Explain cardiac cycle as a20 marks answer

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THE CARDIAC CYCLE

Definition

The cardiac cycle refers to the complete sequence of electrical and mechanical events that occur from the beginning of one heartbeat to the beginning of the next. At a normal resting heart rate of 72 beats/min, one complete cycle lasts approximately 0.833 seconds (duration = 1/heart rate). The cycle is identical in both the left and right sides of the heart, though slight timing differences exist between them.

Guyton and Hall Textbook of Medical Physiology
Ganong's Review of Medical Physiology, 26th Edition

Initiation of the Cardiac Cycle

Each cycle begins with spontaneous generation of an action potential in the sinoatrial (SA) node, located in the superior lateral wall of the right atrium near the opening of the superior vena cava. This impulse travels:

Rapidly through both atria via internodal pathways
Through the AV node (where there is a delay of >0.1 second)
Down the Bundle of His - right and left bundle branches - Purkinje fibers to ventricular myocardium

The AV nodal delay is physiologically important: it allows atria to contract first and "prime" the ventricles before ventricular contraction begins. The atria therefore act as primer pumps, and the ventricles provide the major power for systemic and pulmonary circulation.

Phases of the Cardiac Cycle (Wiggers Diagram)

The cardiac cycle is best understood using the Wiggers diagram, which plots simultaneous changes in left ventricular pressure, aortic pressure, left atrial pressure, ventricular volume, ECG, and heart sounds over time.

Wiggers Diagram - The Cardiac Cycle (Costanzo Physiology)

The cycle is classically divided into 7 phases (labeled A through G in Costanzo's framework):

DIASTOLE

Phase A - Atrial Systole (0 - 0.1 sec)

ECG event: P wave (atrial depolarization)
Mechanical event: Both atria contract simultaneously
Atrial contraction raises left atrial pressure, pushing blood through the open mitral valve into the left ventricle
About 70% of ventricular filling occurs passively during early diastole; atrial contraction contributes the final 30% (atrial kick)
Left ventricular pressure rises slightly due to the additional volume
The venous pulse shows the "a" wave (due to atrial contraction)
Heart Sound: S4 (not normally audible; heard in ventricular hypertrophy where compliance is reduced)
Duration: ~0.1 sec

Phase G - Reduced Ventricular Filling / Diastasis (late diastole)

Passive slow filling of the ventricles as the filling rate decreases
No valve events or heart sounds
ECG: baseline (between T wave of previous cycle and next P wave)

SYSTOLE

Phase B - Isovolumetric Ventricular Contraction (0.1 - 0.16 sec)

ECG event: QRS complex (ventricular depolarization)
The ventricles contract; left ventricular pressure rises sharply
As soon as LV pressure exceeds left atrial pressure (~5-10 mmHg), the mitral valve closes (tricuspid closes on right side)
The aortic valve has not yet opened (LV pressure has not exceeded aortic pressure of ~80 mmHg)
Therefore, all valves are closed - no blood enters or leaves the ventricle
Ventricular volume remains constant (isovolumetric) while pressure rises dramatically
Heart Sound: S1 ("lub") - due to mitral + tricuspid valve closure (mitral closes slightly before tricuspid, causing possible physiological splitting)
Duration: ~0.05 sec

Phase C - Rapid Ventricular Ejection (0.16 - 0.30 sec)

LV pressure exceeds aortic diastolic pressure (~80 mmHg) → aortic valve opens
Blood is rapidly ejected from LV into the aorta
Most of the stroke volume (~70%) is ejected during this phase
LV pressure and aortic pressure both rise to their peak (~120 mmHg)
Ventricular volume decreases dramatically
ECG: ST segment
Left atrial pressure begins to rise as pulmonary venous return continues to fill the atria
The venous pulse shows the "c" wave (bulging of tricuspid valve toward atrium)

Phase D - Reduced Ventricular Ejection (0.30 - 0.43 sec)

Ventricular repolarization begins (T wave on ECG)
Ventricular pressure begins to fall as myocardium relaxes
The aortic valve is still open - blood continues to be ejected at a reduced rate
Ventricular volume continues to fall (reaches minimum = End-Systolic Volume, ESV ≈ 50 mL)
Blood "runs off" into the arterial tree faster than ejection, so aortic pressure begins to fall
Left atrial pressure continues to rise (the "v" wave on venous pulse - due to venous return during closed AV valves)
ECG: T wave (ventricular repolarization)

EARLY DIASTOLE

Phase E - Isovolumetric Ventricular Relaxation (0.43 - 0.50 sec)

After complete ventricular repolarization, LV pressure drops rapidly
When LV pressure falls below aortic pressure, the aortic valve closes
Aortic valve closure produces the dicrotic notch (incisura) on the aortic pressure tracing
All valves are again closed - ventricular volume is constant (isovolumetric) while pressure falls
Heart Sound: S2 ("dub") - due to aortic + pulmonic valve closure (aortic closes slightly before pulmonic)
- During inspiration, pulmonic valve closure is delayed (increased venous return to right heart prolongs RV ejection) → physiological splitting of S2
Duration: ~0.07 sec

