Vasopressors increase preload and ventricular filling pressures including PWP. As a consequence, congestive heart failure may be aggravated. CO almost universally falls and myocardial oxygen requirements increase as afterload and MAP rise.

Why do vasopressors increase preload?

It increases systemic vascular resistance (SVR). It does cause venoconstriction, which may increase preload. Its dominant effect on cardiac output is often to cause a reduction (but this may depend on the heart's ability to tolerate increased afterload).

How do vasopressors increase blood pressure?

Vasopressors can tap into the function of the adrenergic system by stimulating the receptors of its nerve cells. The alpha receptors are mostly located in blood vessel walls, and their stimulation prompts tightening of the arteries, which can raise blood pressure.