Homework SourseA person notices his or her heart beat because he or she sen
A person notices his or her heart beat because he or she senses blood being pumped by the heart. Excessive caffeine intake can lead to irregular heart rhythms (arrhythmias) that patients perceive as “skipped beats.\" Given that caffeine is a stimulant, which of the following mechanisms best explains the reason for the feeling that the heart skipped a beat?
| a Action potentials propagate into the ventricles before the contractile cells have repolarized from the previous heartbeat. |
| b Spontaneous action potentials in the SA node overlap such that the repolarizing phase of one action potential cancels out the depolarizing phase of the next. |
| c Purkinje fibers initiate spontaneous action potentials, which cause the ventricles to contract early. |
| d Action potentials are not delayed sufficiently at the AV node. |
Solution
Irregular heart arrhythmias or skipped beats occur due to premature ventricular contraction (PVC). The elctric impulses do not pass through ventricles normally. Isolated PVC are insignificant and do not require treatment. Transient PVC may result due to excess caffeine intake or consumption of alcohol, illicit drugs and smoking. These substances stimulate the sympathetic nervous system and cause PVC. The PVC is therefore resulted due to heart beat initiated by purkinje fibers in ventricles rather than in sinoatrial node (SA node). Therefore, option c is correct.
