###### Abstract  
Imaging photoplethysmography (iPPG) is a contact-free monitoring of the cutaneous blood volume pulse by RGB (red-green-blue) cameras. It detects vital parameters from skin areas and is associated to cutaneous perfusion. This study investigated the use of iPPG to quantify changes in cutaneous perfusion after major surgery. Patients undergoing coronary artery bypass grafting (CABG) were scanned before surgery and in three follow-up measurements. Using an industrial-grade RGB camera and usual indoor lighting, a contact-free imaging plethysmogram from the chest was obtained. Changes of the iPPG signal strength were evaluated in view of both the operation itself as well as the unilateral preparation of the internal thoracic artery (ITA) for coronary artery grafting, which is the main blood source of the chest wall. iPPG signal strength globally decreased after surgery and recovered partially during the follow up measurements. The ITA preparation led to a deeper decrease and an attenuated recovery of the iPPG signal strength compared to the other side of the chest wall. These results comply with the expected changes of cutaneous perfusion after CABG using an ITA graft. iPPG can be used to assess cutaneous perfusion and its global changes after major surgery as well as its local changes after specific surgical procedures.  
Keywords:imaging photoplethysmography; cutaneous blood volume; cutaneous perfusion; coronary bypass operation; internal thoracic artery +
Footnote †: journal: Applied Soft  
iPPG has been shown to measure physiological data such as heart rate, heart rhythm and respiratory rate in experimental and clinical environments with a high accuracy [2; 3; 4; 5]. As a special feature, its signal intensity responds directly to changes of the arterial blood pressure [6; 7]. This haemodynamic sensitivity is linked to the peripheral vasculature of the skin, as the propagation of light into the tissue is limited to the outer skin layers. The iPPG signal is affected by autonomic nervous activity and vasomotor control in the peripheral vascular bed, such as in reactions to hot and cold thermal stimuli [8; 9], topically induced cutaneous vasodilation [10; 11], the vasodilatory effects of regional anaesthesia and reactive hyperaemia after vascular occlusion [11]. The blood volume pulse detected by iPPG is seen as an indirect measurement of skin perfusion [1; 12; 13]. This idea has been implemented in studies assessing burn wounds [14] and free flaps in plastic surgery, [15] or to reveal changes in the microcirculation of the skin associated with migraine [10; 16].  
Skin perfusion is a valuable prognostic parameter and is used to guide the therapy of critically ill patients [17; 18]. Clinical signs of a poor skin perfusion, like mottling or a prolonged capillary refill, indicate whole-body perfusion deficits during critical illness [19; 20], predict complications after major surgery [21] and are noted in treatment protocols for circulatory and septic shock [17; 22]. Even when perceptible signs of cutaneous malperfusion are absent, a reduction of skin perfusion is common after major surgery as a result of neurohumoral responses, an increased activity of the sympathetic nervous system and a consecutive peripheral vasoconstriction.  
In this study we investigated the suitability of iPPG to detect these typical changes in the microcirculation of the skin following major surgery. Patients undergoing coronary artery bypass grafting (CABG) with the use of a unilateral internal thoracic artery (ITA) graft were investigated. As the ITA is the main supplying vessel of the chest wall, the chest wall perfusion is inevitably affected after this procedure [23; 24; 25]. Hence, two issues are addressed: the global effect of major surgery on the peripheral circulation as well as the regional impact of ITA preparation on the chest wall perfusion.