Cardiac Cycle & Pressure Volume Loop

Key guiding questions:

1. What are the two main phases of the cardiac cycle and their durations?
2. What are the key events that occur during ventricular systole?
3. How does ventricular systole affect the opening of the aortic and pulmonary valves?
4. What are the key events that occur during ventricular diastole?
5. What is relationship between the ventricular pressure vs volume during various the cardiac cycle?

FOCUSED NOTES

Cardiac Cycle

DURATION: 0.8 s

• At the beginning of cardiac cycle, characteristic features during diastole are:
  • All the 4 chambers are relaxed and filled with blood due to venous return.
  • As A-V valve opens, atrium and ventricle of each side comes in continuity to each other and the pressure in each cavity is equal.

Phases

1. Atrial systole: atrial contraction phase (0.1sec)

• • Follows impulse generation in the S.A. Node.
  • Atrial muscle contracts and the pressure increases.
  • Propels 30% of additional blood into the ventricles.

2. Ventricular systole: (0.27 sec)
   • Isometric contraction: 0.05sec

• • • Ventricles contract and the pressure exceeds the atrial pressure.
    • Closure of AV Valvesà first heart sound.
    • Bulging of AV valves into the atrium.

• • Ventricular systole proper: (0.25 sec)- pressure in ventricles exceeds the arterial system.
    • Rapid ejection phase: (0.1sec)

• • • Intra ventricular pressure rises to the maximum and increases the ventricular output.
    • 2/3^rd of the stroke volume is ejected.
    • AV valves come back to original position & a sharp fall in atrial pressure occurs.

1. • Slow ejection phase (0.15sec)

• • • Ventricular contraction subsides and the pressure decreases.

3. Ventricular diastole: (0.5sec)
   • Proto diastole: (0.04sec)

• • • Ventricular pressure drops more rapidly. Closure of Semilunar valvesà second heart sound.

1. • Isometric relaxation: (0.08sec)

• • • Initial part of ventricular diastole.
    • Intra ventricular pressure drops, ventricular muscle continues to relax without change in the ventricular volume.

1. • Ventricular diastole proper: Passive filling of ventricles (70%)

• • • Initial rapid filling phase: opening of AV Valves and continued relaxation of ventricles.
    • Slow filling phase (Diastasis): continuous venous return filling both atrium & ventricles.

4. Atrial diastole: (0.7 sec)

• • Atrial muscle relaxes and the pressure gradually increases due to venous return.

Ventricular Pressure-Volume Relationship (LOOP)

1. Phase a (diastole): Ventricular filling
2. Phase b (systole): Isovolumetric contraction
3. Phase c (systole): Ejection
4. Phase d (diastole): Isovolumetric relaxation

• Point 1: volume at the end of ventricular filling (diastole) therefore represents the end-diastolic pressure and end-diastolic volume (EDV)
• Point 1: aortic valve closes & ejection ceases therefore represents the end-systolic (residual) volume (ESV).
• Width of the loop: represents the difference between EDV and ESV = stroke volume (SV).
• The area within the loop = ventricular stroke work.

Extra Edge

• End diastolic volume: volume of blood in the ventricles at the end of diastole (Maximum volume of blood)
• Stroke volume: volume of blood ejected by the ventricles per stroke at rest.
• End systolic volume: EDV-SV.

Cardiac Cycle & Pressure Volume Loop MCQ’s