The Cardiac Cycle

Part A: Events During the Cardiac Cycle

The 7 Phases of the Cardiac Cycle

Phase A - Atrial Systole (Atrial Contraction)

• The P wave marks depolarization of the atria, triggering atrial contraction.
• Atrial contraction raises left atrial pressure, propelling blood into the already-filling left ventricle through the open mitral valve.
• This constitutes the final phase of ventricular filling (the "atrial kick"), contributing ~20-25% of stroke volume at rest (up to 40% during heavy exercise).
• The rise in atrial pressure is reflected back to the great veins and appears as the "a" wave on the jugular venous pulse (JVP).
• Ventricular volume increases to reach End-Diastolic Volume (EDV) = ~140 mL.
• A faint S4 ("atrial gallop") may be heard if the ventricle is stiff and resists filling.

Phase B - Isovolumetric Ventricular Contraction (IVC)

• Triggered by the QRS complex, the ventricles begin to contract.
• As soon as left ventricular pressure exceeds left atrial pressure, the mitral valve snaps shut - producing S1 (first heart sound). S1 may be slightly split because the mitral valve closes slightly before the tricuspid valve.
• The aortic valve remains closed (LV pressure has not yet reached aortic pressure).
• With both valves closed, no blood can enter or leave the ventricle - volume remains constant (isovolumetric).
• LV pressure rises steeply but volume stays fixed at 140 mL (EDV).
• This is the phase of maximum myocardial oxygen consumption per unit time.

Phase C - Rapid Ventricular Ejection

• When LV pressure rises above aortic pressure (~80 mmHg diastolic), the aortic valve opens.
• Blood is rapidly ejected into the aorta driven by the pressure gradient.
• LV pressure rises to its peak systolic value (~120 mmHg).
• ~70% of the stroke volume is ejected during this rapid phase.
• Ventricular volume decreases sharply; aortic pressure rises rapidly to its peak (~120 mmHg systolic).
• Simultaneously, the left atrium begins filling with blood returning from the pulmonary veins - left atrial pressure slowly increases (seen as the "c" wave → rising towards "v" wave on JVP).

Phase D - Reduced Ventricular Ejection

• The T wave marks ventricular repolarization - the ventricle begins to relax.
• The aortic valve remains open; blood continues to be ejected but at a slower rate.
• Ventricular volume continues to decrease (reaching End-Systolic Volume, ESV = ~70 mL).
• Aortic pressure begins to fall as blood runs off into the peripheral arterial tree faster than it is being added from the ventricle.
• Left atrial pressure continues to build as pulmonary venous return accumulates.

Phase E - Isovolumetric Ventricular Relaxation (IVR)

• When LV pressure falls below aortic pressure, the aortic valve closes - producing S2 (second heart sound). Aortic valve closes slightly before the pulmonic valve.
• The point of aortic valve closure produces the dicrotic notch (incisura) on the aortic pressure tracing.
• Physiological splitting of S2 occurs during inspiration: decreased intrathoracic pressure increases venous return to the right heart, prolonging right ventricular ejection and delaying pulmonic valve closure, causing the aortic and pulmonic components of S2 to separate.
• With both valves again closed, the ventricle relaxes but volume remains constant (isovolumetric) at 70 mL (ESV).
• LV pressure falls rapidly.

Phase F - Rapid Ventricular Filling

• When LV pressure falls below left atrial pressure, the mitral valve opens.
• Blood rushes from the atrium into the relaxed, highly compliant ventricle - ventricular volume rises rapidly.
• Ventricular pressure stays low because the compliant ventricle accommodates volume with little pressure rise.
• The rapid rush of blood into the ventricle generates S3 (third heart sound). In adults >40 years, S3 = "ventricular gallop" and indicates volume overload (e.g., heart failure, mitral regurgitation).
• The large "v" wave on the JVP tracing collapses as blood empties from the atrium into the ventricle (the "y descent").
• Aortic pressure continues to fall as blood runs off into the arterial tree.

Phase G - Reduced Ventricular Filling (Diastasis)

• This is the longest phase of the cardiac cycle.
• Ventricular filling continues but at a much slower rate as the pressure gradient between atrium and ventricle equalizes.
• Both atrial and ventricular pressures rise slowly and together, driven by the slightly higher pulmonary venous pressure.
• The cycle then resumes with the next P wave and atrial systole.
• Clinical note: Tachycardia primarily shortens or eliminates diastasis. If heart rate is very high, this phase may disappear entirely, compromising ventricular filling, reducing EDV, and reducing stroke volume (via Frank-Starling mechanism).

Summary Table of All 7 Phases

Part B: Volume and Pressure Changes During the Cardiac Cycle

Left Ventricular Pressure Changes

Key Volume Values (Typical at Rest)

Aortic Pressure Waveform Features

• Peak systolic pressure: ~120 mmHg (Phase C)
• Dicrotic notch (incisura): Occurs at aortic valve closure (Phase E) - a brief upward deflection followed by a fall in pressure
• Diastolic pressure: ~80 mmHg (lowest point before next systole)
• Pulse pressure: Systolic - Diastolic = 120 - 80 = 40 mmHg
• Mean arterial pressure (MAP): ~93 mmHg (= Diastolic + 1/3 Pulse Pressure)

Jugular Venous Pulse (JVP) Waves During the Cardiac Cycle

The Left Ventricular Pressure-Volume Loop

• Point 1 (bottom right): End-diastole - mitral valve closes. LV volume = 140 mL, LV pressure = low (~8 mmHg)
• 1 → 2 (IVC): Pressure rises steeply with no volume change (vertical line on right)
• Point 2: Aortic valve opens when LV pressure = aortic diastolic pressure
• 2 → 3 (Ejection): Pressure continues rising then falls; volume decreases significantly. Width of loop = stroke volume (70 mL)
• Point 3 (top left): End-systole - aortic valve closes. LV volume = 70 mL (ESV)
• 3 → 4 (IVR): Pressure falls steeply with no volume change (vertical line on left)
• Point 4: Mitral valve opens when LV pressure falls below LA pressure
• 4 → 1 (Filling): Volume increases back to EDV with minimal pressure change (compliant ventricle)

Right Heart vs. Left Heart

Effect of Heart Rate on the Cardiac Cycle

• At 72 beats/min, systole = ~0.4 of the cycle; diastole = ~0.6 of the cycle
• At very high heart rates, diastole is disproportionately shortened (especially diastasis)
• Tachycardia reduces diastolic filling time → reduces EDV → reduces stroke volume (Frank-Starling)
• This is why very fast heart rates (e.g., >150 bpm) can reduce cardiac output despite more beats per minute