Valsalva Maneuver

The Valsalva maneuver is closed expiration against a closed glottis, thereby, increading intrathoracic pressure which decreases venous return to both ventricles. The resultant decrease in cardiac output decreases the intensity of all innocent murmurs and most pathologic murmurs.

A key point is that a murmur that becomes more prominent with decreased venous return is never an innocent murmur.