We explore the use of photoplethysmography to monitor and detect changes in arterial and venous circulation for potential use in surgical flap monitoring. The typical disruptions in circulation that are seen in a flap are emulated by occlusion tests conducted in controlled settings. Arterial and venous occlusions are performed on a limited number of subjects, and associated changes in the PPG signals captured at the specific region of interest are analysed. A set of parameters that can be used to distinguish between arterial and venous occlusions are identified and quantified. These parameters may be used to detect thrombosis and differentiate between an arterial and a venous thrombosis in free flaps during postoperative monitoring after reconstructive surgery.