U01.07.012 Pressure-volume loops and cardiac cycle

The cardiac cycle consists of various phases that regulate the heart’s function, ensuring efficient circulation. The pressure-volume (PV) loop illustrates the relationship between left ventricular pressure and volume during different phases of the cycle.

Phases of the Left Ventricle in the Cardiac Cycle

Phase	Description
Isovolumetric Contraction	The period between aortic valve opening and closing.
Systolic Ejection	The period between aortic valve closing and mitral valve opening.
Isovolumetric Relaxation	The period just before the mitral valve closes.
Rapid Filling	The period just before the mitral valve closing.
Reduced Filling	The period just before the mitral valve closes.

Key Cardiac Parameters

Parameter	Definition
Stroke Volume (SV)	Volume of blood pumped per heartbeat (SV = EDV – ESV).
End-Diastolic Volume (EDV)	The volume of blood remaining in the ventricle after contraction.
End-Systolic Volume (ESV)	Volume of blood remaining in the ventricle after contraction.
Ejection Fraction (EF)	Percentage of blood ejected per heartbeat (EF = SV/EDV).
Preload	Initial stretching of cardiac myocytes before contraction.
Afterload	Resistance the heart must pump against (e.g., aortic pressure).
Contractility	Strength of contraction independent of preload and afterload.

Heart Sounds

Heart Sound	Description
S1	Closure of mitral and tricuspid valves (loudest at the mitral area).
S2	Closure of aortic and pulmonary valves (loudest at the left upper sternal border).
S3	Heard in early diastole during rapid ventricular filling. Can be normal in children, young adults, athletes, and pregnancy, but abnormal in heart failure.
S4	Heard in late diastole (“atrial kick”). Associated with high atrial pressure and ventricular noncompliance (e.g., hypertrophy).

Jugular Venous Pulse (JVP) Waves

Wave	Cause
a wave	Atrial contraction (absent in atrial fibrillation).
c wave	Right ventricular contraction (tricuspid valve bulging into the atrium).
x descent	Downward displacement of a closed tricuspid valve during ventricular ejection (reduced in tricuspid regurgitation).
v wave	Right atrial filling against a closed tricuspid valve.
y descent	Right atrium emptying into the right ventricle (prominent in constrictive pericarditis, absent in cardiac tamponade).

Effects of Valvular Disease on the PV Loop

Condition	Effect on PV Loop
Aortic Stenosis	↑ Left ventricular pressure, ↑ ESV, ↓ SV, ventricular hypertrophy.
Mitral Regurgitation	No true isovolumetric phase, ↓ ESV, ↑ EDV due to regurgitation, ↑ SV.
Aortic Regurgitation	No true isovolumetric phase, ↑ EDV, ↑ SV, large pulse pressure.
Mitral Stenosis	↑ Left atrial pressure, ↓ EDV due to impaired filling, ↓ SV.

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