Final answer:
Vasoconstrictors should not be added to local anesthetic mixtures in patients with cardiovascular disease, allergies or hypersensitivity, and in areas with compromised blood supply.
Step-by-step explanation:
Vasoconstrictors should not be added to local anesthetic mixtures in certain situations. One of these situations is in patients with cardiovascular disease, as vasoconstrictors can adversely affect blood flow and potentially cause a heart attack or stroke. Another situation is in patients who are allergic or hypersensitive to vasoconstrictors. The use of vasoconstrictors should also be avoided in areas with compromised blood supply, as it can further reduce blood flow and lead to tissue damage.