Introduction
Notwithstanding the increasing number of interventional coronary procedures performed worldwide, coronary artery bypass graft (CABG) still remain the treatment of choice for patients with moderate to complex coronary artery disease . Large observational evidence suggests a survival benefit for the use of multiple arterial grafts , but it is estimated that approximately 95% of patients who undergo CABG in the United States and approximately 90% in Europe receive one internal thoracic artery (ITA) . A recent metaanalysis supports the superiority of the use of a second arterial graft over a venous graft and suggests the equivalence in long-term and perioperative outcomes between the right ITA and radial artery (RA), although the higher incidence of deep sternal wound infection remains a concern after bilateral ITA grafting if skeletonization is not used.
In the last decades the patient population with indication to CABG is older and presents more comorbidites that may potentially increase perioperative mortality and morbidity. Among all the most dreadful complication is represented by cerebrovascular accidents (CVAs) . CVAs can have origin during three well-defined hospital time frames, namely, intraoperatively, early-postoperatively or late after surgery, and for various causes, the most relevant of which is the manipulation of ascending aorta . Cardiopulmonary bypass installation, aortic cross clamping, and side-beating clamping can be all responsible for CVAs. Possible preventive measures for CVAs are represented by (1) preoperative aorta CT scan, that allows an alternative surgical planning in presence of a diseased ascending aorta; (2) intraoperative high focused ultra sound scanning, that allows a “last minute” change of the surgical planning in the presence of intraoperative evidence of ascending aorta disease; (3) no-touch aorta CABG technique, known as anaortic-CABG, aimed at avoiding any kind of aortic manipulation using only pedicled grafts or Y/T-graft configurations ( Fig. 10.1 ) plus off-pump technique .
It is evident that the best way to avoid the release of debris from a diseased ascending aorta is to avoid its manipulation; hence, anaortic-CABG should represent the surgical strategy of choice to prevent CVAs. Historically, the concept of anastomosing another bypass graft to a pedicled left ITA in presence of a severely atherosclerotic ascending aorta was introduced by Mills in 1982 using a segment of SV. Later this technique gained more popularity, and in 1994, Tector reported the first series of patients who received both the ITAs using the T-graft technique. A few years later the RA has been widely used with the left ITA as a composite Y-graft with excellent results by Royse et al. . This author has recently reported the 21-years survival comparison of left ITA-RA Y-graft versus either the more conventional left ITA plus SV on ascending aorta or other total arterial revascularization techniques, finding that left ITA plus SV has worse survival than left ITA-RA Y-graft, while this shows the same survival than other total arterial revascularization techniques.
An additional advantage of the left ITA used as a composite Y/T-graft is the increased efficiency of use of the second graft, particularly when it is an arterial conduit. Patients with three-vessel disease can receive full arterial myocardial revascularization with a Y/T-graft alone using both ITAs or the RA and the left ITA .
Despite these evident advantages, cardiac surgeons are usually cautious to adopt anaortic-CABG techniques with Y/T connections from LITA for three main reasons:
- 1.
concerns on the adequacy of the left ITA as single inflow for all the left ventricle;
- 2.
the crucial importance and the technical complexity of the Y/T-anastomosis on the left ITA; and
- 3.
the complexity of the use of an off-pump technique plus sequential coronary artery anastomoses during anaortic-CABG.
The adequacy of the left internal thoracic artery as single inflow for all the left ventricle
The human body has three different types of arteries classified as somatic, splanchnic, and limb arteries . Among these, somatic arteries such as ITAs have the most favorable features to be used as coronary artery grafts like a higher nitric oxide (NO) production ( Fig. 10.2 ).
The left ITA is universally accepted as the best conduit for CABG, its use resulting in excellent long-term graft patency, reduced risk of recurrent angina, and improved survival . The ITA histological structure ( Fig. 10.3 ) is peculiar having an intimal layer limited by a well-formed internal elastic lamina and a media layer featured by a network of circularly and longitudinally interlacing elastic lamellae between which smooth muscle cells are dispersed in a spiral fashion . The high elastic content of the media layer could partially explain the low ITA propensity to vasospasm . Also the ITA endothelium is itself unique with a significantly higher basal production of NO compared to other arterial grafts such as the RA. NO production serves many important functions such as keeping blood and platelets in a nonthrombotic status, avoiding intravascular activation of inflammatory cells and, possibly the most important, regulating the vascular tone and accordingly the ITA blood flow. It is widely recognized that endothelial cells are the biosensors of fluid dynamic shear forces that reduce arterial diameter when blood flow rate decreases and enlarge the diameter when the flow rate increases . The tractive force induced by blood flow acting on the endothelial cell surface is called wall shear stress (WSS) ( Fig. 10.4 ). This force modulates the levels of two potent endothelium-derived vasoactive mediators, the vasodilator NO , and the vasoconstrictor endothelin-1 (ET-1) . Moreover, different flow patterns have different effects on endothelial cells proliferation and endothelial synthesis of vasoactive mediators.
It has already been shown that the ITA is able to adapt its dimension to flow demand in the late postoperative period with a vascular remodeling that increases LITA diameter on follow-up angiograms at 3–10 month postoperatively . More recently it has been showed that in composite Y/T-graft the proximal left ITA is able to actively adapt its dimension to the flow demand, probably through the release of endothelial vasoactive mediators such as NO, consequence of higher values of WSS. This process of adaptation begins immediately after the Y/T-connection as consequence of a passive increase of blood flow due to the lower vascular resistance in the Y/T-graft system. The significant higher blood flow velocity into the proximal left ITA can thus be explained by the lower resistance of the parallel vascular circuit represented by the composite Y/T-graft, as expressed by Kirchhoff’s second law:
1/Rtot=1/R1+1/R2
WSS=1/44mAPV/D
The flow pattern could have a role in the production of vasoactive substances by vascular endothelial cells. Rapid oscillations of flow in magnitude and direction can affect endothelial cell synthesis and release of NO, contributing to vasoconstriction and cell proliferation. Bach et al. have shown that the in situ left ITA displays a pattern of phasic blood flow with a transition from systolic-predominant to diastolic-predominant peak flow velocity shifting from the subclavian artery to the coronary end ( Fig. 10.5 ). On the contrary in Y/T-grafts it has been recorded a diastolic-predominant peak of flow velocity also close to the subclavian end ( Fig. 10.6 ) . This peculiar flow patter is probably related to the reduced vascular resistance of the parallel vascular circuit represented by the Y/T-graft configuration. However, in both single pedicled left ITA and Y/T-graft left ITA the diastolic/systolic velocity flow ratio shows a gradual transition to a greater diastolic peak velocity moving from the subclavian artery to the coronary artery.
