What is the relation of hypertension and decreased cardiac output?

Depends: In theory, the harder it is for blood to leave the heart (as when BP is very elevated) the less of it will be pumped out per beat or per minute (decreased output). For this scenario to occur though the BP will need to be significantly elevated. In most cases a mild or even moderate elevation in BP will not have a tangible effect on cardiac output.

Complicated.: It is an indirect relationship. Hypertension over time can lead to changes in the heart muscle and heart function. It may eventually lead to heart failure. When this ensues the output of the heart can eventually decline and cause serious symptoms. In the earlier stages if hypertension cardiac output actually goes up.

Cardiac damage: Hypertension, if not treated, leads to myocardial hypertrophy, accelerated atherosclerosis resulting in inadequate blood supply to heart muscle that leads to impaired heart function and decreased cardiac output/heart failure.

Hypertension: High blood pressure usually implies an increased peripheral vascular resistance for a time the heart can keep up with the resistance late in the course the heart begins to not be able to generate the stroke volume against the resistance and cardiac output may fall. If abnormal ventricular function is present, cardiac output may fall earlier.