## 4 Discussion  
Contact-free photoplethysmograms were acquired by iPPG on the chest wall of patients before and after CABG. They showed an overall decrease of the signal strength after surgery, followed by a gradual but incomplete recovery until day six. They also indicated a specific impact of the unilateral ITA preparation on the perfusion of the respective side of the chest wall, which was illustrated by a deeper drop and slower recovery of the signal strength.  
These results are consistent with the cardiovascular effects of major surgery on the cutaneous circulation, and the local outcome of ITA preparation on the perfusion of the chest wall. Depending on its extent, surgery initiates a reaction of the autonomous nervous system along with an upraise of circulating catecholamine levels and local vasoconstriction [31]. Aiming at the perfusion of vital organs, the cardiovascular reaction to a surgical trauma etiologically resembles that of shock to some degree [21]. In such conditions the peripheral circulation is the first to be limited and the last to be fully restored [18]. Accordingly, the number of perfused small vessels in the peripheral circulation is significantly reduced after cardiac surgery, even at a greater distance from the surgical site, and is not fully restored after 24 h [32]. The local effects of ITA preparation are demonstrated in a reduced sternal perfusion, which is partially balanced out by a collateral vascularization but is still evident seven days after surgery [33]. The ITA is also the main feeding vessel of the skin on the anterior chest wall with a restriction to the respective side of the body [25]. The resection of an ITA thus gives reason for a reduced cutaneous perfusion, though a collateral vascularization exists as well.  
The relation of the iPPG signal to the microcirculatory sequelae after major surgery and to the local impact of ITA preparation is explained by its signal origin. The cutaneous blood volume pulse is seen as the key source of iPPG [34; 35; 36]. This is underlined by the peak signal strength of iPPG corresponding to the absorption maximum of haemoglobin in the spectral range of green light. Since the propagation of green light into the skin is circumscribed to about 1 mm [37; 38], the hemodynamic information content of iPPG is specific for the cutaneous microcirculation. Though not a direct measure, iPPG signals are strongly associated to cutaneous perfusion [13].  
The results of this study prove iPPG is suitable to reveal the effects of major surgery on the perfusion of the skin. Remarkably, iPPG was able to reveal even small alterations of local blood supply accounting for only 2.8% of iPPG's signal variance. These outcomes depend on the specificity of iPPG for the cutaneous microcirculation, its sensitivity to autonomous nervous activity and vascular regulation and on the association of the cutaneous blood volume pulse to perfusion.  
Of note, iPPG was evaluated by relative changes of its signal strength during the course of the study. Owing to their high variability between subjects, absolute values of iPPG parameters as presented here do not allow to draw conclusions about the actual amount of cutaneous perfusion. Further studies are needed to specify iPPG in its circulatory context and to clarify artefacts and confounders affecting the iPPG measurement.