Cardiovascular Physiology: Pressure–Volume Loops & the Cardiac Cycle
The pressure–volume (PV) loop maps one heartbeat as LV pressure vs. volume, traced counterclockwise through four phases. Mastering its corners, valve-opening points, and how preload, afterload, and contractility reshape it unlocks the cardiac cycle, heart sounds, and JVP waveforms tested on Step 1.
The PV Loop in One Frame
A pressure–volume (PV) loop plots left-ventricular (LV) pressure (y-axis) against LV volume (x-axis) for one heartbeat, traced counterclockwise. Its four sides map directly onto the cardiac cycle: two isovolumetric limbs (all four valves closed, volume fixed, pressure changing) and two volume-changing limbs (ejection and filling).
Two relationships you must know: the width of the loop equals stroke volume (SV = EDV − ESV), and the area inside the loop equals ventricular stroke work. Two boundary lines frame the loop — the end-systolic PV relationship (ESPVR), whose slope reflects contractility, and the end-diastolic PV relationship (EDPVR), which reflects ventricular compliance.
- Loop is traced counterclockwise; start = bottom-right corner = EDV (mitral valve just closed).
- The two isovolumetric phases = all four valves closed → vertical limbs, volume constant, pressure changing.
- Valve sequence: Mitral closes → isovolumetric contraction; Aortic opens (LVP > aortic diastolic ≈ 80 mmHg) → ejection; Aortic closes (→ S2) → isovolumetric relaxation; Mitral opens → filling.
- SV = EDV − ESV ≈ 120 − 50 = 70 mL; EF = SV/EDV ≈ 58% (normal ≈ 55–70%).
- Loop width = stroke volume; loop area = stroke work.
- ESPVR slope = contractility (end-systolic point, upper-left corner); EDPVR = compliance (end-diastolic point, lower-right corner).
- Most-tested manipulation: ↑ contractility → ESPVR steepens → ↓ ESV → ↑ SV and ↑ EF; ↑ afterload → ↑ ESV → ↓ SV; ↑ preload → ↑ EDV → ↑ SV (Frank–Starling).
- Peak dP/dt (max rate of pressure rise, an index of contractility) occurs during isovolumetric contraction.
The Four Phases
| Phase | Valve boundaries | Volume | Pressure | Timing / sound |
|---|---|---|---|---|
| 1. Isovolumetric contraction | Mitral closes → Aortic opens | Constant (EDV) | Rapidly rising | Early systole — S1 |
| 2. Ventricular ejection | Aortic opens → Aortic closes | Falls (EDV → ESV) | Peaks, then falls | Systole |
| 3. Isovolumetric relaxation | Aortic closes → Mitral opens | Constant (ESV) | Rapidly falling | Early diastole — S2 |
| 4. Ventricular filling | Mitral opens → Mitral closes | Rises (ESV → EDV) | Low, slowly rising | Diastole — S3 / S4 if present |
Three Levers That Reshape the Loop
- ↑ Preload (↑ venous return) stretches the ventricle to a larger EDV — the bottom-right corner slides right along the EDPVR — so by the Frank–Starling mechanism the loop widens and SV rises (ESV roughly unchanged).
- ↑ Afterload (↑ aortic pressure / systemic vascular resistance) forces the aortic valve to open against a higher pressure; ejection stops earlier, leaving a larger ESV, so the loop grows taller and narrower and SV falls.
- ↑ Contractility (sympathetic tone / inotropes) steepens the ESPVR, emptying the ventricle to a smaller ESV, so SV and EF both rise.
Acute Effects on Cardiac Volumes
| Intervention | EDV | ESV | SV | EF |
|---|---|---|---|---|
| ↑ Preload | ↑ | — | ↑ | ↑ (modest) |
| ↑ Afterload | — | ↑ | ↓ | ↓ |
| ↑ Contractility | — | ↓ | ↑ | ↑ |
Heart Sounds & Jugular Venous Waveforms
Overlaying the cardiac cycle (the Wiggers diagram) explains the auscultatory and venous findings:
- S1 = mitral + tricuspid closure, marking the start of isovolumetric contraction (systole).
- S2 = aortic + pulmonic closure, marking the start of isovolumetric relaxation (diastole); the dicrotic notch (incisura) on the aortic tracing is aortic-valve closure.
- S3 = early-diastolic rapid ventricular filling (normal in youth/pregnancy; in older adults signals a dilated, volume-overloaded ventricle).
- S4 = late-diastolic atrial kick into a stiff, non-compliant ventricle.
The jugular venous pulse tracks right-atrial pressure: a wave = atrial contraction; c wave = tricuspid bulging during RV isovolumetric contraction; x descent = atrial relaxation; v wave = atrial filling against a closed tricuspid; y descent = tricuspid opening / atrial emptying.
Classic Step 1 associations:
- S3 → heart failure / dilated cardiomyopathy, mitral regurgitation, high-output states.
- S4 → LV hypertrophy from hypertension, aortic stenosis, or HCM (stiff ventricle).
- Aortic stenosis → pressure-overload PV loop (↑ peak systolic pressure, ↑ afterload), concentric LVH, pulsus parvus et tardus, narrow pulse pressure.
- Mitral regurgitation → loss of true isovolumetric contraction/relaxation (blood escapes into the LA), so the loop's vertical limbs disappear; ↑ EDV, reduced afterload, early ↑ (total) EF.
- Aortic regurgitation → loss of isovolumetric relaxation (aortic valve leaks in diastole), right-shifted enlarged loop, wide pulse pressure.
- Cannon a waves → AV dissociation (complete heart block, VT).
- Large v waves → tricuspid regurgitation; absent/blunted y descent → cardiac tamponade; prominent x and y (M/W pattern) → constrictive pericarditis.
- S3 = "Ken-TUCK-y" (S1–S2–S3): the extra sound falls after S2 → early diastole; think ventricular gallop and a dilated, volume-overloaded heart.
- S4 = "TEN-nes-see" (S4–S1–S2): the extra sound falls before S1 → late diastole; think atrial gallop and a stiff, non-compliant ventricle.
Both are low-pitched — heard best with the bell at the apex (left lateral decubitus).
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