To increase blood pressure in patients with low cardiac output, the following agent must be used

• Vasoconstrictors

Explanation:

Vasoconstrictors work by making the muscle layer of the blood vessel to contract which causes an increase in blood vessel tightness and becomes narrow. This raises blood pressure. This kind of response is common during stress and the body copes with this by producing some common vasoconstrictors like norepinephrine. The rationale is that when the cardiac output is low, the blood in vessels is flowing at low rate as well, so when the vessel is narrowed, the blood flows at higher rates.