Transmural redistribution of coronary resistance during embolization: a clue to intramyocardial small artery architecture.

The aim of this study is to assess vascular architecture and the single vessel resistance distribution of the subepicardial (epi) and subendocardial (endo) layers of the left ventricular wall. For this purpose, coronary embolization was performed by injecting three boluses of 25 microns plastic microspheres into the maximally vasodilated left circumflex artery of six dogs; transmural blood flow was assessed by 15-microns radioactive microspheres. A branching tree model of the intramyocardial small artery system--whose general characteristics were described in a previous study (Pelosi et al., 1987, Microvasc. Res. 34, 318-335)--was developed and its parameters were tested against the increase in epi and endo coronary resistance observed during embolization. Embolization produced a significantly greater increase in epi resistance after each bolus injection (from 10.3 +/- 1.1 to 195.6 +/- 32.5 mm Hg/ml/min/g after the last bolus, P less than 0.001) as compared to endo resistance (from 10.3 +/- 2.0 to 56.5 +/- 8.2 mm Hg/ml/min/g after the last bolus, P less than 0.001); as a consequence, the epi/endo resistance ratio increased from 1.09 +/- 0.11 to 1.50 +/- 0.13, 2.48 +/- 0.51, and 3.66 +/- 0.42 (P less than 0.05 for all embolizing injections). The experimental relation (occlusion function) between the number of embolizing beads injected and the epi and endo resistance values was used to define the parameters of the epi and endo branching tree model. Assuming that the embolizing microspheres lodge in vessels of similar resistance (that is similar diameter and length) in the two layers, the experimental results indicate that the intramyocardial small artery system can be depicted as a symmetric dichotomous branching tree with twofold more terminal vessels in the endo than in the epi.