Pressure-volume relationships

PV Loops - The Heart's Dance

• A → B: Isovolumetric contraction (all valves closed).
• B → C: Systolic ejection (aortic valve opens).
• C → D: Isovolumetric relaxation (all valves closed).
• D → A: Diastolic filling (mitral valve opens).

• Stroke Volume (SV): Width of the loop (EDV − ESV).
• Work: Area within the loop.

⭐ Increased contractility causes a wider and taller loop, indicating ↑ stroke volume and ↑ peak systolic pressure.

Cardiac Cycle Phases - Loop de Loop

• A → B: Isovolumetric Contraction
  • Mitral valve closes (S1 sound).
  • Ventricular pressure ↑, volume is constant (isovolumetric).
• B → C: Systolic Ejection
  • Aortic valve opens.
  • Ventricular volume ↓ as blood is ejected.
• C → D: Isovolumetric Relaxation
  • Aortic valve closes (S2 sound).
  • Ventricular pressure ↓, volume is constant.
• D → A: Diastolic Filling
  • Mitral valve opens.
  • Ventricle fills with blood from the atrium.

⭐ The width of the PV loop represents the stroke volume (end-diastolic volume - end-systolic volume).

PV Loop Variables - By the Numbers

• Stroke Volume (SV): Width of the loop.
  • $SV = EDV - ESV$
  • Normal: ~70 mL
• Ejection Fraction (EF): Index of contractility.
  • $EF = (SV / EDV) \times 100$
  • Normal: >55%
• Cardiac Output (CO):
  • $CO = SV \times Heart Rate$
• Pulse Pressure (PP):
  • $PP = Systolic - Diastolic Pressure$

⭐ The area within the PV loop represents the ventricular stroke work, a measure of the energy the heart imparts to the blood with each beat.

Altered States - Shifting Shapes

• ↑ Preload (e.g., IV fluids): Loop widens rightward. Increases end-diastolic volume (EDV) and stroke volume (SV) via Frank-Starling mechanism. No change in contractility.
• ↑ Afterload (e.g., hypertension): Loop becomes taller and narrower. Increases aortic pressure and end-systolic volume (ESV), leading to a decreased SV.
• ↑ Contractility (e.g., dobutamine): Loop widens leftward. Decreases ESV and increases SV, raising ejection fraction. The ESPVR shifts upward and left.

⭐ Increased afterload causes the heart to expend more energy to eject less blood, significantly increasing myocardial oxygen demand.

High‑Yield Points - ⚡ Biggest Takeaways

• The width of the PV loop represents stroke volume (EDV − ESV).
• Increased preload (↑EDV) widens the loop to the right, increasing stroke volume.
• Increased afterload (↑aortic pressure) narrows and heightens the loop, decreasing stroke volume.
• Increased contractility shifts the ESPVR up and left, widening the loop and increasing stroke volume.
• The area within the loop approximates ventricular stroke work.
• Isovolumetric phases are the vertical lines of the loop.