title: Understanding coronary artery bypass transit time flow curves: role of bypass graft compliance.
abstract: OBJECTIVE
  Low mean bypass graft flow (Q) and high pulsatility index (PI) measured by the transit time flow measurement method are not specific for anastomotic stenosis, but occur with competitive flow and poor coronary run-off. We hypothesized that graft compliance is responsible for these changes and that flow measured at the proximal end of the coronary bypass can be viewed as a sum of the graft capacitive flow and flow that passes through the distal anastomosis.
  
  
  METHODS
  Transit time flow measurements (TTFMs) of 15 left internal thoracic artery (LITA) to LAD bypass grafts and 10 saphenous vein grafts (SVGs) to either the right coronary artery (RCA) or posterior descending artery (PDA) were analysed. The TTFM was performed on the proximal and distal end of the graft, and proximally with distal occlusion of the graft. Low mean bypass graft flow PI and diastolic filling (DF) measured distally and proximally were compared, and graft compliance was estimated.
  
  
  RESULTS
  Diastolic filling was higher distally in every single case (LITA-LAD: distal DF 76 ± 12% vs proximal 66 ± 13%, P = 0.005; SVG-RCA/PDA: distal 72 ± 15% vs proximal 63 ± 12%, P = 0.018). There were no significant differences in Q and PI. Subtracting the distal from the proximal flow gave a result identical to the proximal TTFM in distally occluded grafts, confirming the presence of graft capacitive flow. Graft compliance estimated from the flow of distally occluded grafts was 0.99 ± 0.47 μl/mmHg for LITA grafts and 0.78 ± 0.42 μl/mmHg for SVG grafts.
  
  
  CONCLUSIONS
  The study confirmed that the TTFM measured at the proximal end of the coronary bypass could be viewed as a sum of graft capacitive flow and the flow that passes through the distal anastomosis. Graft capacitive flow increases the systolic and decreases the diastolic TTFM when measured at the proximal end of the graft. It explains the higher DF when the TTFM is measured at the distal end of the graft and the increase in the PI at the proximal end when Q decreases. As the influence of graft capacitive flow on the PI in low Q can be eliminated by performing the TTFM at the distal end of the graft, we believe that the value of PI is clinically irrelevant.
authors.affiliation: Department of Cardiovascular Surgery, University Medical Center Ljubljana, Ljubljana, Slovenia.
authors.forename: Matija
authors.initials: M
authors.lastname: Jelenc
authors.forename: Blaž
authors.initials: B
authors.lastname: Jelenc
authors.forename: Tomislav
authors.initials: T
authors.lastname: Klokočovnik
authors.forename: Nikola
authors.initials: N
authors.lastname: Lakič
authors.forename: Borut
authors.initials: B
authors.lastname: Geršak
authors.forename: Ivan
authors.initials: I
authors.lastname: Kneževic
mesh_terms: D001783:Blood Flow Velocity; D003187:Compliance; D001026:Coronary Artery Bypass / Q000009:adverse effects / Q000379:methods*; D003326:Coronary Circulation; D003331:Coronary Vessels / Q000503:physiopathology / Q000601:surgery*; D005260:Female; D006083:Graft Occlusion, Vascular / Q000209:etiology / Q000503:physiopathology; D006801:Humans; D007387:Internal Mammary-Coronary Artery Anastomosis / Q000009:adverse effects; D008297:Male; D008955:Models, Cardiovascular; D011673:Pulsatile Flow; D012307:Risk Factors; D012501:Saphenous Vein / Q000503:physiopathology / Q000637:transplantation*; D013997:Time Factors; D016896:Treatment Outcome; D018924:Vascular Capacitance
keywords: Coronary artery bypass; Coronary bypass graft compliance; Transit time flow measurement