and relaxation
of the atria and ventricles that maintain effective circulation throughout the
body.
At a normal resting heart rate of 75 beats/minute, one cardiac
cycle lasts approximately:
Duration of one cardiac cycle=75/60=0.8 seconds
The cardiac cycle integrates:
- Electrical activity (ECG)
- Mechanical events
- Pressure changes
- Volume changes
- Valve movements
- Heart sounds
Understanding the cardiac cycle is fundamental for:
- Cardiovascular physiology
- Hemodynamics
- ECG interpretation
- Auscultation
- Valvular heart disease
- Heart failure
- Cardiac catheterization
Overview of the Cardiac Cycle
The cardiac cycle consists of two major phases:
|
Phase |
Description |
Approximate Duration |
|
Ventricular relaxation and filling |
0.5 s |
|
|
Ventricular contraction and ejection |
0.3 s |
Basic Sequence of Events
Simplified Flow
Atrial systole
↓
Isovolumetric ventricular contraction
↓
Rapid ventricular ejection
↓
Reduced ventricular ejection
↓
Isovolumetric relaxation
↓
Rapid ventricular filling
↓
Reduced ventricular filling
(diastasis)
↓
Next atrial systole
Diagrammatic Overview of Cardiac Cycle
Simplified Integrated Cardiac Cycle
ECG: P
QRS T
| | |
------------------------------------------------
Atria: Contract ---------------- Relax
------------------------------------------------
Ventricle: Contract ---------- Relax
------------------------------------------------
AV Valves: Open ---- Closed ---- Open
------------------------------------------------
Semilunar: Closed -- Open -- Closed
------------------------------------------------
Important Physiological Parameters
|
Parameter |
Normal Value |
|
Heart rate |
60–100/min |
|
Cardiac cycle duration |
0.8 sec |
|
End-diastolic volume (EDV) |
120–130 mL |
|
End-systolic volume (ESV) |
50–60 mL |
|
Stroke volume (SV) |
70 mL |
|
Ejection fraction (EF) |
55–70% |
Stroke volume is calculated as: SV = EDV - ESV
Example: SV=120 mL−50 mL=70 mL
Ejection fraction: 
Phases of the Cardiac Cycle
1. Atrial Systole
Definition
Atrial systole is atrial contraction that completes ventricular filling.
Duration
Approximately 0.1 second
Events During Atrial Systole
Electrical Event
- Begins shortly after the P
wave on ECG
Mechanical Event
- Atria contract
- Blood pushed into ventricles
Hemodynamics
- Adds final 20–30% of
ventricular filling (“atrial kick”)
Valve Status
|
Valve |
Status |
|
AV valves |
Open |
|
Semilunar valves |
Closed |
Pressure Changes
- Atrial pressure rises
- Produces the a-wave in
atrial pressure tracing
Clinical Significance
Loss of atrial systole occurs in:
- Atrial fibrillation
- Complete heart block
This particularly affects:
- Elderly patients
- Stiff ventricles
- Diastolic dysfunction
2. Isovolumetric Ventricular Contraction
Definition
Early ventricular systole during which ventricles contract with all
valves closed.
Duration ~0.05 second
Events
Electrical Event
Occurs immediately after: QRS complex
Mechanical Event
- Ventricular muscle contracts
- Pressure rises sharply
- Volume remains constant
Valve Events
|
Valve |
Status |
|
AV valves |
Closed |
|
Semilunar valves |
Closed |
Closure of AV valves produces: First Heart Sound (S1)
Why “Isovolumetric”?
Because: ΔV=0, No blood enters or leaves ventricles.
Pressure Changes
- LV pressure rises rapidly
- RV pressure rises rapidly
3. Rapid Ventricular Ejection
Definition
Occurs when ventricular pressure exceeds arterial pressure.
Events
Semilunar Valves Open
- Aortic valve
- Pulmonary valve
Blood Ejected
Most stroke volume expelled during this phase.
Hemodynamic Features
|
Parameter |
Change |
|
Ventricular pressure |
High |
|
Aortic pressure |
Rising |
|
Ventricular volume |
Falling rapidly |
ECG Correlation
Occurs during: ST segment
4. Reduced Ventricular Ejection
Definition
Late systole where ventricular contraction weakens.
Events
- Ventricular pressure begins
falling
- Blood continues to flow due to
momentum
- Ejection rate decreases
ECG Correlation
Associated with: T wave (ventricular repolarization)
5. Isovolumetric Relaxation
Definition
Early diastolic phase during which ventricles relax while all valves
remain closed.
Events
Semilunar Valve Closure
Produces: Second Heart Sound (S2)
Valve Status
|
Valve |
Status |
|
AV valves |
Closed |
|
Semilunar valves |
Closed |
Hemodynamics
- Ventricular pressure falls
rapidly
- Volume remains constant
Dicrotic Notch
Closure of the aortic valve causes a transient rise in aortic pressure
called the:
- Incisura
- Dicrotic notch
6. Rapid Ventricular Filling
Definition
Occurs when ventricular pressure falls below atrial pressure.
Events
AV Valves Open
- Mitral valve
- Tricuspid valve
Blood Flow
- Rapid passive filling of
ventricles
Hemodynamics
|
Parameter |
Change |
|
Ventricular volume |
Rises rapidly |
|
Ventricular pressure |
Slight rise |
|
Atrial pressure |
Falls |
Clinical Correlation
Rapid filling may produce:
Third Heart Sound (S3)
Usually:
- Normal in children
- Pathological in adults
7. Reduced Ventricular Filling (Diastasis)
Definition
Slow passive filling phase before next atrial systole.
