Core mental model
The cardiac cycle is one pressure-gradient story.
Electrical activity changes muscle state, muscle state changes pressure, pressure opens or closes valves, and valves determine flow, volume and sounds.
The Wiggers unlock
Valves are passive.
A valve opens when upstream pressure exceeds downstream pressure. It closes when the gradient reverses.
Wiggers chain
ECG -> contraction / relaxation -> pressure change -> valve movement -> blood flow -> volume change -> heart sound
0.8 s
cycle at 75 bpm
120
mL EDV
50
mL ESV
70
mL stroke volume
ECG
Electrical first
P wave precedes atrial contraction. QRS precedes ventricular contraction. T wave precedes ventricular relaxation.
Valves
Pressure decides
Mitral and aortic valve events are predicted by left atrial, left ventricular and aortic pressures.
Sounds
Closure makes sound
S1 = AV valve closure. S2 = semilunar valve closure. S3/S4 reflect filling mechanics.
Valve board
Four left-sided pressure switches
pressure gradient = valve event
Mitral opens
LA pressure > LV pressure
Rapid ventricular filling begins.
Mitral closes
LV pressure > LA pressure
S1 and isovolumetric contraction.
Aortic opens
LV pressure > aortic pressure
Ventricular ejection begins.
Aortic closes
Aortic pressure > LV pressure
S2, dicrotic notch and isovolumetric relaxation.
Cycle phase map
Atrial systole
After P wave. AV open, semilunar closed. Atrial kick completes filling and EDV is reached.
Iso contraction
After QRS. AV valves close -> S1. All valves closed; pressure rises, volume constant.
Rapid ejection
Aortic valve opens. Most stroke volume exits; aortic pressure rises.
Reduced ejection
Contraction weakens, outflow slows, ventricular pressure begins to fall.
Iso relaxation
Semilunar valves close -> S2. All valves closed; pressure falls, volume constant.
Rapid filling
Mitral valve opens. Passive filling is fast; S3 may occur.
Diastasis
Slow filling. Shortened most by tachycardia.
Diastole
Relaxation and filling. Critical for LV coronary perfusion.
Equation board
SV = EDV - ESV
70 = 120 - 50 mL
EF = SV / EDV
Normal LV EF commonly about 55-70%
CO = HR x SV
About 5 L/min at rest: 70 bpm x 70 mL
Pressure-volume loop translation
EDV
Maximum volume, reached at end-diastole.
ESV
Minimum volume, remaining after systole.
Width
Loop width equals stroke volume.
Area
Area inside the loop equals stroke work.
Heart sounds and atrial waves
S1
AV valves close. Start of ventricular systole, just after QRS.
S2
Semilunar valves close. End of systole, after T wave.
S3
Early diastole during rapid filling. Volume overload in older adults.
S4
Late diastole: atrial contraction into a stiff ventricle. Absent in AF.
a wave
Atrial contraction after the P wave.
c wave
AV valve bulges into atrium during early ventricular contraction.
v wave
Atrial filling against a closed AV valve during ventricular systole.
Preload
Stretch before contraction
Related to venous return, EDV and filling. Higher preload usually increases SV through Frank-Starling within physiologic limits.
Afterload
Load to eject against
Related to aortic pressure and SVR. Higher afterload raises ESV, lowers SV and increases myocardial O2 demand.
Contractility
Force at fixed loading
Higher contractility lowers ESV and raises SV and EF. Reduced by MI, systolic HF and negative inotropes.
Tachycardia consequence
Tachycardia disproportionately shortens diastole.
Less diastole means less ventricular filling time and less left coronary perfusion time.
S2 splitting
Inspiration delays P2.
Inspiration increases venous return to the right heart, prolongs RV ejection and widens physiologic S2 splitting: A2 then P2.
Murmur timing logic
Systolic
Between S1 and S2. Think ejection across semilunar valves or backflow through AV valves.
Aortic stenosis, pulmonic stenosis, mitral regurgitation, tricuspid regurgitation, VSD.
Diastolic
After S2 and before the next S1. Think backflow through semilunar valves or filling across AV valves.
Aortic regurgitation, pulmonic regurgitation, mitral stenosis, tricuspid stenosis.
Stenosis
Turbulence when the valve should be open.
Regurgitation
Turbulence when the valve should be closed.
Common exam traps
Valves are not active
They open and close passively by pressure gradients.
All valves never open
During both isovolumetric phases, all valves are closed.
ECG comes first
Electrical events precede mechanical events.
S1/S2 are closure
S1 is AV closure. S2 is semilunar closure.
Preload vs afterload
Preload is filling/stretch; afterload is ejection load.
S4 needs atria
S4 disappears in atrial fibrillation.
S3 vs S4
S3 = volume overload. S4 = stiff ventricle.
Diastole is vulnerable
Tachycardia shortens diastole more than systole.
How to reconstruct any phase
Ask pressure first
Is ventricular pressure rising or falling? Which pressure is higher: atrial, ventricular or arterial? That determines valves.
Then ask volume and sound
If all valves are closed, volume is constant. If a valve closes, expect a sound. If a valve opens, expect flow.