The long-term success of coronary artery bypass grafting (CABG) is negatively influenced by graft failure (Lopes et al., 2012; Blachutzik et al., 2016). Patency of conventionally harvested saphenous vein grafts is 80–90% 1 year after the CABG procedure and decreases to no more than 60% after 10 years (Campeau et al., 1979; Fitzgibbon et al., 1996; Harskamp et al., 2013a; de Vries et al., 2016). The left internal thoracic artery is considered first choice conduit because of prolonged high patency (Otsuka et al., 2013) but equivalent results (e.g., 83% patency after 16 years) can be achieved with saphenous vein grafts harvested by so-called “no-touch” technique (Souza et al., 2001, 2006; Johansson et al., 2009; Samano et al., 2015). When “no-touch” vein grafts are excised, a sheet of perivascular tissue is retained around the vein (Souza, 1996). Suggestions have been made that improved integrity of the endothelial and medial smooth muscle cell layers (Ahmed et al., 2004; Vasilakis et al., 2004; Loesch et al., 2006), higher expression and activity of endothelial NO synthase (Tsui et al., 2001; Dashwood et al., 2009), greater density of vasa vasorum (Dreifaldt et al., 2011), and paracrine release of vasoactive substances from the perivascular tissue (Fernández-Alfonso et al., 2017) contribute to the superior patency of “no-touch” compared to conventional saphenous vein grafts. Nevertheless, the impact of perivascular tissue and harvesting techniques on the pathophysiology of graft occlusion is still not well-understood.

Vein grafts used for CABG surgery experience dramatic changes in mechanical forces—in particular increased transmural pressure and linear velocity of the blood—when they are transplanted from the venous to the arterial circulation. Elevated shear stress and circumferential wall stress of arteries compared to veins of equal caliber are important factors that influence vessel wall remodeling and neointima formation (Fitts et al., 2014). Increased physiological shear stress—which is typically 10–70 dynes/cm² in arteries compared to 1–5 dynes/cm² in veins (dela Paz and D'Amore, 2009; Fitts et al., 2014)—usually leads to outward remodeling. However, damaged endothelial cell layer caused by excessive physical forces can lead to endothelial dysfunction or even allow shear forces to act directly on the smooth muscle cells (Fry, 1969; Fitts et al., 2014). The mechanotransduction pathways and pathophysiological consequences of increased mechanical stress remain controversial and may depend on the flow pattern, but effects of shear stress on smooth muscle cell phenotype (i.e., shift from contractile to synthetic phenotype) and proliferation have been demonstrated and may contribute to inward remodeling and luminal obstruction of transplanted vein grafts (Kipshidze et al., 2004; Shi and Tarbell, 2011). It is well-accepted that surgical procedures and graft characteristics affect the rate of graft failure (Harskamp et al., 2013b), and it is expected that long-term patency is improved if the transplanted graft is functionally and structurally intact at the time of anastomosis.

In addition to providing structural support, the secretory activity of perivascular tissue can have functional implications for the vascular wall (Fernández-Alfonso, 2004). Crosstalk between perivascular tissue and resistance arteries in many vascular beds contributes to metabolic regulation of blood flow whereby local perfusion is adjusted to the metabolic demand. Proposed paracrine factors released from perivascular tissue include NO, leptin, adiponectin, prostanoids, angiotensin 1–7, and hydrogen sulfide (Simonsen and Boedtkjer, 2016), which in addition to vasomotor effects may modify the structure of the blood vessel wall, for instance, through changes in cell migration, proliferation, and apoptosis. Perivascular cardiomyocyte-rich tissue surrounding coronary arteries modifies both vasocontraction and—relaxation (Aalbaek et al., 2015). Even though the cause-effect relationship is still unclear, the relevance of altered crosstalk between perivascular tissue and cells of the vascular wall for disease is supported by altered anti-contractile influences of perivascular tissue in models of type 2 diabetes (Bonde et al., 2017), obesity (Yudkin et al., 2005; Greenstein et al., 2009; Aghamohammadzadeh et al., 2013), inflammation (Bhattacharya et al., 2013), and hypertension (Li et al., 2013).

There is evidence for structural damage to the vascular wall of conventionally harvested vein grafts (Ahmed et al., 2004; Loesch et al., 2006). However, functional evidence of altered vasomotor performance has not been provided and consequences of different harvesting techniques for the vasomotor function of excised grafts have not been systematically evaluated. In the current study, we hypothesized that harvest of saphenous vein grafts by “no-touch” technique improves the immediate contractile function because (a) the adherent perivascular tissue releases vasoactive paracrine factors that modify smooth muscle contractions or (b) the atraumatic “no-touch” surgical technique reduces damage and preserves smooth muscle contractile capacity. We find that saphenous vein grafts harvested by “no-touch” technique have increased capacity for force generation and that this greater contractile function is unrelated to acute release of paracrine factors from the perivascular tissue. We propose that improved graft viability with associated greater dynamic range of tone regulation contributes to the superior long-term patency of “no-touch” vein grafts used for CABG surgery.

“No-touch” vein harvest technique was implemented at Aarhus University Hospital in 2011, 5 years prior to the present study. All conduits were harvested by the same team of cardiac surgeons experienced in both “no-touch” and conventional harvest techniques. The study was carried out in accordance with Danish legislation and the protocol approved by the Mid-Jutland Regional Committee on Health Research Ethics (enquiry no. 167/2017). According to Danish legislation, written informed consent was not required because the procedures involve excess resected tissue from a surgical procedure where all post-surgical tissue and data handling was anonymized.

