Pressure Volume Loops in Cardiac Cycle

Overview

Pressure volume loops are graphical representations where:

• Y-axis: Pressure in the left ventricle
• X-axis: Volume of the left ventricle
• Each loop corresponds to one cardiac cycle (ventricular systole and diastole).

Key Points in the Loop

End Diastolic Point

• Located at the lower right corner of the loop.
• Mitral Valve Closes: Marks the end of diastole.
  • Produces the first heart sound (S1).
• End Diastolic Volume (EDV):
  • Normal: ~120 ml
  • Ventricle is a relaxed sack full of blood; pressure is low.

Beginning of Systole

• Isovolumetric Contraction Phase:
  • Mitral and aortic valves closed; pressure rises without change in volume.
  • Duration: ~0.05 seconds.

Ejection Phase

• Aortic Valve Opens:
  • Ventricular pressure exceeds aortic pressure (~75 mmHg).
  • Diastolic Blood Pressure (DBP): Lowest aortic pressure at the start of ejection.
• Duration: ~0.25 seconds.
• Peak Left Ventricular and Aortic Pressure:
  • Systolic Blood Pressure (SBP): ~120 mmHg.
  • Pulse Pressure: Difference between SBP and DBP (120 - 75 = 45 mmHg).
• Aortic Valve Closes: Marks end of systole.
  • Produces the second heart sound (S2).
• End Systolic Volume (ESV):
  • Normal: ~50 ml

Calculations

• Stroke Volume:
  • EDV - ESV = 120 ml - 50 ml = 70 ml
  • Volume of blood ejected per heartbeat.
• Ejection Fraction:
  • Stroke Volume / EDV = 70 ml / 120 ml ≈ 58%
  • Percentage of blood volume ejected per heartbeat.

Isovolumetric Relaxation

• Duration: ~0.15 seconds.
• Ventricular pressure falls with no change in volume.

Filling Phase

• Mitral Valve Opens:
  • Initiated when ventricular pressure drops below atrial pressure.
  • Blood flows from atrium to ventricle, both passively and actively.
  • Volume increases to EDV (~120 ml), slight pressure increase.
• Duration of Relaxation Phase: ~0.55 seconds.

Stroke Work

• Definition: Work done by the heart per stroke.
• Proportional to the area inside the pressure volume loop.

Conclusion

• Pressure volume loops display the relationship between left ventricular pressure and volume during a cardiac cycle.
• Provides critical information such as stroke volume, end diastolic volume, and pressures during systole and diastole.