cardiac contraction.
The cardiac contraction is controlled by electrical impulses generated in two nodules of highly specialized heart tissue called sinus node (NS) and atrio-ventricular node (NAV). The first NS acts causing simultaneous contraction of both atria and sending a signal to the NAV. NAV then acts causing the contraction of the ventricles. The NS is activated automatically at the rate typical of each species. In dogs can vary from 70 to 120 times per minute, while cats do between 120 and 150. If the NS fails in his drive activates the NAV eventually producing the so-called escape beats cause ventricular contraction. The bategades escape are slower (60-70) beats per minute. If any two nodes goes wrong, if there is a physical defect in communication between two nodes, or if we have an electrolyte imbalance that will change the sensitivity of nodal tissue to stimuli, they become cardiac arrhythmias. These arrhythmias may accelerate as (tachycardia) or slow (bradycardia) heart rate. 
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