Q1 LONG ANSWER 10M ★ High-yield

Describe the cardiac cycle. Explain the pressure and volume changes in the left ventricle during one cycle. Add a note on the jugular venous pulse.

Physiology · 1st Professional — Pre-Clinical

Definition

Cardiac cycle = the sequence of mechanical and electrical events that occur in the heart from the beginning of one heart beat to the beginning of the next.Duration at HR 75/min = 0.8 second.

Event	Duration
Atrial systole	0.11 s
Atrial diastole	0.69 s
Ventricular systole	0.27–0.3 s
Ventricular diastole	0.5 s
Total cycle	0.8 s

Phases of the Cardiac Cycle

A. Atrial systole (0.11 s)

Contributes only 20–30% of ventricular filling — the "atrial kick"
70–80% of filling has already occurred passively
Produces the a wave of the JVP and the fourth heart sound
Its loss (as in atrial fibrillation) is well tolerated at rest but limits exercise

B. Isovolumetric (isometric) contraction (0.05 s)

Ventricle contracts → AV valves close → first heart sound → All four valves shut → Pressure rises steeply; volume unchanged → Ends when LV pressure exceeds aortic pressure (80 mmHg)

C. Ejection phase (0.22 s)

Rapid ejection (0.11 s) — 70% of stroke volume expelled; LV pressure peaks at 120 mmHg
Reduced ejection (0.11 s) — remaining 30%; pressure begins to fall

D. Protodiastole (0.04 s)

Ventricular pressure falls below aortic pressure
Semilunar valves close → second heart sound
Produces the incisura (dicrotic notch) on the aortic pressure curve

E. Isovolumetric relaxation (0.08 s)

All valves closed again; volume constant at ESV = 50 mL
Pressure falls rapidly to below atrial pressure

F. Ventricular filling (0.5 s)

Rapid filling (0.11 s) — AV valves open; produces the third heart sound
Slow filling / diastasis (0.19 s) — the phase most shortened by tachycardia
Last rapid filling — the next atrial systole

Volume Changes

Parameter	Value
End diastolic volume (EDV)	130 mL
End systolic volume (ESV)	50 mL
Stroke volume (EDV − ESV)	70–80 mL
Ejection fraction (SV/EDV × 100)	60–65%

Pressure Changes

Chamber / vessel	Systolic	Diastolic
Left ventricle	120 mmHg	0–8 mmHg
Aorta	120 mmHg	80 mmHg
Left atrium	—	8–12 mmHg
Right ventricle	25 mmHg	0–5 mmHg
Pulmonary artery	25 mmHg	10 mmHg
Right atrium (CVP)	—	0–8 mmHg

Jugular Venous Pulse

Wave	Cause
A wave	Atrial systole — absent in atrial fibrillation; giant in tricuspid stenosis
C wave	Bulging of the tricuspid valve into the atrium during isovolumetric contraction
X descent	Atrial relaxation + downward pull of the AV ring
V wave	Atrial filling against a closed tricuspid valve
Y descent	Tricuspid valve opens → rapid ventricular filling

Applied Aspects

Tachycardia shortens diastole more than systole → less filling time and less coronary perfusion time → poorly tolerated in mitral stenosis and ischaemic heart disease
Mitral stenosis → obstructed filling → ↑ left atrial pressure → pulmonary congestion
Aortic stenosis → ↑ afterload → LV hypertrophy
Atrial fibrillation → loss of atrial kick → about 20% fall in cardiac output; absent a wave in JVP
Cannon a waves — atrium contracts against a closed tricuspid valve; seen in complete heart block and ventricular tachycardia

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