USMLE Step 1Cardiovascular physiologyEP 05 of 16

Wiggers diagram explained: the cardiac cycle step by step

A Wiggers diagram is a few traces on one time axis. One rule decodes it: blood moves down a pressure gradient, and each valve is a one-way door that obeys the gradient.

5:30 video
3 min read
4 sources
Published 8 Nov 2026

Video lesson · 5:30 · Watch on YouTubeSeries: Pressure–Volume Loops & Cardiac Physiology · @medservatory

Chapters (16)

By the end of this lesson

Relate left-ventricular, aortic and atrial pressures, ventricular volume, heart sounds and the ECG to the valve events in one cardiac cycle.

The short version

Valves open and close purely according to pressure gradients; they are one-way doors, not actively driven.
The cycle has two halves, diastole (relax and fill) and systole (contract and eject), split into seven phases.
S1 is closure of the AV valves (mitral first) and starts isovolumetric contraction; S2 is aortic valve closure (pulmonic follows) and starts isovolumetric relaxation.
The two isovolumetric phases are the ones with every valve closed and ventricular volume unchanged.
Ejection follows the aortic valve opening; filling follows the mitral valve opening.

Pressure rising, nothing leaving: why the Wiggers diagram matters

Picture the left ventricle at 55 mm Hg and climbing fast, while the aorta sits at 80 mm Hg. Pressure is rising, yet no blood is leaving the heart. How can that be?^[1]

The Wiggers diagram answers it. By the end of this lesson you should be able to read one cardiac cycle on it: which valve moves, which sound you hear, and where the ECG fits, in the right order. The 55 and 80 are illustrative round numbers, not patient data.^[1]

What a Wiggers diagram plots

A Wiggers diagram is only a few lines stacked on one time axis, so you can read straight down the page at any instant.^[1]^[3]

Aortic pressure
Left ventricular pressure
Left atrial pressure
Ventricular volume
Heart sounds
The ECG

The cycle has two halves. Diastole is when the ventricles relax and fill. Systole is when they contract and eject. Textbooks split the cycle into seven phases, ending with reduced filling, and phases two to four are systole.^[1]

This lesson uses round model volumes: end-diastolic volume 120 mL and end-systolic volume 50 mL, so stroke volume is 70 mL.^[4]

Model stroke volume

SV = EDV − ESV = 120 − 50 = 70 mL

Round model numbers, consistent with the pressure–volume loop lessons; not a clinical normal.

The rule behind every valve: follow the pressure gradient

Blood moves down a pressure gradient, and a valve is a one-way door that obeys the gradient.^[1]^[2]

The mitral valve opens when atrial pressure beats ventricular pressure, and closes when ventricular pressure beats atrial pressure.
The aortic valve opens when ventricular pressure beats aortic pressure, and closes when it does not.

Every phase below is just this rule applied to the next instant on the diagram.^[1]^[2]

The cardiac cycle phases, valves and heart sounds in order

Start at the end of diastole. The P wave is atrial depolarisation, and the atria contract, pushing one last extra bit of blood into the ventricle with the mitral valve open. That is phase one, atrial contraction.^[1]

Phases of one cardiac cycle (phases 2–4 are systole)
Phase	What happens	Valves	Sound or marker
1. Atrial contraction	Atria contract after the P wave and add a last bit of blood	Mitral open	P wave; S4 if the ventricle is stiff
2. Isovolumetric contraction	Pressure climbs steeply; volume cannot change	All closed	QRS fires it; mitral closure is S1
3. Rapid ejection	LV pressure passes aortic pressure; ejection begins; aortic pressure climbs to its systolic peak	Aortic open	Silent in healthy valves
4. Reduced ejection	Volume falls toward 50 mL as the ventricle starts to relax	Aortic open until it shuts	Aortic closure is S2; dicrotic notch
5. Isovolumetric relaxation	Volume flat, pressure falling fast	All closed	S2 starts it
6. Rapid filling	Mitral opens when LV pressure drops below atrial; most filling is passive	Mitral open	S3 can come early in filling

Phase seven, reduced filling, completes diastole and leads back into atrial contraction.^[1]

Isovolumetric contraction, isovolumetric relaxation and the heart sounds

The QRS complex fires the ventricles. Ventricular pressure rises past atrial pressure, and the mitral valve shuts, with the tricuspid just after. That closure is the first heart sound, S1. Phase two, isovolumetric contraction, follows: every valve is closed, so volume cannot change, but pressure climbs steeply.^[1]^[2]

