The RCEM Basic Science curriculum states the following as required knowledge:
Pressures, volumes and key events through the cardiac cycle
- Atrial systole
- Isovolumetric contraction
- Ventricular ejection
- Isovolumetric relaxation
- Ventricular filling
Knowledge of the following is expected for each named phase above
- Chamber pressure [mmHg]
- Valvular opening / closure
- Relevant heart sound
- Relevant ECG deflection
At the start of the cardiac cycle, towards the end of diastole, the whole of the heart is relaxed. The atrioventricular (AV) valves are open because the atrial pressure is greater than the ventricular pressure. The semilunar valves are closed, as the pressure in the pulmonary artery and aorta is greater than the ventricular pressures.
The cardiac cycle starts when the sinoatrial node (SAN) initiates atrial systole. Atrial depolarisation causes the P wave on the ECG. Blood flows from the atria to the ventricle. At rest, atrial systole only contributes about 15 – 20 % of the final ventricular volume, as most of the filling has occurred passively in diastole due to venous pressure. The atrial contribution increases with heart rate as diastole shortens. There are no valves between the veins and atria and atrial systole causes a small pressure rise in the great veins (the a wave).
The end-diastolic volume is usually about 120 – 140 mL, and the pressure is less than 10 mmHg (and higher in the left ventricle than the right due to the thicker and stiffer left ventricle).
In ventricular systole, the ventricular pressure rises sharply and the AV valves close as soon as this is greater than atrial pressure (causing the first heart sound). Splitting of the first heart sound is always pathological and may occur in conduction defects. Ventricular depolarisation causes the QRS complex on the ECG. For a short period, as the forces are developing, both the AV and the semilunar valves are closed and there is no ejection. This is isovolumetric contraction. The increasing pressure makes the AV valves bulge into the atria, causing a small atrial pressure wave (the c wave) followed by a fall with the opening of the semilunar valves and ventricular ejection (the x descent).
When the ventricular pressure exceeds that in the pulmonary artery and the aorta, the semilunar valves open and blood is ejected, initially rapidly and then more slowly. Rapid ejection can sometimes be heard as a murmur. During the second half of ejection, the ventricles stop contracting, the ventricular pressure starts to decrease and the muscle starts to repolarise; this causes the T wave on the ECG. When the pressure in the ventricles is less than that of the great arteries, the semilunar valves close (causing the second heart sound), and there is a small increase in aortic pressure (the dicrotic notch on arterial waveform). The amount of blood ejected is the stroke volume, and is usually about 70 mL (therefore about 50 mL is left; the end-systolic volume). The proportion of blood that is ejected is the ejection fraction and this is normally about 0.6. Splitting of the second heart sound (A before P) is often heard in young, healthy people especially during exercise and inspiration although a large split may be due to conduction defects or high outflow pressure.
In the last two-thirds of systole and continuing into diastole, atrial pressure increases due to filling from the great veins (the v wave). In early diastole, the ventricles are rapidly relaxing, but the AV valves remain closed as the ventricular pressure is still greater than atrial pressure (isovolumetric relaxation).
Eventually, when atrial pressure is greater than ventricular pressure, the AV valves open, the atrial pressure falls (y descent) and the ventricles refill, initially rapidly (assisted by elastic recoil of the ventricles) and then more slowly (due to venous flow alone). A third heart sound may be heard in rapid ventricular filling, often in young healthy people, but also with raised end-diastolic pressure e.g. in heart failure. A fourth heart sound is associated with forceful atrial systole e.g. due to loss of ventricular compliance in ventricular hypertrophy and is always abnormal. Diastole is usually twice the length of systole at rest, but decreases with increased heart rate.