How/why does sickle cell anemia cause problems when under anaesthetics?

Stress.: While under general anesthesia the body is under increased stress both from the anesthesia and the procedure. This increased stress can increase the rate at which cells sickle potentially causing problems.

Increased sickling: Multiple factors can result in increased sickling during anesthesia and surgery. Chemical changes in the acidity of the blood, oxygen carrying capacity of the blood, body temperature, etc all increase the risk of sickling. Therefore transfusions are strongly recommended prior to surgery to reduce the total number of red cells with hemoglobin s prior to surgery in an attempt to minimize the risk.

Sickling: When someone with sickle cell anemia is in crisis their red blood cells are reconfigured into a sickle shape. This sickled shape cannot carry oxygen adequately. This will lead to hypoxemia.

Sickled red cells: cause vaso-occlusion by clustering and the patients are frequently anemic. They are high risk especially for major procedures.