If the sinoatrial node fails to send an electrical impulse, or if the electrical impulse that is sent is blocked, how will the heart continue to beat? What will the heart rate be?

Question

This question focuses on the intrinsic conduction system of the heart and what the backup conduction looks like when the primary conduction fails.

Answer 1

If I may add, if both the sa and the av node failed then the ventricle produces its own back up rate.

Answer 2

The normal intrinsic mechanism of the heart is created by action potentials that originate from the sinoatrial (SA) node. The action potentials travel from the SA node to the AV (atrioventricular) node, and then through the atrioventricular bundle/bundle of His to the right and left bundle branches, then finally to the Purkinje fibers to excite the ventricular muscle and cause strong contraction. This is what pumps the blood to the lungs from the right ventricle and to the body from the left ventricle.

At times, that SA node impulse can be blocked or redirected back into the atria, which means that impulse never reaches the atrioventricular node or ventricles to create a heartbeat. Obviously, this is not ideal!

Although the SA node is the dominant pacemaker in this system, the AV node also has intrinsic pacemaker qualities that allow it to act as a backup. When the SA node is firing, the faster SA node rate overpowers that of the AV node. When the AV node does not receive that impulse from the SA node for a few seconds, then the backup mechanism kicks in and the AV node starts the electrical activity.

However, the AV node fires at a slower rate than is ideal. The SA node generally creates a normal heart rate and fires around 60 to 100 beats per minute. The AV node fires at 40 to 60 beats per minute. When the AV node has taken over the job as the primary pacemaker of the heart, this is termed "junctional rhythm".