When the intrathoracic pressure is raised above that of the great extrathoracic veins which pass the blood back to the chest, definite changes based on elementary physical principles follow.
If a person shuts off the outlet of air by closing the larynx or nose and lips and makes a forced expiratory effort (blowing), the intrathoracic pressure rises, and the blood cannot flow into the chest; hence, the right side of the heart does not fill, the blood is soon pumped out of the lungs, and the left ventricle likewise becomes empty, arterial circulation is momentarily suspended, and the systolic blood pressure falls to zero.
—Place within the lips a tube connected with a manometer and make a blowing effort till the manometer registers from 40 to 60. If this can be held for a sufficient length of time there follows a general venous engorgement, the subject
GRIMES E. EFFECT OF INTRATHORACIC PRESSURE ON ARTERIAL TENSION. Arch Intern Med (Chic). 1931;47(6):876-882. doi:10.1001/archinte.1931.00140240049004