What is measured from the SA node triggering to atrial contraction?

The P-wave represents the electrical activity that occurs when the atria depolarize, which is directly initiated by the action of the sinoatrial (SA) node. When the SA node triggers, it generates an electrical impulse that spreads through the atrial myocardium, leading to atrial contraction. This sequence is reflected on the electrocardiogram as the P-wave.

Understanding the significance of the P-wave is critical for interpreting ECG readings, as it indicates normal atrial depolarization. The presence and morphology of the P-wave can provide insights into the rhythm and health of the heart. For example, a lack of a visible P-wave can indicate issues such as atrial fibrillation, while changes in the P-wave shape can signal atrial enlargement or other pathologies.

Other choices describe different aspects of the cardiac electrical cycle: the QRS complex indicates ventricular depolarization and is associated with ventricular contraction, the ST segment relates to the time between ventricular depolarization and repolarization, and the QT interval encompasses both ventricular depolarization and repolarization. Therefore, these options relate to different phases of the heart's electrical activity, while the P-wave focuses specifically on atrial contraction initiated by the SA node.