Doc after cabg my uncle developed hiccups, also his saturation dropped below 80.are hiccups reason behind it or else.?

Probably not: Hiccups are common after major surgery. Though they are uncomfortable, typically they're not associated with a drop in oxygen saturation. Usually low oxygen levels are caused by fluid or pain for example. However a distended stomach (with air) or an inflammatory process on the diaphragm can cause hiccups and decreased air movement.

Part of it: Decreased post op oxygen saturation can come from congestion, bronchitis, collapse_atelectasis, fluid buildup when one is hiccuping, the diaphragm is jumping and not as effective in moving air in and out of the airway.