Understanding Cardiac Pressure-Volume Loops: S1, S2, Stroke Volume and Ejection Fraction

Explore how pressure-volume loops map the heart’s cycle by plotting left ventricular pressure against volume. This video explains the sequence from end diastolic volume through isovolumetric contraction, ejection into the aorta, and isovolumetric relaxation, highlighting the first and second heart sounds (S1 and S2). It shows end diastolic volume around 120 mL, end systolic volume about 50 mL, stroke volume of 70 mL, and an ejection fraction near 58%. Pulse pressure is about 45 mmHg. The loop area relates to stroke work, and the video ties these concepts to physiological variations among individuals. A quick recap emphasizes the PV loop as a compact view of cardiac function.

Overview

Pressure-volume loops are a powerful way to visualize how the left ventricle changes pressure and volume during a single heartbeat. The video uses a concrete example to illustrate core measurements: end diastolic volume (EDV) around 120 mL, diastolic pressure around 75 mmHg, and systolic pressure around 120 mmHg. As blood is ejected, the LV volume decreases while pressure remains nearly equal to the aortic pressure until the aortic valve closes. The end systolic volume (ESV) is about 50 mL, and the stroke volume (SV) is the difference between EDV and ESV, here 70 mL. The pulse pressure in this example is 45 mmHg. The loop’s area represents stroke work, i.e., the work the heart performs each beat.

Phases of the Cardiac Cycle

• End diastolic point: mitral valve closes with the first heart sound S1 as the ventricle is filled and pressure remains low.
• Isovolumetric contraction: both mitral and aortic valves are closed, LV pressure rises without a change in volume, lasting about 0.05 seconds.
• Aortic valve opens and ejection begins: LV pressure rises further and LV and aortic pressures are essentially equal as blood is ejected into the aorta; SBP reaches about 120 mmHg.
• End systolic point: aortic valve closes with the second heart sound S2; LV volume is about 50 mL.
• Isovolumetric relaxation: after S2, ventricular muscle relaxes, pressure falls with all valves closed, lasting roughly 0.15 seconds. The mitral valve then opens as atrial pressure exceeds LV pressure, allowing filling to resume.

Derived Measurements

• Stroke volume (SV) = EDV – ESV = 120 mL – 50 mL = 70 mL.
• Ejection fraction (EF) = SV / EDV = 70 / 120 ≈ 58%.
• Pulse pressure = SBP – DBP = 120 – 75 = 45 mmHg.
• Stroke work is proportional to the loop area, linking the PV trajectory to the heart’s energy expenditure per beat.

Clinical Relevance and Variability

The PV loop integrates systolic and diastolic events into a single visual and quantitative framework. It helps explain how heart size, loading conditions, and disease can alter stroke volume, EF, and the energy the heart must expend to pump blood. The video underscores that loop dimensions reflect cardiac performance and loading, not just raw pressures or volumes.

Recap

In summary, pressure-volume loops chart the dynamic LV pressure-volume relationship across a heartbeat, yielding important metrics such as SV, EF, SBP, DBP, and pulse pressure while illustrating the work the heart performs with each beat.