CABG anatomy and technique relevant to ICU care

"Anatomy and surgical technique relevant to ICU post-operative care" is a  Level 1 topic from Section 2.1.18 in the second edition of the CICM Syllabus for the Second Part Examination, and the specific anatomy being covered in this chapter is the anatomy of the coronary grafts. The goal is to add a little bit of context to the cardiothoracic ICU experience of CICM trainees left to receive handover from the surgical team after a patient has been transferred from theatre. To only ever expect them to faithfully document the handover without applying their own interpretation would be a failure of the training program, and the following set of summaries and shameful oversimplifications will hopefully help transform the immensely complex and eponym-rich field of cardiothoracic surgery more comprehensible to the CICM exam candidate.

The author hastens to add that he is neither a cardiac surgeon, nor is he close enough to a cardiac surgeon to ask for peer review, nor was his own training overmuch enriched by solid cardiothoracic teaching, and so he cannot even be said to be experiencing the impostor syndrome in the course of writing this material, him being an actual impostor. For readers experiencing some well-justifiable suspicion of any such material, legitimate resources can be offered in support. Chapter 7 (from Kirklin/Barratt-Boyes Cardiac Surgery) and "Coronary artery bypass grafting using cardiopulmonary bypass" from Mastery of Cardiothoracic Surgery (2013) were instrumental here.

Coronary artery bypass grafting

To CABG somebody means to bypass stenotic sites in the coronary circulation by adding a fresh blood supply below the level of the stenosis. For this, one may:

• Plug another artery into the distal vessel, eg. one of the mammaries
• Extend a graft from the aorta using an artery, eg. a radial artery
• Extend a graft from a nearby coronary artery 
• Use a length of vein instead of an atery, eg. the saphenous vein

The graftingterminology is also potentially confusing:

• A "simple" graft is from aorta to thing
• A Y graft is from aorta to two things
• A sequential graft is from aorta, to thing, to another thing
• A "skip" graft is from aorta, to thing, and then to the same thing

A "skip" graft is apparently a variant of a sequential graft, except nobody seems to be clear on how it is different to a sequential graft, other than that it  Miller (1977) seems to be the only resource that clearly explains what this is, and even there it is unclear:

"Skip grafts represent a variant of sequential grafts where the side-to-side anastomosis is constructed to the same artery between a proximal and more distal stenosis."

The diagram, offered to clarify this, is moderately helpful:

It would be more bserve: the sequential graft accesses two vessels distal to an occlusion, whereas a skip graft specifically "skips" over a lesion in the distal middle of a vessel.

The meaning of graft anatomy

In an unforgivably glib oversimplification, one might say that the anatomy and origin of the grafts has relevance because of some characteristic complications that are seen with specific sites and techniques. A curious reader may find value in summary articles such as Alzghari et al (2023) or He (1999) which cover this topic with greater depth; whereas the reader whose burnout has killed their curiosity will be glad for a quick painless summary, such as what follows:

• LIMA/RIMA: best longevity, fewest complications (sternal healing)
• RA: next best longevity, functional hand complications
• SVG: least longevity, but most versatile and commonest (leg haematoma is the most common early problem)

Internal mammary arteries (also known as internal thoracic arteries) provide the best long term results and are used whenever possible. The data for this assertion comes from ancient studies of the 1980s, where for example Loop et al (1986) found 10-year survival difference of 93.4%  with LIMA-to-LAD vs 88% with vein grafts (for single vessel disease). This totally observational data was enough to weld this idea into the minds of surgeons everywhere, such that their court of peers agrees about the need for a randomised controlled investigation but at the same time admits that "there is no professional nor individual equipoise in the surgical community for such a study". The LIMA to LAD graft is the accepted gold standard for CABG, and wherever possible, the LAD will be grafted with a LIMA (whereas it seems like anything goes for the other arteries).

One possible disadvantage of these is "steal". Consider: the LIMA receives its blood from the left subclavian artery. If the vascular bed of the left upper limb is suddenly dialted (eg. by exercise), the blood flow will follow the path of least resistance into the arm, and not into the heart. This "steal" of blood flow by the upper limb is referred to as "coronary-subclavian steal syndrome" and is a phenomenon that manifests as exercise-induced angina. 
If we can use one, why not use two? Bilateral IMA grafts have the illusion of being amazing because of long term graft patency but come at the expense of higher sternal morbidity (e.g. breakdown, lack of perfusion to the sternum), longer surgical times (bypass related issues) and for patients who are not going to live that long, the benefit is probably not there. Taggart et al. in 2019 have shown no difference in 10 year outcomes between single IMA and bilateral IMAs (BIMAs). 

Radial artery grafts have been attractive as they are also arterial and have the potential for long term patency-  Possati et al (2003) found 9.6% were still patent at 10 years, and Gaudino et al (2018) found a hazard ratio of 0.44 in favour of radial grafts when compared to SVG (i.e. at 5 years the SVGs were 56%  more likely to be occluded). Complications related to hand function are rare enough for RA grafts to be common, but common enough for RA grafts to be associated with some surgical reluctance, and a superstitious tendency to use the nondominant arm (because just in case). 
The use of a radial artery graft can occasionally evoke arterial spasm. This has attracted the use of vasodilators, of which there is a range (eg. some surgeons prefer calcium channel blockers, some prefer systemic GTN, others intraoperative papaverine or verapamil). If one has the choice, it appears nifedipine or amlodipine are best.

Historically, the gastroepiploic artery was used as another arterial graft, arteries being superior to veins, but it hasn’t been used for a while because they are a small artery, fragile and are associated with gastrointestinal complications.

Saphenous vein grafts are the most common because most people have a long useable length of valveless vein,  but their disadvantage is a 50% risk of graft failure at 10 years (reported occlusion rates as high as 25% at 1 year and 75% at 10 years)  Also occasionally it ends up being the most junior member of the team harvesting the vein. Sometimes a long vein graft is used to graft multiple vessels, so one might see a vein graft to D1 to D2 to OM1, otherwise occasionally referred to as "around the world".

Speaking of abbreviated cardiac anatomy details, the CICM exam candidate is again reminded of what the heart looks like when it is not on TTE, specifically to contextualise the common CABG conduits. The main reason to include this textbook image is to give a s special shout out to the clearly censorship-averse proofreader who allowed such freedom in the labelling of the "coronory trees". 

Alternatively, 

Thus, we can conclude that the factors which go into choosing a graft are numerous, individualised, and occasionally dependent on the surgeon. They may include:

• Incidence of spasm (RA is more reactive than IMA)
• Length of the vessels needed (eg. midportion of the IMA is less reactive than the ends)
• Incidence of atherosclerosis (IMA less than RA)
• Condition of the patient (younger patients will get the most benefit from arterial grafts)
• Urgency of the surgery (arterial harvesting and grafting takes longer, so if you are in a real hurry, SVG may be best)
• Position of the stenosis: LIMA can reach the LAD, RIMA can reach the RCA, but beyond that the RA or SVG are required

The meaning of the graft choices discussed above, for the ICU junior medical staff, are of course profoundly uninteresting. Who cares if they tend to become slightly more occluded ten years from now. What about tonight? Will these grafts fall apart during my shift? Considering that the rate of chest re-opening in some institutions approaches about 3.4%, for an institution doing twenty hearts per week that would equate to approximately eight resternotomies per 12-week rotation, which means that statistically speaking, after two or so terms working a typical ICU roster, one will inevitably be involved in something like this. It is therefore of interest to know that there appears to be no difference in the rate of early complications among the different coronary artery bypass conduits.