Later the gradient flips. Aortic pressure beats ventricular pressure, and the aortic valve snaps shut, with the pulmonic just after. That closure is the second heart sound, S2. It also leaves a small notch on the aortic tracing, the dicrotic notch (incisura).^[1]^[2]^[3]

Phase five, isovolumetric relaxation, then begins: valves closed again, volume flat, pressure falling fast. When ventricular pressure drops below atrial pressure, the mitral valve opens and rapid filling begins.^[1]

Two more sounds are only named here. A third sound can come early in filling; it is normal in children, but in adults it often means a dilated ventricle. A fourth sound comes with atrial contraction into a stiff ventricle.^[1]^[2]

Back to the puzzle: pressure rising with no flow

Reading the opening snapshot

Left ventricular pressure is 55 mm Hg and rising
Aortic pressure is 80 mm Hg: 55 is below 80, so the aortic valve has not opened
The mitral valve already shut when ventricular pressure passed atrial pressure after the QRS
Every valve is closed, so volume cannot change: pressure climbs with no flow

Pressure rises with nothing leaving because the aortic valve is still shut. That is isovolumetric contraction.^[1]

Practice question

Original · USMLE style

Your turnQB-MS-S1-CARD-005-01

A healthy 24-year-old woman is undergoing a research protocol with simultaneous catheters in the left ventricle, left atrium and ascending aorta. At one instant, left ventricular pressure is 62 mm Hg and has been falling for the last 20 ms. Left atrial pressure is 9 mm Hg and aortic pressure is 88 mm Hg. Left ventricular volume is not changing. A second heart sound was recorded 30 ms earlier.

Common questions

What is a Wiggers diagram?

It plots aortic, left ventricular and left atrial pressure, ventricular volume, heart sounds and the ECG on one time axis, so every valve event in one cardiac cycle can be read at a glance.

What are the phases of the cardiac cycle?

There are seven: atrial contraction, isovolumetric contraction, rapid ejection, reduced ejection, isovolumetric relaxation, rapid filling and reduced filling. Phases two to four are systole.

What causes S1 and S2 on a Wiggers diagram?

S1 is closure of the AV valves, mitral first, and starts isovolumetric contraction. S2 is closure of the semilunar valves, aortic first, and starts isovolumetric relaxation.

What is isovolumetric contraction?

It is the phase after the QRS in which every valve is closed. Ventricular volume cannot change, but pressure climbs steeply until it passes aortic pressure.

What is the dicrotic notch?

It is the small notch on the aortic pressure tracing left by aortic valve closure, the same event that produces S2. It is also called the incisura.

Transcript

Read the full narration, by chapter701 words

0:00Pressure rising, nothing leaving

The left ventricle has just reached 55 millimetres of mercury, and it is climbing fast. The aorta sits at 80. Yet nothing is leaving the heart. Rising pressure, no flow. How?

0:13What a Wiggers diagram shows

By the end of this lesson, you'll read one cardiac cycle on a Wiggers diagram: which valve moves, which sound you hear, and where the ECG fits, in the right order. A Wiggers diagram is only a few lines stacked on one time axis. Aortic, left ventricular and left atrial pressure, then ventricular volume, heart sounds, and the ECG.

0:37The rule: valves obey pressure gradients

One rule decodes all of it: blood moves down a pressure gradient, and a valve is a one-way door that obeys the gradient. The mitral valve opens when atrial pressure beats ventricular pressure, and closes when ventricular pressure beats atrial. The aortic valve opens when ventricular pressure beats aortic pressure, and closes when it doesn't.

0:59Diastole, systole and the seven phases

The cycle has two halves: diastole, when the ventricles relax and fill, and systole, when they contract and eject. Textbooks split it into seven phases, ending with reduced filling. Phases two to four are systole.

1:13Model volumes: EDV, ESV, stroke volume

We'll use round model numbers: end-diastolic volume 120 mL, end-systolic volume 50, so stroke volume 70.

1:23Phase 1: atrial contraction and the P wave

Start at the end of diastole. The P wave is atrial depolarisation, and the atria contract, pushing one last extra bit of blood into the ventricle. That's phase one, atrial contraction, with the mitral valve open.

1:39Phase 2: QRS, mitral closure, S1, isovolumetric contraction

Now the QRS complex fires the ventricles. Ventricular pressure rises past atrial pressure, and the mitral valve shuts. That closure is the first heart sound, S one. Phase two is isovolumetric contraction. Every valve is closed, so volume cannot change, but pressure climbs steeply. That solves our puzzle: pressure rising with no flow, because the aortic valve is still shut.

