S1 and S2 sounds

The two major sounds of the normal heart sound like "lub dub". The "lub" is the first heart sound, commonly termed S1, and is caused by turbulence caused by the closure of mitral and tricuspid valves at the start of systole. The second heart sound, "dub" or S2, is caused by the closure of the aortic and pulmonic valves, marking the end of systole. Thus the time period elapsing between the first heart sound and the second sound defines systole (ventricular ejection) and the time between the second sound and the following first sound defines diastole (ventricular filling). Please listen to an example of a normal heart sound:

These are the normal heart rate values. A heart rate above the normal values is referred to as tachycardia, a heart rate below the normal values is referred to as bradycardia:

  • Infants: 100 to 160 beats per minute
  • Children 1 to 10 years: 70 to 120 beats per minute
  • Children over 10 and adults: 60 to 100 beats per minute
  • Athletes: 40 to 60 beats per minute

Below is a normal heart sound recorded from a 1 year old boy with no heart disease. A fast heart rate is common in children and in normal subjects following a period of physical activity:

If you have difficulty identifying systole (the interval between S1 and S2) use the second hand to feel the pulse at the neck. The rising phase of the pulse corresponds to the beginning of systole and occurs immediately following S1. In other words, you hear S1 just before the raising phase of the pulse and you hear S2 during declining phase of the pulse.