Cardiac Cycle- Exam Ready Notes

 

Cardiac Cycle

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
Diastole	Ventricular relaxation and filling	0.5 s
Systole	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

------------------------------------------------

Heart Sounds:      S1             S2

 

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
S4	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

Frank-Starling Mechanism

Within physiological limits: ↑EDV→↑Stroke Volume

Greater filling causes:

• Greater myocardial stretch
• Stronger contraction

Ventricular Compliance

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