Which statement is true about sinus rhythms?

• A. They result solely from external electrical stimulation.
• B. They arise from disturbances in impulse discharge or conduction from the SA node.
• C. They occur at a heart rate above 100 beats per minute only.
• D. They are always regular with no variability.

Answer: (B)

Sinus rhythms are primarily initiated by the sinoatrial (SA) node, which is the natural pacemaker of the heart. The statement that sinus rhythms arise from disturbances in impulse discharge or conduction from the SA node recognizes that while sinus rhythms typically originate from this node, they can be affected by various factors that might disturb its normal function. This includes changes in autonomic tone, electrolyte imbalances, or medication effects that can influence how the SA node generates impulses or affects conduction through the heart's electrical pathways.

This statement captures the complexity of sinus rhythms, acknowledging that they can still reflect underlying issues even if they are classified as "sinus." In contrast, other options either oversimplify the nature of sinus rhythms or mischaracterize their behavior under physiological conditions. For instance, sinus rhythms do not solely result from external stimulation, can have heart rates below 100 beats per minute, and while they tend to exhibit a regular pattern, they can also display variability due to factors like respiratory sinus arrhythmia.