Cardiac cycle

Also known as: Wiggers diagram, systole, diastole, pressure–volume loop, isovolumetric contraction

The cardiac cycle is the sequence of electrical and mechanical events in a single heartbeat: atrial systole, ventricular systole (isovolumetric contraction and ejection) and ventricular diastole (isovolumetric relaxation and filling). At 75 beats/min it lasts about 0.8 s.

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Overview

Cycle length at 75/min≈0.8 s

Ventricular systole / diastole≈0.3 s / ≈0.5 s

RA / RV pressure2–6 / 15–30 over 2–8 mmHg

PA pressure15–30 / 4–12 mmHg (mean 10–20)

LA (PCWP) / LV pressure6–12 / 100–140 over 3–12 mmHg

Aortic pressure100–140 / 60–90 mmHg

The Wiggers diagram plots several measurements of one heartbeat against the same time axis. These are the aortic, left ventricular and left atrial pressures, the ventricular volume, the ECG and the heart sounds. Every valve event happens where two pressure traces cross: a valve opens or closes as soon as the pressure difference across it reverses.

At 75 beats/min, atrial systole lasts about 0.1 s, ventricular systole about 0.27–0.3 s and ventricular diastole about 0.5 s. When the heart rate rises, diastole shortens much more than systole. This limits ventricular filling and coronary blood flow at high heart rates, because the left ventricle receives most of its blood supply during diastole.

Parts & subdivisions

1. Atrial systole (late diastole)
After the P wave, the atria contract and push the last of the filling volume into the ventricles. This is about 20% of filling at rest, and more at high heart rates or when the ventricles are stiff. It produces the a wave in the atrial pressure trace. End-diastolic volume reaches about 120–130 mL. An S4 can be heard if the ventricle is stiff. This contribution is lost in atrial fibrillation.
2. Isovolumetric contraction
After the QRS complex, the ventricles start to contract. As soon as ventricular pressure exceeds atrial pressure, the AV valves close, producing S1. All four valves are now shut, so volume stays constant while pressure rises steeply. This continues until left ventricular pressure exceeds aortic pressure (about 80 mmHg) and right ventricular pressure exceeds pulmonary pressure (about 10 mmHg). The bulging AV valves produce the c wave.
3. Rapid ejection
The semilunar valves open, normally silently. About two-thirds of the stroke volume is ejected in the first third of systole. Left ventricular and aortic pressures peak at about 120 mmHg. The atrial pressure falls (the x descent).
4. Reduced ejection
During the T wave the ventricles repolarise, active force falls and ejection slows. Blood keeps moving forward by momentum for a short time even after ventricular pressure has dropped below aortic pressure.
5. Isovolumetric relaxation
When ventricular pressure falls below arterial pressure, the semilunar valves close, producing S2 and the dicrotic notch (incisura) on the aortic trace. All valves are shut again. End-systolic volume is about 50 mL. The atria keep filling, and atrial pressure peaks as the v wave.
6. Rapid filling
When ventricular pressure falls below atrial pressure, the AV valves open. Blood rushes into the ventricles, helped by active relaxation and elastic recoil, and the atrial pressure drops (the y descent). An S3 may be heard. It is normal in children, athletes and pregnancy, but in older adults it usually indicates heart failure or volume overload.
7. Reduced filling (diastasis)
Filling slows as pressures in the atria and ventricles equalise. The next P wave then starts the cycle again.

Function

Coordinates atrial and ventricular contraction with valve opening and closing so that blood flows in one direction.
Produces the stroke volume and the arterial pressure waveform.

Physiology

Pressure–volume loop. Plotting left ventricular pressure against volume over one beat gives a loop with four corners. Mitral closure is at end-diastolic volume, aortic opening at the start of ejection, aortic closure at end-systolic volume, and mitral opening at the start of filling.

The width of the loop is the stroke volume, and its area is the stroke work. The slope of the end-systolic pressure–volume relationship reflects contractility. The end-diastolic pressure–volume curve reflects ventricular compliance.

Clinical significance

Diastolic dysfunction (HFpEF)

In heart failure with preserved ejection fraction, the ventricle relaxes slowly and is stiff. Filling pressures rise and the lungs become congested even though the ejection fraction is normal. These patients depend heavily on the atrial contribution to filling and on having enough time to fill, so tachycardia and atrial fibrillation are poorly tolerated.

Tachycardia and mitral stenosis

In mitral stenosis, the left ventricle can fill only through a narrow valve and needs a long diastole. When atrial fibrillation with a fast ventricular rate begins, diastole is cut short and the atrial contribution is lost. Left atrial pressure rises sharply and pulmonary oedema can follow.

Test yourself

Left ventricular myocardium receives most of its coronary blood flow during:

Atrial systole
Isovolumetric contraction
Rapid ejection
Diastole

Answer: Diastole

During systole the contracting myocardium squeezes the intramural vessels shut, especially in the subendocardium. Most left ventricular perfusion therefore happens in diastole, driven by the recoiling aorta. Tachycardia shortens diastole and so reduces coronary perfusion time.