Phase F - Rapid Ventricular Filling (0.50 - 0.65 sec)

When LV pressure falls below left atrial pressure (~5-8 mmHg), the mitral valve opens
Blood rushes rapidly from the left atrium into the left ventricle down the pressure gradient
Ventricular volume increases rapidly
Left atrial pressure falls sharply as the "v" wave descends (the "y" descent)
Heart Sound: S3 - heard in rapid filling phase; normal in children and young adults; pathological in adults (indicates ventricular volume overload, e.g., heart failure)
Duration: ~0.15 sec

Key Quantitative Parameters

Parameter	Normal Value
Duration of cardiac cycle (at 72 bpm)	0.833 sec
Systole	~0.3 sec
Diastole	~0.5 sec
End-Diastolic Volume (EDV)	~130 mL
End-Systolic Volume (ESV)	~50 mL
Stroke Volume (SV = EDV - ESV)	~70-90 mL
Ejection Fraction (EF = SV/EDV)	~65% (normal >55%)
Peak LV systolic pressure	~120 mmHg
Peak RV systolic pressure	~25 mmHg
Aortic diastolic pressure	~80 mmHg

Pressure-Volume Loop

The P-V loop is a graphical representation of one complete cardiac cycle:

d → a: Passive ventricular filling (diastole) - volume increases, pressure rises slightly
a → b: Isovolumetric contraction - pressure rises sharply, volume constant
b → c: Ventricular ejection - volume decreases as blood enters aorta
c → d: Isovolumetric relaxation - pressure falls rapidly, volume constant

The width of the loop = stroke volume; the area of the loop = stroke work.

Heart Sounds Summary

Sound	Cause	Phase	Clinical Note
S1 (lub)	Mitral + tricuspid closure	Start of ventricular systole	Best heard at apex/LLSB
S2 (dub)	Aortic + pulmonic closure	End of ventricular systole	Physiologically split on inspiration
S3	Rapid ventricular filling	Early diastole	Normal in children; pathological in adults (LV failure)
S4	Atrial contraction against stiff LV	Late diastole (atrial systole)	Pathological; heard in hypertrophy/ischemia

Venous Pulse Waves

The jugular venous pulse reflects events in the right atrium and has three positive deflections:

"a" wave - right atrial contraction (atrial systole)
"c" wave - tricuspid valve bulging toward atrium at onset of ventricular systole
"v" wave - venous return filling the atrium while tricuspid valve is closed (during ventricular systole)

Corresponding descents: x descent (atrial relaxation + tricuspid being pulled down during ejection) and y descent (tricuspid valve opening, atrial emptying).

Effect of Heart Rate on Cardiac Cycle

When heart rate increases, the duration of the cardiac cycle decreases. Diastole is shortened proportionally more than systole. At very high heart rates:

Incomplete ventricular filling occurs (reduced EDV → reduced SV)
At 3× normal heart rate, systole comprises ~65% of the cycle vs ~40% at rest

This is why tachycardia can reduce cardiac output despite a higher rate - insufficient filling time.

Right vs. Left Heart Timing

Although both sides go through identical phases, there are slight asynchronies:

Right atrial systole precedes left atrial systole
Right ventricular contraction starts after left ventricular contraction
However, right ventricular ejection begins before left because pulmonary artery pressure (25 mmHg) is much lower than aortic pressure (80 mmHg)
The pulmonic valve closes after the aortic valve - basis of physiological S2 splitting

Clinical Correlations

Condition	Alteration in Cardiac Cycle
Aortic stenosis	Prolonged isovolumetric contraction; delayed aortic valve opening
Mitral stenosis	Prolonged rapid filling phase; loud S1; opening snap
Heart failure	Reduced ejection fraction; S3 gallop; elevated EDV
Aortic regurgitation	Wide pulse pressure; increased EDV; dicrotic notch absent
Atrial fibrillation	Loss of "a" wave; no S4; variable stroke volumes

Summary

The cardiac cycle is a precisely coordinated sequence of electrical and mechanical events divided into systole (isovolumetric contraction + ejection) and diastole (isovolumetric relaxation + ventricular filling). Coordinated valve opening and closure, regulated by pressure gradients, ensures unidirectional blood flow. The Wiggers diagram and P-V loop together provide a complete picture of cardiac mechanics. Understanding the cardiac cycle is fundamental to interpreting ECGs, heart sounds, venous pulse waveforms, and the pathophysiology of cardiac diseases.

Sources: Guyton and Hall Textbook of Medical Physiology | Costanzo Physiology, 7th Ed. | Ganong's Review of Medical Physiology, 26th Ed. | Morgan & Mikhail's Clinical Anesthesiology, 7th Ed.

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