By injecting a few cubic centimeters of saline into the coronary artery and using thermodilution principles, mean transit time (Tmn) of the injectate can be calculated and is inversely proportional to coronary blood ow. Because microvascular resistance equals distal coronary pressure (Pd) divided by myocardial ow, the product Pd Tmn provides an index of myocardial resistance (IMR). In this in vitro study in a physiologic model of the coronary circulation, we compared IMR to true myocardial resistance (TMR) at different degrees of myocardial resistance and at different degrees of epicardial stenosis. Absolute blood ow was varied from 42 to 203 ml/min and TMR varied from 0.39 to 1.63 dynes sec/cm5. Inverse mean transit time correlated well to absolute blood ow (R2 0.93). Furthermore, an excellent correlation was found between IMR and TMR (R2 0.94). IMR was independent on the severity of epicardial stenosis and thus speci c for myocardial resistance. Thus, using one single guidewire, both fractional ow reserve and IMR can be measured simultaneously as indexes of epicardial and microvascular disease, respectively, enabling separate assessment of both coronary arterial and microvascular disease.