Response to Letters Regarding Article, “Vascular Reactivity and Flow Characteristics of Radial Artery and Long Saphenous Vein Coronary Bypass Grafts: A 5-Year Follow-Up”
Our study showed that 5 years after aortocoronary bypass graft surgery, radial artery grafts had preserved flow-mediated dilatation, whereas saphenous vein (SV) grafts had not.1 In agreement with Brugaletta et al, we feel it would be ideal to know the preexisting vasomotor response of the study grafts in all patients. Unfortunately, not all the patients who were studied at 3 months agreed to participate at 5 years.1,2 Four patients participated in Doppler studies at both follow-ups (radial artery=2 and SV=2), and we believe that the numbers are too small to make any conclusions from these data. Although not ideal, we consider these small numbers do not invalidate the findings of our study.
We agree with Dr Doraiswamy's suggestion that the structural changes that take place in SVs when placed in an arterial system may explain the lack of effect of nitrates in our study. In the Radial artery versus Saphenous Vein Patency (RSVP) trial at 3 months,2 SV grafts dilated to nitrates indicating that, early on, SV grafts are responsive conduits. However, with chronic exposure to high pressure in the myocardial environment, the SV grafts may become passive conduits. The stretch to which they are exposed may reduce their ability to autoregulate. Extreme flow shear stresses then result in a cyclic process of injury and repair. Early endothelial damage may result, followed by a more chronic process of injury and repair, resulting in replacement of the muscle media layer with fibrous tissue (demonstrated by a histopathologic study of chronic vein grafts3). We are the first to show a progression of this process through the gradual loss of endothelial and then medial function in SV grafts.
Although Brugaletta et al state that SVs do not dilate as much to acetylcholine or nitrates as radial arteries, much of their evidence is from in vitro experiments. We cannot assume that SV ring segments in vitro behave similarly to SV grafts in vivo. Human in vivo evidence is lacking, which is why we believe that our study is important.
Statins, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, β-blockers, calcium channel antagonists, and long-acting nitrates were stopped in our study, with no difference in their use between groups. Hypertensive patients were excluded from the study because of our inability to discontinue their medication.
We agree that a 10-year follow-up of the patients enrolled in the RSVP trial would be of interest and important, and we hope to provide these data.
Carolyn M. Webb, PhD
Peter Collins, MD, FRCP, FACC
National Heart & Lung Institute
Imperial College London
Neil E. Moat, MS, FRCS
Chee F. Chong, FRCS
Department of Cardiac Surgery
Royal Brompton & Harefield NHS Foundation Trust
- © 2011 American Heart Association, Inc.