title: Novel index for invasively assessing the coronary microcirculation.
abstract: BACKGROUND
  A relatively simple, invasive method for quantitatively assessing the status of the coronary microcirculation independent of the epicardial artery is lacking.
  
  
  RESULTS
  By using a coronary pressure wire and modified software, it is possible to calculate the mean transit time of room-temperature saline injected down a coronary artery. The inverse of the hyperemic mean transit time has been shown to correlate with absolute flow. We hypothesize that distal coronary pressure divided by the inverse of the hyperemic mean transit time provides an index of microcirculatory resistance (IMR) that will correlate with true microcirculatory resistance (TMR), defined as the distal left anterior descending (LAD) pressure divided by hyperemic flow, measured with an external ultrasonic flow probe. A total of 61 measurements were made in 9 Yorkshire swine at baseline and after disruption of the coronary microcirculation, both with and without an epicardial LAD stenosis. The mean IMR (16.9+/-6.5 U to 25.9+/-14.4 U, P=0.002) and TMR (0.51+/-0.14 to 0.79+/-0.32 mm Hg x mL(-1) x min(-1), P=0.0001), as well as the % change in IMR (147+/-66%) and TMR (159+/-105%, P=NS versus IMR % change), increased significantly and to a similar degree after disruption of the microcirculation. These changes were independent of the status of the epicardial artery. There was a significant correlation between mean IMR and TMR values, as well as between the % change in IMR and % change in TMR.
  
  
  CONCLUSIONS
  Measuring IMR may provide a simple, quantitative, invasive assessment of the coronary microcirculation.
authors.affiliation: Division of Cardiovascular Medicine, Stanford University Medical Center, Stanford, CA 94305, USA. wfearon@stanford.edu
authors.forename: William F
authors.initials: WF
authors.lastname: Fearon
authors.forename: Leora B
authors.initials: LB
authors.lastname: Balsam
authors.forename: H M Omar
authors.initials: HM
authors.lastname: Farouque
authors.forename: Anthony D
authors.initials: AD
authors.lastname: Caffarelli
authors.forename: Robert C
authors.initials: RC
authors.lastname: Robbins
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Fitzgerald
authors.forename: Paul G
authors.initials: PG
authors.lastname: Yock
authors.forename: Alan C
authors.initials: AC
authors.lastname: Yeung
mesh_terms: D000818:Animals; D001783:Blood Flow Velocity; D001794:Blood Pressure; D003326:Coronary Circulation / Q000502:physiology*; D023921:Coronary Stenosis / Q000175:diagnosis* / Q000000981:diagnostic imaging; D003331:Coronary Vessels / Q000502:physiology / Q000503:physiopathology; D003935:Diagnostic Techniques, Cardiovascular; D016014:Linear Models; D008833:Microcirculation / Q000000981:diagnostic imaging / Q000502:physiology*; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D013552:Swine; D018084:Ultrasonography, Interventional; D014655:Vascular Resistance / Q000502:physiology*