Events
- Pressure gradient decreases
- Filling slows
Importance
At high heart rates: Diastasis shortens markedly
This reduces:
- Ventricular filling
- Cardiac output
Wiggers Diagram
The Wiggers diagram integrates:
- ECG
- Pressure changes
- Ventricular volume
- Valve events
- Heart sounds
It is the single most important diagram in cardiovascular physiology.
Simplified Wiggers Diagram
ECG: P QRS T
Atrial P: a
c v
LV Pressure:
/\^^^^^^^^^\__
/ \
Aortic Pressure:
/^^^^^^^^^\___
LV Volume:
____\ /____
Heart Sounds:
S1 S2
Components of the Wiggers Diagram
1. Atrial Pressure Curve
Contains:
|
Wave |
Cause |
|
a-wave |
Atrial contraction |
|
c-wave |
Bulging AV valve |
|
v-wave |
Venous filling |
2. Ventricular Pressure Curve
- Rises sharply during systole
- Falls rapidly during diastole
3. Aortic Pressure Curve
Normal: 120/80 mmHg
Features:
- Systolic peak
- Dicrotic notch
4. Ventricular Volume Curve
Shows:
- EDV
- ESV
- Stroke volume
Pressure Changes During Cardiac Cycle
|
Chamber/Vessel |
Systolic Pressure |
Diastolic Pressure |
|
Left ventricle |
120 mmHg |
0–10 mmHg |
|
Aorta |
120 mmHg |
80 mmHg |
|
Right ventricle |
25 mmHg |
0–5 mmHg |
|
Pulmonary artery |
25 mmHg |
10 mmHg |
Pressure-Volume Loop
The pressure-volume loop graphically represents ventricular mechanics.
Key Components
1. Ventricular Filling
- Volume increases
- Pressure slightly rises
2. Isovolumetric Contraction
- Pressure rises steeply
- Volume constant
3. Ejection
- Volume decreases
4. Isovolumetric Relaxation
- Pressure falls rapidly
- Volume constant
Simplified Pressure-Volume Loop
Pressure
^
|
| /\
| / \
| / \
| / \
|_____/________\____> Volume
Cardiac Output and Cardiac Cycle
Cardiac output depends on:
- Stroke volume
- Heart rate
CO=HR × SV
Example: CO=75×70=5250 mL/min
Normal cardiac output: Approximately 5 L/min
Heart Sounds and Cardiac Cycle
|
Heart Sound |
Cause |
Timing |
|
S1 |
Closure of AV valves |
Beginning systole |
|
S2 |
Closure of semilunar valves |
Beginning diastole |
|
S3 |
Rapid ventricular filling |
Early diastole |
|
Atrial contraction against stiff
ventricle |
Late diastole |
Valve Dynamics During Cardiac Cycle
|
Phase |
AV Valves |
Semilunar Valves |
|
Atrial systole |
Open |
Closed |
|
Isovolumetric contraction |
Closed |
Closed |
|
Ventricular ejection |
Closed |
Open |
|
Isovolumetric relaxation |
Closed |
Closed |
|
Ventricular filling |
Open |
Closed |
Effect of Heart Rate on Cardiac Cycle
Tachycardia
- Diastole shortens more than
systole
- Reduced coronary perfusion
- Reduced ventricular filling
Bradycardia
- Increased filling time
- Increased EDV
- Increased stroke volume
Coronary Blood Flow During Cardiac Cycle
Left Coronary Artery
Most perfusion occurs during: Diastole
Reason: Systolic compression of coronary vessels
Right Coronary Artery
Perfusion occurs during: Both systole and diastole
Clinical Correlations
1. Mitral Stenosis
- Elevated left atrial pressure
- Large a-wave
- Diastolic murmur
2. Aortic Stenosis
- Increased LV systolic pressure
- Reduced stroke volume
3. Mitral Regurgitation
- Large v-wave
- Volume overload
4. Heart Failure
Changes:
- Increased EDV
- Reduced EF
- Elevated filling pressures
5. Atrial Fibrillation
Loss of: Atrial kick
Consequences: Reduced ventricular filling
Advanced Concepts
Within physiological limits: ↑EDV→↑Stroke Volume
Greater filling causes:
- Greater myocardial stretch
- Stronger contraction
Compliance: Compliance=
Reduced in:
- Hypertrophy
- Restrictive cardiomyopathy
- Aging
Chronology of Events
|
Event |
Timing |
|
P wave |
Atrial depolarization |
|
Atrial systole |
After P wave |
|
QRS complex |
Ventricular depolarization |
|
S1 |
AV valve closure |
|
Ventricular ejection |
After QRS |
|
T wave |
Ventricular repolarization |
|
S2 |
Semilunar valve closure |
Summary Table of Cardiac Cycle
|
Phase |
Valves |
Main Event |
Heart Sound |
|
Atrial systole |
AV open |
Active filling |
S4 (sometimes) |
|
Isovolumetric contraction |
All closed |
Pressure rise |
S1 |
|
Rapid ejection |
Semilunar open |
Blood ejection |
— |
|
Reduced ejection |
Semilunar open |
Slow ejection |
— |
|
Isovolumetric relaxation |
All closed |
Pressure fall |
S2 |
|
Rapid filling |
AV open |
Passive filling |
S3 |
|
Reduced filling |
AV open |
Slow filling |
— |
Key Takeaways
- The cardiac cycle is a
coordinated sequence of electrical and mechanical events.
- Pressure gradients determine
valve opening and closure.
- The Wiggers diagram integrates
all cardiac events into one framework.
- Understanding the cardiac cycle
is essential for interpreting:
- ECGs
- Heart sounds
- Hemodynamics
- Valvular diseases
- Heart failure
- Diastole is especially important
for:
- Ventricular filling
- Coronary perfusion