What is produced by the initiation of an impulse in the SA node?

The initiation of an impulse in the sinoatrial (SA) node produces the P wave on an electrocardiogram (EKG). The SA node, often referred to as the natural pacemaker of the heart, generates electrical impulses that initiate the heartbeat. When an impulse is generated by the SA node, it causes depolarization of the atria, which is represented as the P wave on the EKG.

The P wave signifies atrial contraction or depolarization. As the impulse spreads through the atria, it leads to this characteristic deflection on the EKG tracing. Observing the P wave helps clinicians assess the proper functioning of the heart's electrical conduction system and can provide insights into various cardiac conditions.

The other options, though relevant to the EKG tracing, do not directly result from the SA node impulse. The QRS complex reflects ventricular depolarization, the T wave corresponds to ventricular repolarization, and heart sounds are associated with the mechanical action of the heart valves closing and are not directly depicted by the EKG tracing itself. Thus, the P wave is specifically related to the electrical activity initiated by the SA node.