This study was undertaken to compare the efficacy of retrograde cardioplegia for myocardial perfusion with that of antegrade cardioplegia at the same flow rate.Colored microspheres were used in rat hearts to assess the capillary flow of cardioplegia solution. Myocardial perfusion was evaluated with magnetic resonance imaging in pig hearts. Phosphorus 31 magnetic resonance spectroscopy was used to determine the efficacies of the cardioplegic techniques in sustaining myocardial energy metabolism.At the same flow rate, the number of colored microspheres delivered to the capillaries by retrograde cardioplegia (15 +/- 1 microspheres/mm2) was significantly lower than that delivered by antegrade cardioplegia (29 +/- 2 microspheres/mm2). Furthermore, only 19% +/- 3% of the colored microspheres delivered to the capillaries by retrograde cardioplegia were found in the arteriolar portions of the capillaries, whereas most (80% +/- 3%) remained in the venular portions. Moreover, magnetic resonance images showed that contrast-enhanced signal-time courses obtained from different regions of the myocardium during retrograde cardioplegia varied significantly. Localized phosphorus 31 spectra showed that retrograde cardioplegia required a higher flow rate than did antegrade cardioplegia to sustain normal myocardial energy metabolism.We conclude that retrograde cardioplegia provides significantly less capillary flow than does antegrade cardioplegia. Its microvascular perfusion varies significantly among the various small areas of the myocardium. As a result, its efficacy in sustaining normal myocardial energy metabolism is lower than that of antegrade cardioplegia.