2- Heart rate, heart sound and murmurs.

Objectives:
1. Describe the heart rate.
2. Explain the mechanism of the heart sounds.
3. Explain the mechanism of the murmurs.
Heart rate:
It is number of heart beat. The contraction of atria (atrial systole) is followed by contraction of the ventricle (ventricular systole). SA node able to generate action potential with rate of 60 –90 beats/minute in young adult. When SA node does not generate action potential, AV node generates it at rate of 40 – 60 beats /minute and Purkinje fiber at rate of 20 – 40 beats /minute. Increase in heart rate will reduces the duration of ventricular diastole and so reduce the time available for ventricular filling that will reduce the stroke volume.

Normal, and abnormal heart rate:
A normal heart rate is varies between 60 and 90 beats/min. which is called normocardia. A fast heart rate, more than 100 beats/min. is called tachycardia. The electrocardio¬gram is normal except that the rate of heartbeat is increased. The general causes of tachycardia are:
1-Increased body temperature.
2-Stimulation of the heart by the sympathetic nerves.
The term "bradycardia" means a slow heart rate, usually defined as less than 60 beats per minute (eg Athletes and vagal stimulation).