LITERATURE REVIEW
2.5.5. THEBESIAN VESSELS
passes intimately near the left and right posterior aortic cusps. In doing so, it sends offbranches that extend to the upper limits of the valve flaps to dichotomize at the base to send fine branches along the line ofclosure. The right anterior cusp is usually supplied by myocardial branches ofthe LCX in this area, but may also receive supply from Kugel's artery. Kugel's artery also shares a close relationship to the posterior and left lateral surfaces of the aorta. Here, it may send anastomosing branches to the RCA and LCA.
The anatomical value ofKugel's artery, when present, is recognized. In addition, clinically, the intimate relationships it shares with the aortic and mitral valves, comrnisures and base ofthe aorta suggests that it may well play a part in the pathogenesis oflesions in the area or provide additional blood flow when needed.
After having answered the question, Thebesius gave" due credit and much praise to the Creator for having such foresight to anticipate the need and usefulness of these little vessels", (Wearn,
1928).
Although these vessels have since become known as the "Thebesian vessels", it was really Vieussens who first discovered their presence. Of greater importance though, was the fact that Vieussens used and injection method via the arteries, and Thebesius injected the veins, yet both described the same openings. According to Wearn, (1928) these findings were subsequently followed by investigations confirming (Lancisi 1740, Senac, 1749; Langer, 1880) and denying the presence of these vessels (Cruveilhier, 1834, Theile 1843; Lannelongue, 1867).
In 1898, Pratt suggested that the Thebesian veins were connected directly with the coronary veins and indirectly with the arteries through capillaries. Moreover, he explained the lack ofinfarcts in markedly sclerosed hearts, by the presence of the Thebesian circulation.
The embryology of the Thebesian vessels have not been studied extensively, although the literature suggests them to be the remains of the primitive, inter trabecular circulation of the embryo, (Wearn, 1928). According to Wearn, (1928), Minot (1900) found that in the earliest stage, the trabeculae of the heart was made up of muscle cells covered with endothelium, but without capillaries of their own. He believed that the "sinusoidal" circulation of the embryonic heart was sufficient to nourish the trabecular muscle.
However, Grant and Regnier (1926) reported that the sinusoids were not initially connected to the
heart, but grew into them at a later stage. As the coronary veins grew and branched into the myocardium and provided evidence showing direct connection, other than through the capillary bed, between the coronary arteries and the chambers of the heart. In addition, connections between larger coronary veins and the Thebesian veins were demonstrated. The study also showed that although the size and number ofvessels varied between sides, as much as 90% ofthe arterial flow may escape via the Thebesian vessels.
Aninvestigation by Archie in 1974 however, declared the openings in the atrial and ventricular walls to be an unlikely source ofsupply to the subendocardium because ofinappropriate pressure differences in both systole and diastole. Furthermore, Griggs et aI., (1972), commented thatif these vessels were a source of additional supply, then the subendocardium, instead of being the myocardial area most vulnerable to ischemic damage, would be best protected.
It is interesting to note, before dispelling the possibility of any positive role these Thebesian vessels may have to play in the coronary circulation, to consider the findings based on case reports in Wearn's (1928) study. The reports describe two instances of post mortem findings showing complete closure of both coronary arteries at the aortic orifices. In each case, a heart without openings for the coronary arteries had maintained a sufficient circulation that enabled the individual to survive and earn a living for a significant number ofyears. Wearn (1928) postulates that the existence of the Thebesian vessels, "being the only other entrance to the coronary circulation", was responsible for the nourishment necessary in maintaining the myocardium.
He further supports these claims by pointing out how often, individuals that live comparatively
when allowed sufficient time to adapt, the Thebesian vessels can take over the function of coronary arteries. Needless to say, with these contrasting points of view being presented, the obvious issues therefore, of where and how these hearts get sufficient circulation to function so efficiently, remains unclear.
EXTRA-CORONARY COLLATERALS
"Healingisa matter oftime ... but sometimes itisa matter ofopportunity. "
Hippocrates (460-375 BC)
2.6.1. INTRODUCTION
For most conventional intra-cardiac operations, maintaining a quiet, dry operative field is essential. Cardiac arrest may be achieved by cross-clamping the aorta, thereby interrupting coronary flow. Sometimes however, the heart continues to beat or fibrillate despite aortic cross- clamping. This suggests that anoxia ofthe myocardium cannot be achieved when coronary flowis interrupted or that there may be supplementary flow from another source, (Brazier and Hottenrot, 1974).
Further evidence suggesting the possibility of an extraneous flow contribution may be observed during valve replacement or coronary artery revascularisation. Occasionally, outpouring of arterial blood from the coronary ostia occurs when the aorta is opened, or when the coronary artery is incised. This persists in the arrested heart, "despite adequate venting ofthe left ventricle and decompression of the coronary sinus to prevent retrograde flow", (Brazier and Hottenrot, 1974). In addition, it is not uncommon to observe that some patients maintain excellent ventricular function, despite complete occlusion of all 3 major coronary arteries.
The possibility of the existence of an extra-coronary collateral circulation was postulated by Thebesius in 1708, (Bloor and Liebow, 1965), in his description of the venae cordis minimae _
Langer (1880), described branches of the coronary arteries anastomosing with vessels in the mediastinum, parietal pericardium, diaphragm and hila of the lungs.
Origin of L. S~onchiQI A. _
Area ofAnastomosis of Branches of Corona.r!l A.
with Branches of L. Bronchial A.
Figure 43: Demonstration of coronary to bronchial anastomosis (Adapted/rom Moritz et al., 1932)
The literature reveals little,ifany, detailed description ofextra-cardiac sources ofcoronary artery anastomoses. Reports by Gross (1921), Spalteholz (1924) and Karsner (1933) have made reference to the extra-cardiac coronary anastomoses.