INTRODUCTION TO ECG !!

ELECTRICAL STIMULATION OF THE HEART
The electrical "wiring" of the heart is outlined in the figure below. Normally the signal for cardiac electrical stimulation starts in the sinus node, also called the sinoatrial (SA) node. This node is located in the right atrium near the opening of the superior vena cava. It is a small collection of specialized cells capable of automatically generating an electrical stimulus (signal). From the sinus node this stimulus spreads first through the right atrium and then into the left atrium. Thus the sinus node functions as the normal pacemaker of the heart.

The first phase of cardiac muscle activation is electrical stimulation of the right and left atria. This in turn signals the atria to contract and pump blood simultaneously through the tricuspid and mitral valves into the right and left ventricles. The electrical stimulus then spreads to specialized conduction tissues in the atrioventricular (AV) junction, which includes the AV node and bundle of His, and then into the left and right bundle branches, which transmit the stimulus to the ventricular muscle cells.

The AV junction, which acts as a sort of electrical "bridge" connecting the atria and ventricles, is located at the base of the interatrial septum and extends into the interventricular septum . The upper (proximal) part of the AV junction is the AV node.The lower (distal) part of the AV junction is called the bundle of His. The bundle of His then divides into two main branches: the right bundle branch, which distributes the stimulus to the right ventricle, and the left bundle branch,which distributes the stimulus to the left ventricle .

The electrical stimulus spreads simultaneously down the left and right bundle branches intothe ventricular myocardium (ventricular muscle) by way of specialized conducting cells called Purkinje fibers. These fibers are located in the ventricular myocardium.

Under normal circumstances, when the sinus node is pacing the heart (normal sinus rhythm), the AV junction appears to serve primarily as a shuttle, directing the electrical stimulus into the ventricles. However, under some circumstances the AV junction can also act as an independent pacemaker of the heart. For example, if the sinus node fails to function properly, the AV junction can act as an escape pacemaker. In such cases an AV junctional rhythm (and not sinus rhythm) is present. This produces a distinct ECG pattern
Just as the spread of electrical stimuli through the atria leads to atrial contraction, so the spread of stimuli through the ventricles leads to ventricular contraction, with pumping of blood to the lungs and into the general circulation.




http://cal.vet.upenn.edu/projects/anestecg/images/heartdr.JPG

also note that the A-V node provides the delay that is required to prevent simultaneous contraction of atria and ventricles...ensures ventricles contract later