Consider this example from our “Cardiac Cycle and Cardiac output” lecture.
We start at the P-wave of the ECG, which corresponds to the depolarisation of the atria. From the diagram, we can see that the atrial pressure and ventricular volume increases as a direct consequence of the contraction of the atria. Passive and active filling of the ventricles would also therefore increase ventricular pressure and volume. Aortic pressure would still be falling because the aortic pressure is greater than the left ventricular pressure, keeping the aortic valve shut, and in essence “isolating” the aortic system so that changes in the other areas don’t effect pressure there.
The QRS complex corresponds to depolarisation of the ventricular muscle. Therefore, directly after this event, we can expect a number of things to occur.
- The AV valves snap shut – corresponding to the ‘lub’ sound.
- Ventricular pressure increases dramatically, opening the aortic and pulmonary valves.
- Ventricular volume decreases, with blood flowing into the pulmonary trunk and the aorta.
The T-wave corresponds to the repolarisation of the ventricles. We therefore expect:
- Aortic pressure to exceed ventricular pressure, therefore making the aortic and pulmonary valves snaps shut – the ‘dub’ sound.
- Ventricular pressure falls below atrial pressure, thus filling of the ventricles resumes again.
- Aortic pressure gradually falls due to blood entering the peripheral arteries and arterioles.