Comprehensive and Detailed Explanation From Exact Extract:
The left ventricular end-diastolic diameter (LVEDD) is measured at end-diastole, which is conventionally defined as the onset of the QRS complex on the electrocardiogram (ECG). This corresponds to the end of ventricular filling and just before ventricular contraction begins.
Measuring LVEDD at this point ensures consistency and accuracy for assessment of ventricular size and function. Measurement at the onset of the P wave would be too early (atrial contraction). The first frame after aortic valve closure corresponds to end-systole, and after mitral valve closure is during systole.
This timing is standard as per guidelines outlined in the "Textbook of Clinical Echocardiography, 6e", Chapter on Cardiac Chamber Quantification#20:60-65Textbook of Clinical Echocardiography#.