GERARD S. KAKOS AND H. NEWLAND OLDHAM, JR. 
351 
DISCUSSION 
This study was designed to evaluate the ef- 
fects that direct coronary artery revasculari- 
zation might produce in the proximal, distal and 
adjacent coronary circulation in the presence 
or absence of collateral vessels. The canine 
model chosen proved both convenient and adapt- 
able once mastered by persistent application of 
the outlined operative approach. 
In Part 1, an experimental system without 
a large, functioning collateral circulation was 
chosen to obviate any mechanical alterations in 
the peripheral vascular bed ^ which might arti- 
ficially influence the peripheral coronary col- 
lateral resistance and also to replicate the type 
of coronary collateral backflow currently re- 
ported in clinical investigations on this subject.^ 
In addition, Khouri, et al.^ have demonstrated 
experimentally that myocardial collaterals may 
become non-functional once forward flow is 
restored. 
Coronary artery flow ultimately is controlled 
by the myocardial oxygen demand and by the 
availability of this oxygen to the myocardial tis- 
sue. After direct bypass improved ventricular 
function should be expected if myocardial de- 
pression existed and was due to ischemia.* In the 
presence of an adaptable peripheral coronary 
resistance, coronary artery bypass graft flow 
is dependent on the pressure gradient from 
the aortic graft to the recipient coronary artery. 
Assuming that both the anastomosis and the 
caliber of the graft are technically adequate, 
this pressure gradient is directly influenced by 
the timing of the pulse wave and arterial pres- 
sure contour transmitted through the graft, 
and by the flow and pressure pulse presented 
by the proximal portion of the bypassed cor- 
onary artery. Both pulse wave timing and pres- 
sure contour have been described as essentially 
normal when a short ascending aorta-to-cor- 
onary artery graft is utilized.^ These findings 
were substantiated in this study with no evi- 
dence of delay or depulsation of the pressure or 
flow contours by the vein segment. The present 
investigation further demonstrated no altera- 
tions in flow, pressure or resistance in the cor- 
onary artery distal to the vein graft insertion 
as compared to the control (non-bypassed) state 
regardless of either the route of blood flow or 
the volume contributed by the vein graft or 
proximal coronary artery. Major changes, how- 
ever, were produced in the proximal coronary 
artery flow after vein bypass. Bypass of a non- 
critically obstructed coronary reduced blood 
flow through this vessel by 50 % . Concomitantly, 
the vein graft carried only one-half of its maxi- 
mum load (excluding hyperemia). If the proxi- 
mal coronary artery obstruction was critical, 
i.e. accompanied by a demonstrable decrease in 
distal flow and pressure, vein bypass function- 
ally produced total or near total occlusion of this 
stenotic vessel. Under these circumstances, the 
paucity of collateral flow prevented any change 
in adjacent coronary artery flow as has been re- 
ported in selected clinical cases. It would ap- 
pear that proximal coronary artery flow and 
pressure are related directly to bypass graft 
flow. Because of this interdependence, the pres- 
ence of a bypass graft will reduce flow in the cor- 
onary artery segment proximal to its insertion. 
In addition, if this segment contains a critically 
stenotic area, institution of bypass flow will pro- 
duce either a functionally total occlusion or pre- 
dispose this stenotic area to early thrombosis by 
severely limiting its volume flow. Distal coronary 
artery flow and pressure, nonetheless, remain 
totally independent of these relationships and 
are restored in a physiological manner by di- 
rect aorta to coronary artery revascularization. 
In Part II, a system will extensive collateral 
circulation was chosen, again to redefine any 
effects they might have on the circulation within 
both the bypassed coronary artery and vein 
graft per se. Animals without demonstrable 
collateral vessels had functionally independent 
LAD and CCA vascular beds, and flow and pres- 
sure in a graft from the aorta to the acutely 
ligated CCA were not altered by temporary 
LAD occlusion. This was not true, however, in 
animals which had collateral communications 
with the CCA. After chronic CCA occlusion, the 
initial flow through a bypass graft inserted 
distal to the obstruction was only one-fourth 
that after acute obstruction. This was related 
to collateral flow into the CCA, as demonstrated 
by a high peripheral CCA pressure. Tempo- 
rary occlusion of the LAD eliminated one source 
of collateral inflow and was accompanied by a 