2:09Phase 3: rapid ejection

Phase three: ventricular pressure passes aortic pressure, the aortic valve opens, and rapid ejection begins. Healthy valves open silently, so there's no sound here. Aortic pressure reaches its systolic peak.

2:25Phase 4: reduced ejection, aortic closure, S2, dicrotic notch

Phase four, reduced ejection, carries volume down toward 50 mL as the ventricle starts to relax. Then the gradient flips. Aortic pressure beats ventricular pressure, and the aortic valve snaps shut. That closure is the second heart sound, S two. It also leaves a small notch on the aortic tracing, the dicrotic notch, or incisura.

2:50Phase 5: isovolumetric relaxation

Phase five is isovolumetric relaxation. Valves closed again, volume flat, pressure falling fast. When ventricular pressure drops below atrial pressure, the mitral valve opens.

3:05Phase 6: rapid filling, S3 and S4

Phase six, rapid filling, begins, and most filling is passive. A third sound can come early in filling. It's normal in children, but in adults it often means a dilated ventricle. A fourth sound comes with atrial contraction into a stiff ventricle.

3:23Take-home: the two isovolumetric phases

So here's the take-home: the two isovolumetric phases are the ones with every valve closed, and S one and S two are the valve closures that start them.

3:34Practice question

Now a practice question. A healthy 24-year-old volunteer has catheters in her left ventricle, left atrium and aorta. At one instant, left ventricular pressure is 62 mm Hg and has been falling for 20 milliseconds. Left atrial pressure is nine, and aortic pressure is 88. Ventricular volume is not changing, and a second heart sound was recorded 30 milliseconds earlier. Which phase of the cardiac cycle is this?

4:07Wrong options struck first, then the answer

Let's strike the wrong ones first. A, rapid ejection, needs ventricular pressure above aortic pressure, but 62 is below 88. C, atrial contraction, needs an open mitral valve so the ventricle sits at or just below atrial pressure, nowhere near 53 above it. D, isovolumetric contraction, also has every valve closed, but pressure is rising and S one precedes it. Here, pressure is falling after S two. And E, rapid filling, needs ventricular pressure below atrial pressure, so the mitral valve can open. Sixty-two is far above nine. That leaves B, isovolumetric relaxation. Both valves are shut, volume is flat, pressure is dropping, and S two just happened.

5:01Recap

Quick recap. One: valves open and close purely by pressure gradients. Two: S one starts isovolumetric contraction, and S two starts isovolumetric relaxation. Three: ejection follows the aortic valve opening, and filling follows the mitral valve opening.

5:21Next lesson

Next, we ask what happens when the ventricle is stretched further before it contracts: the Frank-Starling curve.

Sources

Numbers in the text, e.g. [1], point here

Klabunde RE. CV Physiology: Cardiac Cycle (hd002); Ventricular Contraction (hd002b); Ejection (hd002c); Ventricular Relaxation (hd002e); Ventricular Filling (hd002f); Heart Sounds (hd010); Cardiovascular pressures (hd001). cvphysiology.com/heart-disease/hd002 · cvphysiology.com/heart-disease/hd002b · cvphysiology.com/heart-disease/hd002c · cvphysiology.com/heart-disease/hd002e · cvphysiology.com/heart-disease/hd002f · cvphysiology.com/heart-disease/hd010 · cvphysiology.com/heart-disease/hd001
UTMB Pediatric Cardiology Core Curriculum: cardiac cycle and heart sounds. utmb.edu/pedi_ed/CoreV2/Cardiology/cardiologyV2/cardiologyV23.html
Cardiac cycle (overview, phases and isovolumic periods). en.wikipedia.org/wiki/Cardiac_cycle
Series model values (EDV 120 mL, ESV 50 mL, SV 70 mL): Medservatory EP 01 to EP 04 on the pressure-volume loop.

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. All diagrams, the heart sketch, the Wiggers diagram and all handwriting are drawn in code (original). No third-party artwork is used in the video. Thumbnail illustration: heart, Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (cropped, recoloured, outlined). Previous: EP 04 · Contractility and the ESPVR · Next: EP 06 · The Frank-Starling Curve

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-CARD-005.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

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← Previous · EP 04Contractility on the pressure-volume loop: the ESPVR explained Next · EP 06 →The Frank-Starling curve: preload, inotropy and heart failure