During the strain phase of the Valsalva maneuver, intrathoracic pressure increases significantly due to forced expiration against a closed glottis. This elevated intrathoracic pressure compresses the thoracic veins, leading to decreased venous return to the heart, which causes a reduction in preload (the volume of blood filling the ventricles during diastole). This reduction in preload is transient and results in decreased stroke volume and cardiac output.

This physiologic response is exploited during echocardiographic evaluation to unmask pseudonormal filling patterns of the left ventricle and to assess diastolic function. For example, during the strain phase, the early mitral inflow velocity (E wave) decreases due to reduced preload, and the E/A ratio can normalize or reverse if diastolic dysfunction is present.

The strain phase does not decrease afterload; in fact, afterload can transiently increase during other phases, but the hallmark of the strain phase is decreased preload.

This explanation is detailed in the "Textbook of Clinical Echocardiography, 6e," which explains the hemodynamic changes during the Valsalva maneuver and its clinical application in echocardiographic assessment of diastolic function .