Myocardial resistance assessed by guidewire‐based pressure‐temperature measurement: In vitro validation

Abstract

By injecting a few cubic centimeters of saline into the coronary artery and using thermodilution principles, mean transit time (T mn ) of the injectate can be calculated and is inversely proportional to coronary blood flow. Because microvascular resistance equals distal coronary pressure (P d ) divided by myocardial flow, the product P d ⅐ T mn 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 flow was varied from 42 to 203 ml/min and TMR varied from 0.39 to 1.63 dynes ⅐ sec/cm 5 . Inverse mean transit time correlated well to absolute blood flow (R 2 ‫؍‬ 0.93). Furthermore, an excellent correlation was found between IMR and TMR (R 2 ‫؍‬ 0.94). IMR was independent on the severity of epicardial stenosis and thus specific for myocardial resistance. Thus, using one single guidewire, both fractional flow 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.

Epicardial Stenosis Severity Does Not Affect Minimal Microcirculatory Resistance

“…As demonstrated previously, the inverse of T mn strongly correlates to absolute coronary blood flow. [11][12][13] Therefore, in the absence of an epicardial stenosis and collateral flow, IMR is equal to the product of P d and T mn at maximum hyperemia and correlates well to TMR both in vitro and in animals. 10,11 With an epicardial stenosis, however, accurate determination of IMR requires knowledge of coronary wedge pressure.…”

“…It is called the index of microcirculatory resistance (IMR). 10,11 IMR is calculated from the simultaneous measurement of distal coronary pressure and thermodilution-derived mean transit time (T mn ) of a bolus of saline injected at room temperature into the coronary artery during maximal hyperemia. As demonstrated previously, the inverse of T mn strongly correlates to absolute coronary blood flow.…”

Impact of Smoking on Coronary Microcirculatory Resistance in Patients With Coronary Artery Disease

“…Coronary physiology measurements: Coronary physiology measurements were performed for all intermediate stenoses, as described previously. 8,[14][15][16][17][18][19] All patients received an intravenous bolus injection of 5,000 IU of heparin and intracoronary isosorbide dinitrate (1-2 mg) before angiography. A coronary 0.014-inch pressure-temperature sensor guidewire (Certus, St. Jude Medical, St. Paul, MN, USA) threaded through 5 Fr diagnostic catheters was used for measurements after a diagnostic coronary angiogram (CAG) (Figure 1).…”

“…8,9) The method of calculating IMR by estimating FFR cor enables easy evaluation of both epicardial stenosis severity and microvascular functions for intermediate coronary stenoses. In this study, 27 patients (27.8%) with FFR myo < 0.80 required incorporation of the estimated coronary FFR cor without P w measurement to calculate IMR.…”

“…7) IMR has been found to be a reproducible index and to be independent of epicardial stenosis severity. [7][8][9][10] The aim of this study was to investigate the relationship between microvascular function and smoking in patients with CAD by comparing 3 coronary physiology parameters, fractional flow reserve (FFR), coronary flow reserve (CFR), and IMR, between patients with and without a smoking history.…”