Adaptive mechanisms of arterial and venous coronary bypass grafts to an increase in flow demand

Abstract

Objective: To compare the mechanisms by which arterial and venous grafts increase their flow during pacing induced tachycardia, early and later after coronary bypass surgery.

Design: 43 grafts (13 epigastric artery, 15 mammary artery, 15 saphenous vein) evaluated early (9 (3) days (mean (SD)) after bypass surgery were compared with 41 other grafts (15 epigastric, 11 mammary, 15 saphenous vein) evaluated later after surgery (mean 23 months, range 6 to 168 months) by quantitative angiography and intravascular Doppler velocity analysis during atrial pacing. Controls were 17 normal coronary arteries.

Results: Baseline graft flow tended to be lower later after surgery than early (41 (16) v 45 (21) ml/min, NS). Blood flow increased during pacing by 30 (16)% early after surgery, less than later after surgery (+46 (18)%, p < 0.001) and less than in normal coronary arteries (+54 (27)%, p < 0.001 v early grafts; NS v late grafts). There was no difference between venous and arterial grafts. No significant vasodilatation was observed during pacing early after surgery in arterial and venous grafts. Later after surgery, significant vasodilatation was observed only in arterial grafts (mammary and epigastric grafts), from 2.41 (0.37) to 2.53 (0. 41) mm (+5.1% v basal, p < 0.001). Early after surgery and in venous grafts later after surgery, the increase in flow was entirely due to an increase in velocity. In later arterial grafts, the relative contribution of the increase in velocity to the increase in flow during pacing was lower in arterial grafts (70 (22)%) than in venous grafts (102 (11)%, p < 0.001) and similar to normal coronary arteries (68 (28)%).

Conclusions: Early and later after surgery, arterial grafts and venous grafts both increase their flow similarly during pacing. Early arterial grafts and venous grafts increase their flow only through an increase in velocity. Later after surgery, arterial grafts act as more physiological conduits and increase their flow in the same way as normal coronary arteries, through an increase in velocity and calibre mediated by the endothelium.

Figure 1

Comparison between baseline graft flow in epigastric artery (EA), internal mammary artery (IMA), and saphenous vein grafts (SV) in the early and late postoperative period.

Figure 2

Changes in mean graft diameter, expressed in per cent of basal diameter, observed after pacing and after isosorbide dinitrate (ISDN) injection, in the early (left) and late (right) postoperative period (*p < 0.01 and **p < 0.001), in saphenous vein grafts (♦), epigastric artery grafts (•) and mammary artery grafts (▪).

Figure 3

Comparison of increase in flow during pacing, expressed as per cent of basal flow between arterial (ART) and venous (VEN) graphs early and late after surgery, and normal coronary arteries (COR ART).

Figure 4

Comparison of the contribution of the increase in velocity to the increase in flow during pacing between early and late grafts (EA, epigastric artery grafts; IMA, left internal mammary artery graft; ART, arterial grafts (epigastric + mammary); VEN, saphenous vein grafts) and normal coronary arteries (COR ART).