Rate

Generally the Heart rate is maintained by the regular impulses generated by the sino-atrial node(SA node),which is located to the right of the superior vena caval opening in the wall of the right atrium.From here the impulses reach the atrio-ventricular node(AV node or Tawara's node) through the three internodal pathways.

1) Anterior internodal tract (Bundle of Bachmann + Descending branch)
2) Middle internodal tract (Bundle of Wenckebach)
3) Posterior internodal tract (Bundle of Thorels)

The AV node is present to the right of the interatrial septum,just above the tricuspid valve.From here the impulses pass through Bundle of His,which divides into right and left bundle branches.The left bundle further divides into anterior and posterior fascicles.All of these end as a network of Purkinje fibers.

The entire conduction system in the heart facilitates rapid conduction of impulses to the entire myocardium.If there is no such fast conducting system,the myocardium will get excited,but with lot of delay as the impulses take some time in spreading to all the adjacent cells(myocytes).There is a natural AV nodal delay for the impulse and it helps the coordinated contraction of atria followed by the ventricles.

Hierarchy of Pacemakers:

The Human body always have backups for many things.Our heart is a very good example of that.If the SA node fails to generate the impulses,as rest of the conductive tissue and myocytes are excitable,they take over the job in the order of AV node,His bundle,its branches and myocytes(ventricle).Their firing rate is as follows:

SA node : 60-80/min
AV node : 40-60/min
Ventricle : 20-40/min

Calculation of Heart Rate:

For a quick reading and convenience of the human eye,the EKG is universally recorded at a paper speed of 25mm/sec.At this speed,the marker moves one large square in 0.2sec and five large squares in 1 second on the EKG paper.That means in one minute 300 large squares or 1500 small squares are covered.Hence these numbers are used for easy calculation.

Formulas:

HR = 1500/Number of small squares in R-R or P-P interval
or
HR = 300/Number of large squares in R-R or P-P interval

If the HR is irregular then we can count the number of QRS complexes for 10 seconds and multiply by 6.

QUICK RULES:

R-R or P-P interval < 3 large squares(HR>100/min,Sinus tachycardia)
R-R or P-P interval > 5 large squares(HR<60/min,Sinus bradycardia)
R-R or P-P interval = 1 large square(HR is 300/min)
R-R or P-P interval = 2 large squares(HR is 150/min)
R-R or P-P interval = 3 large squares(HR is 100/min)
R-R or P-P interval = 4 large squares(HR is 75/min)
R-R or P-P interval = 5 large squares(HR is 60/min)