A series of well-designed and well-conducted randomized clinical trials from a single center in Sweden with up to 16 years of angiographic follow-up provides strong evidence of superior patency rates of “no-touch” compared to conventional saphenous vein grafts (Souza et al., 2001, 2006; Johansson et al., 2009; Samano et al., 2015). The current study evaluates whether the acute vasomotor function of “no-touch” saphenous vein grafts differs from that of conventionally harvested vein grafts and was conducted in order to identify functional properties that might help to understand the improved patency.

Our studies show that saphenous veins harvested by “no-touch” technique have greater contractile capacity than equivalent grafts harvested by conventional technique (Figures 1, 3, 5). The stronger contractions—evident under both isometric (Figures 1, 3) and isobaric (Figure 5) experimental conditions—are observed in response to the vascular agonists serotonin and noradrenaline (Figures 3A,B, 5) and also when smooth muscle cell membrane depolarization is induced by elevating extracellular [K⁺] (Figure 3C).

Vasoactive effects of paracrine factors released from perivascular tissue have been described for multiple arterial preparations from humans and rodents (Gollasch, 2012) and in rat inferior vena cava (Lu et al., 2011). In contrast, we find that removal of the perivascular tissue surrounding human saphenous vein segments only has marginal acute effects on their contractile properties (Figures 1, 2) suggesting that the perivascular adipose tissue surrounding saphenous veins has very little net vasomotor effect. This is in congruence with a rather loose association between the perivascular tissue and the saphenous veins compared, for instance, to coronary arteries. Nevertheless, additional studies are required to determine whether the perivascular tissue is important for long-term graft patency.

Because the greater capacity for vasocontraction of “no-touch” relative to conventional vein grafts is evident also after the perivascular tissue has been removed by microdissection (Figures 1, 2, 3, 5), our studies suggest that the sheath of perivascular tissue is important for atraumatic excision and handling rather than as a source of paracrine secretion. It is commonly observed that stretch or forceful dilation of contracted blood vessels damages their function; and to minimize this effect, vasodilators (e.g., papaverine) are routinely used during CABG graft harvesting (He et al., 1993). In the investigated saphenous vein segments, we observed that if veins were not fully relaxed during the normalization procedure or during the passive stretch component of the diameter-tension protocol, they immediately lost contractile function. Thus, our findings are consistent with the perivascular tissue being critical for permitting the surgeon to avoid directly grasping the vein with surgical instruments that can lead to cell damage and associated vasospasm (Souza and Samano, 2016; Fernández-Alfonso et al., 2017), and protecting against direct damage induced by sharp dissection during stripping of conventional vein grafts.

The proposed importance of the perivascular tissue as a mechanical shield against surgical handling suggests that the amount of retained perivascular tissue can be reduced to a thin sheath, which will minimize the size of the surgical wound on the harvest site and the risk of wound infection. Infection is a challenge of the open surgical techniques compared to endoscopic harvesting techniques (Kopjar et al., 2016; Souza and Samano, 2016). Still, clinical verification of the amount of perivascular tissue necessary for improved long-term graft function and clinical outcome is necessary as there is currently no direct evidence that the capacity for vein graft contraction is causatively related to its long-term patency.

Venous transmural pressure in healthy humans is typically around 20 mmHg but can increase to around 60 mmHg during sustained upright posture or in patients with insufficient venous valve function (Koster et al., 2013). Maximal active tension development at transmural pressures higher than the physiological level (Figure 1) provides basis for a Starling-like mechanism that will limit vein compliance during combined venous build-up of blood and activation of venous smooth muscle cells. The increased active tension development will support local intravascular pressure increases that can facilitate return of blood to the heart through the valve-containing venous circulation.

Previous studies have shown that the storage conditions used between harvest and implantation can affect the quality of CABG vein grafts. In particular, endothelium-dependent vasorelaxation has been found improved by storage in buffered salt solutions (Wise et al., 2015). In the current study, the excised grafts were stored for a few minutes in the storage solution used for the surgical procedure (i.e., heparin-saline or heparinized papaverine-containing autologous blood solution) before they were transferred to a buffered salt solution (i.e., PSS) that was used for transport, dissection, and experiments. Using buffered salt solution for storage and handling, we avoid that variation in storage conditions differentially affect the function of vein grafts harvested by conventional and “no-touch” technique.

Although we primarily consider the enhanced contractile capacity of “no-touch” vein grafts an important sign of improved viability, the greater capacity for vasocontraction also provides enhanced ability for dynamic regulation of precapillary resistance, local perfusion, and capillary pressure. A greater viability of the medial vascular smooth muscle cells of “no-touch” vein grafts is consistent with previous histological investigations (Ahmed et al., 2004; Vasilakis et al., 2004; Loesch et al., 2006). Because cell damage in the vascular wall is associated with structural disintegration and platelet deposition, it is expected that improved graft viability in the acute phase can reduce long-term graft failure.

In conclusion, we demonstrate for the first time that saphenous vein grafts harvested by “no-touch” technique have stronger vasocontractile capacity than vein grafts harvested by conventional technique. In the “no-touch” veins, the increase in active tension development at transmural pressures higher than the physiological level provides basis for a Starling-like mechanism that could limit build-up of blood and counteract edema formation. Whereas, acute vasomotor effects of paracrine factors released from the perivascular tissue were negligible, our findings support that the atraumatic “no-touch” surgical technique improves graft viability, which is consistent with the greater long-term patency.

IM, FdP, and EB: conceived the project; LV and LB: performed and analyzed experiments; All authors contributed to the design of experiments and interpretation of data; EB: wrote the manuscript; All authors revised the manuscript and accepted the final version.