Systole is the phase of the cardiac cycle during which the heart muscle contracts to pump blood out of the ventricles. It is one of two alternating phases of the heartbeat, the other being diastole (the period of relaxation and filling). Blood pressure measurements are expressed as systolic pressure over diastolic pressure (for example, 120/80 mmHg).
During systole, electrical activation from the heart’s conduction system triggers coordinated contraction of the ventricular muscle. Pressure rises rapidly inside the ventricles. When ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the aortic and pulmonary valves open, and blood is ejected. The left ventricle pumps blood into the aorta for systemic circulation; the right ventricle pumps blood into the pulmonary artery towards the lungs.
The proportion of blood ejected from the left ventricle with each contraction is expressed as ejection fraction (EF). A normal ejection fraction is 55% or above. Reduced systolic function (a low EF) means the heart is contracting weakly and is a marker of heart failure and increased risk of sudden cardiac arrest.
During cardiac arrest, coordinated systole ceases entirely. In ventricular fibrillation, the ventricles quiver chaotically rather than contracting in a coordinated sweep, producing no effective pumping action. CPR provides a manual substitute for systole by externally compressing the chest to maintain partial circulation until the heart can be restarted.
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