4. Assessment of the ascending aorta and techniques to reduce aortic manipulation
Recognizing patients with a calcified ascending aorta is essential to preventing neurologic complications. In the era before routine use of perioperative imaging, this diagnosis was made by visual inspection, digital palpation or when the cross-clamp was found to incompletely occlude the aorta (6). These methods have been largely replaced by TEE and epiaortic ultrasound (EAU) (52,53). Katz (54) classified aortic atheroma using intraoperative TEE in five different grades: I= no disease or minimal intimal thickening, II = extensive intimal thickening, III = sessile atheroma < 5mm thick, IV = protruding atheroma ≥ 5mm thick, and V = mobile atheroma in the lumen (Figure 3). Katz and Ribakove (54,55) demonstrated how careful aortic assessment by intraoperative TEE guides the operative strategy in terms of cannulation site, choice of aortic cannula and conduct of CPB. Their results showed that the degree of aortic atheroma detected by TEE correlated strongly with perioperative stroke (p=0.001) and that modifications of surgical technique to avoid atheroma had a favorable effect on patients with grade V disease (54).
Although intraoperative TEE is superior to manual palpation in detecting advanced aortic arch atherosclerosis, visualization of the distal ascending aorta can be suboptimal. To overcome these limitations epiaortic ultrasound (EAU) is a preferable intraoperative imaging modality to detect atheroma from the aortic root to the distal ascending aorta and proximal arch (52,56).
Zingone (57) reported a significant reduction in postoperative neurologic complications after the routine introduction of EAU. Pathological findings at EAU guided changes in surgical strategies including the aortic cannulation and cross-clamp sites, the use of DHCA and off-pump techniques. Finally, pre-operative computed tomography (CT) of the chest (with or without contrast) is a valuable tool to fully characterize the degree of aortic atheromatous changes (2018 ESC/EACTS Guidelines Class IIa, Level of Evidence C) (30). Lee (58) reported a significant reduction in the rate of neurologic complications post elective cardiac procedures after the introduction of a pre-operative non contrast CT chest in high risk patients (3.04% pre-CT vs 0.73% post-CT, p=0.05), thanks to ad hoc changes in operative strategy. Similar results were described by Sandner (59): in 435 matched pairs of isolated CABG postoperative stroke was significantly lower in the preoperative CT cohort (0.92% vs 3.22%, p=0.017).
Once severe atheromatous changes of the ascending aorta are detected, a variety of surgical techniques have been employed to minimize the neurologic risk:
Early reports by Ribakove (55) described aortic debridement under DHCA via femoral cannulation in patients with grade V atheroma undergoing isolated CABG; alternatively debridement and patch repair of the ascending aorta has been reported by Robicsek (60) and Ott (61) to perform proximal anastomoses. In a series of 500 patients by Wareing (62) epiaortic ultrasound detected moderate to severe atheroma of the ascending aorta in 68 cases. In the majority of patients an alternative site for cannulation and clamping was sufficient to avoid overt neurologic complications; otherwise when clamping wasn’t possible hypothermic fibrillation was employed and very occasionally the ascending aorta and proximal hemiarch needed replacement under DHCA (64,65).
A valid alternative to these aggressive strategies is complete avoidance of any aortic manipulation. Over the course of the last four decades “no-touch” techniques have evolved from pump-assisted with peripheral cannulation to totally anaortic off-pump grafting.
Mills and Suma (6,66) were among the first to describe and promote an “aortic no-touch technique” for management of the severely diseased ascending aorta during CABG. Although still utilizing distal arch or peripheral cannulation to accomplish low-flow hypothermic cardiac fibrillatory arrest, the authors successfully introduced the use of T or Y grafts off the internal mammary artery (IMA) or the right gastroepiploic artery to avoid manipulation of the ascending aorta (6). Variations of the “no-touch technique” were described by Holland (67) with the use of the innominate or carotid arteries as proximal inflow for coronary grafts and by Murphy (68) with the idea of a composite left IMA-saphenous vein extension graft to reach a distal marginal branch. Finally, Hendel and Thomson (69) implemented “no-touch” strategies with the adoption of femoral cannulation, fibrillation with venting and core cooling to 25°C and construction of Y-vein-grafts from the left IMA or extension grafts from the right IMA.
Avoiding both aortic manipulation and cardiopulmonary bypass adopting off-pump techniques offer further neurologic protection when intraoperative TEE shows extensive aortic atheroma (70-72). In a large series of consecutive isolated CABGs, Grossi et al (70) selectively performed OPCAB in patients with severe aortic disease detected by intraoperative TEE with a significant reduction of both in-hospital mortality (p=0.08) and perioperative stroke (p=0.01). Using propensity score matching Mishra (71) and Sharony (72) showed significant survival benefit (p<0.001 and p=0.01 respectively) and postoperative stroke reduction (p=0.05 and p=0.03 respectively) when off-pumpvs on-pump coronary grafting was performed in patients with severe aortic atherosclerosis documented by TEE.