Reply to Nezic et al. Interpretation of string sign in radial artery patency bypass grafts.

Abstract

In response to the Letter-to-the-Editor [1] by Nezic et al., we have reviewed problems with definitions and interpretation of graft patencies. Commonly used definitions are anatomic, physiologic or a combination. The use of a simple anatomical definition of patency and complete occlusion in the Radial Artery Patency Study (RAPS) and Radial Artery and Radial Artery Versus Saphenous Vein Patency (RSVP) trials [2] and composite definition of graft failure in the Radial Artery Patency and Clinical Outcomes (RAPCO) trial [3] have led to conflicting interpretations of similar outcomes of radial artery trials (RAPCO vs RAPS and RSVP) [2–4]. Complete occlusion as used in RAPS and RSVP is a reproducible anatomic observation, supplemented by flow measurements. However, such a dichotomous anatomic definition limits sensitivity and classifies high-grade graft stenoses as patent although the conduit may be dysfunctional. The string sign is defined as a diffuse longitudinal narrowing less than 1 mm; this may be complete or partial [RAPS]. The string is seen most commonly in arterial grafts and complicates the interpretation of outcome. The assumption that the arterial graft with the string may recover and regain normal physiologic flow has been reported in 17% of LITA in situ grafts [5], but reversible ischaemia in only 2 of 14 patients with a string in a series of 137 after radial artery grafting [6]. Variable TIMI flow in grafts with a string sign adds to further confusion. It appears that about half of those grafts with a string had TIMI 1 and the other half TIMI 2 or 3 flow [7], thus suggesting that there may be several aetiological mechanisms. Diffuse narrowing may be the result of competitive collateral flow or low flow when sutured to a small target artery. In addition, a string sign may be the result of a poor harvesting technique or inherent graft disease.

The answer to Nezic et al.’s question is that a free arterial graft with a string sign is anatomically patent but has a high probability at least 50% chance that it is dysfunctional and that there is little prospect of any improvement. Most surgeons would interpret such a conduit as a failure. In an attempt to circumvent any potential problem with the definition and to use one that most surgeons would accept in the RAPCO Trial, graft failure was defined as total occlusion, a stenosis greater than 80% and those conduits with a string sign. The argument for including grafts with stenoses between 70% and 100% in the failure group is that this level of narrowing is thought to be related to flow restriction and myocardial ischaemia and is the trigger for re-vascularisation used by many surgeons.