Electrical and Pressure Changes in the Heart

The electrical changes shown on an ECG cause the contractions of the heart, so they can be matched to the pressure changes in the chambers and arteries during one cardiac cycle. The graph shows pressure in the left atrium, left ventricle and aorta, with the ECG trace above on the same time axis.P wave → atrial systole. Shortly after the P wave, the atria contract, giving a small rise in pressure in the atria. This pushes the last of the blood into the ventricles.QRS complex → ventricular systole. Shortly after the QRS complex, the ventricles contract and ventricular pressure rises steeply.• When ventricular pressure rises above atrial pressure, the atrioventricular valve closes.
• When ventricular pressure rises above aortic pressure, the semilunar valve opens and blood is ejected. Aortic pressure rises, and the aorta wall stretches.T wave → ventricular diastole. The T wave shows the ventricles repolarising, just before they relax. Ventricular pressure falls.• When ventricular pressure falls below aortic pressure, the semilunar valve closes. Blood briefly flows back against the closed valve, producing a small bump in the aortic trace called the dicrotic notch.
• When ventricular pressure falls below atrial pressure, the atrioventricular valve opens and the ventricle starts to fill again.So the semilunar valve is open only while ventricular pressure is above aortic pressure. Aortic pressure never falls to zero, because the elastic recoil of the aorta wall maintains it between beats. The right side of the heart shows the same pattern at lower pressures.