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Cardiac Cycle & Pressure-Volume Loops

Name: Cardiac Cycle & Pressure-Volume Loops
Creator: Veli Bakalov, MD

Every click, gallop, and neck-vein wave mapped onto the Wiggers diagram

Cardiovascular·Physiology·4 min read·8 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Valve order each beat: mitral closes (S1) → isovolumetric contraction → aortic opens → ejection → aortic closes (S2, dicrotic notch) → isovolumetric relaxation → mitral opens → filling.
The two isovolumetric periods are the only times all four valves are closed; isovolumetric contraction has the highest myocardial O2 consumption.
Stroke volume = EDV − ESV (PV-loop width); ejection fraction = SV/EDV; diastole fills about 2/3 of the cycle at rest and the atrial kick adds about 20%.
S1 = mitral + tricuspid closure; S2 = aortic + pulmonic closure; S2 splits on inspiration; fixed split = ASD; paradoxical split = aortic stenosis or LBBB.
S3 (early diastole) = dilated, volume-overloaded ventricle (systolic HF, MR), normal in children and pregnancy; S4 (late diastole) = atrial kick into a stiff ventricle (LVH, HTN, AS), absent in atrial fibrillation.
JVP: a = atrial contraction, c = RV contraction bulging the tricuspid, v = atrial filling; x = atrial relaxation, y = RA emptying.
Cannon a waves = atrium contracting against a closed tricuspid in AV dissociation (complete heart block, VT); large v wave = tricuspid regurgitation; no a wave in atrial fibrillation.
PV loop runs counterclockwise: ↑ preload widens it rightward (↑ SV); ↑ afterload raises ESV and lowers SV; ↑ contractility steepens the ESPVR and lowers ESV.

Phases of the cardiac cycle

Systole ejects, diastole fills. The cycle alternates systole (contraction and ejection) with diastole (relaxation and filling); at rest diastole occupies about 2/3 of the cycle. The Wiggers phases are atrial contraction, isovolumetric contraction, rapid then reduced ejection, isovolumetric relaxation, and rapid then reduced filling. Every valve event is set by a pressure crossover between two chambers.

Isovolumetric contraction. After the mitral valve closes (S1), the ventricle contracts with all valves closed: pressure climbs steeply while volume stays at end-diastolic volume (EDV). It is the phase of highest oxygen consumption and ends when LV pressure exceeds aortic pressure and the aortic valve opens.

Ejection. Blood leaves the ventricle, rapidly and then more slowly, and volume falls from EDV to end-systolic volume (ESV). Stroke volume (SV) = EDV − ESV; ejection fraction = SV/EDV; cardiac output = SV × heart rate.

Isovolumetric relaxation. When LV pressure falls below aortic pressure the aortic valve closes (S2, seen as the dicrotic notch on the aortic tracing). The ventricle relaxes with all valves closed, so pressure falls at a constant volume (ESV) until it drops below left atrial pressure.

Filling. The mitral valve opens and filling proceeds in three steps: rapid passive filling (when an S3 may occur), reduced filling (diastasis), and atrial contraction (S4, the a wave), which tops the ventricle up to EDV. Filling is passive early and active late; the atrial kick adds about 20%.

Mitral closes (S1)Isovolumetric contraction begins at EDV
Aortic opensEjection
Aortic closes (S2)Isovolumetric relaxation begins at ESV
Mitral opensFilling, ending with the atrial kick

Valve events: mitral closes first and opens last

The two isovolumetric periods are the only moments when all four valves are closed. Aortic events bracket ejection; mitral events bracket filling.

Heart sounds

S1 and S2. S1 is closure of the AV valves (mitral and tricuspid) at the onset of systole. S2 is closure of the semilunar valves (aortic and pulmonic) at the onset of diastole.

Splitting of S2. Normally S2 splits on inspiration, A2 before P2, because increased venous return to the right heart delays pulmonic closure. Abnormal patterns:

Wide splitting: delayed RV emptying (pulmonic stenosis, right bundle branch block)
Fixed splitting: atrial septal defect
Paradoxical splitting (P2 before A2, split on expiration): delayed aortic closure in aortic stenosis or left bundle branch block

S3 vs S4

Feature	S3 (ventricular gallop)	S4 (atrial gallop)
Timing	Early diastole, rapid passive filling	Late diastole, just before S1
Mechanism	Blood rushing into a dilated, volume-loaded ventricle	Atrium contracting against a stiff, noncompliant ventricle
Pathologic causes	Dilated systolic heart failure, mitral regurgitation, high-output states	LVH, hypertension, aortic stenosis, diastolic dysfunction, HCM
Can be normal	Children, young adults, pregnancy	Common in older adults (stiffer ventricles); otherwise pathologic
In atrial fibrillation	Can still occur	Absent (needs organized atrial contraction)

Both are low-pitched; S3 = dilated and floppy (systolic failure), S4 = stiff and thick (diastolic dysfunction).

Jugular venous pulse

Waves. The a wave is right atrial contraction; the c wave is RV contraction bulging the closed tricuspid valve back into the right atrium; the v wave is passive atrial filling against the closed tricuspid valve.

Descents. The x descent is atrial relaxation; the y descent is tricuspid opening and right atrial emptying into the ventricle.

Cannon a waves: the atrium contracts against a closed tricuspid valve during AV dissociation (complete heart block, ventricular tachycardia)
Large v wave: tricuspid regurgitation
Absent a wave: atrial fibrillation
Large a wave: resistance to RA emptying (tricuspid stenosis, RV hypertrophy, pulmonary hypertension)

Pressure-volume loops

Reading the loop. The LV pressure-volume loop runs counterclockwise. The bottom-right corner is mitral closure at EDV; the right vertical side is isovolumetric contraction; the top-right corner is aortic opening; the top edge is ejection; the top-left corner is aortic closure at ESV; the left vertical side is isovolumetric relaxation; the bottom-left corner is mitral opening, followed by filling along the bottom. Loop width (EDV − ESV) is the stroke volume, and loop area approximates stroke work.

Preload, afterload, contractility. Increased preload (higher EDV) widens the loop to the right and raises SV (Frank-Starling). Increased afterload makes the aortic valve open at a higher pressure, raising ESV and lowering SV (a taller, narrower loop). Increased contractility steepens the end-systolic pressure-volume relationship (ESPVR), lowering ESV and raising SV. The ESPVR slope is contractility: a steeper line means a stronger, better-emptying ventricle.

How the loop changes

Change	EDV	ESV	SV	Loop shape
↑ Preload	↑	≈	↑	Wider to the right
↑ Afterload	≈	↑	↓	Taller and narrower
↑ Contractility	≈	↓	↑	Steeper ESPVR; wider to the left
Aortic stenosis	≈	↑	↓	Much higher systolic pressure
Aortic regurgitation	↑↑	≈	↑	No true isovolumetric relaxation
Mitral regurgitation	↑	↓	↑ (partly backward)	No true isovolumetric contraction
Mitral stenosis	↓	≈	↓	Narrow; high left atrial pressure

≈ = little or no change in the idealized loop.

Acute hypertension is a pure afterload rise: the aortic valve opens later at higher pressure, more blood is left behind (↑ ESV), and stroke volume falls.