The index of microcirculatory resistance (IMR) appears to be a better independent predictor of acute and short-term myocardial damage in patients undergoing percutaneous coronary intervention (PCI) for ST-segment elevation myocardial infarction (STEMI), according to the Feb. 5 issue of the Journal of the American College of Cardiology.
The index of microcirculatory resistance (IMR) is a new measure of microvasculature function using a pressure sensor/thermistor-tipped guidewire.
William F. Fearon, MD, from the division of cardiovascular medicine at Stanford University Medical Center in Stanford, Calif., and colleagues, measured the IMR in 29 patients after undergoing primary PCI for STEMI.
After successful stenting of the culprit lesion, a coronary pressure wire from Radi Medical Systems in Uppsala, Sweden, was calibrated outside the body, equalized to the pressure reading from the guide catheter with the pressure sensor positioned at the ostium of the guide catheter, and then advanced to the distal two-thirds of the culprit vessel, which in the vast majority was beyond the stented region, the authors wrote.
Through this process, the thrombolysis in MI (TIMI) myocardial perfusion grade, TIMI frame count, coronary flow reserve, and ST-segment resolution were also recorded, according to the researchers.
Fearon and colleagues reported that the IMR was achieved by multiplying the mean distal coronary pressure by the hyperemic transit time (mm Hg x s, or U).
The researchers said that IMR correlated considerably with the peak creatinine kinase (CK), while the others measures of microvascular dysfunction did not. In patients with an IMR greater than the median value of 32 U, their peak CK was significantly higher compared with those having values ≤32 U (3,128 ng/ml vs. 1,201 ng/ml), the authors wrote.
The IMR also correlated significantly with three-month echocardiographic wall motion score (WMS), while the other measures of microvascular function did not, according to researchers. They found that the WMS at three-month follow-up was significantly worse in the group with an IMR >32 U compared with ≤32 U (28 vs. 20).
Fearon and colleagues found that on multivariate analysis, IMR was the strongest predictor of peak CK and 3-month WMS. The IMR was the only significant predictor of recovery of left ventricular function on the basis of the percent change in WMS, the researchers concluded.