Kaplan Nursing Entrance Practice Exam

If the SA node fails, what is the likely effect?

1. The heart rate will increase significantly

2. The AV node will take over at a slower pace

3. The ventricles will cease to contract

4. The heart will function without any issues

The correct answer is: The AV node will take over at a slower pace

When the SA node, which is the heart's natural pacemaker, fails, the atrioventricular (AV) node can assume pacemaking responsibilities. However, the rate set by the AV node is slower than that of the SA node. The normal intrinsic rate of the SA node is about 60-100 beats per minute, while the AV node typically generates impulses at a rate of 40-60 beats per minute. This transition serves as a backup mechanism to maintain heart function when the primary pacemaker fails. The other potential effects, such as the heart rate significantly increasing, the ventricles ceasing to contract, or the heart functioning without any issues, do not accurately represent the physiological response in the event of SA node failure. The heart adapts to the failure of the SA node, but it does so by slowing down rather than speeding up. Additionally, while the ventricles do not cease to contract, they will be driven by the slower pacing of the AV node rather than the usual faster rhythm provided by the SA node.