When should nitrates be avoided?

Nitrates should be avoided when systolic blood pressure is below 90 because one of the major effects of nitrates is to cause vasodilation, which can lead to a further decrease in blood pressure. In patients with already low blood pressure, administering nitrates could precipitate hypotension, resulting in dizziness, fainting, or even shock. This makes it critical to monitor blood pressure before administering nitrates and to avoid their use in cases of hypotension, particularly when systolic readings are below the threshold of 90 mmHg.

Other scenarios listed might have their own considerations, but they do not present the same immediate risk of exacerbating low blood pressure that contraindicates the use of nitrates.